VLBI Observation of Spacecraft for Navigation –Approaches with Group Delay and Phase Delay –

Very long baseline interferometry (VLBI) has high sensitivity in coordinate measurement in the celestial sphere. Joint use of VLBI and range and range rate measurement can enhance the accuracy of the spacecraft navigation. Group delay and phase delay are observables of VLBI observation. The latter has an advantage of better delay resolution than the former, although its drawback is difficulty to get absolute delay due to phase ambiguity. We have observed spacecraft NOZOMI with Japanese and Canadian VLBI stations. As an approach to the phase ambiguity problem, we have successfully connected the phase delay over 24 hours, and celestial coordinates of NOZOMI was estimated by astrometry analysis. Group delay observable is good at absolute delay measurement even though delay resolution is limited by bandwidth of the signal from the spacecraft. For increasing the delay resolution, spectrum filtering and correlation processing with replica of the signal are tested in VLBI observation of spacecraft HAYABUSA.