A review of methods of measuring system parameters in time-variant space communication channels

The objective and methods of measurement on linear, timevariant communication channels are discussed with a view of assessing their applicability for the diagnostics of certain unconventional channels of interest to space communication. These may include channels formed during the early, powered phase of the space vehicle flight and during its re-entry into the Earth´s atmosphere. No attempt is made here to elucidate the physical phenomena which are responsible for the formation and properties of such channels and the problem of their measurement is approached from the input-output point of view. It is assumed that the channels in question have been rendered semi-transparent through a suitable choice of carrier frequencies, but that the transmitted waveforms are impressed with identifiable channel characteristics, such as the multipath and Doppler spread. It is concluded that due to the uniqueness and complexity of such channels only the most rudimentary measuring methods, leading to the estimates of gross channel parameters, can be contemplated at this time. The need for further research is indicated.