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AIM

10/12/17

1−1−17

Navigation Aids

part of the data message embedded in the GPS signal.

The GPS coordinate system is the Cartesian

earth−centered, earth−fixed coordinates as specified

in the World Geodetic System 1984 (WGS−84).

2. System Availability and Reliability.

(a) 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:

http://www.navcen.uscg.gov/. Additionally, satel-

lite status is available through the Notice to Airmen

(NOTAM) system.

(b) GNSS operational status depends on the

type of equipment being used. For GPS−only

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.

3. 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-

ance−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.

(a) 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

baro−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.

(b) 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

position.

4. 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

(a) 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.