Research On Modulation And Frequency Compatibility Of GNSS Signals

Global navigation satellite systems(GNSS) are developing rapidly. GPS has been built and put into use. GLONASS and Galileo systems are being improved and optimized. In order to improve the autonomy of satellite navigation and positioning, China has started the construction of Compass system. Japan and India have also begun to build their own navigation satellite systems. As the soul of GNSS, navigation signal plays a key role in determining the system performance. The chip pulse shaping of PRN code will influence the code tracking performance of the signal. As a result, the pseudo-range measurement accuracy will be influenced. The QPSK(Quadrature Phase Shift Keying) and BPSK(Binary Phase Shift Keying) modulations which are adopted by navigation satellite systems at present will not affect the chip pulse form of PRN code. The transmission efficiency of MSK(Minimum Shift Keying) modulation is nearly the same as the QPSK modulation. However, the MSK modulation will change the chip pulse form of PRN code. In addition, the impact on MSK modulated signal is smaller than QPSK modulated signal caused by satellite HPA(High Power Amplifier). Therefore, this thesis will evaluate the impact on code tracking performance brought by MSK modulation and contrast the influence on MSK and QPSK modulated signals caused by HPA, in order to research the feasibility of using MSK modulation in the L band of navigation satellite system.At the same time, as the increasing number of GNSS and finite frequency resource, navigation signals have to locate in the same frequency bands. Compatibility has been hot topic during the international frequency coordination. The compatibility between Compass system and other GNSS is important for the future development of Compass system. For these two issues, the main contents and contributions of this thesis are as follows:1. This thesis established the time-domain and frequency-domain models of MSK-BOC(Minimum Shift Keying-Binary Offset Carrier) signal for the L frequency band of GNSS. Using the signals in L band of GNSS, the impacts of MSK modulation on autocorrelation function and power spectral density were analyzed. It is shown that the high frequency components of main lobe of the power spectral density for MSK modulated signal is more than R and BOC modulated signals; the energy of sidelobe of MSK modulated signal is smaller than R and BOC modulated signals; the restraint outside the band of MSK modulated signal is better than R and BOC modulated signals; the impact of front-end bandwidth on autocorrelation function of MSK modulated signal is smaller than R and BOC modulated signals; when the front-end bandwidth of receiver is close to the main lobe bandwidth of the power spectral density of MSK modulated signal, the slope near zero point of autocorrelation function of MSK modulated signal is greater than R and BOC modulated signals.2. This thesis analyzed the influences of MSK modulation on code tracking accuracy and envelope of multipath error of navigation satellite signal, under different receiver correlator spacings, front-end bandwidths, and carrier to noise ratios; the impact of MSK modulation on Gabor bandwidth was analyzed, under different receiver bandwidths. The simulation results show that when the receiver bandwidth is close to the main lobe bandwidth of the power spectral density of MSK modulated signal, the Gabor bandwidth of MSK modulated signal is larger than R and BOC modulated signals; when the receiver correlator spacing is relatively wide or receiver bandwidth is close to the main lobe bandwidth of the power spectral density of MSK modulated signal, the code tracking error and envelope of multipath error of MSK modulated signal are smaller than R and BOC modulated signals. These analysis results can provide theoretical support for the L frequency band of GNSS to adopt MSK modulation, from the code tracking performance point of view.3. This thesis evaluated the influence on MSK modulated signal brought by the nonlinear of HPA. The time-domain waveforms of MSK and QPSK modulated signals were analyzed; on the basis of nonlinear channel model containing HPA on the satellite, this thesis comparatively evaluated the correlation losses, power losses, and code tracking errors under different receiver bandwidths, adopting MSK and QPSK modulations respectively. It is shown that the phase of time domain waveform of QPSK modulated signal changes discontinuously; the waveform of MSK modulated signal has continuous phase and constant envelope; when the receiver bandwidth is close to or larger than the main lobe bandwidth of the power spectral density of MSK modulated signal, the correlation loss, power loss, and code tracking error of MSK modulated signal are smaller than QPSK modulated signal. These analysis results can provide theoretical support for the L frequency band of GNSS to adopt MSK modulation, from the nonlinear point of view.4. Basing on the latest signal parameters of Compass system and Galileo ICD(Signal In Space Interface Control Document), this thesis analyzed the compatibility between Compass, GPS, and Galileo signals. Firstly, the situation of navigation band was analyzed which was occupied by Compass, GPS, and Galileo systems. Secondly, considering the relative positions of navigation satellite signals, this thesis simulated the spectral separation coefficient(SSC) between the Compass, GPS, and Galileo signals. In the following, on the basis of system constellation, satellite operation cycle, Doppler frequency offset, satellite antenna gain, receiver bandwidth, etc., this thesis simulated the interferences between Compass, GPS, and Galileo signals in the worst case. These analysis results can provide technological support for the frequency coordination between Compass system and other GNSS.