Antenna servo design for tracking low-Earth-orbiting satellites

Abstract

The upcoming NASA missions will require tracking of low-orbit satellites. As a consequence, NASA antennas will be required to track satellites at higher rates than for the current deep-space missions. This paper investigates servo design issues for the 34-m beam-waveguide antennas that track low-orbit satellites. This includes upgrading the servo with a feedforward loop, monopulse controller design, and tracking error reduction either through proper choice of elevation pinion location or through application of a notch filter or adjustment of the elevation drive amplifier gain. Finally, improvement of the signal-to-noise ratio through averaging of the oversampled monopulse signal is described. HE National Aeronautics and Space Administration (NASA) antenna network, called the Deep Space Network (DSN), is op- erated by the Jet Propulsion Laboratory. It consists of several antenna types located at three sites Goldstone, CA, Canberra, Australia, and Madrid, Spain and serves as a communication tool for space explo- ration. Future NASA missions will include low-orbiting satellites, which will require significantly higher antenna tracking rates. Thus the servos for the new-generation 34-m beam-waveguide (BWG) antennas should be upgraded to be able to follow commands at higher rates. These upgrades are illustrated in this paper, with the new DSS-13 BWG antenna controller design. The existing proportional and integral (PI) controllers of the antennas satisfy the requirements for deep-space X-band (8.4-GHz) tracking. For high-rate command following a simple and reliable choice is a feedforward controller, described in this paper. For track- ing, a monopulse controller is a fast-rate alternative to the existing conical scanning (conscan) technique. The design and performance of a monopulse controller is discussed. It is shown that its perfor- mance is improved through either proper choice of the location of the elevation pinion or the implementation of a notch filter or the ampli- fier gain adjustment. Finally, the improvement of the signal-to-noise ratio (SNR) of the monopulse signal is presented. By averaging the redundant monopulse samples, the SNR improvement ranges from 7 up to 17 dB.