Development Of Real-time Error Correction System Of The Satellite Virtual Atomic Clock

China Area Positioning System (CAPS) uses the transponder of in orbit communication satellite as a relay, which repeaters the pseudorandom noise code and navigation message data generated on the ground. After receiving the signal the user measures the pseudo-range, and implements the navigation and positioning through satellite virtual atomic clock (SVAC) model.This paper analyzed the major principle and the original error source of SVAC. Considering the error of SVAC was calculated by precise code while certain system error existed between the precise code and coarse code, we constructed the SVAC error correction system with the basis of course code to correct the system error. The signal was monitored by using the differential pseudo-range of SVAC in CAPS ground navigation master control station, which obtained the system error through the real-time measurement of the circle delay, downlink geometric path delay and SVAC time correction, and got the final SVAC error after modification. According to the procedure of signal processing, the system was divided into three modules: RF module, baseband processing module and error modification module.Dominated by the baseband processing module, this paper introduced the main algorithms of the baseband part from different aspects such as acquisition, tracking, synchronization and pseudo-range extraction. Based on the combination of FPGA+DSP, it first illustrated the hardware circuit of baseband processing module and introduced the function and related communication of chips; then through the software compilation the algorithm completed with simulation and realization was downloaded to hardware circuit , finally it collected and saved the measured data result with analysis of the error in Matlab.By debugging the baseband part, we obtained the experiment result as following: this paper attained the precise synchronization of CAPS signal with the coarse code which also completed the extraction of navigation message data. From the circle delay we can draw the following conclusion: the one-day experiment data presented the change of sinusoids, and the regularity was consistent with the orbit of the satellite moving; the residual of the data changed around 10ns with its variation less than 5ns within 2 hours. Regarding the ranging precision of CAPS with the course code about 10ns, it meant the experiment was feasible and the result met the technical requirements of the system, which laid the foundation of calculating the SVAC error for the next module.