2009 年 55 巻 2 号 p. 151-157
Lunar gravimetry on the far side is one of the most important missions in SELENE. A stable frequency transmitted from a ground station is relayed to Main Orbiter by Rstar, and -Main Orbiter returns back the signal to Rstar. Then the signal is changed to X-band in Rstar, and transmitted to the ground station. This measurement is called 4way Doppler method. When the 4way Doppler measurement is performed successfully, the Rstar receives the signal from the ground station and transmits a different frequency signal synchronized with the received signal to Main Orbiter by using a PLL. In this case, the PLL is in “Locked” status. Main Orbiter also relays the signal by the same way. Usually the status of “Locked” or “Unlocked” is sent to the ground station as telemetry information to confirm the completion of the 4way link. However, the ground station is not able to get the telemetry information from Main Orbiter on real time, because the orbiter is on the far side during 4way Doppler measurements. The new methods have been proposed to know the status of the PLL from variations of the Doppler rate and/or frequency shift of the received signal itself on the ground station. They were applied to actual operations of SELENE and were very useful.