GPS World, January 2017
6 4 2 0 2 4 6 rejection performance of correlator beamforming as evidenced by the C N 0 standard deviations FIGURE 13 shows representative results for satellite PRN06 for the other characteristic indicator of multipath codeminus carrier CmC divergence The de trended CmC standard deviations for all satellites are summarized in TABLE 2 Note that de trending is used to remove the codecarrier divergence due to the ionosphere As shown in Table 2 in terms of CmC divergence on average multipath error is reduced by a factor of five for traditional beamforming and almost a factor of four for correlator beamforming Finally the effect of multipath rejection in the position domain was evaluated FIGURE 14 shows a horizontal error scatter plot for the three receiver implementations while FIGURE 15 shows the time series of the individual position components TABLE 3 lists the root mean square RMS position errors and percent error reduction compared 62 GPS WORLD WWW GPSWORLD COM JANUARY 2017 5 3 1 1 3 5 Single element Correlator beamforming Traditional Beamforming 5 3 1 1 3 5 East error meters North error meters FIGURE 14 Horizontal position error scatter plot for the three receiver implementations to the single element case On average traditional beamforming reduces RMS position error by 80 percent compared to a single element antenna For correlator beamforming the average reduction is nearly as good an impressive 70 percent but achieved without any of the complexities associated with needing an individual RF front end for each antenna element Moreover the simplified architecture of a correlator beamforming GNSS receiver translates directly into decreased power consumption and reduced size weight and cost of the resulting antenna electronics unit Each attribute is highly desirable especially for portable and personal mobile applications CONCLUSIONS The CRADA effort between AFIT and Locata Corporation took Locatas commercially successful 24 GHz systems and proceeded to investigate the feasibility of applying this new correlator beamforming technology to GNSS receivers The CRADA focused on demonstrating an easily modified GNSS receiver to potentially deliver a low cost solution for mitigating multipath specifically targeting short delay and carrier multipath The results presented here show that the multipath rejection performance nearly equals that of a traditional beamforming GNSS receiver Considering the simpler architecture of a correlator 0 10 20 30 40 50 60 Time minutes 8 De trended code minus carrier meters Single element Correlator beamforming Traditional beamforming FIGURE 13 De trended code minus carrier for PRN06 Satellite PRN Singleelement reference Correlator beamforming Traditional beamforming PRN02 444 066 053 PRN06 189 064 041 PRN12 112 054 037 PRN19 237 077 050 TABLE 2 De trended CmC standard deviations in meters
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