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15-9190-01004-03 Rev. B
Garmin GTN 625/635/650 Pilot’s Guide
Foreword
Getting
Started
Xpdr Ctrl
Com/Nav
FPL
Direct-To
Proc
Wpt Info
Map
Traffic
Terrain
Weather
Nearest
Music
Utilities
System
Messages
Symbols
Appendix
Index
GPS Signal Message Description
Acquiring
The GPS receiver is acquiring satellites for navigation.
In this mode, the receiver uses satellite orbital data
(collected continuously from the satellites) and last
known position to determine the satellites that should
be in view.
3D Nav
The GPS receiver is in 3D navigation mode and
computes altitude using satellite data.
3D Diff Nav
The GPS receiver is in 3D navigation mode and
differential corrections are being used.
LOI
The “LOI” (Loss Of Integrity) annunciator (bottom left
corner of the screen) indicates that satellite coverage
is insufficient to pass built-in integrity monitoring
tests.
Table 15-3 GPS Signal Messages
The GPS Status Page also indicates the accuracy of the position fix, using
Horizontal Figure of Merit (HFOM), Vertical Figure of Merit (VFOM), and
Estimated Position Uncertainty (EPU). HFOM and VFOM represent the 95%
confidence levels in horizontal and vertical accuracy. The lowest numbers are
the best accuracy and the highest numbers are worse. EPU is the horizontal
position error estimated by the Fault Detection and Exclusion (FDE) algorithm,
in feet or meters.
NOTE: Operating outside of an SBAS service area with SBAS enabled
may cause elevated EPU values to be displayed on the satellite status
page. Regardless of the EPU value displayed, the LOI annunciation is the
controlling indication for determining the integrity of the GPS navigation
solution.
NOTE: The FDE Prediction program is used to predict FDE availability.
This program must be used prior to all oceanic or remote area flights for
all operators using the GTN as a primary means of navigation under FAR
parts 91, 121, 125, and 135. The FDE program is part of the GTN trainer,
available for download from the GTN product information page on Garmin’s
web site, www.garmin.com.
If the GTN has not been operated for a period of six months or more,
acquiring satellite data to establish almanac and satellite orbit information can