The Application of Differential VLBI to a Sample-Return Mission

To study the precise short-arc orbit determination for multiple spacecrafts in a sample-return mission,the VLBI switching observation mode,integer ambiguity resolution, differential process method and spacecrafts' signal requirements are investigated.Based on the same-beam VLBI cross-correlation phase,we produced the switching VLBI observation data,and calculated the differential phase between two spacecrafts to derive the differential delay.The precise orbits for each spacecraft are derived from the differential delay data. By comparing with the same-beam VLBI.we found that the group delay and phase delay of switching VLBI were as precise as that of same-beam VLBI,with the RMS of 46 mm and 1.6 mm respectively.If using Doppler/ranging data only,the accuracy of the 4-hour short-arc orbit determination is several hundreds meters.By combining the VLBI group delay data,the accuracy of short-arc orbit determination is improved to 75 m.Meanwhile, by combining the VLBI phase delay data,the accuracy of the short-arc orbit determination is improved to～10 m.