Investigation on unexpected variations of differential phase delay of Chang'E-3

The Chinese lunar spacecraft Chang'E-3 (CE3) was launched on December 1, 2013, and it kept transmitting differential one-way range (DOR) signals during the whole mission. Differential phase delays calculated from those DOR signals are examined. An interesting phenomenon is that differential phase delays of different frequency pairs change in similar patterns before CE3′s landing on the Moon, whereas they change differently after the landing. Moreover, differential phase delays and signals’ power experience sudden and large-amplitude variations from time to time. Our analysis shows that, before the landing, signal reflection mainly happened on the spacecraft surface while CE3 was moving in space. Thus, the delay of reflected signals was a few nanoseconds with amplitude variation much larger than 0.12 ns, causing differential phase delays to vary in similar patterns. However, after CE3′s landing, reflected signals from lunar surface were received by ground stations when CE3 was sitting still on the Moon. Accordingly, the delay of reflected signals was more than 20 ns and with amplitude variation within a few tens of picoseconds, causing differential phase delays to change in different patterns. Further investigation reveals that those sudden, large-amplitude variations in differential phase delays and signals’ power were caused by CE3 maneuvers and onboard device movements.