Research On GPS Software Receiver Implementation And Positioning Algorithm

Global Positioning System is widely applied to a wide range of applications, such as geomatics, navigation and guidance, environment monitoring which require high-precision positioning. The increasingly stringent requirements necessitate in-depth research on precision improvement of stand-alone GPS and related algorithms. The hardware receiver is well developed, however, it's unrealistic to apply new algorithms to improve precision due to the cost of revising the firmware or the limited hardware resource, researcher would possibly experience a long development period. Software receiver can efficiently fix this problem. The baseband signal processing will be finished by software approach as long as the RF (radio frequency) signal is correctly converted to IF (intermediate frequency) signal. Software approach provides great flexibility that facilitate the development. Therefore, research on software receiver has great impact on modeling validation and algorithm verification.This paper focuses on GPS software receiver implementation and development of high-precision algorithms for stand-alone GPS. Specifically as following:1. The GPS software receiver's peripheral circuit will be discussed thoroughly. A SuperStar II OEM receiver is hacked, then IF signal from GP2015 will be analyzed, other signal preprocessing module will also be introduced.2. Demodulation of IF signal and decoding of navigation data. Two basic modules, Acquisition and Tracking, will be analyzed. In acquisition, serial search in time domain and FFT search in frequency domain are addressed and compared. In tracking, emphasis will lie in Costas Loop.3. Extraction of navigation message massage from tracing output. Firstly, message structure will be introduced, then parity check will be discussed. Finding the pseudorange is a critical issue in positioning, its formation will be carefully examined. Finally, user position can be calculated from the decoded ephemeris and pseudorange by Iterative Least Square (ILS). 4. High-precision positioning algorithms based on nonlinear filtering and smoothing algorithm will be proposed to improve the stand-alone accuracy and robustness. Two smoothers will be introduced, then they are combined with Unscented Kalman Filter (UKF) to derive iterative algorithm. In order to improve the robustness, UKF will be replaced by Square-Root UKF (SR-UKF) in the forward filtering. Finally, proposed algorithms will be validated by static testing.This paper logically examines the fundamental of GPS positioning, starting from raw IF signal acquisition, to satellite acquisition and tracking, to demodulation and decoding of navigation message, determination of user position. It's helpful for one who is willing to further study in this field to have better understanding of GPS fundamental.