Same-beam VLBI observation of SELENE

The Japanese lunar mission, SELENE (Kaguya) consists of a main satellite and two small satellites, Rstar and Vstar. In same-beam VLBI observations of Rstar and Vstar, phase fluctuations caused by atmosphere, ionosphere and instruments were reduced to a low level of 1-2 deg, and the differential phase delay between Rstar and Vstar was obtained with a very low error of 2 pico-seconds. We corrected the long-term atmospheric and ionospheric delays by using GPS techniques and analyzed other possible influence such as phase-frequency characteristic of the receivers and phase variation in the main beam of telescopes. We performed orbit determination for Rstar and Vstar, the accuracy was much improved from a few tens meters when using only Doppler and range data, to a level of about 10 m when same-beam VLBI data are also used. In addition, the lunar gravity field model was also improved by combing VLBI data.