AIM

4/3/14

1−1−19

Navigation Aids

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 capab-

ility (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 capabil-

ity 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

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 re-

ceivers 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

airspace they were trying to avoid by using an

outdated database. If you don’t have a current

database in the receiver, disregard the moving map

display for critical navigation decisions.

(3)

In addition, waypoints are added,

removed, relocated, or re−named 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