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Navigation Aids

part of the data message embedded in the GPS signal.
The GPS coordinate system is the Cartesian

−centered, earth−fixed coordinates as specified

in the World Geodetic System 1984 (WGS



System Availability and Reliability.


The status of GPS satellites is broadcast as

part of the data message transmitted by the GPS
satellites. GPS status information is also available by
means of the U.S. Coast Guard navigation
information service: (703) 313

−5907, Internet:

. Additionally, satel-

lite status is available through the Notice to Airmen
(NOTAM) system.


GNSS operational status depends on the

type of equipment being used. For GPS


equipment TSO

−C129 or TSO-C196(), the opera-

tional status of non

−precision approach capability for

flight planning purposes is provided through a
prediction program that is embedded in the receiver
or provided separately.


Receiver Autonomous Integrity Monitoring

(RAIM). RAIM is the capability of a GPS receiver to
perform integrity monitoring on itself by ensuring
available satellite signals meet the integrity require-
ments for a given phase of flight. Without RAIM, the
pilot has no assurance of the GPS position integrity.
RAIM provides immediate feedback to the pilot. This
fault detection is critical for performance-based
navigation (PBN)(see Paragraph 1

−2−1, Perform-


−Based Navigation (PBN) and Area Navigation

(RNAV), for an introduction to PBN), because delays
of up to two hours can occur before an erroneous
satellite transmission is detected and corrected by the
satellite control segment.


In order for RAIM to determine if a

satellite is providing corrupted information, at least
one satellite, in addition to those required for
navigation, must be in view for the receiver to
perform the RAIM function. RAIM requires a
minimum of 5 satellites, or 4 satellites and barometric
altimeter input (baro

−aiding), to detect an integrity

anomaly. Baro

−aiding is a method of augmenting the

GPS integrity solution by using a non-satellite input
source in lieu of the fifth satellite. Some GPS
receivers also have a RAIM capability, called fault
detection and exclusion (FDE), that excludes a failed
satellite from the position solution; GPS receivers
capable of FDE require 6 satellites or 5 satellites with


−aiding. This allows the GPS receiver to isolate

the corrupt satellite signal, remove it from the
position solution, and still provide an integrity-as-
sured position. To ensure that baro

−aiding is

available, enter the current altimeter setting into the
receiver as described in the operating manual. Do not
use the GPS derived altitude due to the large GPS
vertical errors that will make the integrity monitoring
function invalid.


There are generally two types of RAIM

fault messages. The first type of message indicates
that there are not enough satellites available to
provide RAIM integrity monitoring. The GPS
navigation solution may be acceptable, but the
integrity of the solution cannot be determined. The
second type indicates that the RAIM integrity
monitor has detected a potential error and that there
is an inconsistency in the navigation solution for the
given phase of flight. Without RAIM capability, the
pilot has no assurance of the accuracy of the GPS


Selective Availability.  Selective Availability

(SA) is a method by which the accuracy of GPS is
intentionally degraded. This feature was 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.

b. Operational Use of GPS. 

U.S. civil operators

may use approved GPS equipment in oceanic
airspace, certain remote areas, the National Airspace
System and other States as authorized (please consult
the applicable Aeronautical Information Publica-
tion). Equipage other than GPS may be required for
the desired operation. GPS navigation is used for both
Visual Flight Rules (VFR) and Instrument Flight
Rules (IFR) operations.

1. VFR Operations


GPS navigation has become an asset to

VFR pilots by providing increased navigational
capabilities and enhanced situational awareness.
Although GPS has provided many benefits to the
VFR pilot, care must be exercised to ensure that
system capabilities are not exceeded. VFR pilots
should integrate GPS navigation with electronic
navigation (when possible), as well as pilotage and
dead reckoning.