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An analysis of a coherent two-way (transponder) Doppler communication system composed of a ground transmitter, a spacecraft transponder, and a ground receiver has been made to determine the effects of VCO phase and thermal phase noise at the ground receiver output (See Fig. 2). This system is typical of those used on lunar and planetary space programs. The analysis shows that the phase noise contribution of the ground receiver's VCO is inversely proportional to the ground receiver's noise bandwidth [See (15)]. Further it has been shown that the spacecraft VCO phase noise, spacecraft thermal phase noise, and ground receiver thermal phase noise are all functions of the ratio of transponder-to-ground-receiver noise bandwidths [See (13), (4) and (5)]. All the relationships were derived using complex integration techniques.