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Since 1960s, the gravitational potential of the Moon has been extensively studied from Doppler tracking data between a ground station and spacecraft orbiting in front of the Moon (e. g., Lorell and Sjogren, 1968; Bills and Ferrari, 1980; Konopliv et al. , 1993; Lemoine et al. , 1997). Because direct radio communication is interrupted while spacecraft is orbiting behind the Moon, however, the coverage of tracking data has been limited mostly to the nearside of the Moon so far. In order to compensate for such lack of tracking data, we propose satellite-to-satellite Doppler measurement by using a relay subsatellite in Japanese mission to the Moon in 2003. A complete coverage of Doppler tracking from an orbiter at sufficiently low altitude will significantly improve lunar gravity model and will contribute for future geophysical study of interior and tectonics on the Moon. Further, we propose differential VLBI experiment between the subsatellite and a propulsion module landed on the surface of the Moon. The differential VLBI is about 10 times more accurate than conventional Doppler measurement for long-wavelength gravity field. Besides, differential VLBI is sensitive to the displacement perpendicular to the line of sight. Thus the VLBI experiment provides precise estimates of the lunar gravity potential at low degree. The last proposal for selenodetic experiments is a laser altimeter. Global topography model has been already developed from the analysis of Clementine LIDAR data (Zuber et al. , 1994), but it is suggested that the model includes appreciable anisotropy between NS and E-W directions due to highly eccentric orbit of Clementine spacecraft (Bills and Lemoine, 1995). The laser altimeter experiment from an orbiter in nearly circular orbit will provide a new reference for the isotropic lunar topography model.

We are investigating the project RISE (Radio Interferometry for SElenodesy) which could measures angular distances between a radio transmitter on a lunar orbiter or on the Moon and quasars by differential VLBI. RISE is an equipment used for the selenodesy mission, which will be performed together with a laser altimeter and a relay satellite. We can measure amplitudes of the physical librations and gravitational harmonic coefficients of the Moon through this technique with an accuracy one or two orders higher than before. It can put stronger constraint on the bulk composition of the Moon through the moment of inertia. It is necessary to develop radio transmitters which can emit several carrier waves at different frequencies in S and X bands with bandwidths of about 1 kHz and with powers of a few hundreds of milliwatts. VLBI antennas forming networks on the Earth receive the radio waves from the lunar orbiter, the lunar surface and the quasars alternatively. It is also necessary to develop a special receiving system with very narrow frequency bands in order to improve the sensitivity and to simplify the data reduction. The narrow band receiving system is appropriate for RISE since there is very limited power on the Moon.