15-9190-01004-03 Rev. P GTN 625/635/650 Pilot’s GuideForewordGettingStartedAudio &Xpdr CtrlCom/NavFPLDirect-ToProcWpt InfoMapTrafficTerrainWeatherNearestServices/MusicUtilitiesSystemMessagesSymbolsAppendixIndex15.2.3 Circle of UncertaintyThe Circle of Uncertainty depicts an area where the ownship location isguaranteed to be when the aircraft location cannot be accurately determined.The area of the Circle of Uncertainty becomes larger as GPS horizontal accuracydegrades and smaller as it improves. The Circle of Uncertainty is shown onlywhen the aircraft is on the ground. The Circle of Uncertainty area is transparentso that features within it may still be seen.Area Within theCircle of UncertaintyOwnship SymbolFigure 15-8 Circle of Uncertainty15.2.4 GPS FaultsThe GTN communicates various fault conditions that can affect the accuracyof the GPS. These include loss of integrity, loss of navigation, and a loss ofposition.Loss of IntegrityA loss of integrity is when the integrity of the GPS position does not meet therequirements for the current phase of flight. This only occurs before the finalapproach fix (if an approach is active).The GTN indicates a loss of integrity by displaying the amber "LOI"annunciation at the bottom of the screen.Loss of NavigationA loss of navigation can be caused by any of the following conditions:• Aircraft is after the final approach fix and GPS integrity does not meet theactive approach requirements• Insufficient number of satellites supporting aircraft position (i.e., morethan 5 seconds pass without adequate satellites to compute a position)• GPS sensor detects an excessive position error or failure that cannot beexcluded within the time to alert• On-board hardware failure
