GPS World, December 2009
Receiver Design WIRELESS FIGURE 8 Verifying independence of adjacent offsets upper full view lower zoomed top view point by even a single offset will result in a run sufficiently different from its predecessor to warrant its inclusion in a statistical population Application to Receiver Optimization Once the independence and lower bound on observable error have been established for a particular set of post processing parameters the MOPP method becomes a powerful tool for fi nding unexpected corner cases in the receiver implementation under test An example of this is shown in FIGURE 9 using the 95th percentile horizontal error as the statistical quantity of interest For this IF file the baseline level for the 95th percentile horizontal error is approximately 67 meters The trajectory generated by offset 192 however exhibits a 95th percentile horizontal error with respect to all other trajectories of approximately 129 meters or nearly twice as large as the rest of the Å FIGURE 9 Identifying a rare corner case upper full view lower top view data set Clearly this is a significant but evidently rare corner case one that would have required a substantial amount of drive testing and a bit of luck to discover by conventional methods When an artifact of the type shown above is identified the deterministic nature of software post processing makes it straightforward to identify the particular conditions in the input signal that trigger the anomalous behavior The receivers diagnostic outputs can be observed at the exact instant when the navigation solution begins to diverge from the truth trajectory and any affected algorithms can be tuned or corrected as appropriate The potential benefits of this process are demonstrated in FIGURE 10 Limitations While the foregoing results demonstrate the utility of the MOPP approach this method naturally has several limitaÅ www gpsworld com December 2009 GPS World 33
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