GPS World, April 2014

Receiver Design GALILEO stored in memory by a front end bit grabber it can also output standard receiver parameters code delay Doppler frequency carrier to noise power density ratio C N 0 phase and navigation message The software receiver LV IXOO FRQ JXUDEOH H WUHPHO ÀH LEOH DQG UHSUHVHQWV an important tool to assess performance and accuracy of selected techniques in different circumstances Code Delay Estimation The code delay estimation is performed in the software receiver by a parallel correlation unit giving as output a multi correlation with a certain chip spacing This approach presents some advantages mostly the fact that the number of correlation values that can be provided is thousands of times greater compared to a standard receiver channel Use of multiple correlators increases multipath rejection capabilities essential features in mass market receivers especially for positioning in urban scenarios The multi FRUUHODWLRQ RXWSXW LV H SORLWHG WR FRPSXWH WKH UHFHLYHG signal code delay with an open loop strategy and then to compute the pseudorange In the simulations performed the multi correlation has a resolution of 1 10 of a chip which is equivalent to 30 meters for the signals in question to increase the estimate accuracy Whittaker Shannon interpolation is performed on the equally spaced points of the correlation function belonging to the correlation peak The code delay estimate accuracy is reported in FIGURES 1 and 2 The results are obtained with Monte Carlo simulations on simulated GNSS signals with sampling frequency equal to 163676 MHz In particular a GPS L1 C A signal is considered affected by constant Doppler frequency equal to zero for the observation period WR DYRLG WKH HIIHFW RI G QDPLFV 7KH JXUHV VKRZ WKH standard deviation of the code estimation error that is the difference between the estimated code delay and the WUXH RQH H SUHVVHG LQ PHWHUV SVHXGRUDQJH HUURU VWDQGDUG deviation for different values of C N 0 To evaluate the quality of the results the theoretical delay locked loop DLL tracking jitter is plotted for comparison as where B n is the code loop noise bandwidth R c is the chipping rate B fe is the single sided front end bandwidth T c is the coherent integration time and c is the speed of light Q WKH WZR JXUHV WKH UHG FXUYH VKRZV WKH WKHRUHWLFDO tracking jitter for a DLL which can be considered as term of comparison for code delay estimation To correlate the results a E L spacing equal to D 02 chip is chosen and the code delay error values of the software receiver VLPXODWLRQ DUH OWHUHG ZLWK D PRYLQJ DYHUDJH OWHU DYHUDJLQJ VHFRQGV RI GDWD IRU H DPSOH values spaced 16 milliseconds an equivalent closed loop 4 3 2 1 0 Theoretical DLL jitter Monte Carlo open loop simulation 35 40 45 50 55 60 C N 0 G ï 7UDFNLQJ MLWWHU P FIGURE 1 Comparison between code delays estimation accuracy T c 1 ms T 16 ms B 1 Hz D 02 chip Theoretical DLL jitter Monte Carlo open loop simulation 35 40 45 50 55 60 4 3 2 1 0 C N 0 G ï 7UDFNLQJ MLWWHU P FIGURE 2 Comparison between code delays estimation accuracy T c 4 ms T 64 ms B 1 Hz D 02 chip bandwidth of about 1 Hz can be obtained In particular in Figure 1 a coherent integration time equal to 1 millisecond ms and 16 non coherent sums are considered while in Figure 2 a coherent integration time equal to 4 ms and 16 non coherent sums spanning a total time T 64 ms are considered In both cases the software UHFHLYHU UHVXOWV DUH H WUHPHO JRRG IRU KLJK C N 0 The code delay error estimate is slightly higher than its equivalent in the DLL formulation The open loop HVWLPDWLRQ HUURU QRWDEO LQFUHDVHV LQ WKH UVW FDVH EHORZ 40 dB Hz due to strong outliers whose probability of occurrence depends on the C N 0 In fact this effect is smoothed in the second case where the coherent integration time is four times larger thus improving the signal to noise ratio Nevertheless the comparison between open loop multi www gpsworld com April 2014 GPS World 37