15-9
190-01004-03 Rev. P
GTN 625/635/650 Pilot’s Guide
System
15.2.3 Circle of Uncertainty
The Circle of Uncertainty depicts an area where the ownship location is
guaranteed to be when the aircraft location cannot be accurately determined.
The area of the Circle of Uncertainty becomes larger as GPS horizontal accuracy
degrades and smaller as it improves. The Circle of Uncertainty is shown only
when the aircraft is on the ground. The Circle of Uncertainty area is transparent
so that features within it may still be seen.
Area Within the
Circle of Uncertainty
Ownship Symbol
Figure 15-8 Circle of Uncertainty
15.2.4 GPS Faults
The GTN communicates various fault conditions that can affect the accuracy
of the GPS. These include loss of integrity, loss of navigation, and a loss of
position.
Loss of Integrity
A loss of integrity is when the integrity of the GPS position does not meet the
requirements for the current phase of flight. This only occurs before the final
approach 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 Navigation
A 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 the
active approach requirements
• Insufficient number of satellites supporting aircraft position (i.e., more
than 5 seconds pass without adequate satellites to compute a position)
• GPS sensor detects an excessive position error or failure that cannot be
excluded within the time to alert
• On-board hardware failure
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