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large and no integrity is provided. To ensure that
baro-aiding is available, the current altimeter setting
must be entered into the receiver as described in the
operating manual.
5. RAIM messages vary somewhat between
receivers; however, generally there are two types.
One type indicates that there are not enough satellites
available to provide RAIM integrity monitoring and
another type indicates that the RAIM integrity
monitor has detected a potential error that exceeds the
limit for the current phase of flight. Without RAIM
capability, the pilot has no assurance of the accuracy
of the GPS position.
6. Selective Availability. Selective Availability
(SA) is a method by which the accuracy of GPS is
intentionally degraded. This feature is designed to
deny hostile use of precise GPS positioning data. SA
was discontinued on May 1, 2000, but many GPS
receivers are designed to assume that SA is still
active. New receivers may take advantage of the
discontinuance of SA based on the performance
values in ICAO Annex 10, and do not need to be
designed to operate outside of that performance.
7. The GPS constellation of 24 satellites is
designed so that a minimum of five is always
observable by a user anywhere on earth. The receiver
uses data from a minimum of four satellites above the
mask angle (the lowest angle above the horizon at
which it can use a satellite).
8. The DOD declared initial operational capability
(IOC) of the U.S. GPS on December 8, 1993.
The FAA has granted approval for U.S. civil
operators to use properly certified GPS equipment as
a primary means of navigation in oceanic airspace
and certain remote areas. Properly certified GPS
equipment may be used as a supplemental means of
IFR navigation for domestic en route, terminal
operations, and certain instrument approach procedures
(IAPs). This approval permits the use of GPS
in a manner that is consistent with current navigation
requirements as well as approved air carrier
operations specifications.
b. VFR Use of GPS
1. GPS navigation has become a great asset to
VFR pilots, providing increased navigation capability
and enhanced situational awareness, while
reducing operating costs due to greater ease in flying
direct routes. While GPS has many benefits to the
VFR pilot, care must be exercised to ensure that
system capabilities are not exceeded.
2. Types of receivers used for GPS navigation
under VFR are varied, from a full IFR installation
being used to support a VFR flight, to a VFR only
installation (in either a VFR or IFR capable aircraft)
to a hand-held receiver. The limitations of each type
of receiver installation or use must be understood by
the pilot to avoid misusing navigation information.
(See TBL 1-1-6.) In all cases, VFR pilots should
never rely solely on one system of navigation. GPS
navigation must be integrated with other forms of
AIM 2/14/08
1-1-26 Navigation Aids
electronic navigation (when possible), as well as
pilotage and dead reckoning. Only through the
integration of these techniques can the VFR pilot
ensure accuracy in navigation.
3. Some critical concerns in VFR use of GPS
include RAIM capability, database currency and
antenna location.
(a) RAIM Capability. Many VFR GPS receivers
and all hand-held units have no RAIM
alerting capability. Loss of the required number of
satellites in view, or the detection of a position error,
cannot be displayed to the pilot by such receivers. In
receivers with no RAIM capability, no alert would be
provided to the pilot that the navigation solution had
deteriorated, and an undetected navigation error
could occur. A systematic cross-check with other
navigation techniques would identify this failure, and
prevent a serious deviation. See subparagraphs a4 and
a5 for more information on RAIM.
(b) Database Currency
(1) In many receivers, an up-datable
database is used for navigation fixes, airports, and
instrument procedures. These databases must be
maintained to the current update for IFR operation,
but no such requirement exists for VFR use.
(2) However, in many cases, the database
drives a moving map display which indicates Special
Use Airspace and the various classes of airspace, in
addition to other operational information. Without a
current database the moving map display may be
outdated and offer erroneous information to VFR
pilots wishing to fly around critical airspace areas,
such as a Restricted Area or a Class B airspace
segment. Numerous pilots have ventured into
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