Aeronautical Information Manual (AIM), page 70

Index   69 -- Page 70 -- 71

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
given phase of flight. Without RAIM capability, the
pilot has no assurance of the accuracy of the GPS

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.
(b) GPS receivers used for VFR navigation
vary from fully integrated IFR/VFR installation used
to support VFR operations to hand-held devices.
Pilots must understand the limitations of the receivers
prior to using in flight to avoid misusing navigation
information. (See TBL 1-1-6.) Most receivers are
not intuitive. The pilot must learn the various
keystrokes, knob functions, and displays that are
used in the operation of the receiver. Some
manufacturers provide computer-based tutorials or
simulations of their receivers that pilots can use to
become familiar with operating the equipment.

(c) When using GPS for VFR operations,
RAIM capability, database currency, and antenna
location are critical areas of concern.
(1) RAIM Capability. VFR GPS panel
alerting capability. This prevents the pilot from being
alerted to the loss of the required number of satellites
in view, or the detection of a position error. Pilots
should use a systematic cross-check with other

navigation techniques to verify position. Be

suspicious of the GPS position if a disagreement

exists between the two positions.

(2) Database Currency. Check the cur-
rency of the database. Databases must be updated for

1-1-16 Navigation Aids

Page 70 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   69 -- Page 70 -- 71