Transponder frequency offset compensation with recurrently assigned unique words

Claim 1

A single-channel-per-carrier satellite communications system having a source earth station and a destination earth station for establishing a plurality of satellite channels therebetween through a satellite transponder having a local oscillator used in common by said satellite channels for translation from up link frequencies to corresponding down link frequencies, said source earth station comprising a plurality of modulation systems corresponding respectively to said satellite channels, said destination earth station comprising a plurality of demodulation systems corresponding respectively to said satellite channels, said satellite communications systems comprising: said modulation systems being divided into a plurality of groups corresponding respectively to a plurality of groups of said satellite channels, and said demodulation systems being likewise divided into a plurality of groups corresponding respectively to said groups of said satellite channels, the satellite channels of each group being covered by a range of frequency drifts caused by said satellite transponder, and a unique word being assigned to each modulation system and to each corresponding demodulation systems being exclusively unique in each group, the unique words assigned to each group being identical to those assigned to the other groups, each of said modulation systems transmitting the assigned unique word on a modulated up link carrier to said satellite transponder, each of said demodulation systems comprising; demodulator means for demodulating a modulated down link carrier received from said transponder with a variable frequency carrier to produce a demodulator output; unique word detector means for detecting whether said demodulator output contains a unique word assigned to the demodulation system; variable frequency generator means responsive to said unique word detector means for generating a local carrier which varies in frequency continuously when said assigned unique word is not detected by said unique word detector means and assumes a constant frequency value which is assigned to the demodulation system when said assigned unique word is detected and applying said local carrier to said demodulator means as said variable frequency carrier; and means for establishing frame start timing in response to the detection of said assigned unique word by said unique word detectors means.