Test Results of a New DGPS RAIM Software Package

Frank van Diggelen, Alison Brown and Joseph Spalding

Abstract:	integrity using differential GPS (DGPS). A new Receiver Autonomous Integrity Monitoring (RAIM) software package, called “NavSafe,” is presented. The specific requirement met by NavSafe is the provision of integrity guarantees for buoy positioning by the US Coast Guard (USCG). This task requires a navigation accuracy of 10 m (R951. NavSafe provides alarms whenever the required integrity is not achieved. Test results show that differential GPS can provide the accuracy required by the USCG for marine navigation tasks such as buoy laying. However, navigation outages do occur, even during zero-baseline laboratory tests. The new RAIM software reliably computes the variance of the pseudo-range errors and detects biases when they occur, thus guaranteeing a predetermined level of navigation accuracy for DGPS. The new RAIM software algorithm reduces integrity monitoring to a simple alarm structure. There are three alarm states, red, yellow, and green, analogous to a traffic light. A green alarm state guarantees that the required navigation accuracy has been met. A yellow alarm indicates a possible navigation outage but with a high probability of false alarm. A red alarm confirms that the required navigation accuracy has not been met. The algorithm adapts to current satellite geometry and provides a measure of the noise on the corrected pseudo-range measurements. The user specifies the required navigation accuracy by setting the maximum Radial Position Error WIPE), the maximum Probability of Missed Detection (PMD), and the maximum Probability of False Alarm (PFA). A user-friendly graphical interface displays relevant navigation and satellite data, the alarm state, any biases on the pseudo-range measurements, the effect of the bias on the navigation solution, and the statistical distribution of the navigation solution. The user selected set-points (maximum RPE, etc.) may be changed on-line using hot keys. A function has been derived which links the false alarm rate to the satellite geometry. This integrity geometry measures the change in HDOP after a biased channel has been removed from the navigation solution. It will be shown that integrity geometry is as important to integrity monitoring as HDOP is to navigation. The integrity geometry allows users to determine in advance what level of integrity monitoring performance they can expect in a particular area at a particular time.
Published in:	Proceedings of the 49th Annual Meeting of The Institute of Navigation (1993) June 21 - 23, 1993 Royal Sonesta Hotel Cambridge, MA
Pages:	641 - 646
Cite this article:	van Diggelen, Frank, Brown, Alison, Spalding, Joseph, "Test Results of a New DGPS RAIM Software Package," Proceedings of the 49th Annual Meeting of The Institute of Navigation (1993), Cambridge, MA, June 1993, pp. 641-646.
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