Study On The Algorithm For Weak Signal Acquisition And Tracking In Gps Software Receiver

The industry of Satellite navigation,of which GPS is representative,has become one of the information industrys that develop the most rapidly in the world. The extensive applications make a request for advanced GPS receiver that can work in weak signal environment, e.g. inside a building,in the built-up area, in the forest ,etc. It is difficult to acquire or track the weak signal for most commercial receivers.Then,the technology of GPS receiver for weak signal should be researched.GPS software receiver,which is based on Software Defined Radio,has many advantages in the flexibility,economical consideration and universality.It has become the important research platform for design and simulation of the receiver, testing experiment and validation of the algorithm.The dissertation focuses on the algorithm for Weak Signal Acquisition and Tracking in GPS Software Receiver, the primary work and conclusions are as follows:1. The spread spectrum modulation of GPS signal is introduced,and the correlation properties of C/A code,as the basis of the acquisition,are studied in detail.The model of GPS intermediate frequency (IF) signal is studied for simulating digital signal as the data source with controllable properties and foreseeable output.And the doppler effect upon the carrier wave and C/A code in the GPS signal is analyzed.2. The basic theory of GPS signal acquisition is studied,as well as the traditional serial acquisition methods and the parallel searching and acquisition strategy of code-phase on the basis of circular correlation.The threshold for confirming of acquisition is gived.The effect of predetection integration time and frequency separation is analyzed.Then three methods of weak signal acquisition are compared:coherent integration,non-coherent integration and differentially coherent integration.The test results that differentially coherent integration is a good method with better performance and fewer operation.In order to increase S/N, differentially coherent integration combining with coherent integration is proposed.The simulating indicates that the new method can acquire the signal with C /N₀=23dB-Hz.3. For the frequency resolution obtained from acquisition is too coarse for the tracking loop,the approaches through phase relation and DFT are used and the frequency resolution is improved to several hertz.In order to track weak signal with long data source,The way of finding the first navigation phase transition is studied.4. The principle and realization of the carrier tracking loop and the code tracking loop is studied.For determining the state of the loop,lock detectors are analyzed.The Kalman filter methods for tracking weak signals are mainly studied.The models of carrier-phase and code-phase measurements and dynamics are built,and the traditional tracking loops are replaced by Kalman filters.Then the feasibility of Kalman filter-based tracking is validated using real data source.And the performance of the method is tested through simulatin. The result is that it can track the signal with C /N₀=24dB-Hz by 20ms's input data selection,which is better 4dB than the traditional tracking loops.