Research On GPS L2C Acquisition And Tracking Algorithms

In the development of GNSS(Global Navigation Satellite System),pilot signal becomes more and more important.Without navigation message bit,pilot signal can be integrated for longer time,and pure PLL(Phase Lock Loop)can be used for tracking which needs lower tracking C/N0 threshold.GPS L2 C combines CM and CL through time-division technique.CM is 10230 chips long and holds message bit,while CL is 767250 chips long,being the pilot channel.The long code period guarantees lowers cross-correlation power,improving acquisition performance in multi-path and weak signal environment.But this also makes acquisition more difficult.Based on the above knowledge,acquisition and tracking of L2 C is analysed in this paper,which falls in the following three aspects:(1)In respect to the condition that no accurate model for time-division signal acquisition is analysed,the integration expression of L2 C under carrier Doppler is deduced and compared with that for conventional continuous signal.The impact of code Doppler for L2 C acquisition is analysed through theory and simulation.Simulation shows that the theoretical expression corresponds to the reality.(2)The Hyper Code algorithm improves acquisition speed through folding local code,getting the correlation of multi code phase at the same time.But the best folding time is not given through quantity analysis in comtemporary essays.This problem is researched in this paper under the principle of least mean acquisition time.A table of folding time corresponding to C/N0 is given.Simulation shows that comparing to no folding,under the signal processing structure given and the folding time worked out,the mean acquisition times degrades 80% at most while gurantees no gain.(3)In respect to the two channels of L2 C,combined tracking is analysed in this paper and a pure PLL combined tracking algorithm based on bit sign guessing is proposed.It is compared to tracking by pilot channel only and combined tracking algorithm based on linearly combining of discriminator outputs.Simulation shows that,using the algorithm proposed,the tracking phase error variance degrades 30% compared to tracking pilot channel only.The degradation is about 24% when C/N0 is 20dB-Hz and integration time is 2ms and 8% when C/N0 is 20dB-Hz and integration is 20 ms separately compared to the combined tracking algorithm based on linealy combining of discriminator output.Performance of the two combined tracking algorithms is almost the same under high C/N0.