PORIMA - Precise Predicted Orbit Based Receiver Integrity Monitoring Algorithms

Hua Su, Walter Ehret

Abstract:	Precise GPS satellite prediction orbits such as IGS ultrarapid orbits can reach better accuracy than these of the GPS broadcast ephemeris. Currently the accuracy of the predicted orbit is better than 10 cm and the predicted satellite clock corrections are at about the 2-5 ns level. Therefore the precise predicted GPS orbits can be used as reference to monitor the GPS SIS (Signal-in-Space) integrity and accuracy. Based on these analyses and results, the paper presents a new user integrity monitoring method for possible SoL (Safety-of-Life) applications. In order to use this user integrity monitoring method, user receivers should load the precise predicted GPS orbits available before the mission starts. Satellite range error thresholds will be determined based on the differences between the broadcast ephemeris and the precise predicted orbits. The user satellite range errors at an epoch can be obtained after the user navigation solution or PVT (Position, Velocity and Time Solution). If the user satellite range errors solved by user receivers are over the thresholds based on the two orbit differences above, the related satellites will be labeled as “bad” satellites and those measurements should be removed from navigation solutions or PVT and protection level computation. In such a way, satellite failures such as satellite maintenance anomaly, satellite maneuver, satellite clock jumps and signal distortion etc. can be easily detected. The paper presents as an example the analysis results to detect a GPS PRN 18 maintenance anomaly which occurred on April 10, 2007 [7] using this new user integrity monitoring method. The data analysis shows that the PRN 18 satellite failure was successfully detected based on the IGS Ultra-Rapid orbit updated at 12:00 (UTC) on April 10, 2007. The measurements from two IGS sites, Auckland, New Zealand and Victoria, Canada were processed and analyzed. The satellite range thresholds for the sites Auckland and Victoria were computed respectively. The GPS PRN 18 satellites failures were detected from both sites. After removing the satellite PRN 18 measurements, the accuracy of the navigation solution and performance of protection levels are improved and stable. According to the analysis and test results presented in the paper, it can be concluded that the performance of the receiver integrity monitoring aided by the precise predicted orbit is improved. For a single frequency receiver, the performance of this approach is better than RAIM and is near the SBAS level. For dual frequency receivers, the performance can be significantly improved. The advantage of this method is that users, especially aviation users, can use this method to monitor the GNSS SIS integrity in real-time without any additional ground or space integrity channel, which is especially useful for the areas not covered by current SBAS systems.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	2953 - 2961
Cite this article:	Su, Hua, Ehret, Walter, "PORIMA - Precise Predicted Orbit Based Receiver Integrity Monitoring Algorithms," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 2953-2961.
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