CATSI EDM – A New Sensor for the Real-Time Passive Stand-off Detection and Identification of Chemicals

Jean-Marc Thériault1, Paul Lacasse2, Hugo Lavoie1, François Bouffard1, Yan Montembeault3,Vincent Farley*3, Louis Belhumeur3 and Philippe Lagueux3

1Defence Research and Development Canada (DRDC) Valcartier, 2459 Pie-XI Blvd North, Val-Bélair, Qc, Canada, G3J 1X5

2AEREX Avionique Inc., 324, avenue St-Augustin, Breakeyville, Qc, Canada, G0S 1E1 3Telops, Inc., 100-2600 St-Jean-Baptiste, Quebec, Qc, Canada, G2E 6J5

ABSTRACT

DRDC Valcartier recently completed the development of the CATSI EDM (Compact ATmospheric Sounding Interferometer Engineering Development Model) for the Canadian Forces (CF). It is a militarized sensor designed to meet the needs of the CF in the development of area surveillance capabilities for the detection and identification of Chemical Warfare Agents (CWA) and toxic industrial chemicals (TIC). CATSI EDM is a passive infrared double-beam Fourier spectrometer system designed for real-time stand-off detection and identification of chemical vapours at distances up to 5 km. It is based on the successful passive differential detection technology. This technique known as optical subtraction, results in a target gas spectrum which is almost free of background, thus making possible detection of weak infrared emission in strong background emission. This paper summarizes the system requirements, achievements, hardware and software characteristics and test results.

Keywords CATSI EDM, passive standoff detection, remote sensing, area surveillance, detection of CWA, detection of TIC

1. INTRODUCTIONIn the early 2000s, DRDC Valcartier demonstrated the high potential of passive infrared differential technology for standoff detection and identification of Chemical Warfare Agents (CWA) and Toxic Industrial Chemicals (TIC) (Refs. 1-3). Based on this success and the requirement to fill the need for an Area Detection and Identification system capability for the Canadian Forces, DRDC Valcartier initiated the development of the CATSI Engineering Development Model. In early 2005, a contract was awarded to Telops Inc for the development and fabrication of two prototypes while DRDC retained responsibility for the algorithm development and final system performance testing.

CATSI EDM is a state of the art passive infrared double-beam Fourier spectrometer sensor designed for real-time stand-off detection and identification of chemical vapours at distances up to 5 km. It successfully combines well balanced dual-beam Fourier Transform Infrared (FTIR) interferometer outputs with two adjacent field of views to optically suppress the spectral background and the instrument self emission spectrum. This technique is called optical subtraction. It resulted in a target gas spectrum almost free of background, which makes possible detection of weak Infrared (IR) emissions in strong background emissions. The sensor combines this technology with state of the art calibration, detection and identification algorithms to provide real time situation awareness of chemical threats for contamination avoidance and early warning.

This paper provides a brief résumé of the sensors requirements and presents hardware and software characteristics and some test results.

* vincent.farley@telops.com; phone (418) 864-7808; fax (418) 864-7843; www.telops.com

2. PERFORMANCE AND TECHNICAL REQUIREMENTS The CATSI Engineering Development Model project was devised to produce an improved, ruggedized and pre-production model version of the CATSI instrument required to meet Canadian Forces military scenarios. Its performance and technical requirements were derived from the preliminary operational requirements of the chemical agent sensor area detection and identification system and founded on the technology, algorithms and lessons learned from the CATSI instrument. The CATSI EDM fundamental design criteria defined the sensors broad performance characteristics as compact, self contained, crew transportable and fully automatic. In addition, the design approach called for the use of commercial-of-the-shelf components as much as possible to keep the cost down.

The CATSI EDM has been designed to meet the following salient performance criteria:

A service life expectancy of 10 years;

Operable by the 5th to 95th percentile of the Canadian Forces population;

24 hour continuous operation;

360° azimuth and -10° to 40° elevation spatial scanning;

Automatic detection and identification (D&I) of gaseous CWAs and TICs at distances up to 5 km;

Provide situation awareness to commander with alarms, and provide the message ATP 45 NBC4 (Allied Technical Publication 45 Nuclear Biological and Chemical 4);

Operable with NBC individual protective equipment and night vision goggle;

Operate on rechargeable batteries and various power sources;

User friendly software interface;

Minimum operator and 1st line maintenance;

Mean time between unscheduled maintenance of 720 hrs; and

Set up without tools.

The CATSI EDM was designed to meet the following technical requirements:

10 mrad, single pixel field of view;

Dual field of view;

Radiometrically balanced Michelson double beam interferometer with double beam input and corner cube reflectors and symmetrical beamsplitter;

Maximized beam intensity transmission and modulation efficiency for the spectral band from 650 cm-1 to 1450 cm-1 and automatically selectable spectral resolution at 4, 8 and 16 cm-1;

Optimized telescope module to accommodate both differential and direct detection operation and maximized transmission;

High resolution view finder display bore sighted with Field Of View (FOV);

Operate with a local console or a remote console;

Have a remote console, which can be located at up to 70 meters from the sensor and can control up to 6 sensors;

Ruggedized and militarized to meet CF global operations; and

Encrypted and updatable CWAs and TICs library.

3. CATSI EDM HARDWARE

3.1 General Description

The CATSI EDM sensor is presented in Figure 1. It consists of an optical head (OH), a Scene Scanning Mechanism (SSM), an adjustable tripod, a local console/Central Processing and Control (CPC) computer, a remote console, an AC-DC converter, a DC-DC converter, a battery pack, an Ethernet switch and cables. The OH is cantilever mounted on the SSM and tripod. The OH and the SSM are controlled by local consol/CPC module or with the remote console. The electrical power, spectral data, command and control information are fed through cables from the CPC to the SSM and through a slip ring and cables to the OH. The CPC runs on a battery pack or DC or AC power sources using applicable power converters. The CATSI EDM is completed with a ruggedized remote console which can be located at up to 70 m from the CPC. The remote console is connected directly to the CPC through a 25 m Ethernet cable or via a 70 m Ethernet cable and a small Ethernet switch module. When using the switch module, the remote console can control up to six CATSI EDM systems. The operator can perform ranging function with the remote console when using a combination of two sensors.

Figure 1. CATSI EDM hardware

3.2 Optical Head

The optical head (OH) contains the following sub modules:

Viewfinder: This is a color camera bore sighted on the Optical Head FOVs. From the remote console it provides a view of the scene currently under observation by the OH. The user can utilize it to set up scanning parameters.

Telescope module: This module consists of an aperture window and a smaller lens objective assembly, two shutters assemblies and one collimating assembly. The collimating assembly encompasses two identical optical paths composed of a miniature lens, a splitting device, a collimating lens and a folding mirror. The telescope module transmits the scene signal to the modulator module.

Modulator: A Michelson Fourier transform Infrared sweeping arm type modulates the two beams IR signal received from the telescope module into many wave lengths and sends them to the detector module.

Detector: This module encompasses a condenser lens, a detector window and a photoconductor (PC) single pixel detector.

Calibration source: The OH uses an external calibration plate. The calibration ensures that the system adapts correctly to its operating medium. The plate is uniformly heated to allow a two step calibration with an acceptable thermal contrast. System calibration occurs at start up and at preset frequency thereafter.

The sensor head optical design is at the heart of the sensor concept. The key requirements for this optical design are:

Minimize the components volume and the optical path (to reduce the overall weight);

Maximize infrared light transmittance from 7.4 to 14.3μm;

Minimize the optical beam diameter at one location for each input port in order to insert a shutter blade that switches the system from the direct to differential mode in less than 5 ms;

A FOV of 10 mrad and a telescope diameter of 100 mm; and

Dual inputs superimposition on a common detector.

Considering all these requirements, different lens trade off analysis were performed in order to minimize the number of lenses, maintain the image quality and to ease the optical alignment. As a result, aspherical lenses with high refraction index (lenses made of germanium, zinc selenide and cadmium telluride) were used in a three-dimensional layout to reduce the system size. The same telescope is also used for both inputs in order to minimize the system weight. Figure 2 shows the optical design ray tracing of CATSI EDM where the modulator mirrors and the input splitter prism have been removed for clarity purpose.

Figure 2. CATSI EDM optical design ray tracing

Fast-shutter positions

The difference between the radiometric signals provided by both inputs is shown in Figure 3.

700 800 900 1000 1100 1200 1300 14000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Wavenumbers [cm-1]

Inpu

ts U

nbal

ance

[%]

Figure 3. CATSI EDM relative unbalance between both inputs

3.3 Local Console

The local console houses the Central Processing and Control computer of the CATSI EDM sensor. It is the main interface between the user and the CATSI EDM OH and it provides the essential functionalities and controls to operate one OH & SSM combination. It offers the following main features:

A switch to turn on and turn off the system.

Central processing and control of all OH and SSM functionalities via a computer, a hard drive, firmware and software. It provides all communication and control functionality, data acquisition and processing features and alarms to operate CATSI EDM.

A screen with an easy to use menu combined with a ruggedized keyboard to set up and operate the system. These include setting of azimuth and elevation scanning parameters, inserting GPS coordinates, performing fine azimuth calibration and system self test, modifying alarms signal features, selecting language of operation, adjusting screen contrast, ordering system standby, and changing system boost factor for detection and system shut down.

Visual and audible alarms.

Operable with NBC protective equipment (including gloves) during the day or at night.

A bilingual interface.

A self test and diagnostic capability.

Conversion and distribution of power to the local console subcomponents, the SSM, the OH and the Ethernet switch.

3.4 Remote Console

The remote console is a ruggedized Dell Latitude XFR D630 lap top computer. It offers all the features to operate, configure and troubleshoot the CATSI EDM sensor. As its name implies, it can be used from remote location up to 70 meters of distance from the local console. It can operate up to six local consoles and associated SSMs and OHs. The main capabilities and characteristics available from the remote console are described under the user software section.

4. CATSI EDM SOFTWARE The CATSI EDM software has been designed to provide an automatic situation awareness of the scene’s gaseous CWA and TICs with minimum operator inputs. The firmware and the software manage, without user input, all the sensor’s functionality modes such as system warm up, initialization, calibration, detection and identification of threats. It also adapts the threshold of the detection algorithm to its scanning environment as we will see later in the description of the detection and identification software.

4.1 User Interface

The local console software provides direct access to the sensor main operating features. The friendly graphical user interface (GUI) software guides the user through the local console display indicates current system operating modes and offers various menus and options. Through a number of console interface buttons and by using the various menus and options available on the local console display, the user can control all the sensors basic operations.

Through the remote console software, the user has access to a much broader range of the sensors functionalities. It operates under Windows XP operating environment. It is programmed in C++ and uses Extended Mark up Language (XML) format for configuration files. The remote console software provides all the functions and GUI necessary to configure and control up to six OH. It provides four levels of user access based on their responsibilities: Super-administrator, Administrator, User and Viewer.

The remote console software offers the following main features:

Provides a friendly window type interface with various degrees of features and capabilities depending on the user role and responsibility;

Provides real time monitoring of the detailed status of up to six OH;

Displays the operating mode status of OH;

Provides warnings, errors, and gas alerts history listing;

Provides OH internal temperature status;

Provides ranging information on identified threat (need at least two OHs);

Allows the creation of new sessions and presets for D&I optimisation;

Provides real time diagnostic monitoring of spectral signal;

Allows setting of search window and scanning parameters using the view finder and boost factor;

Provides a mean to enter the OH GPS coordinates;

Provides real time observation of the field of view through the viewfinder camera;

Offers a scientific diagnostic window for scientific application;

Provides gas alert window; and

Fills up and provides NBC-4 report threat information to the user.

4.2 Calibration Methodology

CATSI EDM is programmed to perform automatically and regularly two points radiometric calibrations without user input to offer optimal detection and identification performance. The sensor operates with default calibration parameters but they can also be modified using the presets available in the administrator mode.

For a Fourier transform spectrometer based on a double–input port interferometer, such as the CATSI EDM system, the total signal S can be developed as in equation 1.

'112122

'2211 DD OLKKLLKOLKLKS (1)

where

K1 and K2 are the radiometric gains of input 1 and input 2 in which enter radiances L1 and L2 respectively; and

O’D is the instrument offset

Since K1 -K2 in CATSI EDM, the gains are almost symmetrical and the linear system associated with the traditional two point calibration (Ref. 4) equation becomes ill-conditioned.

The calibration equation can be rearranged to make explicit the differential radiation L2 - L1. To do so, the scene differential measurement of both reference temperatures (hot and cold) are detailed in equations 2 and 3.

'21

'21 D

hBBD

hBB

hBB

hD OLKKOLKLKS (2)

'21

'21 D

cBBD

cBB

cBB

cD OLKKOLKLKS (3)

From these two measurements, the differential gain and the instrument offset are calculated with equations 4 and 5.

cBB

hBB

cD

hD

LLSSKK 21

(4) hBB

hDLD LKKSO 21

')( (5)

The subscripts and superscripts appearing in the above equations mean: BB-blackbody, D-detector, h-hot blackbody, c-cold blackbody, L-radiance. As the instrument offset is function of the radiance incoming in the instrument (term (K1 + K2)L in equation 5), the calibration of an unknown differential scene needs a direct acquisition just before a differential measurement in order to correct the radiometric offset bias caused by the unbalanced inputs. The differential offset obtained is then defined in equation 6.

'21)( DLD OLKKO

(6)

With the differential gain and offset calculated and the equation 1, the differential radiance between both input ports can be determined using equation 7.

2

)(12

1

KOS

LL LD

(7)

If the direct scene used to correct the differential offset is obtained using the input 2 instead of the input 1, the equation is then modified as in equation 8.

1

)(21

2

KOS

LL LD

(8)

4.3 Detection & Identification Software

The CATSI EDM utilizes a proprietary algorithm named Chemical Agent Spectral SIgnature Detection and Identification (CASSIDI) to perform detection and identification of gaseous Chemical Warfare Agent and Toxic Industrial Agents. The software is built on a three stages algorithm which are detection, confirmation and alarm. In the detection stage, active during the searching mode, CASSIDI receives a continuous stream of spectral signature measurements from the sensors FOV which is compared to the spectral signature library targets signatures. It uses the structural background General Likelihood Ratio Test (GLRT) algorithm to perform that first stage, which thereby identifies potential gas candidate.

When potential candidates are identified, CASSIDI enters in its second stage, the Confirming mode. The sensor increases its spectral resolution, performs measurements within the immediate surrounding area of the detected potential

candidates and applies the non linear fitting GASeous EManations (GASEM) algorithm to the spectral measurements. GASEM calculates quantitative estimates of the gases physical parameters then passes the results to the alarm mode. In the last stage, the algorithm compares the candidate(s) quantitative estimates with predefined thresholds and they are fitted against the sensor spectral signature library candidates. If a number of conditions are satisfied, the alarm mode trig an alarm and provides gas(es) information to the user. At this stage, the sensor will attempt tracking the gaseous cloud displacement. When two or more CATSI EDM sensors are connected to the same remote console, the first one to detect a candidate gas will slave a second sensor and attempt to provide ranging information on the cloud. If the conditions to trigger an alarm are not satisfied, the sensor will simply return in the searching mode.

CASSIDI also uses adaptive detection and identification algorithms by the use of a background buffer. Utilizing previous measurements of the scanned background to evaluate their statistics, the algorithms adapt to the scanning environment. This technique features a feedback loop which allows stage 1 low scores below a certain threshold to be considered as background and incorporated in the background buffer. This is an important feature of the algorithm, which boosts its sensitivity but causes its behavior to change over time and therefore complicates the configuration of its various parameters. Figure 4 shows a simplistic schematic of the CASSIDI algorithm.

Figure 4. CATSI EDM detection and identification algorithm schematics, including classification, background buffers and target signatures

5. TEST RESULTS The CATSI EDM has been subjected to several subsystem and system functional and performance validation tests throughout its development. In September 2007, a user test provided valuable insights and recommendations which guided the remaining of the development effort. In September and November 2008, CATSI EDM was subjected to performance and evaluation testing on simulants and CWAs under laboratory setups and on representative simulants clouds in a field testing trial. During these tests, the sensor was subjected to hundreds of events to evaluate, with statistical confidence, its sensitivity and D&I performance. In laboratory or when faced with a representative asymmetric threat scenarios of CWAs and TICs using simulants at 3 km and 5 km distances, CATSI EDM successfully met the sensitivity requirement and surpassed the required 80% probability of detection with a 95% confidence level.

Figure 5 shows examples of laboratory and field differential spectra measurements where the Sulfur Hexafluoride (SF6)signature is clearly detected.

Confirmation Alarmmeasurementstarget

candidatesquantitative

results yes / no

backgroundmeasurements

Targetsignatures

Backgroundbuffer

DetectionClassification

0,2

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0,7

0,8

0,9

1

800 900 1000 1100 1200 1300

Laboratory Result: Detection of SF6

Diff

eren

tial R

adia

nce

(a.u

.)

Wavenumber (cm -1)

0,2

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0,9

1

800 900 1000 1100 1200 1300

Field Result: Detection of SF6 at 5 km

Diff

eren

tial R

adia

nce

(a.u

.)

Wavenumber (cm -1)

SF6

Figure 5. Example of differential spectra for the Lab and the Field portion (5 km) of the test

6. CONCLUSION Over the past 5 years, DRDC Valcartier and Telops Inc. have developed CATSI EDM, a state of the art passive IR stand-off detection sensor. Designed to be employed in a chemical agent sensor area detection and identification role, CATSI EDM benefits from a rugged design, compatible with the requirements of military field deployable equipment. Its hardware and software provide optimal sensitivity through adaptive processing of the background environment, automatic calibrations, 360° azimuth and -10° to +40° elevation surveillance and real time detection and identification of CWAs and TICs at up to 5 km distances. It is capable of operating in network providing large site protection capabilities. Finally, CATSI EDM demonstrated detection performance exceeding a probability of 80% at a 95% confidence level at up to 5 km distances during field testing.

REFERENCES

1. Thériault, J.-M., Puckrin, E., Bouffard, F. and Déry, B., “Passive Remote Monitoring of Chemical Vapors by Differential FTIR Radiometry: Results at a Range of 1.5 km,” Appl. Opt. 43, 1425-1434 (2004).

2. Lavoie, H., Puckrin, E., Thériault, J.-M. and Bouffard, F., “Passive standoff detection of SF6 at a distance of 5.7 km by differential FTIR radiometry,” Appl. Spectros. 59(10), 1189-1193(5) (2005).

3. Lavoie, H., Puckrin, E. et al., “Detection and Identification of Toxic Chemical Vapours in an Open-Path Environment by a Differential Passive LWIR Standoff Technique,” International Journal of High Speed Electronics and Systems, ISSSR proceeding 18(2), 457-468 (2008).

4. Revercomb, Henry E., Buijs, H., Howell, H. B., LaPorte, D. D. Smith, W. L. and Sromovsky, L. A., “Radiometric calibration of IR Fourier transform spectrometers: solution to a problem with the High-Resolution Interferometer Sounder,” Appl. Opt. 27(15), 3210-3218 (1988).