Deep-Space Ranging Using Pseudonoise Codes

A report discusses aspects of a ranging system in which the distance between the Earth and a spacecraft is determined from the difference between the phases of (1) modulation on a radio signal trans- mitted to the spacecraft and (2) a replica of the modulation transmitted back to Earth by a transponder on the spacecraft, received at Earth a roundtrip- light-time after the original transmission. The system correlates the transmitted and return modulation for different phase shifts. The phase shift for which the correlation is maximum is deemed to be related to the round-trip signal-propagation time and, hence, to the distance. The modulations used in prior such systems were sequential square-wave tones or repeating pseudo- noise tones, but not both in the same system. A proposed improvement would equip a ranging system to use either square-wave or pseudonoise tones. The report presents mathematical analyses and comparisons of the performances of square-wave and pseudonoise ranging. It is shown that in comparison with the existing system using sequential square-wave tones, a system using a set of pseudonoise codes would perform better (in terms of integration time and variance in distance) and could be configured and operated more easily.