Research On High Sensitivity GNSS

Recently, the industrial applications in the civilian sphere of navigation and position based on Global Navigation Satellite System(GNSS) attracted more and more people's attention. Application in the civilian sphere included: l)the needs of navigation in urban environments; 2) the needs of car navigation systems, mobile phones and other portable mobile terminal; 3)the needs of navigation in canyon and deep forest environments. The salient feature was that signal was very weak in those environments and signal strength was 10²0dB weaker than open zone, even 30dB. Also, there were multi-path and interference. Receivers must enhance the ability of weak signal processing to work properly. So the technology of high sensitivity became one hotshot of navigation technologies. The technology of high sensitivity mainly resolved problems of weak signal acquisition, weak signal tracking, real time monitor of signal and others. The thesis researched some of these problems deeply.The thesis analyzes the technology of high sensitivity according to the features of GNSS signal and scenarios. The thesis includes:1)Based on the mathematical expression of GNSS signal, we discussed the autocorrelation properties of spread spectrum code with BPSK modulation, also discussed the cross-correlation properties. We introduced the ideal model of coherent spread spectrum receiver. Then, we discussed the error sources of GNSS receivers including crystal error and we introduced a method to simulate the frequency error sequence of crystal.2)To resolve the problem of frequency estimation in weak signal environment, we analyzed three methods: differential in time-domain, FFT in frequency-domain and grid of frequency & comparison of energy. We introduced three methods of bit synchronization. The Phase difference method was suitable for use with large frequency offset. The likelihood method was suitable for use in weak signal environment, but sensitive to frequency offset. To overcome the problem of sensitive to frequency offset, we proposed a method to estimate the frequency offset and the edge of data jointly.3)We analyzed the feature of ephemeris and almanac, then proposed the method to tracking weak signal based on data prediction and carrier assisted code tracking. When data could be predicted, carrier and code were tracked simultaneously, or carrier was held and code was tracked. Then we compared different code phase discriminators and analyzed the error resulting from thermal noise. We suggest using Early-minus-Latepower discriminator. Then we analyzed the traditional carrier loop--FLL and PLL,and we found that they couldn't achieve high sensitivity. So we proposed a method based on FFT in frequency domain to track weak signal and this method could fulfill our aim.4)We introduced two methods for estimating carrier-to-noise ratio(C/N₀). Also we discussed the method to determine the lock of PLL. Finally, we discussed the structures of receiver and procedures of signal processing.