Study On Regenerative Pseudo Noise Ranging And Its Implementation

The development of gravity measurement and satellite formation flying imposes rigorous requirements on inter-satellite ranging precision. The regenerative pseudo noise (PN) ranging is realized through regenerating the uplink PN ranging signal and remodulating it onto the downlink carrier. The regenerative PN ranging not only can be used to obtain high accuracy, but also can be combined with carrier ranging as an universal inter-satellite ranging solution. Though the theory of regenerative PN ranging has been studied by some researchers, there still have some problems to be resolved. (1) For PN ranging signal waveforms, Only squarewave and MSK have been considered, while other waveforms and the design of the optimum waveform have not been studied. (2) Several regenerative ranging codes have been proposed, but it's difficult to simultaneously meet the performance requirements of tracking accuracy and code acquisition time with the existing regeneration method. (3) Although regenerative PN ranging module has been integrated into some transponder prototypes, the impact of digital implementation on PN regenerating performance has not been analysed in depth. (4) Mutual interference of PN ranging channel and data channel has not been investigated in detail.In this dissertation, the regenerating performance of the uplink PN ranging signal is studied in detail. For the used PN regeneration structure, PN regenerating performances due to thermal noise, including tracking jitter of chip tracking loop (CTL) on PN ranging signal and code sequence acquisition time of correlators on code sequence, are analyzed, and the factors affecting these regenerating performances, such as signal-to-noise ratio, loop bandwidth, the window width of the mid-phase integrator, are presented.In addition to these factors, the PN ranging signal itself is studied to improve regenerating performance. First, the effect of PN ranging signal waveforms is investigated. Under the condition of the same signal bandwidth, the impact of several waveforms on CTL tracking performance is analyzed and simulated. The result shows that different waveforms have different loop detector gains, which results in different tracking performance. From this result, the optimum waveform is found. Second, different ranging codes have different CTL tracking performance or code sequence acquisition performance, In existing ranging codes, JPL1999 is best in tracking precision, while T2/T2B present the shortest acquisition time. However, there's no ranging code which can simultaneously attain high performance of clock tracking and code sequence acquisition. A novel method is proposed to improve the clock tracking accuracy without impairing the code acquisition time performance. In this method, the regenerated code sequence is fed back as the correcting input to the phase detector of the chip tracking loop afterthe ranging code is acquired. The result shows that the loop jitter variance of T2/T2B ranging code is reduced by about 2.6 dB, which results in a good integral performance for this ranging code, since its code acquisition time performance is intrinsically superior.In practical transponders, there are various sources impairing the PN regenerating performance. Some important sources are studied in this dissertation. The error due to oscillator instability is analyzed. The result shows that there is the optimum loop bandwidth to minimize loop phase jitter, and it should be dynamically configurated according to the signal-to-noise ratio (SNR) condition. Besides, with I/Q sub-sampling, digital processing is simplified, but a fixed phase lag between I and Q signal exists after odd/even separation. In addition, the CORDIC technique can be used to improve the efficiency of digital resources. The impact of digital demodulation method implemented with I/Q sub-sampling and CORDIC on the CTL tracking performance is analyzed. The result reflects that the effect owing to the phase lag is quite small under general dopper conditions, and phase compensation may not need to be adopted. Moreover, the degradation of CTL tracking accuracy is investigated due to the interference of data channel with two modulation modes PCM/PSK/PM and SP-L/PM. The result shows that with selection of the appropriate data rate, the degradation can be reduced greatly.In order to validate the above theoretical results, an IF sampling based digital transponder whose carrier recovery loop is enclosed in digital domain is designed on principles of flexibility, extensibility, modularization and low power consumption. In this transponder, signal processing blocks such as all-digital modulation, digital receiver and regenerative PN ranging are implemented, and an inter-satellite PN ranging system is established on the basis of these blocks. Experiment results show good coincidence between the measured code acquisition time and the theoretical one. The test results of CTL phase jitter variance are inverse proportional to SNR in low SNRs, while tend to be constant in high SNRs. The reason is that the thermal noise dominates in low SNRs, while the impact of digital implementation and oscillator phase noise becomes the main source in high SNRs, which accords with theoretical and simulation results. Finally, the PN ranging system is verified, and measured ranging accuracy is about 2 meters, which is within expectation.In this dissertation, PN regenerating performances are studied in depth, and some implementation issues on digital transponder and PN ranging system are investigated. These results are valuable for engineering implementation and system optimization of precision PN ranging, and may promote the development of PN ranging technology.