Synchronization of Earth Stations to a Communications Switching Satellite

Abstract

<p>The problem of synchronizing earth stations to the switching sequence of the SDMA/SS-TDMA communications satellite system described by COMSAT Laboratories is examined in detail. System synchronization is discussed and two techniques for locking the time bases of all earth stations to the sync window of the satellite are described.</p> <p>It is established that synchronization for an individual earth station can be separated into three distinct modes of operation called the coarse search mode, the fine search mode and the tracking mode. Three different methods for achieving coarse search synchronization are described.</p> <p>In the fine search mode, the synchronization loop is analysed assuming that the pertinent earth station transmits sync bursts which are modulated by the sync window. Results of this analysis show that a receiver configuration employing an integrator provides a set of error detection characteristics which relate an error voltage measured in the earth station to the timing error measured at the satellite. It is shown that the average timing error is reduced to zero with successive transmissions of sync bursts around the loop.</p> <p>The timing error due to constant velocity satellite motion can be eliminated by employing a tracking network. The tracking network uses an integrator to adjust the earth station time base clock.</p> <p>A laboratory model, which closely approximates the actual physical model used in the theory, has been designed and constructed. The coarse search, fine search and tracking modes of operation are all included in this experimental study, and a provision which simulates satellite motion is described. Details of experiments, which confirm the theoretical predictions, are provided.</p> <p>The major contributions of the thesis are summarized below.</p> <p>(1) A technique for providing synchronization for all earth stations operating in the system, employing three control stations, is described.</p> <p>(2) A new technique for achieving coarse search synchronization using coded search signals has been developed.</p> <p>(3) Three new implementations of the sync burst have been described.</p> <p>(4) The behaviour of the synchronization loop has been analysed in detail and a concept using error detection characteristics has been explored.</p> <p>(5) A tracking network can be employed to eliminate timing errors caused by constant velocity satellite motion.</p> <p>(6) A laboratory model has been designed and constructed.</p> <p>(7) A special coherent receiver for demodulating bursts of PSK signals has been developed.</p> <p>(8) Results of experiments using the laboratory model are demonstrated to agree closely with theoretical predictions.</p>