Research On The Acquisition Algorithm Of GPS Software Receiver

In recent years, with the developing of navigation and positioning technology, The Global Navigation Satellite System have been quickly and widely used whether in the military or civilian aspects, such as transportation, weather forecasting, social security, natural disaster relief and personal mobile phone positioning. Currently, the world’s satellite navigation system has been America’s GPS, Russia’s GLONASS, China’s Compass system and the European Galileo navigation system (Galileo). With the innovation of technology, traditional hardware receiver has been unable to meet people’s needs. The software receiver which have compatibility and flexibility will become the future trend lower in supporting of the microprocessor manufacturing processes and technologies This paper focuses on the problems in acquisition phase of the GPS software receiver.The GPS software receiver requires to deal with the ranging signal with real time process, including analogy down conversion, analogy to digital change and pseudo-noise sequence (PN) synchronization.The PN synchronization process is usually achieved in two stages:PN acquisition and PN tracking. In order to realize the PN tracking the PN acquisition (or coarse alignment) should be obtained firstly. The acquisition stage ususally requires alignment within certain chip periods. As long as coarse alignment is obtained, PN tracking is accomplished with a phase locked loop (PLL). If the phase error of local PN and received PN is beyond half of a chip period, which is acquired in the PN acquisition session, then the exact PN phase could be quickly achieved in PN tracking session. Otherwise, it will take a great deal of time to realize PN tracking. Similarly, the frequency acquisition speed of carrier wave directly influent the frequency tracking of carrier wave. Therefore, the acquisition stage determines the PN code synchronization time. Therefore, PN acquisition is the key of a successful PN Ranging System.This thesis work mainly focus on the DS-SS PN acquisition of PN Ranging System,including the digital process, acquisition algorithm research and circuit optimization.Firstly, the paper introduces the principle of direct sequence spread spectrum communication in GPS system and the basic development of GNSS used.It studies the GPS signal structure, the signal generation and its eorrelation The components of the software receiver is explained.The paper compared the receiver hardware and software receiver from the structure, function and other aspects.Secondly,the acquisition principles is analysised.It focuses on the three commonly used acquisition algorithm, namely sliding correlation acquisition, parallel frequency acquisition and matched filter acquisition. The algorithms is compared from Computational algorithms,complexity and hardware implementation.It analysises the pros and cons Three important factors, is summaried. namely Satellite motion generated by the Doppler effect, the search step when searching for signals and the length of acquiring data We constructed signal detection, judge algorithm and digital down conversion moduleFinally, based on the above analysis of the methods,the improved fast FFT acquisition algorithm is proposed This paper parallelly searched the code phase of the C/A code, serially searched the Doppler shift, greatly reducing the search space. The correlation operation is made during the received signal and the local pseudo-code in parallel by using the FFT algorithm. In the using FFT process, the using circumference relevant operator reduced the number of FFT operation, thereby reducing the amount of computation to improve the acquising speed.