GPS World, March 2011

Road TRANSPORTATION FIGURE 6 Position error 2D of each receiver as a function of driving environment FIGURE 7 GPS satellites used by receivers on occasion down to no satellites used Although the satellite count difference between all inview Type A and C1 C2 receivers was forced by means of that resemble these conditions are in fact possible Examples side wall of the freeway in below ground road sections FIGURE 8 shows the position outputs of all four receivers on a satellite image of a short segment of the freeway The biases can be seen in the outputs of all four receivers with offset with the magnitude of more than a lane width FIGURE 9 illustrates a time series of the positioning error components of all four receivers It clearly shows error ramp ups from the Type B receiver at frequent intervals These coincide with the satellite count drops of Type B shown in Figure 7 No such error ramp ups are observed for larger errors are observed in the height direction Local Road Eastbound This segment includes data gathered on an eastbound multi lane local road with 40 mph posted Type A has used around 3 more satellites most of the time Type A C1 and C2 have also used around 3 satellites less compared to the all in view Type A receiver FIGURE 10 shows the vehicle position as reported by all three receivers and the reference system output for a short road segment in this drive It clearly illustrates the lateral offsets of both C1 and C2 solutions The C2 receiver Blue generated about a lane width offset towards north whereas the C1 receiver output is biased by around two lane widths to the south FIGURE 11 positioning biases evident in Figure 10 It clearly shows large more than 5 meter biases in North and East position error components for C1 and C2 receivers Local Road Northbound remained almost unchanged for Type A receivers For dropouts of 4 or fewer satellites during the run Figure 12 shows the position output of all receivers on a short road can be readily observed in the output of C1 and C2 FIGURE 13 shows the time series view of the positioning heading dependent biases The short term biases seen in the Type B receiver output coincide with the change in the number of satellites used shown in Figure 7 This illustrates the implications of different estimation methods used in the stepwise changes in its position estimate whereas Type A output tends to gradually converge to different states Urban Canyon Results of the urban canyon segment of the drive are shown in FIGURES 14 AND 15 A statistical analysis isolate from the errors that are the result of multipath and www gpsworld com March 2011 GPS World 39