Lowest Cost Alternative to Auto-Tracking Using GPS-TRAK, Augustin-Sullivan Distribution, & Single Axis Antenna Techniques

Abstract

The first telemetry tracking system was desired in 1959 for the space program. Cost was of little concern. The tracking technique used was 3 channel monopulse which is still today, after all these years, the optimum in performance for any type of tracking requirement. Telemetry tracking really got off the ground in the early 1970's with the move from P-Band to S-Band for telemetry. In the design of early tracking systems, performance was on the top of the list, and cost was on the bottom of the list in establishing the design criteria. By the beginning of the 1980's cost was approaching performance in importance. Today, with the demise of the cold war and a considerable reduction in global threats coupled with the state of the world economy, cost has now reached the top of the list. The cost of a telemetry tracking system can be reduced by more than a factor of two by going to a single axis tracking technique. The lowest cost single axis approach heretofore has been the use of a cosecant squared (CSC²) distribution. To improve the efficiency of a single axis system and increase the overhead coverage capability, the use of a dual beam antenna has been widely used as another type of single axis approach. The dual beam technique involves additional costs since two tracking antennas are required. Except for satellite tracking, almost all telemetry tracking is performed at low elevation angles and, like it or not, multipath is there. The multipath fade varies from a few dB, to over 20 dB depending upon the reflecting terrain. Most general purpose systems should be designed for at least a 10 dB multipath fade. For all telemetry tracking applications, the multipath effect is completely negligible at elevation angles greater than 10 degrees. The Augustin-Sullivan Distribution, in effect, fades away the multipath margin as the multipath effect decreases. Because of the multipath phenomenon, an antenna beam should not be shaped at the one dB point as is the case with a CSC² distribution, but only needs to be shaped from somewhere between the 15 - 20 dB level based on the mission requirements. This involves a gain reduction from a pencil beam on the order of 1/2 dB or less, rather than the 3 dB reduction associated with the CSC² distribution. The Augustin-Sullivan distribution does not start shaping the beam until shaping is retired, and shapes the beam for constant altitude coverage from the horizon to zenith. For the first time, coverage is provided from the peak of the beam to directly overhead with a single antenna and a single axis rotator. When GPS information is available from the tracked vehicle, the Augustin-Sullivan distribution, with a single axis rotator and using the GPS-TRAK technique, results in the lowest possible cost alternate to autotracking.