Injected transfer orbit determination of lunar probe Chang’E 5T1 using short-arc rocket GPS measurements

The injected transfer orbit of lunar probe Chang’E 5T1 (CE-5T1) is determined immediately after the probe separates from its launcher. As the first orbit in the lunar flight, the CE-5T1 injected transfer orbit is crucial to the consequence of rocket vehicle launch mission and the probe’s subsequent midway orbital manoeuvre. In this paper, we discuss the problem of using rocket GPS measurements to determine the probe velocity increment due to mechanical separation, and subsequently the injected transfer orbit determination of CE-5T1. Motivated by the post-mission analysis of lunar probe Chang’E 3 (CE-3), we give theoretical evidence to explain the physical phenomenon of semi-major axis sudden change at the probe separation instant through the derivation of the Vis-Viva equation. In succession, we focus on the description of the procedure used for the orbit determination performed on separated arcs of rocket GPS measurements through the use of momentum conservation to determine the probe separation velocity. Finally, actual flight data of the CE-3 and CE-5T1 missions are used for the validation.