Pseudo-Code Structure Design And Research On The Acquisition Technology Of GNSS In Weak Signal Environments

The Global Navigation Satellite System plays an important role in the national defense and economic construction. All the world’s major powers are working to develop their own independent satellite navigation system. Under the condition of multiple GNSS systems working together in the future, the integrity, stability and location accuracy of GNSS positioning service will be its core competitiveness. Especially in recent years, with the expansion of GNSS applications, the demand of receiving GNSS in harsh environment increases dramtically. Therefore, how to improving the positioning performance of GNSS signal in weak signal reception environment becomes a hot topics in the study of GNSS gradually.In all factors that affect the positioning performance of GNSS in weak signal environment, the signal structure design and the weak signal acquisition technology are the two key factors. Signal strcture design determines the inherent properties of GNSS signal, and the ability of acquisition determines the receiving sensitivity. So the dissertation focuses on these two aspects of research, the design of the GNSS signal and the acquisition technology in weak signal environment. The main achievements and contributions are as follows:Firstly, in terms of signal strcture design, a novel code design method based on the concept of unequal-length codes is put forward to solve the main problems exist in the code design of GNSS currently. This novel short pseudo-code optimization method from the perspective of signal property improves signal acquisition and tracking performance in weak signal environments. In order to fully explain the advantages of unequal-length code design, the theoretical analysis is deduced elaboratedly from the time domain and frequency domain, followed by the Monte Carlo simulation to verify the correctness of the theoretical analysis. The results show that under the same acquisition conditions, the detection probability of the signal using unequal-code length pseudocodes is significantly higher than that of the signal using equal length codes.As for as the tracking error caused by the multi-access interference (MAI) in the case of the weak signal environment, the unequal-length pseudo-code design also significantly improved the effect of code tracking accuracy. Especially when different GNSS signal Doppler frequency shift near consistent, the code tracking error caused by the multiple access interferences are difficult to eliminate. However the unequal-length code design will eliminate the tracking error because the code-lengthes of different signals are co-prime, thus the cross-correlation between different signals will present a random variation, and thus the fixed code tracking deviation caused by the MAIs will be eliminated after the loop filter. The theoretical analysis and simulations verify the unequal-code length code design can improve the performance of code tracking accuracy effectively in weak signal environment.Secondly, in terms of the acquisition technology, an improved differential combination post-correlation processing technology is proposed for the conventional GNSS signals in weak signal environments. In order to give readers a more comprehensive understanding of the weak signal acquisition technology and the innovation of this dissertation, the current acquisition technologies in weak signal are reviewed firstly. Among these three post-correlation processing technologies, the differential combination has the ability to eliminate the effect of residual phase of carrier and without obvious square loss, while it is not very sensitive to data bits flip, thus it is a good choice for acquisition in weak signal environment.However the conventional differential combination technique still has some problems, the data bit flip and the estimation error of Doppler shift will result in acquisition difficulties, and which become worse in weak signal environment. Therefore, the dissertation proposed an improved semi-bit frequency compensation differential technology. The output of the correlator is significantly improved by introducing the Doppler compensation technology. And the processing loss due to data bits flip is removed by semi-bit combination.Theoretical analysis shows that better acquisition performance will be achieved by semi-bit frequency compensation differential technology than that of the conventional one. Monte-Carlo simulations verify the correctness of the theoretical analysis.Thirdly, since non-white radio frequency interference has greater impact on the performance of the signal acquisition and tracking and it is difficult to analysis in weak signal, a systematic method is presented for analyzing the effects of non-white interference in GNSS. The analysis process will be simplified by using this method, because non-white interferences can be converted to equivalent white noise by corresponding whitening coefficients. Since it takes into full consideration of the characteristics of coherent processing and non-coherent processing, this method is more suitable for code tracking than the previous. By introducing new parameters, the effects of non-white interference on code tracking can be evaluated easily. For the interference analysis of prompt channel, it also shows superior to quantify the effects of non-white interference on the post-correlation signal-noise-radio and carrier tracking error. The simulations show that more accuracy analysis result can be achieved by this method. Comparison with conventional methods, it is more general and simpler to apply.