Hiroshi TAKEUCHI

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.

We performed more than 30 VLBI experiments for the tracking of Japan's NOZOMI Mars probe from September 2002 until June 2003. Though the rms scatter of the group delays of VLBI data from the ISAS/JAXA R&RR results are relatively large up to several tens nanoseconds, the both results are consistent with each other. The estimated position based on the phase delays are consistent with the R&RR results less than 40 milli-are second. We perform another VLBI experiments for the tracking HAYABUSA spacecraft. Both the VLBI group delays based on range signal and R&RR results are consistent with within 10 nanoseconds. We also describe an evaluation for reducing propagation delays due to the neutral atmosphere using differential VLBI technique.