An excerpt from JPL Document D-12104 presents additional information on the control system of a steerable microwave antenna that is mounted on the roof of a land vehicle and used to communicate with a geostationary satellite. The antenna and control system were described previously in "Steerable K/Ka-band Antenna for Land-Mobile Satellite Applications" NASA Tech Briefs, Vol. 18, No. 1 (January 1994), page 28. Under control by a computer mounted in the vehicle, a motor turns a rotary table to aim the antenna in azimuth toward the satellite, which transmits a pilot-tone signal that serves as an aiming beacon. (The radiation pattern of the antenna is wide enough in elevation so that steering in elevation is unnecessary.) Once the pilot tone has been acquired, an inertial turn-rate sensor mounted in the vehicle provides most of the information needed by an open-loop control subsystem that keeps the antenna pointed toward the satellite as the vehicle moves. Drift of the inertial-sensor bias is the source of most aiming error. Any such error is detected by sinusoidally dithering the rotary table by ±1° at a frequency of 2 Hz while measuring the resulting dither in the strength of the received pilot tone.

This work was done by Arthur C. Densmore of Caltech and Richard Renner of Cal Corporation for NASA's Jet Propulsion Laboratory. To obtain a copy of the excerpt, "Subsystem Design," access the Technical Support Package (TSP) free on-line at under the Electronic Systems category, or circle no. 189 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). NPO-19863