Validity of RAIM Prediction

摘要

ABAS, aircraft-based augmentation system, is anrnavailable source of integrity-assured position information.rnGPS/RAIM, receiver autonomous integrity monitoring, isrnan ABAS implementation defined by MOPS, minimumrnoperational performance standardsdeveloped by thernRTCA. Most countries require GPS receivers used forrnIFR, instrument flight rules, to meet MOPS requirements.rnThis means that GPS receivers for IFR must have aRAIMrnfunction. RTCA DO-208, a MOPS for GPS receivers,rndefines RAIM FD (fault detection), and RTCA DO-229,rnMOPS for GPS/WAAS receivers, defines RAIM FDErn(fault detection and exclusion). For domestic flight mostlyrnwithin radar coverage, RAIM FD is sufficient to detectrnintegrity failures of GPS and switch to another navigationrnsource or request radar monitoring. To use GPS as arnprimary means of navigation for oceanic and remote flight,rnRAIM FDE is required because there is no otherrnnavigation means.rnOne problem is the fact that availability of GPS/RAIM isrnnot high enough for “sole-means” operations. Even ifrnGPS/RAIM exists at the time of departure, it is notrnensured that it will continue to be exist and provide thernfault detection function until arrival. The availability ofrnGPS/RAIM is a function of time and location. This factrnleads to the need for RAIM prediction. It is possible tornpredict the existence of GPS/RAIM from departure tornarrival and change navigation source on the flight plan ifrnGPS/RAIM is predicted to be unavailable at somernlocation. Most countries that allow using GPS/RAIM asrnprimary means of navigation require operators to performrnRAIM prediction before the flight.rnIn Japan, currently GPS/RAIM is allowed to be used as arnsupplemental navigation source for IFR flight and as arnprimary navigation source for approach at some airports.rnRecently JCAB is considering of allowing the use ofrnGPS/RAIM as a primary navigation source for Terminalrnand Enroute flight phases. The author conducted somernresearch and analysis to support this activity. A potentialrnproblem is the missed alert rate, i.e., how often RAIM isrnpredicted to exist, but during the actual flight, GPS/RAIMrnis unavailable. Such a condition affects the operation ofrnair traffic control because many aircraft might requestrnradar monitoring when they lose GPS navigationrnunexpectedly.rnAs results, the paper describes brief fundamentals ofrnGPS/RAIM, the RAIM prediction process, therncharacteristics of GPS satellite failures, and the validity ofrnRAIM predictions. Based on the measured characteristicsrnof GPS failures, the rate of missed alarms of RAIMrnprediction, i.e., RAIM was predicted, prior to departure,rnto exist but during actual flight GPS/RAIM is unavailable,rnis estimated to occur 2 to 3 times per year. Detailedrnanalysis for the specific case of GPS unexpected failuresrnshowed that the affected area is roughly overall ofrnJapanese FIR and the event lasts up to 1 to 2 hours. Thisrninformation is helpful to understand characteristics ofrnGPS/RAIM failure and to consider allowing the use ofrnGPS/RAIM as a primary navigation source.