Earth Planets Space, Vol. 60 (No. 4), pp. 391-406, 2008
Fuyuhiko Kikuchi, Qinghui Liu, Koji Matsumoto, Hideo Hanada, and Nobuyuki Kawano
RISE Project Office, National Astronomical Observatory, 2-12 Hoshigaoka, Mizusawa-ku, Oshu, Iwate 023-0861, Japan
(Received February 23, 2007; Revised September 21, 2007; Accepted November 9, 2007; Online published April 9, 2008)
The same beam VLBI method (SBV) is newly applied to the multi-frequency VLBI method in the VRAD mission of SELENE (KAGUYA). By simultaneously observing two nearby spacecraft with one antenna, the error sources of VLBI measurement common in two propagation paths can be almost canceled out. In this paper, error estimation and simulation analysis are carried out for a feasibility study to apply the SBV method to the VRAD mission. Differential phase delay can be estimated without cycle ambiguity even if tropospheric fluctuation is large and/or traveling ionospheric disturbance occurs. The sensitivity of the differential phase delay with respect to the average elevation angle and the elongation of two spacecraft is also investigated. Moreover, a method is developed for estimating differential phase delay in switching VLBI observations using the cycle ambiguity derived from SBV observations. This method can be performed in more than 90% of the VRAD mission's total paths. Precise positioning with SBV contributes to accurate estimation of the low degree coefficients of lunar gravity fields by more than one order of magnitude than previous results.
Key words: VLBI, spacecraft, orbit determination, narrow bandwidth, differential phase delay, VRAD, RSAT, SELENE (KAGUYA).