Investigation on peculiar SNR variation at TianMa radio telescope

Qingbao He

doi:10.1038/s41598-019-55072-3

Abstract

Chinese lunar spacecraft Chang’E-3 (CE3) and Chang’E5-T1 (CE5T1) were launched in 2013 and 2014, respectively and very long baseline interferometry (VLBI) observations were performed. Signal to noise ratio (SNR) of the Tianma (TM) station experienced peculiar variation, whereas SNRs of other stations were rather stable. Further, it happened only when observing spacecraft, and showed no such variation when performing astronomical observations. Moreover, it was distinctive on X band signals, whereas it was not noticed on S band signals. Analysis showed that the SNR variation was closely related with changing rate of elevation angle. Further investigations discovered that there were two sets of antenna control software at TM station, and the SNR variation originated from a bug in elevation control software used for observing spacecraft, not for astronomical observations. The bug caused big pointing error (around ± 30 as) on elevation angle of TM, which resulted in the peculiar SNR variation. It was not noticed on S band signals due to its relatively wide and flat main beam comparing with ± 30 as pointing error. However, the bug was fixed in software update in July, 2016, and the SNR of TM showed no such variation in 2017 and 2018.

Highlights

Tianma (TM) radio telescope is a newly-built fully-steerable instrument located in Shanghai, China[1]
Very long baseline interferometry (VLBI) observations were performed to improve the accuracy of spacecrafts orbit determination, which involved telescopes of Tianma (TM, 65 m), Beijing (BJ; 50 m), Kunming (KM; 40 m), and Urumuqi (UR; 25 m)
In the observations of spacecraft, the signal-to-noise ratio (SNR) of TM experienced peculiar variation all the time, with the variation period changing from a few tens of seconds to a few hundreds of seconds and the variation amplitude reaching a few dB, whereas the SNRs of other stations were rather stable

Summary

Introduction

Tianma (TM) radio telescope is a newly-built fully-steerable instrument located in Shanghai, China[1]. CE5T1 was an experimental lunar mission to conduct atmospheric re-entry test, and was launched on October 23, 20146,7 It consisted of a return vehicle and a service module. On October 31, 2014, the return vehicle went back to Earth, while the service module continued its extended tests in space In both missions, very long baseline interferometry (VLBI) observations were performed to improve the accuracy of spacecrafts orbit determination, which involved telescopes of Tianma (TM, 65 m), Beijing (BJ; 50 m), Kunming (KM; 40 m), and Urumuqi (UR; 25 m). Error in pointing of TM could not properly explain why it occurred only when observing spacecraft, and not when performing astronomical observations It could be caused by a new astronomical event.

Findings

Discussion

Conclusion