A Method for Measuring the Group Delay of an On-Orbit Satellite Transponder

As one of the important parameters of the satellite transponder, the group delay of the transponder not only determines the magnitude of the signal transmission delay that is generated by the system or network but is also closely related to the signal transmission distortion, which has a substantial impact on the signal transmission quality. When the satellite is in orbit, it is difficult to measure this index due to the particularity of its position. To overcome this problem, this paper proposes a method for measuring the group delay of an on-orbit satellite transponder. By using two sets of ground station antenna systems that are installed in parallel to conduct the observation of the transponders to be measured simultaneously in the mode of “receiving its own station-disseminated signal,” the measurement results of the group delay of the transponders can be obtained after the calculation. Based on the orbit determination by the transfer tracking system of the National Time Service Center, a group time delay measurement experiment of the C5B transponder of the APSTAR-7 satellite in orbit was conducted. The results of an analysis of the data demonstrate that this method can effectively measure the group delay of a satellite transponder and that the root mean square of the two measurement results is less than 0.2 ns, which provides a useful reference for further optimization and improvement of the orbit determination accuracy of the system and provides a performance evaluation service for satellite companies.