Airborne infrared sensor calibration

Summary form only given, as follows. The employment of sub-milliradian infrared measurement sensors in an airborne platform requires extraordinary techniques to permit the attainment of the parameter accuracy in Earth coordinates vs aircraft coordinates. The airborne platform, in general, contains a GPS/INS system located at the approximate center of gravity. The sensors, in general, are located at the extremities of the aircraft. As a result, static and dynamic (i.e. flexure) errors must be calibrated to permit accurate passive measurements of azimuth and elevation angles to an airborne target. The high frequency, dynamic flexure of the sensor location is usually measured and corrected for by a second INS unit collocated with the sensor. Static offset errors due to aircraft flexure under different aircraft maneuvering conditions can usually be measured by commanding the aircraft through a maneuver sequence and comparing the differences in the two INS units. The results may be stored for subsequent corrections using a table-lookup procedure. Over longer time span (minutes and hours), drift errors of the INS will introduce large attitude uncertainties in the sensor angular output, which if uncorrected, will introduce large positional uncertainities in Earth coordinates of the measured objects. The long term drift errors, however, can be corrected by periodic in-flight observations of known infrared stars. With the improved sensitivities and large areal coverage of the current generation of infrared focal plane arrays, there are sufficient number of stars that could be conveniently observed at periodic intervals at aircraft altitude in both day and night to permit absolute positional calibration of the sensor in real-time. Preliminary ground-based measurements of airborne objects have been obtained with sensors having angular resolution and sensitivity typical of airborne sensors to allow evaluation of the expected calibration performance. By incorporating a sequence of in-flight calibration procedures employing the aircraft GPS/INS, the sensor INS, and periodic star observations under specific sequence, it is expected that the absolute attitude of an airborne IR sensor can be calibrated to within a small part of the instantaneous field of view of the sensor system