Improved pointing accuracy using high-precision theodolite measurements

摘要

Abstract: Geostationary earth observation satellite applications almost always require high pointing accuracy. The ability to meet very high pointing accuracy, at or below microradian levels, is limited by our ability to measure and correct pointing errors in the laboratory environment. Certain types of pointing errors are invariant over time and appear as fixed pattern over the field of view. These errors may not even be sensed by the servo. Such errors can be measured by other means and corrected through the servo in an open loop manner. This paper describes the method developed to measure the fixed pattern errors (FPE) associated with a servo controlled, two degrees of freedom mirror arrangement by autocollimation of a theodolite. This system has been developed for GOES-8 and above S/C instruments. Several interesting optical phenomena were observed during measurement of the FPE. Mathematical models have been developed to help understand and explain these phenomena. The paper also describes how several numerical techniques were used to simplify the laborious testing by minimizing the number of measurements without compromising pointing accuracy. Fourier analysis of the test data show that the FPS's are reduced to within $POM 4.5 microradians. The measurement accuracy obtained in these tests are believed to be at least two times better than the measurement accuracy reported so far using theodolites or other similar instruments. !3