GPS World, January 2011

RECEIVER DESIGN Vehicle Navigation TYPICAL VIEW from vehicle during test Nevertheless some pseudorange errors will be absorbed by the navigation solution hence the statistics of the residuals can be viewed as only a good estimate of the quality of the measurements themselves Solution Availability As previously discussed the navigation processing strategy adopted is the same for all receivers used in the test A single point epoch by epoch least squares solution is computed at a 1 Hz rate If there are insuffi cient satellites in view at a given epoch or the solution fails to converge in 10 iterations no solution is computed In this section the analysis focuses on the percentage of epochs during the downtown portion of the test for which a solution was computed FIGURE 6 shows the percentage of solutions computed for each receiver processing strategy as a function of HDOP and VDOP thresholds respectively Thus for example the assisted HS GPS GLONASS processing strategy yielded navigation solutions with a HDOP less than 6 between 80 percent and 85 percent of the time For larger DOP thresholds it is clear that there is little difference between GPS only processing and GPS GLONASS processing The biggest differences are caused by the processing strategies employed The advantages of HS processing are clear at least in terms of solution availability For this test and the particular geometry of the satellites in view during the test GPS GLONASS processing does yield a noticeable improvement in the VDOP particularly at lower thresholds Note that the standalone HS GPS receiver exhibits greater solution availability than the assisted software HS GPS GLONASS receiver at higher DOP thresholds This is most likely due to the low elevation satellites that were excluded from the assisted HS processing due to their being masked during the initialization period as discussed earlier Overall however there is little difference between GPS only processing and GPS GLONASS processing in FIGURE 6 Percentage solution availability versus HDOP and VDOP thresholds terms of solution availability This fact of course does not yield any information on the quality of the solutions obtained which is discussed later To gain further insight into the impact of GLONASS FIGURE 7 shows the percentage of solutions computed that exhibit redundancy Thus of all solutions computed during the downtown portion of the test Figure 7 illustrates the percentage of those solutions that have redundant measurements For GPS only processing this implies that fi ve or more measurements were used in computing the position while for GPS GLONASS processing a minimum of six measurements were required In this case the advantage of using GLONASS becomes more apparent For all processing strategies the addition of GLONASS yields an increase of 5 to 10 percent in the number of solutions with redundancy Although not studied herein this would have a positive impact on fault detection Residuals Analysis To investigate the quality of the measurements generated by each processing strategy the residuals from the leastsquares solutions are studied Only those epochs for which GPS World January 2011 www gpsworld com 54