Hiroshi TAKEUCHI

VSOP-2 is a next generation space-VLBI (SVLBI) mission for millimeter-wave VLBI astronomy. The VSOP-2 mission has a radio telescope satellite, ASTRO-G, as a space segment, which will be launched in JAXA's fiscal year 2012. The ASTRO-G satellite will be orbiting around the Earth with the apogee and perigee altitudes of 25000 and 1000 km, respectively. In VSOP-2, Precise Orbit Determination (POD) with an accuracy of 10 cm for ASTRO-G is required to conduct an advanced observation mode of phase referencing. To achieve the POD for such a highly elliptical orbit, GPS receiving system and SLR are proposed to be installed on ASTRO-G. In this paper, ASTRO-G's POD plan is reported.

A part of X-band is allocated for use in deep space communication, However, it is congestive yet and will be more difficult to apply for new missions. In this situation, Ka-band use as an alternative bandwidth looks attractive and necessary for coming deep space activities. Since JAXA's 64m deep space antenna has no compatibility to Ka-band, we have discussed a new system for Ka-band based on arrayed smaller aperture antennas. A key factor for its realization is a precise phase difference estimation among array's received signals. We report on how to synthesizing a downlink signal for our arrayed future deep space receiver system.

For the purpose to improve the orbit determination in Japanese space mission, Delta(VLBI) (Very Long Baseline Interferometry) observations have been conducted for NOZOMI and HAYABUSA missions. Although, Delta(VLBI) observation data has not achieved enough contribution to improvement of orbit determination. One of the reasons is low precision of group delay observable due to the lack of signal bandwidth from spacecraft. The alternative observable phase delay has potential to measure the geometrical delay with higher precision than group delay. Though, integer number of phase ambiguity is a problem to use phase delay. In November 2005, HAYABUSA has made touch down approach to asteroid Itokawa, whose orbit has been given in good precision by measurement of other techniques. In this occasion, we could use phase delay observable by avoiding the problem of phase ambiguity. And the precision of Delta(VLBI) observation with phase delay has been tested.

A differential VLBI (Very Long Baseline Interferometry) technology for deep space navigation (Delta-DOR) has been developed in recent years among major Japanese VLBI agencies such as JAXA, NICT (National Institute of Information and Communications Technology), GSI (Geographical Survey Institute) and NAOJ (National Astronomical Observatory of Japan). This technology is also being developed at ESA and China independently of NASA's original system. In order to promote the data exchanges between different agencies, standard processes and message formats for use in the Delta-DOR techniques are proposed in the CCSDS (The Consultative Committee for Space Data Systems). We introduce the main features of this standard, and discuss about the future development plans for upcoming Japanese deep space missions.