Accurate navigation of airborne image sequences for rapid surveys of water depths and currents

An airborne digital framing camera has been used to collect a temporal sequence of images of the nearshore ocean with the objective to retrieve water depths and currents by analyzing the propagation of shoaling gravity waves. To accurately map the imagery of the waves onto a rectilinear grid on the surface, the camera location and attitude is measured with an integrated GPS/INS. A computer-based controller uses the location and attitude measurements in real time to point the camera to a designated geodetic position on the ocean surface as the aircraft flies past. This spotlight pointing is accomplished by mounting the camera in a turret-type positioner and driving the turret attitude motors with the controller output. The system is mounted on the underside of a small, single engine aircraft and is being used to emulate and evaluate concepts for an application to future unmanned aerial vehicles. This paper describes the design of the system and the results of flight tests. At a nominal range of 6 km, the ground location errors of the image data using the raw GPS/INS data are ~20 m (1□), and the accompanying drift errors are ~20 cm/s. These have been reduced to <5 m and ~2 cm/s respectively by correcting the attitude errors by either of two separate techniques. One technique has been to reprocess a more accurate kinematically-derived GPS position for the camera with raw IMU data to re-align the INS, and another has been to use fiducials such as natural features on land or moored buoys in the scene