Servo Controls for Low S/N Satellite Auto-Tracking

Abstract

At very low Signal to Noise receive ratios, a typical Autotrack servo loop exhibits significant servo noise tracking jitter that can degrade the received signal characteristics. The angular jitter can be minimized by a lowering of the autotracking servo loop bandwidth, but at the sacrifice of the servos ability to reject external torque disturbances, such as wind deflections. As satellite downlinks continue to move to higher frequencies, the angular errors caused by either the tracking noise or the wind deflections of a receive antenna will have increasingly negative effect on the quality of the received data. This paper examines the performance of a servo algorithm that maintains the needed wide bandwidth characteristics of wind deflection rejection while achieving the noise smoothing characteristics of a very low bandwidth Autotrack servo response. The control algorithms have been tested on a 13 meter Remote Sensing Satellite tracking system to determine the overall tracking performance of the new implementation. Current results indicate that the new servo configuration achieves the desired results typical of a wide bandwidth system of high wind rejection and low target dynamics tracking errors. At the same time, it provides elimination of slowly changing errors from axis miss-alignments, thermal effects, and gravitational effects and the low RF noise jitter performance typical of a low bandwidth system.