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Recent advancements in IR sensor technology and signal processing techniques have made possible design of IR detection system for providing longer detection ranges (greater than 50 Km) if sensor is operating at higher altitudes. The increase in ranges is possible due to better atmospheric transmittance and clutter free background at higher altitudes. In order to find optimum sensor altitude, the atmospheric transmission have been computed in 3 - 5 micrometer and 8 - 10.5 micrometer spectral band using LOWTRAN 7 for different altitudes and detection ranges. The present paper describes the design consideration in terms of choice of spectral band, detector and optics and S/N trade off analysis. For utilizing optimum sensitivity of the detector two conditions must be satisfied viz. (1) the target image should be more than the diffraction blur and (2) the pixel size should match the image size. For meeting above conditions, the collecting aperture required shall be quite high when the detection range is substantially large. In such situation, the second condition has to be violated to arrive at a practically realizable system. This leads to a supixel resolution limited system and calls for special signal processing techniques. We have worked out a system providing resolution of 0.14 mrad utilizing 480 X 4 MCT linear focal plane arrays (LFPA's) having 200 mm, f/1 optics. The system covers a FOV of 80 degrees in azimuth and 3.8 degrees in elevation. For different target and sensor height and S/N detection ranges have been computed. In a typical situation it has been shown that it is feasible to detect a missile in excess of 100 Km when it is at 3 Km height and sensor is operating at height of 5 Km. The proposed sensor could be deployed from an Airborne platform or Aerostat and need to be gyrostabilized. The IR sensor with tracking algorithm can form part of a multisensor weapon platform.
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