Aeronautical Information Manual (AIM)

AIM

4/3/14

1−1−18

Navigation Aids

INSs combine the components of an IRU with

an internal navigation computer. By programming a

series of waypoints, these systems will navigate along

a predetermined track.

AHRSs are electronic devices that provide

attitude information to aircraft systems such as

weather radar and autopilot, but do not directly

compute position information.

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17. Doppler Radar

Doppler Radar is a semiautomatic self−contained

dead reckoning navigation system (radar sensor plus

computer) which is not continuously dependent on

information derived from ground based or external

aids. The system employs radar signals to detect and

measure ground speed and drift angle, using the

aircraft compass system as its directional reference.

Doppler is less accurate than INS, however, and the

use of an external reference is required for periodic

updates if acceptable position accuracy is to be

achieved on long range flights.

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18. Global Positioning System (GPS)

a. System Overview

System Description. The Global Positioning

System is a satellite−based radio navigation system,

which broadcasts a signal that is used by receivers to

determine precise position anywhere in the world.

The receiver tracks multiple satellites and determines

a pseudorange measurement that is then used to

determine the user location. A minimum of four

satellites is necessary to establish an accurate

three−dimensional position. The Department of

Defense (DOD) is responsible for operating the GPS

satellite constellation and monitors the GPS satellites

to ensure proper operation. Every satellite’s orbital

parameters (ephemeris data) are sent to each satellite

for broadcast as 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).

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)

The operational status of GNSS opera-

tions depends upon the type of equipment being used.

For GPS−only equipment TSO−C129a, the opera-

tional status of nonprecision 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). When GNSS equipment is not using

integrity information from WAAS or LAAS, the GPS

navigation receiver using RAIM provides GPS signal

integrity monitoring. RAIM is necessary since delays

of up to two hours can occur before an erroneous

satellite transmission can be detected and corrected

by the satellite control segment. The RAIM function

is also referred to as fault detection. Another

capability, fault exclusion, refers to the ability of the

receiver to exclude a failed satellite from the position

solution and is provided by some GPS receivers and

by WAAS receivers.

The GPS receiver verifies the integrity

(usability) of the signals received from the GPS

constellation through receiver autonomous integrity

monitoring (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; thus, RAIM needs a minimum of 5 satellites

in view, or 4 satellites and a barometric altimeter

(baro−aiding) to detect an integrity anomaly.

[Baro−aiding satisfies the RAIM requirement in lieu

of a fifth satellite.] For receivers capable of doing so,

RAIM needs 6 satellites in view (or 5 satellites with

baro−aiding) to isolate the corrupt satellite signal and

remove it from the navigation solution. Baro−aiding

is a method of augmenting the GPS integrity solution

by using a nonsatellite input source. GPS derived

altitude should not be relied upon to determine

aircraft altitude since the vertical error can be quite

large and no integrity is provided. To ensure that

baro−aiding is available, the current altimeter setting

must be entered into the receiver as described in the

operating manual.

RAIM messages vary somewhat between

receivers; however, generally there are two types.

One type indicates that there are not enough satellites