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(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.
18.1.7The 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).
18.1.8The 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.
18.2VFR Use of GPS
18.2.1GPS 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.
18.2.2Types 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 ENR 4.1−5.) In all cases, VFR pilots
should never rely solely on one system of navigation.
GPS navigation must be integrated with other forms
of 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.
18.2.3Some critical concerns in VFR use of GPS
include RAIM capability, database currency, and
antenna location.
18.2.3.1RAIM 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 18.1.6
and 18.1.7 for more information on RAIM.
18.2.3.2Database Currency
a)In many receivers, an updateable 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.
b)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
airspace they were trying to avoid by using an
ENR 4.1−24 AIP
15 MAR 07 United States of America
Nineteenth Edition Federal Aviation Administration
outdated database. If you don’t have a current
database in the receiver, disregard the moving map
display for critical navigation decisions.
c)In addition, waypoints are added, removed,
relocated, or renamed as required to meet operational
needs. When using GPS to navigate relative to a
named fix, a current database must be used to
properly locate a named waypoint. Without the
update, it is the pilot’s responsibility to verify the
waypoint location referencing to an official current
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