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In systems employing relay satellites there is invariably a strict requirement for maintaining frequency and phase coherence throughout the system. Although frequency lock can generally be attained by tracking loops and Doppler correction, phase coherency is often destroyed by phase noise, frequency offsets, and differential phase shifts. To evaluate the degree of phase instability, and its effect on decoding and Doppler tracking, it is necessary to perform an accurate phase noise analysis, accounting for all phase noise sources and the manner in which they interact. In relay satellite systems this analysis is further complicated by the interconnection of many tracking loops among ground, satellite and user. In this paper the basic procedures for generating phase stability models are presented and are applied to general forms of relay satellite systems. Consideration is given to both forward and return channel effects, including the use of pilot tone links for satellite referencing. Such phase noise models aid in pinpointing most significant sources, and often suggest design procedures relating to implementation alternatives and bandwidth selection.