Linear Variable Filters for Satellite-Based Chemical Warfare Agent Detection

Previous research has indicated that it may be possible to detect chemical warfare agents in the thermal infrared with instruments such as the Infrared Atmospheric Sounding Interferometer (IASI) on the Meteorological Operational (MetOp) satellites. However, the low likelihood of the IASI overpass coinciding with an attack, coupled with the low spatial resolution of such an instrument would make it difficult to observe chemical attacks of a realistic scale. Additionally, these instruments have high size, weight and power needs and are thus less suited to smaller spacecraft. It was decided to investigate detection of Sarin and Sulphur Mustard using a linear variable filter – a filter for which the transmission properties vary along the filter’s length – as the spectral selection apparatus for a spaceborne infrared detector. Linear variable filters offer the advantage of being inexpensive and compact as well as offering multiple spectral bands, allowing for more affordable instruments to be constructed. These chemical agents were chosen for their use in recent conflicts as well as their high spectral absorption in the thermal infrared. First, to produce input spectral data, dispersion modelling was conducted using the "Urban Dispersion Model" producing atmospheric profiles for releases of Sarin and Sulphur Mustard. These profiles were then inputted to the "Oxford Reference Forward Model", along with other parameters such as temperature, emissivity, angle and altitude of satellite, and other atmospheric trace gases, with associated absorption cross-sections and line parameter data. This model calculated the radiative transfer, allowing the relevant bands of absorption to be selected. Commercially available linear variable filters were investigated, along with suitable detector technologies, and the necessary parameters for an appropriate filter and detector were determined from this investigation. Models for the filter and detector responses have been run for three different scenarios produced with the dispersion model. These include a scenario 1 minute after a 100kg release of Sarin, a scenario 1 minute after a 100 kg release of Sulphur Mustard, and a scenario in which neither gas is present, to allow for comparisons to be made and limits of detection to be established, including the necessary signal-to-noise ratio the instrument must achieve. Analysis of each of these scenarios utilised an idealised filter and detector constructed for the purposes of this work, and these were modelled using an optical model described in previous work by the authors. The results from the detector model suggest that the signal-to-noise ratio achieved by the instrument when observing these scenarios is more than sufficient for almost all wavelengths in the bands of interest. This indicates that it is likely that observations of Sarin and Sulphur Mustard releases could be successful under certain circumstances. Further analysis is required to establish a threshold for detectable concentration or the timeframe on which detections are viable, and there are still some technical barriers regarding the detectors to be overcome, but this work demonstrates the potential of using these filters for chemical warfare agent detection.