Pointing a ground antenna at a spinning spacecraft using conical scan-simulation results

The authors present the results of an investigation of ground antenna pointing errors which are caused by fluctuations of the receiver automatic gain control signal due to thermal noise and a spinning spacecraft. Transient responses, steady-state errors and losses are estimated using models of the digital conscan (conical scan) loop, the fast Fourier transform (FFT), and antenna characteristics. Simulation results are given for the Voyager mission and for the Galileo mission, which uses a spinning spacecraft. The simulation for the Galileo spacecraft predicts 1 mdeg of error with a 1280-s scan and 4 mdeg with a 32-s scan assuming an AGC loop signal-to-noise ratio of 35 to 30 dB.<>