Research And Implementation On Baseband Signal Processing Algorithm Of GPS Software Receiver

In recent years, GPS software receiver is well developed for its advantages on modularization, programming, flexibility, and adaptability. Also it is compatible with a variety of navigation systems. Research based on software receiver, could make the system analysis procedure more convenient and effective. The study on GPS could provide reference for other satellite navigation systems, especially for Chinese BeiDou navigation system.This dissertation works on the research and implementation of GPS software receiver based on software radio, and mainly focuses on algorithm improvement and realization for baseband signal processing and positioning calculation.Firstly, this dissertation introduces the GPS system architecture and principles; studies the signal format, characteristic, and modulation; and illustrates the simulation diagram of GPS system, including signal generation, channel model, and software receiver. Secondly, in order to obtain the navigation data, C/A code phase and Doppler frequency shift, the GPS baseband signal processing is mainly discussed, including signal acquisition, tracking, and so on.The acquisition of GPS signal employs time domain serial search, frequency domain parallel search by FFT algorithm, and a new fast acquisition algorithm based on correlation value and FFT. Simulation results show that this algorithm decreases calculation complexity and achieves signal fast acquisition under equivalent precision. For weak signal acquisition, this dissertation introduces conventional accumulation models. And a new acquisition algorithm based on successive interference cancellation is proposed for weak signal co-existing strong satellite signals. Both the theoretical analysis and simulation results show that this algorithm has higher acquisition ability. Non-Coherent Delay Locked Loop and Phase Lock Loop are used in the GPS signal tracking module.Finally, locating algorithm is analyzed. Navigation data and pseudorange are acquired from baseband processing results, thus user position would be calculated. If the number of visible satellites is more than 4, except for the Iterative Least-Squares algorithm, we can find the 4 best location satellites for positioning by vertex of the largest-volume tetrahedron.This dissertation also studies on the current research hotspots of GPS software receiver, such as:1. Signal fast acquisitionFor GPS C/A code's two-dimensional search, Doppler frequency shift is estimated by polynomial fitting to reduce search scope, and a method combining FFT/IFFT and correlation is used to determine C/A code phase.2. Weak signal detectionFor weak signal acquisition, most conventional GPS receivers couldn't provide sufficient data if the received signal contains strong satellite interference. An acquisition algorithm based on successive interference cancellation is proposed. Acquisition is processed step by step according to satellite signal's carrier-to-noise ratio, and adaptive adjustment of combining time is utilized to achieve successful fast acquisition.3. Find the 4 best location satellites for positioningIf the number of visible satellites is more than 4, and the signals'carrier-to-noise ratios are nearly the same, we can choose 4 satellites for positioning according to the geometric distribution of these satellites.