Preliminary performance analysis of the first backup satellite of BeiDou Navigation Satellite System

To further enhance the capability of BeiDou Navigation Satellite System (BDS) to provide continuous and efficient service, the first backup satellite I6 was launched in March 2016. The differences and similarities between I6 and other IGSO satellites were analyzed, from the perspective of satellite clock prediction, orbit determination accuracy and pseudorange variation. The contribution of I6 to BDS was evaluated by PDOP values and grid availability.The satellite clock measurements obtained by two-way satellite time frequency transfer (TWSTFT) system were used to evaluate clock prediction error of I6.The results show that for 5-hour prediction, clock error of I6 is 2.32 ns in terms of root-mean-square (RMS) and the extrapolated one-hour prediction error is 0.73 ns (RMS). Compared with other IGSO satellites clock, the error forecast level of I6 is equivalent. The orbit determination accuracy of I6 optimized by the attitude control method is not degraded obviously compared with the I3 of BDS when the Sun is close to the I6's orbit plane, which overcomes the decrease of the orbit accuracy of the existing BDS satellites during the shadow. The pseudorange variation of the I6 satellite was analyzed using the data collected by the large aperture parabolic antenna. The results show that the peak-to-peak difference of the pseudorange variation is about 1m in the single observation arc of I6 satellite. It is consistent with other IGSO satellites. I6 is in the hot standby mode of operation, providing services with the original BeiDou satellites after May 2016, changing the BDS constellation composition. The results of PDOP simulation show that the maximum PDOP of KaShi is decreased from 12.82 to 7.26, and the percentage of PDOP greater than 6 is decreased from 29.11% to 17.21%. The results show that I6 increases the availability of six ionospheric grid points.