The Servo System for Antenna Positioning

The needs of broadband satellite communications systems for a highgain, low-noise antenna led to the choice of the horn type structure. High gain and low noise inherently mean a narrow-beam antenna and, therefore, an antenna with a large reflector which has to be pointed very accurately. In this case, the beamwidth is such that at 4 kmc, the strength of the received signal is 3 db clown when the antenna is pointed 0.11° off center. However, in order to conserve signal-to-noise ratio, maximum reduction in received signal strength of 1 db was alloted to errors in pointing.1 This places an over-all accuracy requirement of 0.06 degree maximum error on the antenna pointing system. The dynamic requirements on the control system are generated by the orbits of the satellites to be tracked and by the nature of the antenna mount. With an azimuth-elevation mount, it takes a very high azimuth velocity to track a low-altitude satellite passing close to the zenith, and a very low azimuth velocity to track a satellite pass as it comes up over the horizon. The compromise objective set for the system was that it be able to track smoothly and accurately all satellite passes with maximum azimuth velocities up to 0.5 degree per second. For a satellite with a 2500-mile altitude, typical of the first Telstar satellite orbits, the 0.5 degree per second velocity would limit the tracking to passes having maximum angle of elevation less than 81 degrees. However, for satellites with altitudes of 6000 miles and above, the maximum trackable angles of elevation would be 86°.