Radiotelescope Low Rate Tracking Using Dither

Radiotelescope access to a certain part of the sky requires very low tracking rate in azimuth. At low rates, the magnitude of the dry rolling friction torques at the telescope wheels cause a nonsmooth azimuth motion and the limit cycling of a telescope, which, in turn, deteriorates its pointing precision. The improvement of the low-rate tracking pointing through the dither implementation is discussed in this paper. Dither is a high-frequency external signal injected into a system, with frequency much higher than the system dynamics. It has long been known that the injection of high-frequency signals into nonlinear electrical circuits drastically reduces the system limit cycles. We have recently identified a similar phenomenon in the limit cycling behavior of large wheel-and track radiotelescopes and microwave antennas. The high frequency signal excites local vibrations that overcome the frictionstiction torque between the wheels and the track. The vibrations, however, do not propagate through the telescope structure thus not impacting its pointing performance. We showed through nonlinear simulations that the limit cycle amplitude of the National Radio Astronomy observatory’s 100rneter radiotelescope was dramatically reduced when dither was applied, and that its pointing accuracy at low rates was significantly improved. The implementation of the dither to the radiotelescope control system is comparatively simple as it requires rather trivial modifications of the controller software and hardware. Therefore dither is a reasonable control design option for tracking improvement at low rates. Copyright G 1997 by the American Institute of Aeronautics and Astronautics. All rights reserved.