2-62 Pilot’s Guide 190-02488-01 Rev. B
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POSITION ACCURACY FIELDS
Under 14 CFR parts 91, 121, 125, and 135, the FDE availability prediction program must
be used prior to all oceanic or remote area flights using
GPS 175/GNX 375/GNC 355 as a primary means of navigation.
Circle of Uncertainty
FEATURE LIMITATIONS
• Available only when the aircraft is on ground
• Displays only on the Map page
LABEL POSITION DATA
EPU Estimated Position Uncertainty
HDOP Horizontal Dilution of Precision
HFOM Horizontal Figure of Merit
VFOM Vertical Figure of Merit
Information fields indicate the
accuracy of the position fix.
HFOM and VFOM values
represent 95% confidence levels
in horizontal and vertical
accuracy.
Lower values mean higher
accuracy. Higher values are the
least accurate.
EPU is the horizontal position error estimated by the
fault detection and exclusion (FDE) algorithm, in feet
or meters.
• Depicts area surrounding the ownship when GPS
cannot accurately determine aircraft location
• Expands as GPS horizontal accuracy degrades
• Shrinks as accuracy improves
• Translucent with minor shading so as not to
obstruct other features
Circle of
Uncertainty
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