The exploration of lunar gravity by VLBI observations of SELENE (Kaguya)

SELENE (Kaguya), which was successfully launched on Sep. 14, 2007, consists of the main orbiter, and two small free-flying sub-satellites, called Rstar (OKINA) and Vstar (OUNA). We use multi-frequency VLBI to measure the angular distance between the two sub-satellite radio sources Okina and Ouna in order to improve the accuracy of the low degree gravitational harmonics and the gravity field near the limb. The observations are made at three frequencies in S-band, (2212, 2218 and 2287 MHz), and one in X-band, (8456MHz) with carrier waves. The Japanese domestic VLBI network, VERA, will conduct VLBI observations for the whole mission period of one year. In addition, we will conduct two periods of international observations, each one month in duration, which will also include the international stations, Shanghai, Urumqi, Hobart, and Wettzell. We have succeeded in making VLBI observations of Okina/Ouna with VERA and the international network, and have also succeeded in correlating of signals from Okina/Ouna. We obtained phase delays with an accuracy of several pico-seconds in S-band.