A target location and pointing algorithm for a three-axis stabilized line scanner (AMIDARS)

This paper presents an algorithm to calculate the location of a target, and/or point to it, given the position of the aircraft, its attitude and altitude, and the gimbal angles of the stabilized platform. The methodology was developed for the Airborne Minefield Detection and Recognition System (AMIDARS), a three-axis stabilized infrared line scanner, but it could be easily adapted to other configurations. With the AMIDARS, a point of interest or target can be selected by placing the cursor on the image displayed in real time or in playback mode. The position of this target can then be related to a reference point for which the necessary data has been stored. This reference point is normally the Center of the Field of View (CFOV). The position of the target is then specified as the number of pixels to the side and below the CFOV. The ground dimensions corresponding to the pixel count are calculated based on the direction of the scan line and the scan angle, thus locating the target with respect to the CFOV. Next, the position of the CFOV is computed with respect to a previous set of nadir coordinates (aircraft latitude and longitude) using the aircraft yaw, pitch and roll angles, its altitude, and the gimbal angle measurements. Combining the two ground dimensions, target to CFOV and CFOV to nadir, completely defines the target location. The necessity of this intermediate step is discussed in the paper. The converse problem of pointing to a target of known coordinates, given the present aircraft location, is also addressed. The pointing algorithm uses the same equations derived for target location, but almost in reverse order. Finally, an error analysis is also provided to assess the accuracy of the target location technique.