Astrometric VLBI observation of spacecraft with phase delay

A series of VLBI observations for the NOZOMI spacecraft was performed between the end of 2002 and July 2003 with aim of supporting orbit determination. We have made an astrometric analysis of the VLBI data for celestial coordinates estimation of the spacecraft with phase delay observables. Phase delay has the potential to give 3-4 orders of improvement in delay resolution over group delay measurement, whereas the main difficulty in using phase delay is the ambiguity problem. We observed the NOZOMI spacecraft continuously for a long time with multiple VLBI stations, and we avoided the problem of phase ambiguity by connecting the phase delay. For correlation processing and analysis of VLBI data, a relativistic VLBI delay model for a finite distance radio source was developed, and it was used for the analysis. The coordinates of the spacecraft in the celestial sphere were estimated by a least-squares parameter fitting technique.