Precise positioning of spacecrafts by multi-frequency VLBI

Multi-Frequency VLBI (MFV) is one of the most powerful methods for precise positioning of spacecrafts. The system transmits three carrier waves at S-band and one wave at X-band. These frequencies are set to resolve the cycle ambiguity of carrier wave at X-band from two group delays between carrier waves and one phase delay of the carrier wave at S-band. The procedure to resolve the cycle ambiguity is proposed in this article. Some conditions about frequency variation and prediction of position and the ionosphere are also clarified for resolving the cycle ambiguity. The dedicated recording system for MFV is developed. A preliminary observation of MFV is carried out with this system by using Lunar Prospector. As a result of the experiment, residual phases from predicted ones are within +/-2pi, and the RMS of the residual for the period of several seconds is about 4 degrees. This result supports realization of the MFV.