Computer controlled ground electrooptical systems for deep space operations

Digitally controlled ground electrooptical systems have been designed such that all possible total system uncertainties are accounted for prior to track. The definition of total system here encompasses the satellite, earth, and station as a whole measured to common references. Considerations include utilizing stabilized oscillators to generate compensated Universal Time (UTC) thus reducing system time uncertainties to less than 2 μsec, reducing pointing uncertainties to less than 1 sec of arc with 21-bit encoders and knowledge of station geodetic location to within a small number of feet. Sensor biases and errors both static and dynamic are removed or modeled prior to track. Initially this is accomplished through a precise static (moving at sidereal rates) calibration of the mount to the celestial sphere. Data on several hundred stars are resident in the computer, and where specific stars are selected the system will continually point open loop to that reference as angular deviations are measured. With sufficient observations of a sampled distribution of known stars, the modeling constants are computed and the entire process repeated until angular deviations are zero. Slew measurements are made to determine dynamic constants. The system is then used to track known satellites and over-all system performance observed to insure that modeling is correct. A generalized block diagram is shown in Fig. 1.