The transputer in satellite signal processing

GPS arose from early experimental American military programs interested in clock stability and relativity. It is based on the concept that if the exact range from three fixed positions is known, the position can be calculated, in three dimensions. The range is determined by the propagation delay of the signals from the satellites, which assumes a known transmission time. This is solved by using a fourth satellite, and using the redundancy in the position equations to solve for time also. The author explains the details of satellite signal processing in GPS and the application of the transputer for signal tracking in software. The signal has to be read in to the processor, multiplied by locally generated code, filtered and detected. The easiest way of achieving the last two operations is to down convert to a low frequency and apply a low pass filter, then perform an FFT if the frequency is unknown, or a synchronous down conversion to DC if the frequency has already been determined. The author discusses the approach taken for a single satellite, then demonstrates how vast economies can be achieved when performing the work for multiple signals