Study On Unaided GNSS Signal Acquisition Technologies

With the wide application of Global Navigation Satellite System (GNSS) positioning technique, there is an increasing demand for GNSS positioning in harsh environment, such as indoor positioning, urban canyon positioning or dense foliage positioning. In this circumstance, the received signal is usually very weak. When GNSS is applied in aircraft positioning, satellite positioning or missile positioning, the GNSS receiver receives the weak and high dynamic signal. Moreover, there are requirements for receiver’s power consumption in those environments. So unaided receiving technique is proposed. GNSS signal acquisition plays the most important role in GNSS positioning, and coarsely estimates the acquisition parameters. In this dissertation, the received signal in different environment is modelled and analyzed. Then, the new methods are proposed. The author’s main contributions are outlined as follows:1.To overcome the influence of bit sign reversal and frequency error on integration peak, block zero-padding method based on discrete chirp-Fourier transform (DCFT) has been proposed. Compared with the conventional receiver architecture that uses closed-loop acquisition and tracking, it is the suggested method for an open-loop acquisition. Since calculating the inter-block conjugate products restricts the Block Accumulating Semi-coherent Integration of Correlations (BASIC) in a low signal to noise ratio (SNR) acquisition, the proposed method combines DCFT and block zero-padding. In this way, the post-correlation signal is coherently integrated with the bit sequence stripped off, and the high dynamic parameters are precisely estimated, at the same time. Then, the detection performance of the proposed method has been analyzed, and its detection probability and false alarm probability has been derived. Finally, Simulation results show that when the length of the received signal is fixed, the proposed method can precisely detect the high dynamic parameters in lower SNR than the BASIC.2.To improve the detection probability of GNSS acquisition when SNR is low and the range of frequency error is small, this paper has proposed two methods based on transform domain filtering. Firstly, the acquisition method based on Fast Fourier Transform (FFT) filtering has been proposed. However, compared with the method based on Discrete Cosine Transform (DCT) or Fourier Transform (FT) filtering, the wavelet function can be selected based on signal property. So the code acquisition method based on wavelet domain filtering has been proposed. The signal frequency band in wavelet domain can be divided so that the received signal frequency band can be selected to reconstruct the signal. In this way, the noise can be filtered out and most useful energy can be retained, which improves the detection probability. Moreover, for better retaining the useful signal energy, the proposed method uses the useful signal energy before filtering and after filtering to calculate the energy ratio as the rule of selection of wavelet function. Finally, the simulation results show the detection probabilities under the Gaussian channel and fading channel, and proves that the proposed method can improve the detection probability compared with the normal methods.3.In order to acquire the GNSS signal under the multiplicative and additive noise, the joint acquisition method of mean function and autocorrelation function has been proposed. Firstly, the cyclostationarity of received signal is proved. Then, the proper cyclic frequency points are chosen to estimate the acquisition parameters. The proposed method uses the energy ratio threshold to determine which function (mean function or autocorrelation function) to be used for estimation of the received signal frequency. Compared with conventional methods, the proposed method not only uses the mean function but also uses autocorrelation function. Finally, the simulation results prove that the proposed method is more robust to the change of multiply noise than the conventional method.4.For fast acquisition of the high dynamic GNSS signal under high SNR, the compressed GNSS acquisition method based on adjacent part differential coherence has been proposed. Firstly, the adjacent part differential coherence process between the two neighbor milliseconds is used. Then, Walsh Hadamard Transform is used to compress and detect integration signal in the two steps. Moreover, the detection probabilities of the proposed method are derived. Since the adjacent part differential coherence process reduces the impact of bit sign and frequency error on the integration amplitude and the two-step compressed detection can reduce the acquisition time, the proposed method realizes the fast acquisition in the high dynamic environment. Finally, the simulation results show that the simulated results are the same with the results of the theoretical derivation, the proposed method has better detection probability and the lower mean acquisition time than compared method under the same false alarm probability.Through the research of my dissertation, block zero-padding method based on DCFT, the code acquisition method based on wavelet domain filtering, the joint acquisition method of mean function and autocorrelation function, and the improving compressed GNSS acquisition method using adjacent part differential coherence have been proposed for GNSS signal acquisition in the special environment. The dissertation proves that proposed methods improve the GNSS signal acquisition performance by the theoretical derivation and simulation results. The research results provide the improvement of GNSS acquisition. The proposed methods combined with aided acquisition methods can particularly provide theoretical base for Beidou navigation precision positioning.