VLBI for better gravimetry in SELENE

The Japanese lunar explorer SELENE (SElenological and Engineering Explorer), to be launched in 2007, will for the first time utilize VLBI observations in lunar gravimetry investigations. This will particularly improve the accuracy to which the low degree gravitational harmonics and the gravity field near the limb can be measured, and when combined with Doppler measurements will enable three-dimensional information to be extracted. Differential VLBI Radio sources called VRAD experiment involves two on-board sub-satellites, Rstar and Vstar. These will be observed using differential VLBI to measure the trajectories of the satellites with the Japanese network named VERA (VLBI Exploration of Radio Astrometry) and an international VLBI network. We will use a multi-frequency VLBI method to measure the angular distance between the two sub-satellite radio sources Rstar and Vstar. The observations will be at three frequencies in S-band, 2212, 2218 and 2287 MHz, and one in X-band, 8456 MHz. This method uses low power consumption carrier waves, and is suitable for the positioning of spacecraft. 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 intensive observations, each one month in duration, which will also include the international stations, Shanghai, Urumqi (China), Hobart (Australia) and Wettzell (Germany). These observations will measure the phase delay to an accuracy of better than 0.17 rad (10 deg) in X-band. For a baseline length of 2000 km this is equivalent to a positioning accuracy of about 20 cm on the Moon. Combining the data from the tracking of the 3 SELENE satellites (main orbiter, Rstar and Vstar) at different altitudes and from four-way Doppler measurements, and by making long term observations of the sub-satellites (in excess of one year without any maneuvering), we will be able to improve the accuracy to which the lunar gravitational field is known by an order of magnitude. We have completed the development of on-board instruments, and are carrying out pre-flight tests under a variety of conditions. We have also undertaken test VLBI observations of orbiters such as Geotail, Nozomi and Smart-1 with the international network.