Author: Ing. Jiří Viceník , PhD. et al.

Advantages of Stand-off Detection

No need to get into direct contact with the agent;
Long range (few kilometres);
Fast detection over large area;
Preliminary identification of the agent;
List of detectable agents can be supplemented.

SoD Basics

The only principle useable for stand-off detection of CWA is absorption of optical radiation in far-infra-red (FAR IR) part of optical spectra. Absorption is caused by molecules of the CWA.

Reasons:

All CWAs have strong absorptions bands inside FAR IR part of the spectra;
Atmosphere is “clear” inside the FAR IR, thus enabling long range of such sensors.

Consequences:

Detection of CWA is possible only on condition the agent is present in the air in the vapour form (droplets in the air and liquid on the ground do not contribute directly to selective optical absorption);
Physical properties of CWA vapour cloud and terrain will limit the detection capability of any SoD system.

SoD Potential and Limitation

Potential of SoD :

Long effective range (3 km minimum, 5 km typically; hangs on weather and terrain profile);

Capability to identify the agent;
High sensitivity ( 0.1 µg/l for 1.000 m long measuring path for most agents);
Low false alarm rate.

Limitations of SoD :

Physical properties of the chemical cloud: Molecular weights of all CWAs are very high; as a result, CWA can be found only close to the ground, surely not over 20 m. The situation is illustrated in following picture:

To achieve reliable detection, the measurement (detection) should be done in the dense CWA cloud – close to the ground. SoD system having too big instantaneous field of view (FOV) operates partly through air with lower density of CWA vapors; the probability of detection is much lower. Conclusion: The instantaneous field of view of specific SoD system is essential. System having smaller FOV features correspondingly higher effective range.

Terrain profile along a measuring path: Influence of terrain profile and importance of proper choice of measurement path can be explained using following picture. To detect contamination in the valley, the lowest path can be effective. Aiming at small house cannot detect agent in the valley, but can detect contamination in front of the house. Aiming at the top of the chimney is useless and detection of contamination behind any solid obstacle is not possible, of course.

Limits of technology used in particular SoD system. Any commentary concerning different SoD technologies would require a lot of space. Just one parameter should be emphasized here once again: The instantaneous field of view (FOV) of specific technology.

For any SoD system, two ranges should be specified:

Maximum range: It is the range, at which specific SoD system is able to evaluate the respective optical signals;
Effective range: It is the range, at which specific SoD system is able to detect the agent under realistic scenario and the nominal sensitivity is achieved.

Where is the difference? The maximum range will work on condition the agent is dispersed homogenously in the air. As it was explained above, such assumption is not realistic e.g. CWAs have large molecular weight and can be found close to the ground only. For estimation of the effective range, this phenomenon together with the instantaneous field of view is essential. It can easily happened, that specific SoD has maximal range 5.000 m, however, due to large FOV (e.g. 30 mRad) its effective range is much smaller – 300 m for GB agent. The best system will have “effective range” similar to “maximum range”.

Note: Because different CWAs have different molecular weights, any SoD system has for each agent corresponding effective range.

SoD Tactics

Military attack by CWA

Military attack using CWA will never take place in front line, because the probability, that the CWA will hit both sides, both attacker and attacked forces, is very high. Therefore, CWA can be used mainly to hit support tail of the enemy to achieve following:

Blocking movement of enemy forces;
Blocking food and ammunition supply lines;
Blocking reinforcement.

To achieve such goals, the real CWA military attack should be massive, that means:

The contaminated area should be large – more than 1 x 1 km. If it is not so, it will be easily possible to bypass the contaminated area;
The contamination should be very intense, more than one hundred, maybe several hundred kg of CWA should be used (if it is not so, it will be possible to cross the contaminated area without substantial problems and continue combat activity);
The contaminated area should really intersect the supplying lines.

After dissemination of the CWA the “Primary cloud of CWA” is formed.

The “Primary Cloud” can be characterised as follows:

It will be formed immediately after dissemination of CWA;
Most of the stuff will be in the aerosol form, but an equilibrium between liquid and gaseous form of CWA will be constituted soon;
The vapour concentration will be close to the saturated vapour pressure – very high;
Dimensions of the primary cloud: the footprint > 1 km2; the height will be a few tens of meters;
Stability: From a few minutes to a few tens of minutes (lifetime of primary cloud strongly depends on used dissemination method).

After the sedimentation of the droplets of agent took place, the “Secondary Cloud of CWA” is formed, which can be characterised by following features:

The evaporation from contaminated ground is the dominant process;
As a result of evaporation, the secondary cloud containing only CWA vapour is formed;
Vapour concentration is not as high as in primary cloud.

The size of the cloud will depend on:

Size of contaminated ground;
Weather condition (wind, temperature, convection);
Terrain profile.

An ideal stand-off detection system should be able to detect the “Secondary cloud” (detection of the “Primary cloud” is much more easy).

NBC units (at least in NATO armies) are using detectors of CWA to execute following tasks:

Chemical observation: It is used to protect forces or important military objects against surprise CWA attack;
Chemical reconnaissance: Such activity is intended for location of contaminated area after triggering event;
Checking of marching route: If the troop should move from point “A” to point “B” and the route is not “NBC safe” the commander sends the NBC reconnaissance vehicle in advance. The recce vehicle rides along the planned route and if in the positive detection of CWA takes place, an alternative route is used. The vehicle checks the route also on move using point detectors, of course.

Chemical observation using SoD, “Phase I.”

The SoD system is located at carefully chosen position (e.g. atop a hill) and checks the “chemical” situation in the surroundings using carefully chosen measuring paths, upwind of forces, of course. Such operation of SoD system should generate an early warning and it is often used in combination with remote NBC detectors.

During “Phase I.” of “Chemical Observation”, SoD checks its close surroundings to prevent own contamination. In this example, it is atop small hill and nine directions were chosen. Solid red line in the picture denotes the part of measurement path with high detection probability (close to the ground) while the dashed red line denotes medium detection probability.

The detection along all nine measuring paths takes approx. 1 min (using Falcon 4G).

Chemical observation using SoD, “Phase II.”

On condition the “Phase I.” was negative, the operator will use longer measuring paths to obtain data from larger area. Totally different “fan” of measuring paths can be chosen to achieve the best result. The best result means the best coverage of the area (in the example pictures the “headings” of measurement paths are all the time the same; it is just for simplicity).

Using nine directions, the detection will once again take 1 min. To do the same job using point detectors, it would last 45 to 80 min, because the checked area is larger.

Chemical observation using SoD, “Phase III.”

On condition both the Phase I. and Phase II. were negative, the operator will use measuring paths, which are as long as it is possible. The limitations are given by terrain, weather conditions and SoD technology limits. As in the previous examples, the detection will take 1 min. To do the same job using point detectors, it would last surely more than an hour, maybe even hours.

Long-term chemical observation using SoD

After the “Chem Observation – Phases I. to III.” were negative, the operation of SoD would be more easy: The number of measuring paths can be reduced, the chem. observation is done preferable up-winds. In the previous model country, only four directions enable to have general overview of the chemical situation.

To do the same job using point detectors, at least 10 remotely operated point detectors should be used. If direction of the wind is not stable, the SoD system can be easily re-programmed for respective set of four to five measuring path. To fulfil the “Chemical Observation” under variable wind direction without SoD, it will be necessary to deploy more point detectors, maybe 20 units.

Chemical Reconnaissance

SoD system should be installed atop a NBC reconnaissance vehicle, which moves towards the suspected area and makes short stops approx. each 2 to 3 km. During the stops, the vehicle activates SoD system; the operator selects several proper measuring paths (up to 5 km long). It should be a “fan” of directions in front of the vehicle. In principle, the “Chemical Reconnaissance” is all the time the initial phase of “Chem. Observation”. If all measurements are negative, another 2 - 3 km long step will be done with the required heading. The new position should be inside the previously SoD checked area, of course. After positive detection using SoD takes place, the commander will evaluate the situation. Subsequently, the SIBCRA team is sent to the area, while the vehicle continues monitoring using SoD, or it is withdrawn to prevent its own contamination.

One square in the picture is 1 x 1 km large, so, under ideal conditions from one position up to 24 km2 can be checked from one position.

Besides the avoiding the direct contact with the agent, the major advantage of using SoD for “Chemical Reconnaissance” is its capability to achieve fast detection over large area.

Checking of marching route

The methodology of such activity is the same like during “Chemical Reconnaissance”, but the position of the reconnaissance vehicle with SoD installed is all the time close to the planned marching route. Comparison with the use of “point detectors” for checking of marching route is also very interesting. The reaction time of most point detectors is few tens of seconds, thus it limits speed of the reconnaissance vehicle. Also the checking of surrounding terrain (it can be contaminated and the secondary cloud can hit the planned route few minutes later, or after change of wind direction) is much more difficult.

Conclusion

As it was describes above, Stand-off Detection can make detection of CWA, in comparison with point detection:

More safe;
Faster;
More flexible.

To use the potential of Stand-off detection, it is necessary:

The deployed SoD system should be capable to detect the secondary cloud and it should have high effective range (more than 2 km);
Both the commander and the operator of SoD system should understand the SoD technology;
The use of SoD system should be well planned (position of SoD and measuring paths);
Evaluation of SoD result should be done carefully, that means using precise maps and corresponding software.