Removal of turnaround subcarriers in a spacecraft transponder



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The problem of up-link subcarriers being remodulated on the down-link in a spacecraft transponder is investigated. The analysis emphasizes possible improvement in the efficiency of the Apollo down-link communication system. The improvement in down-link signal power is determined for the case where the turnaround up-link subcarriers are suppressed in the spacecraft transponder and therefore not remodulated. The results indicate that a significant signal improvement is available. Also a general expression is developed for use by the communications engineer in determining what power improvement can be expected if turnaround subcarriers are suppressed in a spacecraft transponder. A comparison is made between direct filtering and a phase-lock cancellation technique to suppress the turnaround subcarriers in the presence of wideband ranging modulation. It is shown that the cancellation technique is far superior in performance when considering the effects on the ranging modulation spectra. An analysis of the cancellation technique and systems determination of its principle components are included.