ION Technical Content
Validity of RAIM Prediction
| Abstract: | ABAS, aircraft-based augmentation system, is an available source of integrity-assured position information. GPS/RAIM, receiver autonomous integrity monitoring, is an ABAS implementation defined by MOPS, minimum operational performance standards developed by the RTCA. Most countries require GPS receivers used for IFR, instrument flight rules, to meet MOPS requirements. This means that GPS receivers for IFR must have a RAIM function. RTCA DO-208, a MOPS for GPS receivers, defines RAIM FD (fault detection), and RTCA DO-229,MOPS for GPS/WAAS receivers, defines RAIM FDE(fault detection and exclusion). For domestic flight mostly within radar coverage, RAIM FD is sufficient to detect integrity failures of GPS and switch to another navigation source or request radar monitoring. To use GPS as a primary means of navigation for oceanic and remote flight, RAIM FDE is required because there is no other navigation means. One problem is the fact that availability of GPS/RAIM isnot high enough for “sole-means” operations. Even if GPS/RAIM exists at the time of departure, it is not ensured that it will continue to be exist and provide the fault detection function until arrival. The availability of GPS/RAIM is a function of time and location. This fact leads to the need for RAIM prediction. It is possible to predict the existence of GPS/RAIM from departure to arrival and change navigation source on the flight plan if GPS/RAIM is predicted to be unavailable at some location. Most countries that allow using GPS/RAIM as primary means of navigation require operators to perform RAIM prediction before the flight. In Japan, currently GPS/RAIM is allowed to be used as a supplemental navigation source for IFR flight and as a primary navigation source for approach at some airports. Recently JCAB is considering of allowing the use of GPS/RAIM as a primary navigation source for Terminal and Enroute flight phases. The author conducted some research and analysis to support this activity. A potential problem is the missed alert rate, i.e., how often RAIM is predicted to exist, but during the actual flight, GPS/RAIM is unavailable. Such a condition affects the operation of air traffic control because many aircraft might request radar monitoring when they lose GPS navigation unexpectedly. As results, the paper describes brief fundamentals of GPS/RAIM, the RAIM prediction process, the characteristics of GPS satellite failures, and the validity of RAIM predictions. Based on the measured characteristics of GPS failures, the rate of missed alarms of RAIM, prediction, i.e., RAIM was predicted, prior to departure, to exist but during actual flight GPS/RAIM is unavailable,is estimated to occur 2 to 3 times per year. Detailed analysis for the specific case of GPS unexpected failures showed that the affected area is roughly overall of Japanese FIR and the event lasts up to 1 to 2 hours. This information is helpful to understand characteristics of GPS/RAIM failure and to consider allowing the use of GPS/RAIM as a primary navigation source. |
| Published in: |
Proceedings of the 2011 International Technical Meeting of The Institute of Navigation January 24 - 26, 2011 Catamaran Resort Hotel San Diego, CA |
| Pages: | 258 - 270 |
| Cite this article: | Sakai, T., "Validity of RAIM Prediction," Proceedings of the 2011 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2011, pp. 258-270. |
| Full Paper: |
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