| GPS Software receiver technology, based on thought of SDR(Software-Defmed Radio), is an effective solution for adapting to new GPS technique and algorithms, coexisting and being compatible with other Global Navigation Satellite System (GNSS). The hardware scheme in this paper is consist of the RF Front-end module based on the GP20f5, IF data acquisition based on CY7C68013 with the USB 2.0 interface. After IF GPS data being saved to PC, the research on such key technologies as signal acquisition, tracking, position solution and so on can be carried out.At signal acquisition aspect, firstly the paper introduce the properties of autocorrelation and cross-correlation of C/A code, analyze key parameters in acquisition, and explain frequency domain parallel search by FFT algorithm and show it's advantage by contrast with time domain serial search, and then a concrete algorithm of "Averaging Correlation" based on FFT is proposed.It has been proved successful that the scheme of acquisition can capture all satellite signals in real data. After that, the precise frequency acquisition can make it more rapid to enter locked state in tracking loop.At signal tracking aspect, first, the elements of code loop and carrier loop was proposed based on the basic phase-locked loop (PLL). Then the influence of carrier frequency error on code loop and influence of code phase error on carrier loop in different noise are discussed in simulation. Finally, code loop and carrier loop are coupled together to track GPS signal in close loop, and analyze of different performances on track loop under different signal intensity is given.At position solution aspect, navigation data decoding starts from bit synchronization and sub-frame synchronization. Then navigation data is extracted after successful parity check. Satellite position can compute by using ephemeris parameters in navigation data; Pseudo-range can be estimated from time offset between the start of sub-frame and the start of local time. Having gotten satellite positions and pseudo-ranges, receiver position can be obtained by iterates position formulation. It can be proved that the GPS software receiver in this paper is successful according to comparing the receiver position coordinates to real position coordinates. |
