SNR periodical variation of Chang’E-3 spacecraft orbiting the Moon

Chang’E-3 spacecraft was orbiting the Moon from December 6–14, 2013, and very long baseline interferometry (VLBI) observations were performed to improve the accuracy of its orbit determination. In the process of recording VLBI raw data, 2 bits quantization was implemented. Interesting phenomenon was that signal-to-noise ratio (SNR) of each VLBI station experienced periodical change and had large variation on amplitude while in the Moon’s orbit, whereas SNR kept in a stable level after Chang’E-3 landed on the Moon. Influence of varying elevation angle on SNR was analyzed and compensation of 2 bits quantization harmonics to SNR calculation was investigated. Most importantly, telescope system noise temperature increase caused by the Moon was computed along the time of Chang’E-3 orbiting the Moon, and well matched SNR changing trend in terms of correlation coefficients.