Assessment of HF Sonar Performance from a Surface Ship · DRDC-RDDC-2016-R053 i Abstract This Scientific Report presents a quantitative analysis of sea-trial data assessing the target

Assessment of HF Sonar Performance from a Surface Ship

Mark V. Trevorrow DRDC – Atlantic Research Centre

Defence Research and Development Canada

Scientific Report

DRDC-RDDC-2016-R053

April 2016

IMPORTANT INFORMATIVE STATEMENTS

This work was conducted under DRDC Project 01CC (torpedoes and torpedo defence). DRDC and DFO

jointly supported use of the CCGS Vector for these sea-trials.

Template in use: (2010) SR Advanced Template_EN (051115).dotm

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2016

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale,

2016

DRDC-RDDC-2016-R053 i

Abstract

This Scientific Report presents a quantitative analysis of sea-trial data assessing the target detection

performance of a High-Frequency (HF, 100 kHz) horizontally-oriented, multibeam sonar from a

surface ship. Forward-looking target detection tests were conducted in two different locations,

featuring upward- and downward-refracting sound speed profiles under sea-states from 0 to 4. Ship

wake effects were evaluated in aft-looking tests with a towed target at ship speeds of 6–7.5 knots.

The HF sonar output was calibrated so that absolute reverberation levels and acoustic target

strengths could be assessed. In forward-looking tests the targets were detected at ranges up to

580 m in low sea-states, but detection range was reduced to roughly 150–300 m under higher sea

states. In the aft-looking sonar tests the target was detectable up to 450 m range. In both cases

signal to noise ratios of 10–25 dB (range dependent) were routinely observed. Near-surface sound

speed gradients were found to produce important acoustic propagation effects in both cases.

Significant ping-to-ping variability in both target echo strength and background reverberation

levels was observed in all tests. A HF sonar performance modeling tool generated predictions in

reasonable agreement with the sea-trial results.

Significance to Defence and Security

A specific assessment of the performance of HF (~100 kHz) active sonars for torpedo detection

and tracking in the vicinity of a ship was planned under the DRDC Torpedo Defence project. The

sonar utilized in these sea-trials has some of the characteristics of systems which might be

employed for close-range target detection and tracking purposes from a ship. Similar sonars have

been previously developed for forward-looking obstacle avoidance and harbour surveillance

purposes.

ii DRDC-RDDC-2016-R053

Résumé

Le présent rapport contient une analyse quantitative des données d’essais en mer pour évaluer la

performance de détection de cibles au moyen d’un sonar multifaisceaux haute fréquence

(100 kHz), orienté horizontalement, à bord d’un navire de surface. Des essais de détection

frontale de cibles ont été réalisés à deux endroits différents, révélant des profils de vitesse du son,

à réfraction vers le haut et à réfraction vers le bas, dans des états de mer de 0 à 4. Les effets de

sillage du navire ont été évalués lors d’essais de détection arrière, sur une cible remorquée, à des

vitesses de 6 à 7,5 noeuds. La sortie du sonar HF a été étalonnée de manière à ce que les niveaux

de réverbération absolue et l’indice de réflexion acoustique puissent être évalués. Lors des essais

de détection frontale, les cibles ont été détectées à une distance pouvant atteindre 580 m lorsque

la mer était calme, mais la portée de détection était réduite à près de 150 à 300 m lorsque la mer

était agitée. Pendant les essais de sonars à balayage arrière, la cible était détectable jusqu’à une

distance de 450 m. Dans les deux cas, on a couramment observé un rapport signal-bruit de 10 à

25 dB (en fonction de la distance). Des gradients son vitesse, près de la surface, ont eu

d’importants effets sur la propagation acoustique dans les deux cas. On a également observé une

variabilité « ping-à-ping » importante du niveau de l’écho des deux cibles et du niveau de

réverbération à chaque essai. Un outil de modélisation du rendement du sonar HF a généré des

prévisions montrant une concordance raisonnable avec les résultats obtenus lors des essais en

mer.

Importance pour la défense et la sécurité

Une évaluation spécifique du rendement des sonars actifs HF (~100 kHz) pour la détection et la

poursuite de torpilles à proximité d’un bâtiment a été prévue dans le cadre du projet de défense

antitorpilles de RDDC. Le sonar utilisé pour ces essais en mer possède certaines caractéristiques

des systèmes qui pourraient être employés à des fins de détection et de poursuite de cibles à

courte distance d’un navire. Des sonars similaires ont déjà été mis au point à des fins d’évitement

d’obstacles frontaux et pour la surveillance des zones portuaires.

DRDC-RDDC-2016-R053 iii

Table of Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Significance to Defence and Security . . . . . . . . . . . . . . . . . . . . . . i

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Importance pour la défense et la sécurité . . . . . . . . . . . . . . . . . . . . ii

Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Sea-Trials, Instrumentation, and Analysis Approach . . . . . . . . . . . . . . 2

2.1 Overview of Sea-Trials . . . . . . . . . . . . . . . . . . . . . . 2

2.2 HF Multibeam Sonar . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Sonar Data Analysis Methods . . . . . . . . . . . . . . . . . . . . 7

3 Forward-Looking Sonar . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Sonar Data Examples . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.1 Saanich Inlet at Low Sea-State . . . . . . . . . . . . . . . . . 10

3.1.2 Strait of Georgia at Sea-State 3 . . . . . . . . . . . . . . . . . 13

3.1.3 Strait of Georgia at Sea-State 4 . . . . . . . . . . . . . . . . . 14

3.1.4 Summary Results . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Transmission Loss Effects . . . . . . . . . . . . . . . . . . . . . 18

3.3 Variability of Target Echoes . . . . . . . . . . . . . . . . . . . . 19

3.4 Propeller Cavitation Effects . . . . . . . . . . . . . . . . . . . . . 20

4 Aft-Looking Sonar . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.1 Sonar Data Examples . . . . . . . . . . . . . . . . . . . . . . . 22

4.1.1 Fixed-Range Towing . . . . . . . . . . . . . . . . . . . . 23

4.1.2 Target Pull-Ins . . . . . . . . . . . . . . . . . . . . . . . 25

4.1.3 Target Detection During S-Turns . . . . . . . . . . . . . . . . 26

4.2 Wake Backscatter and Extinction . . . . . . . . . . . . . . . . . . . 28

4.3 Target Echo Variability . . . . . . . . . . . . . . . . . . . . . . 29

5 Modeling HF Sonar Performance . . . . . . . . . . . . . . . . . . . . . 31

6 Summary Discussions . . . . . . . . . . . . . . . . . . . . . . . . . 37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

HF Sonar Calibrations . . . . . . . . . . . . . . . . . . . . . . 43 Annex A

A.1 Sonar Self-Noise Levels . . . . . . . . . . . . . . . . . . . . . . 44

A.2 Clipping Levels . . . . . . . . . . . . . . . . . . . . . . . . . 45

A.3 Transmitter Source Levels . . . . . . . . . . . . . . . . . . . . . 46

iv DRDC-RDDC-2016-R053

A.4 Transmitter Horizontal and Vertical BeamWidths . . . . . . . . . . . . . 46

A.5 Echo Strength from Reference Target Spheres . . . . . . . . . . . . . . 48

A.6 Receiver Bandwidth . . . . . . . . . . . . . . . . . . . . . . . 50

List of Symbols/Abbreviations/Acronyms/Initialisms . . . . . . . . . . . . . . . 52

DRDC-RDDC-2016-R053 v

List of Figures

Figure 1: Map showing the two sea-trial locations (from www.google.ca/maps). . . . . 2

Figure 2: Comparison of measured sound speed profiles from Saanich Inlet and Strait of

Georgia locations. . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 3: Sketch of (upper) forward-looking and (lower) aft-looking sonar geometry.

Surface waves are omitted for clarity. . . . . . . . . . . . . . . . . . 4

Figure 4: Sonar targets (left). Steel sphere with chain ballast, (right) Sharko towbody. . 5

Figure 5: Photograph of the SM-2000 sonar mounted on the sonar strut in the aft-looking

orientation. The strut pivots outboard and down to lock into a vertical position. 6

Figure 6: Forward-looking HF sonar image to 600 m range, 1711Z April 9 in Saanich

Inlet. Wind speed = 2.5 knots. Range rings are in 120 m increments. Arrow

denotes location of big target sphere, suspended at 4 m depth. . . . . . . . 11

Figure 7: Comparison of target echo and background intensity (dB re sonar units) vs.

range for approach run against the big sphere starting 1710Z April 9 in

Saanich Inlet. Dashed lines show expected -20log10[r] variation in target echo

and sonar clipping and noise thresholds. . . . . . . . . . . . . . . . . 12

Figure 8: Apparent Target Strength (dB re m2) vs. range for approach run against the big

sphere starting 1710Z April 9 in Saanich Inlet. Dashed line shows sonar

clipping threshold. . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 9: Forward-looking HF sonar image to 400 m range, 1735Z April 11 in Strait of

Georgia. Wind speed = 12 knots. Range rings are in 80 m increments. Arrow

denotes location of large target sphere, suspended at 4 m depth. . . . . . . 13


range for run against the big sphere starting 1734Z April 11 in Strait of

Georgia. Dashed lines show expected -20log10[r] variation in target echo and

sonar clipping and noise thresholds. . . . . . . . . . . . . . . . . . . 14



denotes location of large target sphere suspended at 1 m depth. . . . . . . . 15


range for run against the big sphere starting 2108Z April 12 in the Strait of

Georgia. Dashed lines show expected -20log10[r] variation in target echo and

sonar clipping threshold. . . . . . . . . . . . . . . . . . . . . . . 16

Figure 13: Summary of maximum detection ranges from all forward-looking sonar tests

with big and small spheres in Saanich Inlet and Strait of Georgia, April 9–12.

Best fit line to data where wind > 6 knots is shown. . . . . . . . . . . . 17

Figure 14: Comparison of averaged Reverberation Level (dB re 1 µPa) vs. range for runs

against the big sphere in Saanich Inlet and Strait of Georgia on April 9, 11,

and 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

vi DRDC-RDDC-2016-R053

Figure 15: Comparison of apparent TS (dB re m2) vs. range for big sphere on April 9, 11,

and 12 under wind speeds of 2.5, 12, and 16 knots, respectively. Data are

10-pt. adjacent averaged. Dashed line shows estimated true TS of -14.0 dB.

Target depths were 4 m on April 9 and 11 and 1 m on April 12. . . . . . . 19

Figure 16: Frequency distribution of apparent TS over all ranges for detection run against

the big sphere starting 1710Z April 9 in Saanich Inlet. Best fit Gaussian

distribution (mean = -8.1 dB, width = 14.8 dB, r2 = 0.926) shown in red line. . 20

Figure 17: Forward-looking HF sonar image to 600 m range, 2118Z April 9 in Saanich

Inlet during period of ship acceleration from 7.5 to 8 knots. Range rings are in

120 m increments. Arrows denote bands believed to be blade rate modulations. 21

Figure 18: Summary of Sharko depths during aft-looking HF sonar tests April 13 in

Saanich Inlet. Black lines show approximate -3 dB limits of sonar vertical

beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 19: Aft-looking HF sonar image to 200 m range, 1828Z April 13 in Saanich Inlet.

Range rings are 40 m increments. Arrow denotes location of Sharko target, at

16 m depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 20: Temporal variation in target echo and background intensity for stationary tow

at 220 m range in ship wake starting 1618Z April 13 in Saanich Inlet. Ship

speed during this period was 7.2 knots. . . . . . . . . . . . . . . . . 24

Figure 21: Apparent target strength (dB re m2, black squares) and SNR (dB, red circles)

vs. range from all fixed-range towing tests, April 13 in Saanich Inlet. . . . . 24

Figure 22: Comparison of averaged target, background, and SNR vs. range from six

separate pull-in runs in arbitrary sonar units. Dashed lines show -20log10[r]

and -30log10[r] variations. . . . . . . . . . . . . . . . . . . . . . . 25

Figure 23: Apparent target strength (dB re m2) estimated from average of six separate

pull-in runs. Red line shows best fit line (intercept = -19.0 dB,

slope = 0.007 dB/m). . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 24: Aft-looking HF sonar image to 300 m range during S-Turn, 1626Z April 13 in

Saanich Inlet. Range rings are 60 m increments. Arrow denotes location of

Sharko target. . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 25: Apparent target strength (dB re m2) vs. time following Sharko towed target at

210 m range through series of S-Turns, 1624Z-1630Z April 13 in Saanich

Inlet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 26: Comparison of in-wake and non-wake reverberation level vs. range profiles at

two speeds, taken near 1800Z April 13 in Saanich Inlet. . . . . . . . . . 29

Figure 27: Distributions of apparent target strength during fixed-range towing at 220 m

range 1618Z April 13. Best fit Gaussian distribution (mean = -20.2 dB,

width = 4.0 dB, r2 = 0.994) shown in red line. . . . . . . . . . . . . . . 30

Figure 28: Predicted sonar SE vs. target range and depth for the April 9/10 Saanich Inlet

conditions, with assumed wind speed of 5 knots. Target TS = -14 dB. . . . . 33

DRDC-RDDC-2016-R053 vii

Figure 29: Predicted sonar SE vs. target range and depth under April 11 Strait of Georgia

conditions, with assumed wind speed of 10 knots. Target TS = -14 dB. . . . 33

Figure 30: Predicted sonar Pd vs. range at 4 assumed wind speeds for a 1 m deep, -14 dB

target under Strait of Georgia conditions. . . . . . . . . . . . . . . . 34

Figure 31: Predicted sonar SE vs. target range and depth under April 13 Saanich Inlet

conditions, with assumed wind speed of 2 knots. Target TS = -19 dB. Black

line shows approximate target depth vs range. . . . . . . . . . . . . . 35

Figure 32: Comparison between measured wake RL vs. range, taken 1758Z April 13 in

Saanich Inlet, with ESPRESSO prediction at wind speeds of 10, 15, and

20 knots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure A.1: Comparison of sonar self-noise intensity (dB re sonar units) at various sonar

range settings. TVG given in Eq.(A.1) with -92 dB and -82 dB offsets. . . . 44

Figure A.2: Comparison of sonar self-noise intensity (dB re sonar units) at various gain

offsets for the 600 m sonar range setting. . . . . . . . . . . . . . . . 45

Figure A.3: Measured SM-2000 vertical beam-pattern (normalized), with reference curve

based on simple line-array with length of 66 mm. . . . . . . . . . . . . 47

Figure A.4: Measured SM-2000 horizontal beam-pattern (normalized) at medium and high

transmit power. . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Figure A.5: Analytic prediction of Tungsten-Carbide sphere backscatter target strength vs.

frequency for two sphere diameters. . . . . . . . . . . . . . . . . . 49

Figure A.6: Example target echo strength parameters extracted from a 90 sweep using the

60 m sonar range setting. Target sphere was located at 14.3 m range. . . . . 50

Figure A.7: Measured variation in receive response vs. frequency for default

high-resolution 600 m setting. . . . . . . . . . . . . . . . . . . . . 51

viii DRDC-RDDC-2016-R053

List of Tables

Table 1: SM-2000 sonar settings used in these sea-trials. Acoustic and sample

resolution based on sound speed of 1480 m/s. . . . . . . . . . . . . . . 7

Table 2: ESPRESSO input parameters for the SM-2000 sonar models. . . . . . . . 32

Table 3: ESPRESSO predictions of SM-2000 sonar detection ranges. . . . . . . . . 36

Table A.1: Measured sonar amplitudes and estimated transmit source levels. . . . . . . 46

Table A.2: Summary of SM-2000 sonar calibration parameter, using Eq.(A.2). . . . . . 50

DRDC-RDDC-2016-R053 1

1 Introduction

This work explores the capabilities and limitations of high-resolution, hull-mounted sonar for

detection and tracking of near-surface targets. The intended Naval applications generally require

that targets must be detected and tracked at horizontal ranges up to roughly 500 m around the ship

with spatial resolution of order 1 m and update rates near 1 Hz. In order to achieve high spatial

and temporal resolution, use of relatively High Frequency (HF: 10 to 100 kHz) multibeam sonar

systems is required. Such sonars have been previously utilized for seabed mapping, obstacle

avoidance, and harbour surveillance purposes.

The use of HF sonar in this manner is subject to significant environmental constraints. Seabed

reverberation and clutter is an obvious problem in shallow waters. Additionally, previous work

has suggested that the performance of high-resolution active sonars near the ocean surface may be

strongly limited by sea-state conditions and ship wakes [1, 2]. HF sonars are highly sensitive to

the presence of small (10–100 µm radius) air bubbles, which are created by both natural processes

(e.g., due to surface wave-breaking) and in ship wakes. These bubbles generate both strong

acoustic backscattering, which raises the background reverberation levels, and strong acoustic

extinction, which reduces the target echo strength. Several additional physical effects also

complicate the target detection process, namely:

acoustic refraction by near-surface sound speed gradients,

interference between direct and surface-reflected acoustic paths, and

modulation and possible shadowing of surface-reflected acoustic paths by surface waves.

All of these effects are accentuated in the near-horizontal, small surface grazing-angle geometry

of this sonar application.

A focused sonar detection performance sea-trial was conducted in order to assess the impact of

these difficulties. These sea-trials were conducted in relatively deep waters in order to isolate the

near-surface constraints. A new 90 kHz multibeam active sonar was acquired specifically for

these tests. This sea-trial was held April 8 to 14, 2015 based on the ship CCGS VECTOR

operating in B.C. south coast waters [3]. This one-week sea-trial was a collaboration between

DRDC Atlantic Research Centre (Dept. of National Defence) and Institute of Ocean Sciences

(IOS), which is part of the Dept. Fisheries and Oceans (DFO). These tests took advantage of a

unique retractable sonar strut mounted on the CCGS VECTOR. The HF multibeam sonar was

mounted in both forward- and aft-looking configurations. Drifting and towed acoustic targets

were constructed and deployed for these tests.

This Scientific Report is focused on quantitative sonar performance, investigating sonar signal

and noise characteristics under a variety of conditions. Specifically tests were conducted under

sea-states from 0 to 4 and with both upward- and downward-refracting sound speed profiles. Ship

wake effects were evaluated at ship speeds up to 7.5 knots. The HF sonar output was calibrated so

that absolute reverberation levels and acoustic target strengths could be assessed. Finally, a sonar

performance modeling tool was validated against some of the sea-trials results.

2 DRDC-RDDC-2016-R053

2 Sea-Trials, Instrumentation, and Analysis Approach

This section briefly describes the sonar sea-trials and HF multibeam sonar, followed by details on

the post-analysis approach. A detailed summary of the sea-trials is presented in [3]. The brief

descriptions herein are intended to provide context for the analysis of results and sonar modeling

to be reported in the following three sections. Sonar results in the post-sea-trial report [3] were

based solely on sonar data replay without any follow-on quantitative analysis. Following the

sea-trials, acoustic calibration measurements were performed on the HF multibeam sonar in order

to verify the sonar characteristics, quantify performance limitations, and provide quantitative

acoustic output. The HF sonar calibration results are reported in Annex A.

2.1 Overview of Sea-Trials

The sea-trials were conducted at two locations on the southern coast of British Columbia, as shown in

Figure 1. The Saanich Inlet site was just west of the IOS facilities at Patricia Bay, B.C. Saanich Inlet

was chosen for its calm (wind < 10 knots), relatively deep water (180–220 m) conditions with

minimal shipping traffic. The limited fetch prevented development of any significant wave height,

particularly for wind directions across the inlet. Tests were conducted in Saanich Inlet on April 9, 10,

and 13. The southern Strait of Georgia operating area was northeast of Galiano Island in waters

180–250 m deep. The Strait of Georgia location featured higher winds (typically 8–16 knots) and

Sea-States (SS 3 to 4, significant wave heights 1–2 m), providing a more demanding test of the

forward-looking sonar geometry. Tests were conducted in the Strait of Georgia on April 11 and 12. In

the Strait of Georgia location the wind direction was from the SW, and so seas were fetch-limited in

the lee of Galiano Island. The distance from shore was greater than 5 km.

Figure 1: Map showing the two sea-trial locations (from www.google.ca/maps).


Water property measurements were conducted at various locations in Saanich Inlet, Strait of Georgia,

and the Canadian Gulf Islands during these sea-trials in support of a DFO monitoring program.

Figure 2 shows a comparison of sound speed estimated from Conductivity-Temperature-Depth (CTD)

measurements at the sea-trial sites. The two sea-trial locations showed distinctly different near-surface

(< 20 m depth) water properties. The Saanich Inlet location exhibited nominally downward-refracting

conditions whereas the Strait of Georgia location had upward-refracting conditions. All three Saanich

Inlet profiles were taken at the same location, within a few kilometers of the location of the sonar tests.

In Saanich Inlet there is some variation in the near-surface sound speed profiles, but they are

essentially identical below 40 m (profiles extend down to > 200 m). Meteorological measurements

were made with a ship-based system, corrected for ship speed and heading (see [3] for details). No

independent measurement of Sea State (e.g., measurement of significant wave height) was conducted.

Figure 2: Comparison of measured sound speed profiles from

Saanich Inlet and Strait of Georgia locations.

The sea-trials ship, CCGS Vector, is 40 m length overall with displacement of 515 tonnes and top

speed of 12 knots. These sea-trials took advantage of a retractable sonar strut mounted on the

ship’s starboard side, allowing deployment of the HF multibeam sonar at a depth of 3.5 m in

either a forward- or aft-looking horizontal orientation. A Kongsberg Mesotech SM-2000 sonar

system operating at 90 kHz was utilized for these sea-trials (to be described in greater detail in

Section 2.2 below). Ship position, speed, and heading (from Global Positioning System—GPS) as

well as sonar heading (from a gyrocompass) were recorded along with the raw sonar data.

The experimental geometry is sketched in Figure 3. Because of the near-horizontal sonar

geometry there will be generally both direct and surface-reflected acoustic paths from sonar to

target (and back). Both cases will experience strong backscattered reverberation from surface

waves and sub-surface bubble clouds. In the forward-looking tests the target depth was 1 or 4 m,

putting it within or close to the bubble layer. In the aft-looking tests the target was generally

below the ship wake. In both cases the surface-reflected path is subject to scattering and

absorption by bubbles relative to the direct path. In the presence of surface waves, the reflection

point will change its location depending on the local surface slope. Because the experimental sites

were chosen in relatively deep waters any seabed reflections can be ignored.


Figure 3: Sketch of (upper) forward-looking and (lower) aft-looking

sonar geometry. Surface waves are omitted for clarity.

The forward-looking sonar tests examined the detectability of drifting near-surface targets at low

and medium sea states. These tests were conducted in both Saanich Inlet and the southern Strait

of Georgia. The targets were two hollow steel spheres, 91 cm and 71 cm diameter (denoted big

and small, respectively), ballasted to submerge them just below the surface (see Figure 4, left).

The target depth was usually 4 m, except for all tests on April 12 in the Strait of Georgia where it

was 1 m. Sonar data was recorded as the ship made a series of approach runs near the targets at

differing approach headings relative to the wind/sea. The typical Closest Point of Approach

(CPA) was between 15 m and 50 m. The typical ship speed during these approach runs was 3 to

5 knots under sea state conditions ranging from 0 to 4. Over the 4 days of forward-looking tests a

total of 40 runs were made against one or more targets in each run.

The sea-trial summary report [3] made predictions of the rigid Target Strength (TS) of the two

target spheres; specifically -15.0 dB and -12.8 dB for the small and large spheres, respectively.

While these predictions were believed to be approximately valid, it was expected that errors

would arise from the assumption of rigid scattering and echo contributions from the steel chain

ballast (see Figure 4, left). Therefore this report will utilize the sonar calibrations to produce in

situ estimates of the target strengths as part of the overall assessment.

The aft-looking sonar tests were designed to test target detection in the vicinity of the ship wake.

The aft-looking sonar tests were only conducted under calm conditions in Saanich Inlet on

April 13. For these tests the SM-2000 sonar was simply rotated to face aft on the sonar strut.

Unfortunately, for reasons unknown the sonar strut was deployed on April 13 with a

9 counter-clockwise rotation. This posed no difficulty in conduct of the sea-trials or

post-processing because the wake and towed target remained fully within the sonar 90 horizontal

aperture.


Figure 4: Sonar targets (left). Steel sphere with chain ballast, (right) Sharko towbody.

A newly designed acoustic target, nicknamed Sharko (see Figure 4, right), was towed at distances

between 20 and 450 m behind the ship at speeds of 6 to 7.5 knots. Sharko was 2.41 m in length

and 41 cm in diameter (excluding fins) with a weight of approximately 135 kg (in air). It was

internally freely-flooding. Sharko had an internally-recording pressure sensor to determine its

depth. At tow speeds of 6 to 7.5 knots the typical Sharko depth was between 6 and 25 m,

dependent on cable scope. Two types of tests were conducted: fixed-range, where the target was

held at a constant cable scope for periods of 5 minutes, and target pull-ins, where the tow-rope

was reeled-in at a speed of 2 m/s (3.9 knots) starting from approximately 220 m range.

Sharko carried internally two 30 cm diameter target spheres (SonarBell), each of which was

specially designed to have Target Strength (TS) near -6 dB at frequencies near 10 kHz. The TS at

90 kHz was not known, but was expected to be closer to the well-known geometric TS (e.g., from

[4]) of 10log10[radius2/4] = -22.5 dB (re m

2). It was expected that backscatter contributions from

other parts of the Sharko tow-body (e.g., a stainless steel bulkhead plate immediately aft of the

nose cone) would make some contribution at these frequencies. Owing to time and logistical

constraints, independent Sharko TS measurements have yet to be conducted.

In the forward-looking sonar tests there were two additional targets, an empty oil drum and an

acoustic bubble measurement device called the Resonator. The barrel was only used on April 9,

and hence did not provide a comparison between sites. The Resonator was routinely observed in

the sonar data, however its echo strength was found to be highly variable and inconsistent.

Neither of these is used in this present work.

2.2 HF Multibeam Sonar

DRDC acquired a new HF multibeam active sonar, a Kongsberg Mesotech SM-2000 system, for

these sea-tests. The SM-2000 system consists of a sonar head, a power and data telemetry

interface unit, and a sonar processing, display, and data storage computer. The sonar head was

mounted on a sonar strut over the side of the ship (see Figure 5). The strut allowed deployment of

the sonar head at 3.5 m depth in either a forward-looking or aft-looking orientation. It was found

that the sonar strut with the SM-2000 head remained stable at ship speeds up to 8 knots under

calm sea conditions. The sonar head was connected to dry-end electronics (inside the ship’s


science lab) via a 30 m umbilical cable. The processor was also setup to accept and record inputs

from both a gyrocompass (TSS Meridian Surveyor) and GPS.

The SM-2000 sonar operates at 90 kHz, providing 128 overlapping 1.8 wide beams covering an

angular aperture of 90. The vertical beamwidth (full width to -3 dB) is 13.5 on both transmit

and receive. The sonar operates by transmitting over the full horizontal aperture (90) then

beam-forming the received echoes. The maximum operating range is 800 m; reduced range

settings were used to decrease the ping interval and improve range resolution. The nominal

transmit source level (given by the manufacturer) is 205 dB (re 1 µPa at 1 m). The sonar allows

some control over transmit power, pulse lengths, receiver time-varying gain, and receiver

bandwidth. These tests utilized sonar range settings between 200 m and 800 m, pulse lengths

between 0.5 and 1 ms, and pulse intervals between 0.5 and 1 s. Laboratory measurements on the

sonar as part of the acoustic calibrations (see Annex A) generally confirmed the manufacturer

specifications.

Figure 5: Photograph of the SM-2000 sonar mounted on the sonar strut in the aft-looking

orientation. The strut pivots outboard and down to lock into a vertical position.

During all these sea-trials the SM-2000 was configured to provide maximum transmit power and

best range-resolution for a given range setting. The sonar parameters corresponding to the various

range settings are shown in Table 1. Sonar range settings from 200 to 600 m were in the

sea-trials. The varying receiver bandwidth is presumably set to minimize internal noise

limitations in the sonar. This bandwidth constraint also slightly reduced the effective range

resolution, e.g., at the 600 m setting the 800 Hz bandwidth corresponds to an effective pulse

length of 1.25 ms and hence range resolution of 0.928 m. A fixed sound speed of 1480 m/s (an

average value based on CTD measurements, see Figure 2) was used for all sonar processing and

range calculations.


Table 1: SM-2000 sonar settings used in these sea-trials. Acoustic and

sample resolution based on sound speed of 1480 m/s.

Range

Setting (m)

Ping Rate

(Hz)

Pulse

Length (ms)

Acoustic

Resolution (m)

Sample

Resolution (m)

Receiver

Bandwidth (Hz)

60 2.00 0.165 0.122 0.078 4800

100 2.00 0.220 0.163 0.130 3600

150 2.00 0.330 0.245 0.195 2400

200 2.00 0.385 0.286 0.234 2100

300 2.00 0.550 0.408 0.334 1500

400 1.84 0.660 0.490 0.414 1200

600 1.23 0.990 0.735 0.620 800

800 0.92 1.265 0.939 0.793 600

The SM-2000 Time-Varying Gain (TVG, in dB) was set to the factory default of

20log10[r] + 21r, (1)

where r is the range in metres and 1 is the acoustic absorption, having the value 0.019 dB/m.

Note that this value of 1 under-predicts the expected in situ value, which should be close to

0.0256 dB/m under the sea-water conditions encountered in this trial (using relations found

in [5]). A detailed post-calibration of this sonar was performed, reported in Annex A, which

included verification of this TVG function.

2.3 Sonar Data Analysis Methods

The SM-2000 processor recorded all the raw sonar data along with GPS position and

gyrocompass heading data in a single file. Only the raw, unprocessed sonar element data was

recorded. The SM-2000 then regenerates the sonar images upon playback, thereby providing

some flexibility to adjust beam-forming parameters. The manufacturer provided (at special

request) a MATLAB script to perform the beam-forming.

The sonar data were extracted from the raw sonar recordings using a two-stage process. Firstly

the raw receiver stave data vs. time from a single ping (herein called a snapshot) were extracted

and used in a beamforming algorithm to produce a matrix of sonar amplitude vs. range and

beam-angle. At this stage the amplitudes were in arbitrary sonar units, to be converted to acoustic

units later. These were stored in separate snapshot files. Then various follow-on processing steps

(e.g., beam-wise averaging) were performed on these snapshot files. A fixed sound speed of

1480 m/s was used to calculate sonar range. The sonar beams were assumed to be equally spaced

at 90/128 beams = 0.703 increments, with zero angle in the center of the sonar aperture and

positive angles to starboard. The range increment was dependent on the sonar range setting. The

maximum sonar amplitude after beam-forming was near 2000 (sonar units), presumably derived

from a raw A/D resolution of 12-bits (±2048 units). This implies a maximum sonar echo level

near 66.2 dB (re sonar units).


An analysis routine was written to extract estimates of the target sphere and Sharko echo strength

and Signal to Noise Ratio (SNR) from successive ping snapshots. This was performed on a

sequence of snapshot files using appropriate initial values for target range and bearing. The initial

target location was determined through visual inspection of the sonar images. In each snapshot

the maximum target echo amplitude in the vicinity of the previous estimate was found. This was

assumed to be an estimate of the target echo amplitude. No thresholding was applied. Then the

background amplitude was averaged over a region 20 samples in range by 10 samples in angle

(excluding the central peak due to the target). This corresponds to a region roughly 5 m in range

by 7 in angle. The peak and averaged background echo-amplitudes were stored along with ping

number, range, and bearing. A display of the sonar snapshot with the target region highlighted

was used to visually confirm the target tracking process. Estimates showing significant deviation

in range and/or bearing from the overall trend of the approach run were manually removed.

The acoustic calibration information was used to estimate target strengths and reverberation

levels. The backscattered acoustic Target Strength (TS, dB re 1 m2) is an intrinsic property of the

target, and is generally dependent on frequency and incidence angle. For the sphere or Sharko

targets, the apparent TS was calculated using a sonar equation approach [4], i.e.,

TS = 20log10[A] + K + 40log10[r] + 2r - TVG(r), (2)

where A is the target echo amplitude (sonar units) and K is the calibration coefficient (dB). This

latter parameter is a catch-all that includes sonar source level, sensitivity, fixed gains,

analog-to-digital conversion, and beamforming calculations. Estimated values of K (mildly

dependent on sonar range setting) can be found in Annex A, Table A.2.

Note that Eq.(2) implicitly utilizes a spherical spreading acoustic propagation model and only

includes losses due to seawater absorption. Variations in the measured TS due to non-spherical

acoustic propagation and bubble-induced losses will generate range-dependent variations in the

estimated TS. Non-spherical acoustic propagation and bubble-induced losses are to be expected in

this work due to sea-surface reflections, acoustic refraction due to sound speed gradients, and

bubble-induced losses. Here the term Apparent TS will be used to refer to this measured TS which

includes acoustic propagation effects. By searching for data which do not have appreciable

non-spherical acoustic propagation (e.g., at short range) estimates of the true TS can be made.

The above TS equation also assumes no sonar beam deviation losses. This assumption is

considered generally valid because in these tests the target elevation angles relative to the sonar

were always near zero, and thus less than the ±6.5 vertical beamwidth of the sonar. The

exception to this occurs in the aft-looking tests where the Sharko target was towed at depths up to

25 m, creating depression angles up to 5. Where vertical Beam Deviation Loss (BDL)

corrections are required, a simple line-array model [4] can be used, hence:

BDL() = 20log10[sin(x)/x], where x = (L/)sin() (3)

where is the vertical angle, L is the array effective vertical length (66 mm), and is the acoustic

wavelength (0.0164 m). This is for one-way (either transmission or reception) loss. Additionally,

the calibration results (Annex A) showed a significant reduction in sonar sensitivity near the outer


edges of the beam, hence data from the outer 10 of the sonar horizontal aperture were ignored in

any quantitative analysis.

The sonar Reverberation Level (RL, dB re 1 µPa) can be estimated using a similar sonar equation

approach, i.e.,

RL = SL + 20log10[A] + K - TVG(r). (4)

where SL is the sonar Source Level (dB re µPa at 1 m) verified by the calibrations (see Annex A).

This RL is independent of any geometric spreading or beam-pattern corrections.

Occasional interference due to the ship’s navigational Doppler speed sensor was observed. This

sensor operated at 100 kHz using a broadband, structure pulse. The signature of this in the

SM-2000 data was an isolated, high-intensity ring, approximately 4 m in width, with highest

intensity centered on the bearing intersecting the ship’s bow. Depending on the SM-2000 range

setting (and hence ping rate) this interference ring occurred at varying range in the images. Any

contamination from this in the target and background intensity estimates was either avoided or

manually removed.

Echoes from the ship’s hull were consistently observed in the forward-looking tests. These

dominated the port side of the sonar image up to a strong highlight 25.5 m forward and 4.5 m to

port, believed to be due to the breaking bow wave. In areas of stronger sea-surface reverberation,

an acoustic shadow created by the ship’s hull was also observed. This shadowing was not a

problem for forward-looking target detection because the target spheres were usually kept on the

ship’s starboard side (same as the sonar). Any beam-wise averaging avoided use of shadowed

regions. The ship’s hull was not visible in the aft-looking sonar tests.

Occasionally a direct seabed-reflected echo was observed. This appeared as a narrow band in all

sonar beams at a range equal to the local water depth (approx. 180 to 220 m). This was more

commonly observed in Saanich Inlet. Seabed echoes did not interfere with target measurements,

but do show up in averaged background data.

On occasions when the ship accelerated, strong acoustic interference attributed to propeller

cavitation was observed. This usually disappeared when the ship speed reached a steady-state

speed. This did not pose any operational limitations on the data collection, and was avoided in the

data processing by only choosing data from steady-state ship operations. Note that this type of

interference may (in future) be a problem for higher-ship-speed applications. This will be

investigated in Section 3.4 below.


3 Forward-Looking Sonar

This section will present an overview of the HF sonar target detection results. Recorded sonar

data was replayed and analysed following methods described in Section 2.3. Several example data

sets will be presented below, followed by summary results and investigation of particular features

of the data.

Based on a review of the sonar imagery there were a number of qualitative observations which

generally applied to both the forward- and aft-looking sonar tests:

Under low wind and sea-state conditions the targets were detectable at ranges up to 580 m.

The targets were generally detected and followed using their temporal and location

consistency from ping to ping. On a single-ping basis targets were frequently difficult to

detect due to the presence of strong background clutter with similar intensity and spatial

scale. Target echoes were generally 1–2 m in range dimension by approximately 1–2 in

angular width. As the wind and sea-state increased, targets were detectable only at shorter

ranges, in some cases at ranges less than 200 m. While this suggests wind speed is a

significant factor in target detectability, the effect of near-surface acoustic propagation

enhancement was also found to be important.

Qualitatively the sonar performance appeared to be generally reverberation limited, with

only minor evidence for flow-noise limitations on the sonar. In cases with very low

background reverberation, at longer ranges the sonar background was dominated by internal

electronic noise enhanced by the sonar TVG.

A key characteristic of all the sonar data was strong small-scale variability, both spatially

within each snapshot and temporally from ping to ping. This might be described as

fluctuations or scintillation. Both target echoes and the background reverberation exhibited

this variability, even under calm conditions in Saanich Inlet. Under higher sea-state

conditions (e.g., in Strait of Georgia) these fluctuations appear to be modulated by surface

wave effects.

3.1 Sonar Data Examples

In this sub-section, three examples covering different sea-states and locations will be examined.

These examples will focus on the big target sphere. The results from the small target sphere were

quantitatively very similar.

3.1.1 Saanich Inlet at Low Sea-State

Figure 6 shows an example sonar image from Saanich Inlet, collected under calm conditions

(wind speed < 3 knots). The target was the big (91 cm diameter) sphere at a depth of 4 m. The

ship speed at this time was approximately 2.5 knots. The target is distinctly visible at 530 m range

approximately 5 to the right of the sonar main axis. The sonar target appears elongated in the

angular dimension due to the 1.8 beamwidth; the apparent target width decreases at shorter

ranges. This target could be clearly tracked inwards over a period of 7 minutes to a Closest Point

of Approach (CPA) near 40 m. The large intense scattering features on the right side of the image


are the remnant bubbly wake from a previous pass by the ship. A low-intensity ring

corresponding to the direct seabed echo is visible at 200 m range. While in this particular image

the target has high signal strength relative to the local background, there are other regions of

higher sea-surface backscatter in the image against which the target would not be so easily

detectable. There is also an acoustic shadow due to the ship’s hull on the port (left) side of the

image at ranges less than 200 m.

Figure 6: Forward-looking HF sonar image to 600 m range, 1711Z April 9 in Saanich Inlet.

Wind speed = 2.5 knots. Range rings are in 120 m increments. Arrow denotes

location of big target sphere, suspended at 4 m depth.

Results of the target detection and background extraction process are shown in Figure 7. There

are 490 samples spanning ranges from 49 m to 572 m. The most obvious characteristic of the data

is strong fluctuation in both the target and background estimates. There is also a very high SNR

(expressed in dB), varying between 10 dB (at long ranges) to more than 30 dB in some regions.

The overall average SNR was 22.4 dB. The minimum SNR was approximately 10 dB, which

confirms a commonly-held assumption of the active detection threshold of 10 dB (more on this in

Section 5). The simple target detection process used herein became very unstable when the SNR

dropped below 10 dB. The target intensity roughly follows the expected -20*log10[r] variation,

however there are significant variations which are hypothesized to be due to acoustic convergence

and shadow zones created by reflection and refraction near the sea-surface. This will be

investigated later using acoustic models (Section 5). The background approaches the sonar

self-noise level (which increases due to the TVG) at ranges beyond 500 m. The target echoes also

reach the sonar analog-to-digital clipping limit near 66 dB (sonar units) in a few places.


Figure 7: Comparison of target echo and background intensity (dB re sonar units) vs. range for

approach run against the big sphere starting 1710Z April 9 in Saanich Inlet. Dashed lines show

expected -20log10[r] variation in target echo and sonar clipping and noise thresholds.

Figure 8: Apparent Target Strength (dB re m2) vs. range for approach run against the big sphere

starting 1710Z April 9 in Saanich Inlet. Dashed line shows sonar clipping threshold.

Figure 8 shows the conversion of raw sonar amplitude into Apparent TS (ATS). In the absence of

non-spherical propagation loss effects this should yield a constant value with range (ignoring

fluctuations). However, the ATS clearly shows an increase with range in the nearest 270 m, with

several peaks and drop-outs underlying the short-spatial scale fluctuations. The overall average

ATS is -8.0 dB. It is hypothesized that the true sphere TS (near -14 dB) can be seen from 50–80 m

range (more on this in Section 3.1.4).


3.1.2 Strait of Georgia at Sea-State 3

Figure 9 shows a target detected under somewhat higher sea-state conditions. This example is

taken from the Strait of Georgia on April 11, under winds of 12 knots. This environmental

condition is just at the threshold for breaking wave activity [1], so sub-surface bubble layers are

expected to be weak or negligible. In this example the big sphere target at 4 m depth is located at

275 m range; it was first detected at a maximum range of 390 m. Under these conditions the

sea-surface reverberation was confined to a zone up to roughly 180 m from the sonar, allowing

target detection beyond. As the ship approached the target it became more difficult to detect the

target inside this shorter-range zone of higher surface reverberation. This existence of this surface

reverberation maximum can be attributed to the upward-refracting acoustic propagation at this

location. Note also a distinct acoustic shadow on the left (port) side of the image up to 160 m

range created by the ship’s hull.

Figure 9: Forward-looking HF sonar image to 400 m range, 1735Z April 11 in Strait of Georgia.

Wind speed = 12 knots. Range rings are in 80 m increments. Arrow denotes

location of large target sphere, suspended at 4 m depth.

Figure 10 shows the corresponding target and background intensities extracted as the ship

approached from over 350 m range to a CPA near 25 m. Similar to the previous example the SNR

are relatively high (overall average 20.1 dB with standard deviation of 5.0 dB). The target echo

generally follows the -20log10[r] trend line except for a clear drop-out between 200 m and 250 m

range. This drop-out in target echo strength beyond 200 m coincides with a clear decrease in the

background reverberation outward of its peak at 200 m (see Figure 9). This drop-out in target

echo and background is likely due to a near-surface shadow zone starting at 200 m range. The

background shows a strong reverberation peak near 200 m, suggesting the sonar beam is strongly

focused on the sea-surface at this range, but is otherwise of similar level to that seen in Saanich

Inlet (Figure 7). The target echo also shows some clipping at ranges less than 50 m.



run against the big sphere starting 1734Z April 11 in Strait of Georgia. Dashed lines show

expected -20log10[r] variation in target echo and sonar clipping and noise thresholds.

3.1.3 Strait of Georgia at Sea-State 4

Figure 11 shows the corresponding sonar image taken under the highest wind speeds in the

sea-trials, near 16 knots, in the Strait of Georgia. On April 12 the target spheres were suspended

at 1 m depth, expected to enhance the impact of near-surface bubbles on sonar performance. At

this wind speed, bubble layers or clouds were expected to exhibit exponentially decreasing

densities with depth scales of 0.5 to 0.8 m [1]. At wind speeds near 16 knots detection ranges

were generally limited to less than 250 m. A key characteristic of this location was the patchiness

of the observed background reverberation. These patches appear to have spatial dimensions

approximately 40–100 m. In this snapshot the target was readily detected in-between patches of

relatively intense reverberation. If the target had been located inside one of the reverberation

patches it would have been much more difficult to detect. The background reverberation reaches a

consistent peak between 60 m and 100 m range; this can be attributed to the upward-refracting

conditions focusing the sonar beam on the surface. Once again the acoustic shadow extending up

to 60 m range on the port side of the ship is clear.




denotes location of large target sphere suspended at 1 m depth.

Figure 12 shows the corresponding target echo and background intensity measurements. A key

feature here is a drop-out in target detection between 90 m and 110 m range due to a local

maximum in the sea-surface reverberation. The target echo also shows strong ( ±10 dB)

variability with a spatial scale of approximately 10–20 m. Visual inspection of the sequence of

snapshots suggests that these are due to surface waves, which can be seen to move through the

images. Finally, beyond 110 m range the target echo drops below the -20log10[r] trendline, likely

due to absorption of the sonar signal by near-surface bubbles at shorter ranges. The target echoes

showed signs of clipping at ranges less than 80 m.



run against the big sphere starting 2108Z April 12 in the Strait of Georgia. Dashed lines

show expected -20log10[r] variation in target echo and sonar clipping threshold.

3.1.4 Summary Results

The maximum detection range under the varying conditions is one of the most obvious

parameters to calculate. Based on previous investigations [1, 2] it was hypothesized that this

would be strongly controlled by the wind speed. These maximum detection ranges were found as

a by-product of the target echo vs. background investigations (i.e., as shown in Figures 7, 10,

and 12) conducted for every forward-looking sonar run against the two sphere targets. These are

summarized in Figure 13. While there is a clear trend of decreasing detection range under

increasing winds, it is not a simple one. The overall maximum detection range is near 575 m, but

some scatter is present at ranges 450–575 m at low wind speeds (< 4 knots). A threshold for wave

breaking above 6 knots was chosen to separate the wind-induced effect. A best-fit line

(intercept = 17.0 knots, slope = -0.0189 knot/m, correlation coefficient r2 = 0.455) is shown,

however there is significant scatter in the data. This analysis suggests that these targets would not

be detectable at wind speeds higher than 17 knots. Unfortunately the present data set does not

cover the higher sea-state conditions required to confirm this assertion. At wind speeds between

8 and 10 knots the clear difference between the Saanich Inlet and Strait of Georgia locations also

highlights the importance of local acoustic propagation effects.


Figure 13: Summary of maximum detection ranges from all forward-looking sonar tests with

big and small spheres in Saanich Inlet and Strait of Georgia, April 9–12.

Best fit line to data where wind > 6 knots is shown.

Using the calibration results it is desirable to estimate the true acoustic target strength (TS) of the

sphere targets. However, it was clear in all the data sets that the Apparent TS (ATS) generally

varied with range due to acoustic propagation effects (example shown in Figure 8 above, and to

be discussed in Section 3.2 below). The ATS usually showed a roughly constant value, assumed to

be an estimate of the true TS, at ranges less than approximately 100 m. However, at very close

ranges (< 50 m) the sonar data frequently showed clipping (sonar raw intensity > 66 dB). Thus,

averaged ATS were computed over to range interval 50 to 100 m for each of the various approach

runs. It was found that there was no significant difference in these true TS estimates between the

two locations and at varying wind speeds. The overall averaged TS estimates were thus

(-14.0 ±1.4) dB and (-14.4 ±1.5) dB for the big and small spheres, respectively. These estimates

fall in-between the geometric TS value for the two spheres (see [3]) of -15 dB and -12.8 dB. The

fact that these two in situ TS estimates are similar in spite of the difference in sphere diameter

(71 cm vs. 91 cm) suggests that the chain ballast made a contribution to the overall TS.

The background reverberation showed variations up to 10 dB between locations due to the

combined effects of wind speed variations and local acoustic propagation effects. Figure 14

shows a comparison of averaged background Reverberation Level (RL) between the three

example locations. These curves were averaged in angle over a 10 sector near the center of the

sonar aperture and in time over 200 pings. Sonar self-noise was only a minor contributor at ranges

greater than 500 m (i.e., only for the April 9 Saanich Inlet case). Short range (< 20 m) peaks in all

data are due to the sonar beam-pattern and interference from ship’s hull echoes. The low wind

speed Saanich Inlet case shows the lowest overall RL. The two higher wind speed cases in the

Strait of Georgia show peaks and drop-outs attributed to acoustic propagation effects. The fact

that the higher wind speed cases (2108Z April 12) shows slightly lower RL (than the 12 knot

case) beyond 100 m is attributable to bubble layer saturation effects, whereby (at higher wind

speeds) bubbles at shorter ranges attenuate echoes from bubbles at longer ranges.


Figure 14: Comparison of averaged Reverberation Level (dB re 1 µPa) vs. range for runs

against the big sphere in Saanich Inlet and Strait of Georgia on April 9, 11, and 12.

3.2 Transmission Loss Effects

Quantitative analysis of the data uncovered plentiful evidence for non-spherical acoustic

propagation effects. This was seen as large (up to 15 dB) anomalies in the apparent TS, both

positive and negative. Generally these were attributed to the combined effects of surface

reflections, refraction by near-surface sound speed gradients, and acoustic extinction due to

bubbles. Figure 15 shows a comparison between three separate instances of ATS for the big

sphere. The key differences between these environments are wind speed (2.5, 12, and 16 knots)

and sound speed profiles (upward vs. downward refracting, see Figure 2). The April 9 data show

strong positive ATS anomalies, enhancing the target detectability both in terms of SNR and

maximum detection range. The April 11 data show a middle case, with both positive and negative

apparent TS anomalies. The highest wind speed case (April 12) shows much lower ATS, which is

attributed to acoustic extinction by the bubble layer, limiting detection the range to 190 m. All

three data sets show significant variability (even after adjacent-averaging) with peaks and troughs

approximately ±3 dB and spatial scale approximately 30–40 m. It is speculated that these peaks

and drop-outs in ATS are due to acoustic interference effects between the direct and

surface-reflected acoustic paths (as sketched in Figure 3).


Figure 15: Comparison of apparent TS (dB re m2) vs. range for big sphere on April 9, 11,

and 12 under wind speeds of 2.5, 12, and 16 knots, respectively. Data are 10-pt.

adjacent averaged. Dashed line shows estimated true TS of -14.0 dB.

Target depths were 4 m on April 9 and 11 and 1 m on April 12.

3.3 Variability of Target Echoes

One of the most obvious characteristics of these sonar trials was the strong ping-to-ping

variability in both the target echoes and background reverberation. Assuming that calculation of

ATS removes the dominant geometric trends with range, it is useful to consider the statistical

characteristics of the ATS over the entire approach run. Note that using the data in this manner

includes both temporal variations, for example due to surface wave effects, and spatial variations

due to the decreasing range to target. A probability distribution of the target ATS from the

April 9 data (Figure 8) was calculated, as shown in Figure 16. In this case the ATS data appear to

be Gaussian distributed, with a width (2 x standard-deviation) of 14.8 dB. Analysis reported by

Dahl and Plant [6] using a fixed geometry concluded that a Gaussian distribution is appropriate

for logarithmic variables (such as ATS) scattered near the sea-surface. Their levels of variance

were somewhat smaller (standard deviation of 5.6 dB) because their analysis only included

temporal variations in sea-surface scattering. The fact that the Gaussian-fit mean value (-8.1 dB)

is larger than the estimated true TS of the sphere (-14.3 dB, see Section 3.1.4) again suggests that

acoustic propagation effects are (on average) enhancing the target echoes, improving the sonar

detection performance.


Figure 16: Frequency distribution of apparent TS over all ranges for detection run against the

big sphere starting 1710Z April 9 in Saanich Inlet. Best fit Gaussian distribution

(mean = -8.1 dB, width = 14.8 dB, r2 = 0.926) shown in red line.

3.4 Propeller Cavitation Effects

While the ship was traveling at constant speed there appeared to be no interference from

propeller cavitation. The maximum ship speed reached during the forward-looking target

approach runs was less than 5 knots, so propeller cavitation effects were not expected. However,

during periods when the ship accelerated, particularly at speeds above 6 knots, strong interference

effects attributed to propeller cavitation were observed. Figure 17 shows an example. Compare

the background level with similar sonar setup in Figure 6. In this example the ship was

accelerating from 7.5 to 8 knots during a self-noise test in Saanich Inlet on April 9. The signature

of propeller cavitation noise appears as a strong increase in background noise at longer ranges

(due to amplification by the sonar TVG) with some modulation due to the propeller blades.

This modulation appears as bands roughly 40 m wide with a peak to peak spacing of

approximately 80 m. This spacing in range corresponds to a period of 0.11 s. This cavitation

noise has the potential to completely mask target echoes at greater ranges. A quantitative

comparison of background levels between low speed (3 to 5 knots) and this example found an

approximate 15 dB increase in background noise levels at ranges beyond 300 m.

The CCGS Vector has a single, three-bladed, variable-pitch propeller with diameter of 1.8 m.

At this speed the propeller RPM was near 180, corresponding to a blade rate of 9 Hz (period

of 0.11 s). This supports the hypothesis that the cavitation noise interference has a blade rate

modulation. It is believed that at higher ship speeds (> 8 knots) this cavitation interference will

pose a strong limitation on HF sonar performance. Unfortunately during this trial it was

impossible to assess performance at higher ship speeds due to safety limits on use of the sonar

strut.


Figure 17: Forward-looking HF sonar image to 600 m range, 2118Z April 9 in Saanich Inlet

during period of ship acceleration from 7.5 to 8 knots. Range rings are in 120 m

increments. Arrows denote bands believed to be blade rate modulations.


4 Aft-Looking Sonar

HF sonar performance in the ship wake was specifically assessed in a series of tests on April 13

in Saanich Inlet. In all these tests the winds were light (< 3 knots) and seas were calm. The wake

was the dominant acoustic feature in these tests. Tests were conducted at ship speeds from 6 to

7.5 knots, this being the upper limit of safe operation of the sonar strut. It should be noted that

this speed generated only a relatively modest ship wake. Greater wake width, depth, and densities

have been previously observed with this same ship at speeds up to 12 knots [7].

Based on a previous study on the wake of CCGS VECTOR [7], the expected maximum wake

width at this speed was approx. 60 m, with the wake core deepening over the first 180 s up to a

maximum of approximately 6 m. At the ship’s transom the wake core depth can be assumed to be

near 3 m. In that earlier study the wake was observed to persist as a distinct acoustic feature for

periods greater than 6 minutes at low speeds (5 knots). This increased to over 10 minutes for

higher ship speeds. This wake lifetime is much larger than the equivalent sonar ranges explored in

these tests. Given that the Sharko tow depths were between 5 m and 24 m (greater depth at greater

range), then the target was usually below the wake core depth. This geometry implies that

wake-bubbles should not cause any acoustic extinction of the direct-path target echo, but may

cause some extinction of the surface-reflected paths. The vertical beamwidth of the sonar (13.5)

ensures that echoes from both the target and wake bubbles will be combined, so that wake

backscatter sets a threshold for target detection.

4.1 Sonar Data Examples

The Sharko towed target depth varied with the amount of tow-cable scope, as shown in Figure 18.

The Sharko depth generally varied between 5 m and 24 m at ranges from 20–450 m. The target

depth was controlled by the combination of the tow-rope shape and drag and the Sharko weight

relative to the water, and depends strongly on the towing speed relative to the water. The target

pull-ins showed a slightly shallower approach due to the fact that the target speed relative to the

water was 3.9 knots higher than the ship speed.

Figure 18: Summary of Sharko depths during aft-looking HF sonar tests April 13 in

Saanich Inlet. Black lines show approximate -3 dB limits of sonar vertical beam.


4.1.1 Fixed-Range Towing

Fixed-range towing tests were conducted at ranges from 100 m to 450 m. Beyond 450 m range

the target was very difficult to detect visually. Sonar data was collected over a 5 minute period at

each range to assess target echo variability. Figure 19 shows an example with Sharko towed at

150 m range. Recall that the sonar was mounted with a 9 counter-clockwise offset, so that the

wake appears to have clockwise rotation. This presented no difficulties in processing, so

corrections were not applied.

Figure 19: Aft-looking HF sonar image to 200 m range, 1828Z April 13 in Saanich Inlet. Range

rings are 40 m increments. Arrow denotes location of Sharko target, at 16 m depth.

The general characteristic of the wakes observed in these tests was that they were not

homogeneous, but rather consisted of isolated scattering patches of dimension up to 10 m. In the

sonar frame of reference these patches appeared to be advected away from the sonar at the ship

speed. The Sharko target echo was then identifiable as a stationary feature against this moving

background. Note that the apparent intensity of the Sharko target is similar to that of the

scattering patches, making it somewhat more difficult to detect the target in the near-field

(ranges < 60 m) where the density of scattering patches is much higher. The fact that the wake

apparently decreases in intensity at greater range is hypothesized to be a result of acoustic

extinction within the wake, rather than any dissipation of the wake itself. The maximum wake age

(time after wake generation at the ship stern) in this image is 65 s, which is considerably shorter

than the previously measured wake persistence (360 s, see [7]). The maximum wake age

assessed in these fixed-range towing trials (ranges up to 450 m) was near 120 s.

Following a similar target following algorithm as employed in the forward-looking tests,

estimates of the target echo and background intensity were extracted on a ping-by-ping basis from

the sonar snapshots. A typical result spanning 220 s is shown in Figure 20. The figure shows that

both target and background sonar intensities exhibited roughly constant average values overlain


with ping-to-ping fluctuations. The level of short-period fluctuation in this example is

approximately ±2 dB. The mean SNR is 22 dB, which is very good. Target echo fluctuations will

be discussed further in Section 4.3.

Figure 20: Temporal variation in target echo and background intensity for stationary tow

at 220 m range in ship wake starting 1618Z April 13 in Saanich Inlet.

Ship speed during this period was 7.2 knots.

Figure 21: Apparent target strength (dB re m2, black squares) and SNR (dB, red circles)

vs. range from all fixed-range towing tests, April 13 in Saanich Inlet.

Figure 21 shows a compilation of Apparent TS and SNR from all the fixed-range towing tests.

The ATS results show the mean ±1 standard deviation averaged over a period of 3 to 5 minutes.

The ATS results show a 5 dB increase as range increases from 100 to 450 m, attributable to


acoustic propagation enhancements due to acoustic refraction and inclusion of surface-reflected

multi-paths. Also, the level of fluctuation (size of error bars) generally increases with range.

Generally the averaged SNR values were very good (> 20 dB) at ranges up to 350 m, falling to

13 dB by 450 m. The ATS estimate at 100 m range shows a somewhat lower mean value and a

much larger variability due to variations in depth (12–16 m) during the run, which put it on the

edge of the sonar main beam (recall that sonar beam-pattern effects are not compensated for in

the ATS value). Disregarding the anomalous data point at 100 m range, the fixed-range results

suggest a minimum ATS for the Sharko target near -20 dB (re 1 m2).

4.1.2 Target Pull-Ins

The second type of target detection test involved pulling in Sharko at 2 m/s (3.9 knots) relative to

the ship from a start range near 210 m. Six separate pull-in runs were conducted at ship speeds

between 6 and 7.5 knots. A 200 m sonar range setting was used for all the pull-in tests. Sharko

depths during the pull-in runs are shown in Figure 18.

Apart from ping-to-ping variability (similar to that observed in the fixed-range tests), the separate

pull-in runs were all essentially the same in terms of target echo strength and SNR. Thus the

results were averaged over all six runs. This was done by averaging the target and background

intensities within 5 m range bins from 20–200 m. Averaged pull-in results are shown in

Figure 22. This shows relatively smooth variations in target echo and background intensity with

range, approximately following -20 and -30*log[r] variations, respectively. The -20*log[r]

variation in target echo suggests a constant ATS value. The -30*log[r] variation in background

intensity is created by the linear increase in sonar sampling volume with range. Overall the SNR

are good, increasing with range from 14 to 24 dB.

Figure 22: Comparison of averaged target, background, and SNR vs.

range from six separate pull-in runs in arbitrary sonar units.

Dashed lines show -20log10[r] and -30log10[r] variations.


The averaged target echo intensity can be converted to the Apparent TS, shown in Figure 23.

While removing the dominant geometric spreading effects, the ATS profile shows that there is still

some target echo variability even after averaging over multiple runs. The best-fit line shows a

small range-dependence. Recall that the ATS parameter is not corrected for acoustic propagation

effects nor for sonar vertical beam-pattern. Acoustic propagation effects should diminish at short

ranges. Based on the known target depths during the pull-in runs the sonar Beam-Deviation Loss

(BDL, dB, see Eq.(3)) can be calculated. The two-way BDL for the target depths encountered in

these tests increases from 0 to 2 dB over ranges from 20 m to 200 m. Thus, the minor decrease in

ATS can be explained by sonar beam-pattern effects. Also, since the BDL at short ranges is

effectively zero, then the intercept of the best-fit line (-19 dB re m2) can be taken as an estimate

of the true TS of the Sharko target (at bow incidence). This is consistent (within experimental

uncertainty) with the results from the fixed-range towing (near -20 dB, see Figure 21).

Figure 23: Apparent target strength (dB re m2) estimated from average of six separate pull-in

runs. Red line shows best fit line (intercept = -19.0 dB, slope = 0.007 dB/m).

4.1.3 Target Detection During S-Turns

A separate fixed-range target detection test was conducted while the ship conducted a series of

S-turns (through roughly ±45 relative to the mean heading) at speeds of 6.5 to 7 knots. Sharko

was towed at a fixed distance of 210 m behind the ship at a depth near 16 m. The ship turns

forced the Sharko target to tow outside of wake, generally on the inside of the turn, with frequent

wake crossings. Figure 24 shows an example of the sonar data. Generally the signal to noise

properties were at least as good as in previous straight-line tests, although the wake itself appears

to be a stronger target than in previous constant-heading runs (contrast wake strength compared to

Figure 17). It is expected that ship wakes are stronger in turns.

In addition to forcing Sharko outside of the wake, the incidence angle of the sonar beam relative

to Sharko was also forced away from directly on the bow (as it was in the previous tests). It was

speculated that, due to its construction and presence of two acoustic target spheres within the

main body, Sharko would show variation in TS with incidence angle, generally increasing in a


complicated way towards a broadside maximum. This effect is evident in Figure 25 (Apparent TS) by

large positive excursions in ATS at various points through this series of ship turns. While there are

similar ping-to-ping fluctuations as previously observed, these positive excursions are far larger

than previous ATS estimates. Unfortunately, no simple relationship between Sharko position,

wake locations, and ATS could be deduced from this test.

Figure 24: Aft-looking HF sonar image to 300 m range during S-Turn, 1626Z April 13 in

Saanich Inlet. Range rings are 60 m increments. Arrow denotes location of Sharko target.


Figure 25: Apparent target strength (dB re m2) vs. time following Sharko towed target

at 210 m range through series of S-Turns, 1624Z-1630Z April 13 in Saanich Inlet.

4.2 Wake Backscatter and Extinction

It was hypothesized in earlier work that bubbles in the ship wake would both (a) generate

increased backscattered signal relative to the ambient conditions and (b) absorb and scatter the

sonar signal as it passes through (a process collectively known as extinction). The first effect can

be quantified through a comparison between the averaged background reverberation levels vs.

range both inside and outside of the wake, as shown in Figure 26. This shows an increased

backscatter level within the wake of between 5 and 15 dB, varying with range from a maximum

near 40 m to a minimum near 200 m. The result is nearly identical at the two speeds investigated.


Figure 26: Comparison of in-wake and non-wake reverberation level vs. range

profiles at two speeds, taken near 1800Z April 13 in Saanich Inlet.

While there is clear evidence for the increasing background levels inside the wake, there is no

solid evidence for wake extinction effects on the target echo strength. For example with the

Apparent TS measurements in the fixed-range towing tests (Figure 21), the ATS was observed to

generally increase with range. If acoustic extinction effects due to bubbles were present then a

general decrease in ATS should have been observed. Bubble acoustic extinction losses can exceed

0.1 dB/m (see analysis in [1, 2]). The relatively small decrease in ATS with increasing range

observed during the target pull-in runs (Figure 23) was adequately explained by the sonar vertical

beam-pattern effects. The reason for this lack of acoustic extinction effect in the target echo is

simply that the target depth (between 6 and 25 m, see Figure 18) placed it below the wake core

(expected to deepen with distance behind the ship from roughly 3 to 6 m—see [7]). The

direct-path from sonar to target was thus unaffected by wake bubbles. Furthermore, for deeper

targets the surface-reflected acoustic paths have somewhat higher grazing angles, of the

order 10, thus traveling shorter distances through the bubble layers and hence suffering only

minor extinction losses. Note that this conclusion would not necessarily hold for much deeper

wakes, generated for example by larger ships operating at higher speeds.

4.3 Target Echo Variability

In both the fixed-range and pull-in target detection runs significant ping-to-ping variability was

observed. It is hypothesized that this variability is due to acoustic interference between the direct

and surface-reflected acoustic paths, inevitably randomized due to small scale fluctuations in

acoustic path length caused by surface roughness and wake turbulence. In the fixed-range tests

the target depth and bearing were approximately constant, thereby allowing an assessment of the

statistical properties of these fluctuations on a temporal basis alone. Figure 27 shows a typical

example. The distribution of ATS over this period is well-matched by a Gaussian distribution, but

with a considerably smaller width (4 dB) compared to the forward-looking tests (near 15 dB, see

Figure 16). The Gaussian-distributed character of this result is in agreement with finding in Dahl


and Plant [6], but now with a much smaller width. A possible explanation for this smaller width is

that the interfering surface-reflected path is slightly reduced in amplitude by bubble extinction

effects, allowing only partial constructive or destructive interference with the direct path. Other

fixed-range towing tests at different ranges showed Gaussian widths varying from 4 to 10 dB, the

larger values coming from tests showing some variation in target depth.

Figure 27: Distributions of apparent target strength during fixed-range towing

at 220 m range 1618Z April 13. Best fit Gaussian distribution

(mean = -20.2 dB, width = 4.0 dB, r2 = 0.994)

shown in red line.


5 Modeling HF Sonar Performance

Sonar performance models potentially allow a more general prediction of the environmental

limitations faced by a sonar; however these predictions need to validated against at-sea data.

Modeling sonar performance must necessarily include the physics of acoustic propagation,

including boundary scattering and internal refraction, combined with a variety of system and

environmental parameters. In this work it was expected, both from previous experience and from

the initial sea-trials results, that both sonar electronic (self-) noise and sea-surface reverberation

will pose limitations to target detection with this sonar.

This work will utilize the active sonar Signal Excess (SE, in dB) for the target as a function of

range. In this work SE is defined as the excess of the target echo SNR over a Detection Threshold

(DT) based on signal-processing considerations. The target is said to be detectable at ranges

where SE > 0 dB. SNR is the ratio of target echo Sound Pressure Level (SPL) to the combination

of the sonar Noise Level (NL) and Reverberation Level (RL) due to backscatter from the sea

surface. The NL is assumed to be dominated by internal electronic noise in the sonar system.

Although ambient noise can be included in the model (based on wind speed), this was found to be

negligible compared to sonar self-noise at this frequency (90 kHz). Volumetric backscatter, such

as due to zooplankton, will also be ignored. The effects of volume scattering from sub-surface

bubbles, however, cannot be ignored.

The fundamental model for SE uses the active sonar equation (in dB), i.e.,

SE = SL + TS - 2TL - (RL NL) - DT, (5)

where SL is the sonar Source Level, TS is the Target Strength, and TL is the Transmission Loss

from the source to target. This CW sonar is assumed to have no active processing gain. The

reverberation level is calculated based in part on the sonar beamwidths and pulse length.

The denotes a power sum between the two terms.

Using the SE or SNR values, the sonar Probability of Detection (Pd) can be calculated using

models outlined in Trevorrow and Myers [2]. In active monostatic sonar it is usual to assume a

DT = 10 dB (which is in agreement with results from Sections 3 and 4). The calculation also

requires a probability of false alarm, which is set at 0.1%. With these parameters a SE of 0 dB

corresponds to a Pd = 50%. The range where SE consistently drops below 0 dB (or equivalently

Pd drops below 50%), herein denoted the detection range, will be taken as a Measure of

Performance (MoP) for comparison between environments.

The effects of different oceanic environments enter through the acoustic transmission loss and

surface and seabed reverberation terms. The NATO acoustic modeling suite ESPRESSO

(Extensible Performance and Evaluation Suite for Sonar) [8], version 1.05 (November 2008) was

used to perform the basic acoustic propagation, scattering and noise modeling needed to produce

the results shown below. The tool was developed by the NATO Centre for Maritime Research

and Experimentation in La Spezia, Italy. The software implements a number of standard models

for sonar performance prediction. A modified version of BELLHOP, a public-domain ray-tracing

model developed by Porter [9], was chosen as the propagation sub-model. This was implemented


in ESPRESSO as described in [10] for bottom reflection and backscatter, surface reflection and

backscatter, and loss due to near-surface bubbles were developed by the University of

Washington Applied Physics Laboratory (APL-UW), described in [11]. Most of the APL-UW

models have been validated for the 10–50 kHz range, and some have been validated up to

100 kHz. Seawater absorption loss at the sonar operating frequency (following [5]) is also

included. ESPRESSO does not model sea-surface height variations due to surface waves.

In ESPRESSO the environmental inputs consist of a sound speed profile, water depth, seabed

properties, and a surface sea-state controlled by wind speed. The water depth, sediment type, and

sound speed profiles are all assumed to be range-independent. The various sonar input parameters

for modeling the SM-2000 are summarized in Table 2. The assumed TS values were taken from in

situ estimates for the target spheres (-14 dB) and Sharko (-19 dB) targets, as described in previous

sections. The outputs include reverberation levels, SE vs. range and depth, and detection

probabilities. Generally the predictions were generated at 1 m intervals out to 600 m range and

0.5 m intervals in target depth up to 20 m.

Generally the ESPRESSO model showed that the sonar detection performance was limited by

sea-surface reverberation at shorter ranges, switching to noise-limited at longer ranges. This is in

general agreement with the experimental results discussion above. The sound speed profile was

found to by a highly important factor, in some cases creating acoustic focusing and shadow zones

that controlled target detection almost independent of wind speed.

Table 2: ESPRESSO input parameters for the SM-2000 sonar models.

Parameter Value

Operating frequency 90 kHz

Pulse length 1.0 ms

Source Level 205 dB re µPa at 1 m

Sonar Depth 3.5 m

Sonar elevation angle 0 (horizontal)

Vertical beamwidth (both transmit and receive) 13.5

Receiver horizontal beamwidth 1.8

Receiver bandwidth 800 Hz

Receiver spectral self-noise level 50 dB re µPa/Hz

Receiver Directivity Index (DI) 30 dB

Water depth 200 m

Seabed sediment type silty-clay

Target backscatter Target Strength (TS) -14, -19 dB re m2

The sound speed profiles were distinctly different in Saanich Inlet as compared to the Strait of

Georgia, as shown in Figure 2. Generally the Saanich Inlet conditions were downward-refracting,

whereas the Strait of Georgia was strongly upward-refracting. Note also day-to-day variability in

the upper 20 m in Saanich Inlet; it is suspected that this also indicates some degree of spatial

variation along the inlet on any given day. Unfortunately the spatial variation in near-surface

conditions was not measured during the sea-trials. In this modeling, an average profile for


Saanich Inlet on April 9 and 10 was utilized for assessing the forward-looking tests. This was

done because the target detection results (e.g., Figure 13) showed no significant difference

between the two days. The aft-looking assessment used the April 13 profile (shown in Figure 2).

Figures 28 and 29 compare the predicted SE between Saanich Inlet and the Strait of Georgia

conditions. Note a difference in wind speed (5 vs. 10 knots) which is appropriate for the two

locations. Recall that SE > 0 dB defines the regions of target detectability, both of which are far

from uniform in depth. The Saanich Inlet conditions feature a near-surface (4 to 5 m depth) sound

channel that strongly enhances target detectability out to ranges of 600 m and beyond. Otherwise

the Saanich Inlet prediction features a downward-refracted detection zone and a near-surface

(1–3 m) shadow zone that prevents target detection beyond a range of approximately 300 m.

Because the target detectability is controlled by the sound speed profile, these results are

relatively robust with respect to small changes in wind speed. This result explains the observed

sonar detection ranges up to 600 m range against target spheres at 4 m depth.

Figure 28: Predicted sonar SE vs. target range and depth for the April 9/10 Saanich Inlet

conditions, with assumed wind speed of 5 knots. Target TS = -14 dB.

Figure 29: Predicted sonar SE vs. target range and depth under April 11 Strait of Georgia



The Strait of Georgia SE prediction, shown in Figure 29, shows distinctly different behaviour due

to the upward-refracting sound speed profile. The SE values are generally lower throughout due

to the higher sea-surface reverberation level due to the higher wind speed. The effect of

increasing wind speed is to reduce the SE values throughout. Note that in the uppermost 2 m the

target detectability drops away with range; this is due to bubble-induced target echo extinction

losses. For targets at 1 m and 4 m depth the region of target detectability (SE > 0) is broken into

two zones (0 to 200 m and a narrow patch near 350 m) with a shadow zone in between. In this

Strait of Georgia environment it is predicted that targets will not be detectable beyond 370 m

range at any depth. The maximum observed sonar detection range in the Strait of Georgia was

near 400 m under winds near 10 knots (see Figure 13).

Figure 30: Predicted sonar Pd vs. range at 4 assumed wind speeds for a

1 m deep, -14 dB target under Strait of Georgia conditions.

An example of the conversion of SE to Pd is shown in Figure 30 for the Strait of Georgia

conditions. The two zones of target detectability are clear. Using the Pd = 50% MoP, the effect of

increasing wind speed is to dramatically reduce the detection range. The predicted target

detection range in Figure 30 drops from 430 m to 50 m as the wind speed increases from 5 to

20 knots. Recall in the sea-trials that the maximum detection range dropped to below 200 m as

the wind speed reached approximately 15 knots.

The conditions and target depths were distinctly different for the aft-looking wake tests on

April 13 in Saanich Inlet. In this case the winds were light and the Sharko target was towed at

depths of 5–24 m. This analysis ignores the effects of the ship wake, which cannot be easily

modeled in ESPRESSO. Figure 31 shows the predicted SE vs. range and depth under these

conditions. For deeper targets the predicted detectability zone extends to greater horizontal range,

which matches with the towing profile of the Sharko target which is deeper at greater range. The

predicted detection range for the Sharko target at 20 m depth is near 440 m. This is in good

agreement with the experimental result of maximum detection near 450 m (see Section 4.1.1).

Furthermore in the inner 300 m the SE values along the target depth profile are all in excess of

10 dB. This result is in agreement with the relatively high SNR observed in the sea-trials.

Figure 31 predicts relatively good sonar performance in the upper 5 m at ranges up 300 m. This is

considered erroneous due to bubble-induced acoustic extinction effects within the ship wake.


During the sea-trials it was not possible to explore this range-depth zone due to the towing

characteristics of Sharko. It is hypothesized that target detection would not be possible at depths

inside the wake core.

Figure 31: Predicted sonar SE vs. target range and depth under April 13 Saanich Inlet


Black line shows approximate target depth vs range.

Figure 32: Comparison between measured wake RL vs. range, taken 1758Z April 13 in

Saanich Inlet, with ESPRESSO prediction at wind speeds of 10, 15, and 20 knots.

Moreover, ESPRESSO does not provide bubble scattering models appropriate for ship wakes,

i.e., it is not possible to fake the presence of a wake by using a larger wind speed. This is shown

in Figure 32 through a comparison of Reverberation Level (RL). The ESPRESSO predictions,

based on the APL-UW models for wind-driven seas [11], show the effects of bubble extinction in

two ways: (i) a decrease in longer range RL to a saturation value as the wind speed increases to

15 knots and above, and (ii) an initial increase then a decrease in the peak RL value near 20 m

range. In comparison with the sea-trial data it is clear that while ESPRESSO does predict the peak

values (ranges 20–40 m) reasonably well, it does not produce the same longer range dependence

in the RL. This can be attributed to the use of a bubble model that is not appropriate for wakes.


The result is that ESPRESSO predictions for wakes based on a fictional wind speed will, outside

of a near-field zone, over-predict the RL and hence under-predict the sonar performance.

Using the Pd = 50% MoP, the predicted detection range for various target depths in the different

locations can be calculated, as summarized in Table 3. These predictions are all in reasonable

agreement with the sea-trial results. The Saanich Inlet Apr 9/10 predictions show the most

extreme difference between 1 and 4 m depth, due to the refractive sound channel centered at

4–5 m depth. Recall that the Saanich Inlet tests on Apr 9/10 were all conducted with the target

spheres at 4 m depth, so there is no sea-trial data to verify the 1 m prediction. The sea-trial results

in Saanich Inlet under light winds (under 6 knots, see Figure 13) produced an average detection

range of 504 ±45 m, which is in reasonable agreement with this prediction.

Table 3: ESPRESSO predictions of SM-2000 sonar detection ranges.

Location/Date TS target

depth

wind =

5 knots

wind =

10 knots

wind =

15 knots

wind =

20 knots

Saanich Inlet,

Apr. 09/10

-14 dB 1 m

4 m

309 m

626 m

213 m

626 m

98 m

619 m

50 m

556 m

Strait of Georgia,

Apr. 11/12

-14 dB 1 m

4 m

431 m

410 m

247 m

246 m

133 m

238 m

54 m

238 m

Saanich Inlet,

Apr. 13

-19 dB 10 m

15 m

20 m

322 m

422 m

441 m

305 m

421 m

440 m

316 m

421 m

440 m

305 m

417 m

440 m

The ESPRESSO predictions for the Strait of Georgia conditions (wind speeds 10 to 16 knots)

suggest detection ranges between 130 m and 250 m, with improved performance for a 4 m target.

This is somewhat pessimistic as compared to the sea-trial results. In the Strait of Georgia

sea-trials the target spheres were deployed at 4 m depth on April 11 and at 1 m depth on April 12.

Detection ranges up to 400 m under wind speeds near 10 knots were observed on both days, and

on April 12 the targets were detected up to 340 m range under wind speeds near 15 knots (see

Figure 13). A contributing factor is that the target echo included some contribution from the chain

ballast suspended approximately 2 m below the sphere. This would have improved target echoes

in the 1 m depth cases (April 12). Another potential explanation for this discrepancy is that the

Strait of Georgia sound speed measurement (reported in Figure 2) was conducted approximately

5 km to the north-east of the sonar test location; some spatial variation in the near-surface sound

speed profile may have been present. This highlights the importance of conducting in situ

environmental measurements at the same location and time as the sonar tests.

Table 3 shows that the predicted detection ranges for the Sharko target in the aft-looking

under-wake tests were in good agreement with the experimental result (maximum detection near

450 m at depths near 24 m). There is very little variation in detection range due to changing the

model wind speed.


6 Summary Discussions

The HF multibeam sonar operated well in both the forward- and aft-looking target detection tests

from a ship, producing a valuable data set covering a variety of sea states and near-surface

sound-speed conditions. In forward-looking tests hollow steel spheres suspended 1–4 m below the

surface were used as targets. These sphere targets were routinely detected at ranges up to 580 m

in low sea-states in Saanich Inlet, but detection range was reduced to roughly 150–300 m under

moderate sea states (winds up to 16 knots) encountered in the Strait of Georgia. While this

suggests wind speed was a controlling factor in target detectability, the effect of near-surface

sound speed gradients was also found to be important.

Increasing wind speed was observed to produce both an increase in background reverberation and

a reduction in apparent target strength. Both of these effects reduced the signal to noise properties

of the target echoes, resulting in reduced target detection ranges. Observed SNR varied between

10 dB (the lower limit of target detectability) and 30 dB. Extrapolations from the observed sonar

performance suggested that these targets would not be detectable at wind speeds exceeding

17 knots; however sea-trial data necessary to confirm this hypothesis was not collected. If this

turns out to be a valid trend, this potentially places strong wind and sea-state limitations on future

naval applications. However, some evidence for bubble saturation effects, whereby the observed

reverberation level does not increase with increasing wind and sea-state, was observed at wind

speeds from 12 to 15 knots. This has the potential to stop the trend of decreasing detection range

with increasing wind speed. While no difference in detectability was observed between 1 and 4 m

target depth (see Figure 13), modeling suggests improved detection performance against deeper

targets.

It was found that the relatively strong near-surface sound speed gradients in both locations

created acoustic convergence and shadow zones which modified the target detectability. This was

particularly clear in the Saanich Inlet test where a sub-surface refractive sound channel was

observed to bias the apparent target strength by up to +15 dB relative to simple spherical

spreading predictions. In some cases the refraction effects produced local maxima in surface

reverberation, inducing drop-outs in target detectability at some ranges. This observed sensitivity

to near-surface sound speed gradients was not anticipated prior to the sea-trials, and thus only

minimal (e.g., single location, daily) sound speed profiling was conducted. A key lesson-learned

is that timely, local environmental measurements are necessary to understand sonar performance

(at least in littoral regions).

Considerable ping-to-ping variability in both target echo strength and background reverberation

levels was observed. It is hypothesized that this target echo variability is due to acoustic

interference between the direct and surface-reflected acoustic paths, inevitably randomized by the

small scale fluctuations in acoustic path length caused by surface roughness and wave-induced

turbulence. Observed distributions in apparent target strength were found to be

Gaussian-distributed with variation (standard deviation) ±5 to ±8 dB relative to the mean. This

variability is believed to have both temporal and spatial components. The background

reverberation also exhibited a combination of short period, small scale fluctuations and

larger-scale scattering patches. It is recommended that further work be conducted to assess the

causes and implications of this variability.


During the aft-looking sonar tests a towed target was detectable up to 450 m range behind the

ship at speeds from 5 to 7.5 knots. The towed target depth varied from 5 to 24 m, generally below

the ship wake. The wake itself was observed to generate between 5 and 15 dB increase in the

background levels (dependent on range), while still allowing observed target SNR between 15

and 25 dB. The wake was observed to have little influence on the apparent target strength. Similar

to the forward-looking tests, significant levels of ping-to-ping variability were observed in both

the target echo and background scattering from the ship wake. While these results are

encouraging, it should be noted that the ship wake depths and densities at the maximum sea-trial

speed (7.5 knots) were relatively modest. The maximum ship speed was limited by safety

considerations in use of the sonar strut. Greater wake width, depth, and densities were previously

reported with this same ship at speeds up to 12 knots [7]. Larger, higher-speed ships (e.g.,

HALIFAX-class frigates) have much deeper and denser wakes and so a much greater level of

wake backscatter and acoustic extinction is to be expected. Therefore, sonar tests at higher ship

speeds and possibly with larger ships should be undertaken. This will require some additional

engineering changes to the sonar strut on CCGS Vector, for example through the use of

streamlined fairings, or alternate arrangements on bigger ships.

Relatively strong acoustic interference attributed to propeller cavitation was observed in both

forward- and aft-looking tests when the ship accelerated at speeds above 6 knots. This cavitation

generated approximately 15 dB increase in background noise levels, with some evidence for

blade-rate modulation. This measurement occurred near the cavitation inception speed; at higher

ship speeds cavitation effects should increase. Thus, there is strong potential for ship-generated

noise to render the sonar useless at higher speeds. However, it should be noted that the HF sonar

used in these sea-trials was mounted relatively close to the propeller (5 m to starboard); other

hull-mounted applications may provide greater physical separation and hence less interference.

Other bigger ships will also have a higher propeller cavitation inception speed.

A sonar performance model (NATO ESPRESSO) was useful in elucidating environmental

influences on the results. Generally this model produced quantitative predictions in reasonable

agreement with the sea-trials results. It was particularly useful in quantifying differences in sonar

performance caused by the different sound speed profiles between Saanich Inlet and the Strait of

Georgia. This limited validation now provides some confidence in the model, which may now be

used to extrapolate sonar performance to different environments and greater target depths.

These sonar tests were all conducted at relatively low target speeds relative to the sonar, which is

appropriate for obstacle avoidance applications, but misses important acoustic effects occurring

with higher speed ships and against high-speed targets such as torpedoes. For example, in moving

ship applications some parts of the target echo, sea-surface backscatter reverberation, and wake

scattering may be shifted out of the receiver band by own-ship Doppler effects, reducing their

apparent amplitude. Similarly, the Doppler shift of a high speed target might push the echo

completely out of the sonar receiver band. In the default settings for this SM-2000 sonar the

receiver bandwidth was tightly coupled to the sonar pulse length, making it susceptible to these

effects (see Annex A, Section A.6). Any future application against high speed targets would need

to utilize a much wider receiver bandwidth. This can be done with the existing sonar. In future,

adding a Doppler sensing capability to the sonar would potentially allow improved discrimination

between background reverberation and high-speed targets.


This report makes use of acoustic calibrations of the multibeam sonar. While time-consuming,

this effort had several benefits, namely (i) a verification of the manufacturer’s technical

specifications, and (ii) enabling quantitative sonar output which then allowed comparison with

quantitative physical modeling. While most naval applications are concerned only with relative

signal strength (i.e., target echo vs. reverberation), generating absolute acoustic quantities allows

a better assessment of sonar performance. The calibration results generally confirmed sonar

source levels, beam-patterns, time-varying gain functions, and clipping and self-noise thresholds.

Importantly, the calibration results allowed estimates of the true target strengths of the target

spheres (with ballast) and the Sharko towed target. These estimates would have been impossible

to produce accurately from simple geometric formulae. The TS estimates were then used in sonar

performance models to produce realistic predictions of target SNR and detection range.


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References

[1] Trevorrow, M., 2003. Measurements of near-surface bubble plumes in the open ocean with

implications for high-frequency sonar performance, J. Acoustical Soc. Am. 114(5), 2672–2684.

[2] Trevorrow, M., and Myers, V., 2012. Modeling obstacle avoidance sonar performance in the

presence of near-surface bubbles, presented at 11th European Conference on Underwater

Acoustics, Proc. of Meetings on Acoustics 17.

[3] Trevorrow, M., and Vagle, S., 2015. Summary of HF sonar sea-trials on CCGS VECTOR,

April 2015, DRDC-RDDC-2015-R245.

[4] Medwin, H., and Clay, C., 1998. Fundamentals of Acoustical Oceanography (Academic

Press, San Diego).

[5] Francois, R., and Garrison, G., 1982. Sound absorption based on ocean measurements Part II:

boric acid contribution and equation for total absorption, J. Acoust. Soc. Am. 72, 1879–1890.

[6] Dahl, P., and Plant, W., 1997. The variability of high-frequency acoustic backscatter from a

region near the sea surface, J. Acoust. Soc. Am. 101(5), 2596–2602.

[7] Trevorrow, M., Vagle, S., and Farmer, D., 1994. Acoustic measurements of micro-bubbles

within ship wakes, J. Acoustical Soc. Am. 95(4), 1922–1930.

[8] Davies, G., and Signell, E., 2006. Espresso – Scientific User Guide, NATO SP-2006-003.

[9] Porter, M., and Bucker, H., 1987. Gaussian beam tracing for computing ocean acoustic fields,

J. Acoust. Soc. Am. 82(4), 1349–1359.

[10] Meyer, M., and Davies, G., 2002. Application of the method of geometric beam tracing for

high frequency reverberation modelling, Acta Acustica 88(5), 619–622.

[11] APL-UW, 1994. High-frequency ocean environmental acoustic models handbook,

Technical Report 9407, Applied Physics Laboratory, University of Washington.

[12] Foote, K., Chu, D., Hammar, T., Baldwin, K., Mayer, L., Hufnagle, L., and Jech, J., 2005.

Protocols for calibrating multibeam sonar, J. Acoust. Soc. Am. 117(4), 2013–2027.

[13] Cochrane, N., Li, Y., and Melvin, G., 2003. Quantification of a multibeam sonar for

fisheries assessment applications, J. Acoust. Soc. Am. 114(2), 745–758.

[14] Trevorrow, M., and Crawford, A., 2005. Calibration of a high-frequency multibeam sonar

for water volumetric surveys, DRDC Atlantic TM2005-146.

[15] Vagle, S., Foote, K., Trevorrow, M., and Farmer, D., 1996. Absolute calibrations of

monostatic echosounder systems for bubble counting, IEEE J. Oceanic Eng. 21(3), 298–305.


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HF Sonar Calibrations Annex A

An effort was made to acoustically calibrate the SM-2000 sonar. This was done in order to

verify the manufacturer-provided technical specifications,

understand the sonar performance limitations, and

enable quantitative sonar outputs.

This process involved multiple distinct measurements to quantify the acoustic output of the sonar.

Calibration procedures for multibeam sonars have been previously described in References [12–15].

While some of these measurements could be made at ranges up to 3.5 m in the DRDC Atlantic

acoustic test tank, far-field target sphere measurements had to be conducted at the DRDC

Acoustic Calibration Barge.

The following were specific areas of investigation:

1. Background noise levels—measured sonar signal output while the sonar transmitter was

turned off, at varying range and receiver gain settings.

2. Clipping levels—measured using a separate transmitter to generate 90 kHz signals on the

main sonar axis, with varying receiver gain.

3. Transmitter source levels—measured using a calibrated hydrophone on the main sonar axis,

with varying transmitter power levels and pulse lengths.

4. Transmitter horizontal and vertical beamwidths—measured with a single hydrophone while

either rotating the sonar head (horizontal) or varying the hydrophone vertical location

(vertical).

5. Echo strength from reference target spheres—conducted by suspending a sphere in the sonar

beam and rotating the sonar so the target can be seen across the sonar aperture with

successive pings. The apparent angular width of the target is then a measure of the sonar

beamforming resolution.

6. Receiver bandwidth (Doppler capability)—measured receiver response vs. frequency using a

separate transmitter to generate either continuous or pulsed-CW signals.

These will be described in turn below.

The sonar data were extracted from the raw sonar recordings using a two-stage process, as

explained in Section 2.3. The SM-2000 Time-Varying Gain (TVG, in dB) was set to the system

default of

20log10[r] + 21r, (A.1)


where r is the range in metres and 1 is the acoustic absorption, having the value 0.019 dB/m. A

fixed gain offset is available, but this was set to 0 dB for the field trials and most of the acoustic

calibrations. This TVG is limited to a maximum value of 122 dB, then remains constant. This

limit only occurs when using an additional fixed gain offset. Note that the default value of 1

under-predicts the true value, which should be close to 0.0256 dB/m under the sea-water

conditions encountered in these measurements (using relations found in [5]).

It was expected that some calibration parameters would be dependent on the sonar range settings,

for example due to changes in pulse length and bandwidth settings (see Table 1, Section 2.2).

Therefore all of the measurements were repeated using multiple sonar range settings.

A.1 Sonar Self-Noise Levels

The background self-noise levels in the sonar were measured while the sonar transmitter power

was turned off, at varying range and receiver gain settings. In this mode the internal self-noise of

the sonar was amplified by the TVG, so that these measurements also served as verification of the

TVG variation. The raw sonar amplitude was averaged over all 128 beams. A minimum of

20 pings (snapshots) were averaged.

Figure A.1: Comparison of sonar self-noise intensity (dB re sonar units) at various

sonar range settings. TVG given in Eq.(A.1) with -92 dB and -82 dB offsets.

As shown in Figure A.1 the background self-noise was strongly controlled by the TVG, and had a

mild dependence on the sonar range setting. In this figure a fixed offset to TVG curves was

applied to match the raw sonar intensity. There appeared to be two components of the self-noise:

(i) a small transducer noise level amplified by the TVG, and (ii) a fixed (in time or range) lower

limit arising after the TVG. The pre-TVG noise appeared to have two distinct values with a

distinct drop (approximately 5 dB) near 130–150 m range. It is hypothesized that the higher noise

level is due to cross-talk from the digital data telemetry output of the sonar head. Beyond 250 m

range the noise levels all asymptote to a single TVG curve.


The post-TVG noise dominated at short ranges, and was distinctly different for the various sonar

range settings. It strongly increased for the longer range settings, varying from -14 dB for the

60 m setting to +2 dB for the 600 m setting. This increase can be partly explained by the fact that

the different sonar settings have a varying digital A/D gain (not shown in Table 1, Section 2.2),

which increases from 1 to 32 times as the sonar range increases from 100 m to 600 m. This digital

gain effect on self-noise was partially counteracted by the decreased receiver bandwidth of the

longer range settings.

A measurement of self-noise levels at varying gain offsets was performed to verify linearity in the

preamplifier and signal processing chain, with result shown in Figure A.2. The self-noise

increased in almost exact correspondence to the fixed gain offset (0, +10, +20, and +30 dB). The

maximum noise levels near +32 dB (re sonar units) were created by the TVG ceiling. As in the

previous figure, there appeared to be a fixed (post-TVG) self-noise level of order 1 sonar unit.

Figure A.2: Comparison of sonar self-noise intensity (dB re sonar units) at

various gain offsets for the 600 m sonar range setting.

A.2 Clipping Levels

It was not possible to drive the sonar to clipping using only the internal self-noise. Thus a

separate measurement was conducted using an external transmitter positioned on the main sonar

axis at a range of 2.5 m. The standard TVG setup (0 dB offset) was used and the sonar transmitter

was turned off. The clipping level was explored by varying the voltage applied to the external

transmitter. A fixed clipping level of 66 dB (re sonar units) was observed at a variety of external

transmitter power levels. No evidence was seen for so-called soft-clipping due to pre-amplifier or

signal-detector non-linearity at higher amplitudes.


A.3 Transmitter Source Levels

The transmitter source levels were measured using a separate calibrated hydrophone mounted on

the main sonar axis at a distance of 2.4 m. The calibrated hydrophone was a RESON model

TC4035, which had a receiving response at 90 kHz of -214 dB re 1 Volt per µPa. Pulse

amplitudes (root-mean-square) at three distinct transmit power levels (low, medium, and high)

and five range settings (100–600 m) were recorded. Recall that the transmit pulse length

increased at larger sonar range settings (as in Table 1, Section 2.2). The observed pulse lengths

were as specified by the manufacturer. Only minor pulse amplitude droop was observed in the

longer pulses and only at the high power setting. Transmitter source level results are shown in

Table A.1. The measured source levels were corrected for spherical spreading. This result

confirms the manufacturer quoted nominal maximum source level of 205 dB. The medium and

low power source levels appeared to have levels -5 dB and -15 dB relative to the high power

setting, respectively.

Table A.1: Measured sonar amplitudes and estimated transmit source levels.

Power

Level

Range

Setting

Pulse Length

(µs)

Measured Amplitude

(mV RMS)

Estimated Source Level

(dB re µPa at 1 m)

High 100 m 220 141 204.9

200 m 385 139 204.8

300 m 550 136 204.7

400 m 660 135 204.6

600 m 990 130 204.3

Medium 100 m 220 79.3 200.

200 m 385 79.2 200.

300 m 550 78.8 199.9

400 m 660 78.5 199.9

600 m 990 77.3 199.8

Low 100 m 220 25.4 190.1

200 m 385 25.4 190.1

300 m 550 25.4 190.1

400 m 660 25.3 190.1

600 m 990 24.9 190.1

A.4 Transmitter Horizontal and Vertical BeamWidths

The transmitter beamwidths were measured in the DRDC calibration tank with a single

hydrophone mounted at a distance of 2.4 m. This was the maximum distance that could be

achieved in the tank while avoiding reflections from the surface and bottom of the tank. For

horizontal beam-pattern measurements the sonar head was rotated through ±70 (relative to the

sonar main axis) in 1 increments. Horizontal measurements were conducted independently at

both high and medium transmit power. For the vertical beam-pattern measurements the


hydrophone was moved vertically in 1 cm increments, with the corresponding elevation angle

calculated using simple trigonometry. The vertical beam-pattern was measured at the sonar main

horizontal axis.

The measured vertical beam-pattern is shown in Figure A.3. The available vertical angle covered

the main beam lobe to -12 dB, and on the upper side out to the first null near +15. The

measured -3 dB beamwidth is 12.8. The vertical beam-pattern is in good agreement with the

reference curve based on a line-source model with length of 66 mm. This measurement shows a

somewhat smaller beamwidth than the manufacturer quoted value of a 13.5 vertical beamwidth.

Figure A.3: Measured SM-2000 vertical beam-pattern (normalized), with

reference curve based on simple line-array with length of 66 mm.

The measured horizontal beam-patterns at both high and medium transmit power are shown in

Figure A.4. Overall the beam-patterns showed the expected ±45 main transmit lobe. However,

the significant variability (±3 dB) across the main lobe suggests that the measurements were

contaminated by near-field artifacts. Close examination of the transmitted pulses found

complicated phase structures that changed rapidly as the sonar was rotated; this is characteristic

of near-field measurements. Furthermore, the fact that there was good agreement between the

high and medium power levels suggests that these artifacts were not due to measurement noise or

error. The entire horizontal measurement was repeated twice, with essentially identical results.

The far-field scale for a HF transducer array (such as this one) can be given by L2/λ, where L is a

characteristic length and λ is the acoustic wavelength. Using a characteristic length of 0.30 m

(half the array width), the far-field scale for this sonar is 5.4 m. Therefore the only way to avoid

these near-field artifacts would be to repeat the measurement at greater range. This was not done

due to logistical constraints on using the Acoustic Calibration Barge.


Figure A.4: Measured SM-2000 horizontal beam-pattern (normalized)

at medium and high transmit power.

A.5 Echo Strength from Reference Target Spheres

The absolute acoustic calibration was conducted by suspending a 40 mm diameter

Tungsten-Carbide sphere in the sonar beam and rotating the sonar so the target could be seen

across the sonar aperture with successive pings. This was conducted at the Acoustic Calibration

Barge at a nominal range of 14.5 m for a variety of sonar range settings. Two separate runs were

conducted at each of five sonar range settings (60–600 m). Similar calibrations were attempted in

the calibration tank, but results were corrupted by the near-field conditions.

The target sphere method (described in more detail in [15]) is based on analytic models for the

backscatter target strength (TS) of the target spheres. Detailed calculation of TS vs. frequency for

two sizes of target sphere is shown in Figure A.5. The 40 mm diameter sphere was selected to

avoid the strong TS drop-out (due to shear-waves resonances inside the sphere) in the 38.1 mm

sphere at 90.5 kHz. The 40-mm sphere has analytic TSsphere of -39.0 dB at 90 kHz. This value is

used in a sonar equation to estimate a value for an acoustic calibration coefficient, K, hence:

K = TSsphere - 20log10[A] – 40log10[r] - 2αr + TVG(r) + FG (A.2)

where A is the amplitude of the target echo, r is range (m), FG is the fixed gain offset (dB), and α

is the acoustic absorption coefficient (0.027 dB/m for acoustic calibration barge conditions, using

relations in [5]). A FG = 0 dB was used as these calibrations were conducted using the high sonar

transmit power setting, same as in the field tests.


Figure A.5: Analytic prediction of Tungsten-Carbide sphere backscatter

target strength vs. frequency for two sphere diameters.

An analysis routine was written to extract estimates of the target sphere echo strength from

successive ping snapshots. Firstly the maximum target echo in the vicinity of the previous

estimate was found. Then 10-point vectors in range and beam samples were extracted and

separately fit with a Gaussian curve-fitting routine. The average of the range- and beam-wise

Gaussian fit amplitudes was taken as the final echo-amplitude. Two times the beam-wise

standard-deviation, converted to degrees, was taken as the angular beamwidth. Two times the

range-wise standard-deviation, varying from 0.2 to 1.0 m, was found to be consistent with the

sonar pulse length, which increased at longer range settings as shown in Table 1.

Figure A.6 shows an example of the target sphere calibration results. In this example

approximately 190 target echoes were collected over the 90 aperture of the sonar. The sphere

echo amplitude exhibits some variability across the aperture; it is speculated that this is due to a

combination of variations in beam-wise sensitivity and slight variations in the position of the

sphere. The sensitivity drops off on the outer beam edges (angles < 5 and > 85). Within the

main part of the sonar aperture the average amplitude was 437 sonar units (52.8 dB). Note that the

angular width reached a minimum near 1.8 within the central 40, then increased towards the

outer edges of the sonar aperture. This result is somewhat larger than the manufacturer’s

specification (1.5). The angular resolution reached a maximum near 2.9 at the outer edges of the

sonar aperture. Results from other sonar range settings were qualitatively similar. A summary of

the estimated calibration coefficients is shown in Table A.2. Some minor variation in sensitivity

at different range settings is expected due to variations in the sonar setup parameters. An average

value for the calibration coefficient of -112.7 dB will be used for range settings of 200 m and

greater (common in the sea-trials).


Figure A.6: Example target echo strength parameters extracted from a 90 sweep using

the 60 m sonar range setting. Target sphere was located at 14.3 m range.

Table A.2: Summary of SM-2000 sonar calibration parameter, using Eq.(A.2).

Sonar Range

Setting

Average Echo Amp

(sonar units)

Estimated Calibration Coeff.

(dB re µPa)

60 m 421 -114.9

100 m 433 -115.1

200 m 334 -112.9

400 m 319 -112.5

600 m 330 -112.8

A.6 Receiver Bandwidth

These measurements were conducted using an external transmitter positioned on the main sonar

axis at a range of 2.9 m. The standard TVG setup (0 dB offset) was used and the sonar transmitter

was turned off. Two separate approaches were used: (i) CW signals were transmitted, with

resulting amplitude vs. range data averaged over 20–30 pings, and (ii) a 1 ms pulse was

transmitted synchronously with the sonar pulse cycle so that the pulse appeared at the same range

in successive pings. Received pulses were then processed similarly to target sphere echoes

described in the previous section. In both cases the transmitted frequency was varied between

90 and 94 kHz (the receiver band was assumed to be symmetric about the center frequency).

Figure A.7 shows a comparison of results for the CW and pulsed methods for the 600 m default

high-resolution sonar setup. From both tests it was clear that the sonar receive bandwidth was

centered near 90.8 kHz. The CW method shows a much narrower bandwidth, which is attributed


to beam-wise changes in the apparent insonification at frequencies away from the peak. Hence the

pulsed method is assumed to be more accurate. From the pulsed result the -3 dB total bandwidth

was 800 Hz, which is in complete agreement with the quoted bandwidth for this sonar setup

(see Table 1, Section 2.2). Some evidence for sidelobes near 92.2 kHz and 93.2 kHz can be seen

in the figure. Results for other sonar range settings were qualitatively similar.

Figure A.7: Measured variation in receive response vs.

frequency for default high-resolution 600 m setting.

The immediate implication of this result is that low-Doppler signals should be relatively

unaffected. At this sonar range setting, for a target speed of 5 knots the Doppler shift should be

310 Hz, which is well within the -3 dB bandwidth. However, for target speeds of 20 knots

(corresponding Doppler shift of 1240 Hz) or greater the bandpass filter reduces the signal by at

least 15 dB. A wider bandwidth setup will be required for high-speed targets.


List of Symbols/Abbreviations/Acronyms/Initialisms

APL-UW Applied Physics Laboratory – University of Washington

ATS Apparent Target Strength (dB re m2, assuming only spherical spreading loss)

BDL Beam Deviation Loss (dB)

CCG Canadian Coast Guard

CCGS Canadian Coast Guard Ship

CPA Closest Point of Approach

CTD Conductivity-Temperature vs. Depth (oceanographic measurement)

CW Continuous Wave

DFO Department of Fisheries and Oceans

DND Department of National Defence

DRDC Defence Research and Development Canada

DT Detection Threshold (dB)

GPS Global Positioning System

HF High Frequency (underwater acoustic: 10 to 100 kHz)

IOS Institute of Ocean Sciences (Sidney, BC)

MoP Measure of Performance

NATO North Atlantic Treaty Organization

Pd Probability of Detection

SE (acoustic) Signal Excess (dB)

SL (acoustic) Source Level (dB re 1 µPa at 1 m)

SNR Signal to Noise Ratio (dB)

SPL (acoustic) Sound Pressure Level (dB re 1 µPa)

TL (acoustic) Transmission Loss (dB)

TS (acoustic) Target Strength (dB re m2)

TVG Time-Varying Gain (dB)

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Assessment of HF Sonar Performance from a Surface Ship

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Trevorrow, M. V.

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This Scientific Report presents a quantitative analysis of sea-trial data assessing the target

detection performance of a High-Frequency (HF, 100 kHz) horizontally-oriented, multibeam

sonar from a surface ship. Forward-looking target detection tests were conducted in two

different locations, featuring upward- and downward-refracting sound speed profiles under

sea-states from 0 to 4. Ship wake effects were evaluated in aft-looking tests with a towed target

at ship speeds of 6–7.5 knots. The HF sonar output was calibrated so that absolute reverberation

levels and acoustic target strengths could be assessed. In forward-looking tests the targets were

detected at ranges up to 580 m in low sea-states, but detection range was reduced to roughly

150–300 m under higher sea states. In the aft-looking sonar tests the target was detectable up to

450 m range. In both cases signal to noise ratios of 10–25 dB (range dependent) were routinely

observed. Near-surface sound speed gradients were found to produce important acoustic

propagation effects in both cases. Significant ping-to-ping variability in both target echo

strength and background reverberation levels was observed in all tests. A HF sonar performance

modeling tool generated predictions in reasonable agreement with the sea-trial results.

---------------------------------------------------------------------------------------------------------------

Le présent rapport contient une analyse quantitative des données d’essais en mer pour évaluer la

performance de détection de cibles au moyen d’un sonar multifaisceaux haute fréquence

(100 kHz), orienté horizontalement, à bord d’un navire de surface. Des essais de détection

frontale de cibles ont été réalisés à deux endroits différents, révélant des profils de vitesse du

son, à réfraction vers le haut et à réfraction vers le bas, dans des états de mer de 0 à 4. Les effets

de sillage du navire ont été évalués lors d’essais de détection arrière, sur une cible remorquée, à

des vitesses de 6 à 7,5 noeuds. La sortie du sonar HF a été étalonnée de manière à ce que les

niveaux de réverbération absolue et l’indice de réflexion acoustique puissent être évalués. Lors

des essais de détection frontale, les cibles ont été détectées à une distance pouvant atteindre

580 m lorsque la mer était calme, mais la portée de détection était réduite à près de 150 à 300 m

lorsque la mer était agitée. Pendant les essais de sonars à balayage arrière, la cible était

détectable jusqu’à une distance de 450 m. Dans les deux cas, on a couramment observé un

rapport signal-bruit de 10 à 25 dB (en fonction de la distance). Des gradients son vitesse, près de

la surface, ont eu d’importants effets sur la propagation acoustique dans les deux cas. On a

également observé une variabilité « ping-à-ping » importante du niveau de l’écho des deux

cibles et du niveau de réverbération à chaque essai. Un outil de modélisation du rendement du

sonar HF a généré des prévisions montrant une concordance raisonnable avec les résultats

obtenus lors des essais en mer.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful

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e.g., Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which are

Unclassified, the classification of each should be indicated as with the title.)

HF multibeam sonar; bubbles; ship wakes

Documents

Assessment of HF Sonar Performance from a Surface Ship · DRDC-RDDC-2016-R053 i Abstract This Scientific Report presents a quantitative analysis of sea-trial data assessing the target