Evaluation of differential VLBI phase delay observable for spacecraft navigation - ΔVLBI observation of Hayabusa at touchdown to ITOKAWA -

This report describes the evaluation of Δ VLBI delay measurement accuracy for Spacecraft Navigation. At the epochs of Hayabusa´s touchdown to asteroid Itokawa, we made Δ VLBI observation for Hayabusa with Japanese domestic VLBI stations. Since the orbit of Itokawa is accurately known, we could use this occasion for evaluation of the delay calibration accuracy by Δ VLBI. By using a calibration technique of modeling the excess delay with group delays of multiple reference sources, calibration accuracy in the order of several hundreds pico seconds are obtained. Further improvements are expected by using dual-band observation for reference sources. As another approach, phase delay data of reference radio sources were applied for calibration with expecting improved precision and temporal resolution, though it did not show significant improvement for calibration in this case.