GPS World, October 2009
Algorithms Methods INNOVATION multi antenna differential environment This concept was extended to undifferenced SNR data so that carrier phase errors at single antenna GPS stations could be modeled and removed In our implementation we used wavelet analysis to fi rst separate the direct amplitude from the multipath signal then estimated the frequency content t of SNR MP as a function of time Using as the primary input to an adaptive least squares algorithm we then estimated multipath amplitude and relative phase as a function of time Substituting these A d A m and estimates into Equation 5 for carrier phase multipath yielded a multipath error correction profi le A simple example from the Salar de Uyuni a large salt fl at in Bolivia illustrates the process For PRN8 observed during September 2002 with an antenna about 14 meters above the salt surface the SNR due to multipath has very clear oscillations with a constituent frequency of approximately 00021 Hz 470 second period see FIGURE 3 Using frequency estimates as an input the adaptive estimation algorithm estimates direct and multipath signal amplitudes as well as the multipath relative phase which is approximately linear with time due to the relatively constant frequency estimate Figure 3 shows that the modeled SNR MP closely matches the SNR data and the carrier phase correction profi le closely matches the phase errors 40 20 0 20 40 SNRMP 550 500 450 400 1 seconds 02 01 0 Am Ad 4 2 0 2 4 radians 0 500 1000 1500 2000 2500 3000 10 5 0 5 10 millimeters Time seconds FIGURE 3 SNR modeling example from the Salar de Uyuni data set PRN8 ascending arc in seconds since the beginning of the satellite pass Real data are given in black while estimated quantities are colored lines estimation uses SNR due only to multipath i e after the direct signal has been removed in linear amplitude units The goal of SNR modeling is to generate a phase multipath correction profile shown in the bottom panel as a red line overlaying phase residuals Let Topcon steer you where you need to go Precise positioning for your automation topconsolutions com www gpsworld com October 2009 GPS World 35
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