A Preliminary Study of Loran-C Additional Secondary Factor (ASF) Variations International Loran Association 31st Annual Convention And Technical Symposium

A Preliminary Study of Loran-C A Preliminary Study of Loran-C Additional Secondary Factor Additional Secondary Factor

(ASF) Variations(ASF) VariationsInternational Loran AssociationInternational Loran Association

31st Annual Convention And Technical Symposium 31st Annual Convention And Technical Symposium October 27-30, 2002October 27-30, 2002

10/30/0210/30/02 USCGA-JJMA Presentation for ILA-31USCGA-JJMA Presentation for ILA-31 22

AuthorsAuthors

CAPT Richard HartnettCAPT Richard Hartnett US Coast Guard AcademyUS Coast Guard Academy

Gregory JohnsonGregory Johnson John J. McMullen Assoc. Inc.John J. McMullen Assoc. Inc.

Peter SwaszekPeter Swaszek University of Rhode IslandUniversity of Rhode Island

Mitchell J. NarinsMitchell J. Narins US Federal Aviation AdministrationUS Federal Aviation Administration


OutlineOutline

IntroductionIntroduction Methodology of Data CollectionMethodology of Data Collection Error Sources and BoundsError Sources and Bounds August Flight TestsAugust Flight Tests Highlights of ASF Corrected Loran dataHighlights of ASF Corrected Loran data ConclusionsConclusions Future workFuture work


IntroductionIntroduction

Concept:Concept: LORAN-C as “the” backup for GPSLORAN-C as “the” backup for GPS NPA – RNP 0.3NPA – RNP 0.3 Obvious issues: accuracy, availability, Obvious issues: accuracy, availability,

integrity, and continuity integrity, and continuity

USCGA Goal: USCGA Goal: Improve our understanding of those variations Improve our understanding of those variations

in TOA that could be mitigated (ASFs) in TOA that could be mitigated (ASFs)


This report:This report: Report on flight tests (August 2002) to collect Report on flight tests (August 2002) to collect

concurrent LORAN and WAAS GPS position concurrent LORAN and WAAS GPS position data (equipment, methodology, variation data (equipment, methodology, variation characteristics)characteristics)

From ground data estimate TOA ASF in the From ground data estimate TOA ASF in the vicinity of an intended approachvicinity of an intended approach

(post-)Process some approach flight data to (post-)Process some approach flight data to determine feasibility of 1/6 NM accuracy using determine feasibility of 1/6 NM accuracy using local correction local correction


Data Collection MethodologyData Collection Methodology

Use USCGA PC-104 TOA ASF Measurement Use USCGA PC-104 TOA ASF Measurement SystemSystem Run receiver in reference mode at a known location Run receiver in reference mode at a known location

(surveyed or DGPS/WAAS position)(surveyed or DGPS/WAAS position) Save TIC and RAW TOA, SNR, ECD measurements Save TIC and RAW TOA, SNR, ECD measurements

for each station at 2 sec. intervalsfor each station at 2 sec. intervals Collect data over 1-2 hr periodCollect data over 1-2 hr period Post-process to calculate ASF’s at 2 sec. intervalsPost-process to calculate ASF’s at 2 sec. intervals ASF, SNR, and ECD are used to determine if ASF is ASF, SNR, and ECD are used to determine if ASF is

validvalid Average ASF’s over the 2 hr periodAverage ASF’s over the 2 hr period


TOA ASF Measurement TOA ASF Measurement SystemSystem

H-field antenna

Novatel OEM-4WAAS GPS

PC-104Rcvr

1 PPS

DS-345Signal Generator

10 MHz

12.8 MHz 2 Channels RF

Save to disk: TIC, RAW TOA,ECD, SNR, etc.


PC-104 Based TOA ASF PC-104 Based TOA ASF Measurement SystemMeasurement System

CPU board, Gatefield CPU board, Gatefield DSP board, oscillator DSP board, oscillator board (opt), Power board (opt), Power and I/O boardsand I/O boards

Provide stabilized Provide stabilized clock frequencyclock frequency 12.8 MHz from Cesium

clock stabilized signal source

GPS 1PPS signalGPS 1PPS signal H-field antennaH-field antenna

Two crossed loops


ASF CalculationASF Calculation

1 PPS

PCI Strobe

Master Xmit Station Xmit

(Master offset from 1 PPS) mod 200 μsec = 0

Time Interval Counter Raw TOA

Emission Delay Predicted all seawater path TOA

ASF

Emission Delay Correction

Receiver Delay

Loran signal rcvd

(TIC + Raw TOA - Rcvr Delay - Emission Delay - (TIC + Raw TOA - Rcvr Delay - Emission Delay - Predicted TOA ) mod 200 μsec = EDC + ASFPredicted TOA ) mod 200 μsec = EDC + ASF


ASF DefinitionASF Definition

Define TOA ASF as the additional propagation Define TOA ASF as the additional propagation delay due to a non-seawater pathdelay due to a non-seawater path

Need to separate out system timing errors from Need to separate out system timing errors from ASF valueASF value Need to know exactly when stations transmit, relative Need to know exactly when stations transmit, relative

to UTC (use GPS 1 PPS as time reference).to UTC (use GPS 1 PPS as time reference). Master: existing TTM equipment Secondary: LSU/PIG installed TOTM equipment

Difference between measured and calculated Difference between measured and calculated TOA is ASF + time offsetsTOA is ASF + time offsets


Nantucket TOTMNantucket TOTM

Secondary TOTM data for time period of flight Secondary TOTM data for time period of flight tests (Julian days 233-241, 2002)tests (Julian days 233-241, 2002) X-axis = hours starting from midnight of first dayX-axis = hours starting from midnight of first day Y-axis = offset from UTC in nsY-axis = offset from UTC in ns

9960 5930


Carolina Beach TOTMCarolina Beach TOTM

9960 7980


ASF Measurement Error SourcesASF Measurement Error Sources

Timing ErrorsTiming Errors Master and SecondaryMaster and Secondary

Receiver ErrorsReceiver Errors 1 PPS stability1 PPS stability Sampling resolutionSampling resolution Receiver tracking accuracyReceiver tracking accuracy Receiver calibrationReceiver calibration Cross-coupling in H-field antennaCross-coupling in H-field antenna


Error BoundsError BoundsError SourceError Source NotesNotes Error boundsError boundsMaster timing TOTM equip. Unknown, 23 hr avg. only

Secondary timing PIG/LSU TOT monitors 2/4 minute samples, vary +/- 50 ns

1 PPS stability Universal Counter referenced to Cesium

+/- 1 ns for Novatel OEM-4 PowerPak

Sampling resolution 12.8 MHz +/- 39.1ns resolution

Tracking accuracy SNR dependent ~20 ns for strong signal

Receiver calibration Fixed offset Approx. 100 ns

Loop cross-coupling Fixed offset for given heading

+/- 145 ns


Loop cross-couplingLoop cross-coupling

10 15 20 25 30 35 40 45 50 55-2

-1

0

1

2

3

4

5

6Antenna rotation 11 Sept 02; ASF Corrections for 9960

Mic

rose

cond

s

Time (minutes) Seneca - black Caribou - blue Nantucket - green Carolina-B - read Dana - cyan

Nantucket

Carolina Beach

Seneca

Caribou

Dana

Variation in TOAs as aircraft antenna is rotated 720 degrees.


Loop cross-couplingLoop cross-coupling

0 100 200 300 400 500 600 700 800 900-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15TOA Changes Due to Rotation for Nantucket

Mic

rose

cond

s

Data Points0 100 200 300 400 500 600 700 800 900

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15TOA Changes Due to Rotation for Nantucket

Mic

rose

cond

s

Data Points

Close-up of Nantucket data from previous slide


Ohio University King AirOhio University King Air


CG Equipment on King AirCG Equipment on King Air


August 2002 Flight TestAugust 2002 Flight Test

DateDate Location and Airport DesignatorLocation and Airport Designator21 Aug21 Aug Hyannis, MA (HYA)Hyannis, MA (HYA)

22 Aug22 Aug Sandy Hook, NJ (BLM)Sandy Hook, NJ (BLM)

26 Aug26 Aug Cape Elizabeth, ME (PWM)Cape Elizabeth, ME (PWM)

27 Aug27 Aug Bay Bridge area (W29)Bay Bridge area (W29)

Mayport, FL (CRG)Mayport, FL (CRG)

28 Aug28 Aug Plumbrook, OH (5A1)Plumbrook, OH (5A1)

Westerly, RI (WST)Westerly, RI (WST)

29 Aug29 Aug FAA Tech Ctr (ACY)FAA Tech Ctr (ACY)


Flight routesFlight routes

Dana

Malone

Carolina Beach

Seneca


22 August, 9960 Data22 August, 9960 Data

10 20 30 40 50 60 70 80 90 100 110-2

0

2

4

69960 ASF

SenecaCaribouNantucketCarolina-BDana

10 20 30 40 50 60 70 80 90 100 110-10

0

10

20

309960 SNR

10 20 30 40 50 60 70 80 90 100 110-5

0

59960 ECD

Time(minutes)



10 20 30 40 50 60 70 80 90 100 110-2

0

2

4

68970 ASF

DanaMaloneSenecaBaudetteBoise City

10 20 30 40 50 60 70 80 90 100 110-10

0

10

20

308970 SNR

10 20 30 40 50 60 70 80 90 100 110-5

0

58970 ECD

Time(minutes)



10 20 30 40 50 60 70 80 90 100 110-2

0

2

4

65930 ASF

CaribouNantucketCape RaceFox Harbor

10 20 30 40 50 60 70 80 90 100 110-10

0

10

20

305930 SNR

10 20 30 40 50 60 70 80 90 100 110-5

0

55930 ECD

Time(minutes)


8/22 BLM TOA Distributions8/22 BLM TOA Distributions


8/27 W29 TOA Distributions8/27 W29 TOA Distributions


Portland Me, 26 Aug 02; Uncorrected and Corrected LORAN-C verses DGPS

Longitude, DD.dd

Latit

ude,

DD

.dd

-70.7 -70.6 -70.5 -70.4 -70.3 -70.2 -70.1

43.45

43.5

43.55

43.6

43.65

43.7

43.75

43.8

43.85

Uncorrected LORAN-CCorrected LORAN-CDGPS

Portland ME, 26 Aug 02Portland ME, 26 Aug 02

Loran (red), Corrected Loran (green) and DGPS (blue) positions during approach to Portland, PWM.Corrected Loran is the raw TOA’s corrected with measured ASF values, ED offsets, and Master TOT offsets.


Portland ME, 26 Aug 02Portland ME, 26 Aug 02

Radial error measured from WAAS GPS position

13.9931 14.0131 14.0331 14.0531 14.0731 14.09310

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45Portland Me, 26 Aug 02; Uncorrected and Corrected LORAN-C Error

Err

or,

Na

utic

al M

iles

GPS time, HH.hhhh

Uncorrected LORAN-CCorrected LORAN-C

RNAV goal of .165 NM


ConclusionsConclusions

ASF collection procedure:ASF collection procedure: Methodology works Methodology works Further resolution of error sources Further resolution of error sources

Sample flight tests: Sample flight tests: TOA All-In-View receiver (with timing TOA All-In-View receiver (with timing

corrected and static, local ASF correction) corrected and static, local ASF correction) provided <1/6 NM accuracy on approaches provided <1/6 NM accuracy on approaches (to 20 miles out)(to 20 miles out)

Potential for LORAN to achieve RNP 0.3Potential for LORAN to achieve RNP 0.3


Future WorkFuture Work

Examine spatial and temporal ASF variationsExamine spatial and temporal ASF variations Daily data collection at USCGA (multi-week)Daily data collection at USCGA (multi-week) Collect additional spatial data in December and Collect additional spatial data in December and

JanuaryJanuary

Receiver improvementsReceiver improvements Increase sampling rate to 25.6 MHz to increase Increase sampling rate to 25.6 MHz to increase

resolutionresolution Resolve receiver calibrationResolve receiver calibration

Apply ASF corrections to Locus receiver TOAsApply ASF corrections to Locus receiver TOAs


AcknowledgmentsAcknowledgments

Christian Oates, George Sanders, Ken DykstraChristian Oates, George Sanders, Ken Dykstra JJMA, Inc.JJMA, Inc.

Dave Diggle, Brian Branham, Jamie EdwardsDave Diggle, Brian Branham, Jamie Edwards Ohio UniversityOhio University

Ben PetersonBen Peterson P.I.G.P.I.G.

USCG Loran Support UnitUSCG Loran Support Unit USCG NAVCENUSCG NAVCEN

Documents

A Preliminary Study of Loran-C Additional Secondary Factor (ASF) Variations International Loran Association 31st Annual Convention And Technical Symposium