Carrier Phase Measurement Technique Based on Non-cooperative GNSS Signals

摘要

The "Yinhc incident" and the chemical weapon crisis happened in Syria both showed that, the US army may have the power to shut down GPS civil signals in some specific areas. In this situation, traditional technologies of signal processing are no longer available, new methods must be developed to make use of GPS signal resources. A new system named Navigation via Signals of Opportunity (NAVSOP) is presented. This system is able to calculate users' position by making use of hundreds of random signals that are all around us. Then a specific method to realize NAVSOP is proposed, in which the non-cooperative GNSS signals are used as the input signal. The word "non-cooperative" means that the pseudo-code loaded on the carrier is unknown, so the carrier phase measurement technique based on non-cooperative GNSS signals will be the key problem to deal with. GNSS receivers can receive navigation signals with high SNR (signal-to-noise ratio) by using CPRA (controlled radiation pattern antenna), thus users can obtain superior signal source to carry out research. The main research contents include signal acquisition and tracking based on non-cooperative GNSS signals. In this paper the whole process of signal acquisition is expressed mathematically. Effect of noise with different intensities on the acquisition results are researched. And in theory it is proved that, when the SNR of input signals is above -15 dB, the acquisition algorithm proposed can successfully capture the frequency. A threshold is set to decide if the acquisition results are corresponded to Doppler frequency shifts. The optimal value of parameters are determined based on their impact on the performance of the acquisition algorithm. The result of signal acquisition is the estimation of carrier frequency. A phase locked loop (PLL) is utilized to lock the carrier and track its variation. The major error sources of PLL in GNSS receiver are phase jitter and dynamic stress error. Optimal parameters are set to improve the performance of tracking loop and reduce the error. Besides, the order of PLL are determined according to the dynamic environment of user's satellite, and the isolation and isolation frequency are proposed to reduce the influence of the close carrier frequency on the tracking results. The simulation results showed that the error of signal acquisition algorithm is within 3 Hz, and the number of satellites which is correctly acquired is more than 6. When PLL stabilized to track the signal, the tracking error (difference between the frequency of output signals and input signals) is within 15 Hz. The method of signal acquisition and tracking conducted in this paper realized carrier recovery of non-cooperative GNSS signals, it also laid the technical foundation of implement of NAVSOP.