GPS World, January 2017
Single element 670 671 672 673 674 675 676 677 678 679 Time hours of GPS week 60 55 50 45 40 35 JANUARY 2017 WWW GPSWORLD COM GPS WORLD 61 15 10 05 0 05 10 However effects of multipath are still present in these 1 Hz phase estimates Much of this is removed by fitting a third order polynomial to the data FIGURE 11 shows a representative plot of the 1 Hz phase measurements and the fitted polynomials From these polynomials phase offsets are computed and applied at a 1 Hz rate for beamforming RESULTS Several hours of sampled data were processed for all satellites in view Standard receiver outputs such as pseudorange carrier phase and C N 0 from all three software receivers single element traditional beamforming and correlator beamforming were recorded from which multipath mitigation performance results could be derived All three software receiver implementations used the same signal tracking parameters at the final measurement producing state These steady state parameters are as follows Carrier loop pre detection integration time 20 milliseconds PLL order 3 PLL noise bandwidth 18 Hz Correlator spacing 01 C A code chip Code discriminator type Normalized coherent earlyminus late DLL update rate 10 Hz performs data wipe off as shown in Figure 1 DLL noise bandwidth 1 Hz DLL order 1 Carrier aiding of code enabled C N 0 algorithm narrowband power over wideband power ratio NBP WBP FIGURE 12 shows representative C N 0 measurements for satellite PRN06 TABLE 1 lists the C N 0 standard deviations for all satellites after de trending using a second order curve fit For all results obtained C N 0 varies significantly for the single element receiver This variation is consistent with multipath fading As expected multipath fading is nearly absent for the traditional beamforming receiver This clearly shows how beamforming rejects multipath from off beam directions As expected the 10log 10 7 845 dB gain advantage of traditional beamforming over correlator beamforming is clearly apparent Furthermore C N 0 of correlator beamforming remains close to that of the center element However the most striking result is the multipath Satellite PRN Singleelement reference Correlator beamforming Traditional beamforming PRN02 21 044 13 PRN05 22 050 082 PRN06 15 049 082 PRN12 20 041 138 PRN19 20 052 080 PRN25 15 040 082 30 C N 0 dB Hz Correlator beamforming Traditional beamforming FIGURE 12 C N 0 measurements over time for PRN06 240 242 244 246 248 250 252 254 256 Time GPS seconds of week 105 15 Phase offset cycles Str2 Str3 Str4 Str5 Str6 Str7 FIGURE 11 Estimated phase offsets for Streams 2 through 7 with respect to center reference element with third order curve fits TABLE 1 De trended C N 0 standard deviations in dB Hz
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