Ground-based verification of four-way Doppler measurement for the SELENE mission

In Japan’s lunar mission called SELENE (SE-Lenological and ENgineering Explorer), detailed measurements of the lunar gravity field will be made by using two methods. The first is the differential VLBI measurement with two sub-satellites, and the second is the four-way Doppler measurement by using a sub-satellite which relays radio waves from the main lunar orbiter. The lunar far-side gravity field will be directly measured for the first time by using the second method, which will give us new information about the inner structure of the Moon. For the four-way Doppler measurement, the confirmation of the link establishment is an important issue because there is no telemetry relay from the main satellite to the sub-satellite in SELENE. The instruments are now under development, and a compatibility test of the instruments to the ground antenna station was carried out in March 2003. We have applied recently proposed algorithms for the confirmation of the link establishment and confirmed that they work successfully for data obtained during this test. The accuracy of the 2- and 4-way Doppler measurement are also measured and confirmed to be the expected level.