GNSS Signal Acquisition Techniques In Weak Signal Environment

GNSS (Global Navigation Satellite System) has been widely used in the world, butwhen the signal conditions are not ideal, for example, in the interior, forests and urbanenvironment, multipath and interference will lead serious energy decline, normal GNSSreceiver will be difficult to capture and track navigation satellite signals. In recent years,with applications of location-based services (LBS),increased demand for indoor andurban positioning has been attracted more and more attention, and so high-sensitivityGNSS and Assisted-GNSS (A-GNSS) positioning technology has become a currentresearch focus.Signal capture is one of the most critical technologies in the baseband signalprocessing of high-sensitivity positioning receiver.Based on this background, this paperfocuses on code and frequency acquisition in the weak signal environment, the author’smajor contributions are outlined as follows:1. The satellite channel model with the small scale fading characteristics isestablished. Through the quantitative analysis of the relation between channel coherenttime and the coherent integration detection performance, we pointed out that thecoherence integration time not only depend on the frequency offset and the data bit flip,but also on the channel coherence time. Based on the conclusion, we also given thecoherent interation time recommendation under the different mobile speed. At the sametime, the conclusions can be used to explain Ali’s Synthetic antenna gain.2. Focusing on the detection performance degradation caused by frequency offset,two improved code phase and Doppler shift united capture methods for low mobilityequipment is proposed. The method based on the difference of FFT spectrum line isalso popularize to synthetic and real antenna.Therefore,combined with the analysisof frequency offset estimated CRLB, the theoretical detection performance of jointcode and Doppler acquisition algorithm is derived. Simulation results show that thedetection algorithm can effectively improve the frequency estimation accuracy and thedetection probability, and has very low complexity.3.For the satellite signal acquisition with low time-frequency uncertainty, a codeacquisition algorithm based on discrete cosine transform (DCT) domain filtering isproposed. Based on the different time varied charistristic between the synchronous andnon-synchronous PMF output vector, the DCT transform domain filtering and IDCT reconstruction is used to improve the detection signal to noise ratio, thereby increasingthe signal detection probability. In addition, this algorithm is also popularize to themulti-antenna, and the theoretical detection performance under AWGN and flatRayleigh fading channel is also derived. The theoretical analysis and simulationresults show that the detection algorithm can effectively increase the probabilityof detection, especily in fading channel.4.Focusing on the problem that the weak signal’s code acquisition performancewill be decreased when there exists strong interference form other satellites, thequantitive cross-correlation interference analysis is done, and we concluded that theserious cross-correlation interference only occurred at about mKHz,where m isintergeral. Based on the conclusion, a code acquisition method based on eliminating theinterference from the correlation is proposed. The method only need to pre-store a smallcount of cross-correlation vectors with mKHz frequency diffrence, and with thecross-correlation vectors pre-stored,detecting error caused by interference signals canbe eliminated directly from the correlator, which avoid the interference signalsreconstruction and parameter reestimation process. Simulation results show that theproposed detection algorithm can effectively improve the detection probability of weaksignals, and has very low complexity.5. Finally, considering the actual weak signal parameters, this paper summarizedthe applicable scenarios and the performance gain for all the proposed algorithms. Inaddition, the proposed acquisition strategy and capture parameters is suggested underthegiven signal strength and channel conditions.