Phase-Coherent Dual-Frequency Link for High-Precision Doppler Tracking on the Apollo-Soyuz Mission

Abstract

During the Apollo-Soyuz Test Project, a phase-coherent dual-frequency VHF link was scheduled to be established between the Docking Module and the Apollo Command Service Module. The purpose of the radio link was to enable measurements of the relative Doppler between the two modules to an accuracy of the order of one milliHertz. These measurements will be used to detect mass concentrations in the earth's crust of the order of 10 milligal and greater. The transmitter has a high stability master oscillator common to both channels. The receiver has a local oscillator with identical stability characteristics; its two channels employ phase-lock loops. The receiver is followed by a Doppler processor and tape recorder to extract and encode the desired gravity field information. This paper describes the instrumentation system concept and its implementation. It also contains an error budget for the deterioration of the Doppler accuracy due to various instrumental and propagation effects.