| In order to assure the quality of the navigation and position service,monitoring stations are needed to collect,process and analyze the downlink data from the navigation satellite.Based on that,monitoring and assessment on the navigation and position service can begin.However,because of the limitation of the transfer path,most of the monitoring service works at the fixed monitoring station now.Monitoring and assessing the quality of the navigation and position service based on cloud platform can solve this problem.But the limitation of the storage resource becomes the bottleneck which impedes its development.This paper starts on account of this real situation.(1)Aiming for the drawback of the current fixed navigation signal monitoring system,navigation and position service monitoring and assessing system based on cloud platform is introduced and the navigation signal monitoring and assessing technological process with limitation of storage resource is designed.With the background about the limitation of the storage resource,three main parts about the size of data are analyzed and the technological process to compress data is analyzed.At the same time,this paper teases the current navigation signal monitoring index system.Through analyzing the most direct and most important index about the navigation performance,the index system under the storage resource limitation is proposed,which provides basis for the design of the monitoring and assessing system under the limitation of the storage resource.(2)Based on previous analyses,the signal performance loss because of the sampling and quantizing is analyzed.Quantizing provides CNR loss.1bit quantizing introduces loss about 1.96 dB.2bit quantizing introduces loss about 0.55 dB.Different sampling rate will case difference of the amplitude of the correlation function.But the symmetry will not be influenced even if the sampling rate is lower than the Nyquist sampling rate.However,while the sampling rate is smaller than Nyquist sampling rate,frequency domain aliasing is introduced.Thus,anti-aliasing filter is used before sampling,which will promote the tracking accuracy.However,filtering will causes the loss of the high-frequency component,will causes the correlation function becoming smoother.Considering that whether correlation function is smooth or not is not very important in acquiring part,the anti-aliasing filtering strategy is still the better choice.When different signal components are sampled at the same time,the target signal performance loss because of other signal components is analyzed,mainly about CNR loss.It can be concluded that when the number of signal components is not very large,the CNR loss is about 0.2 dB,resulting from the consideration about correlation.However,when the number of signal components is pretty large,the CNR loss will come to several dB.Aiming to solve this problem,the orthogonal multiplexing technology based on frequency-division and phase-division,which is also a method to mitigate the cross-correlation,is introduced.It refers to the design of OFDM,while the frequency bandwidth is half of that.(3)Duty ratio signal model is introduced.Considering that at the time when signal is not sampled,losing lock will appear in tracking loop.Aiming at this problem,considering that the navigation and position service monitoring period is not very long,the movement of receiver is simplified as uniform variable speed movement.Furthermore,least squares estimate is introduced to estimate the carrier frequency and code phase,to narrow down the uncertainty of them.In simulation test,when the CNR is 50 dBHz and the duty ratio is 50%,the conventional loop has lost lock.With least squares estimate,the loop can still work even the duty ratio is 40%.What's more,after analyzing the main reason of the lock losing is that the loop dynamic condition exceeds the loop scope to pull in,a method to double the loop pull in scope is informed,by adding two carriers with different phase. |
PDF Full Text Request