Aeronautical Information Manual (AIM), page 69

Index   68 -- Page 69 -- 70

canceled by the FAA.

1-1-15. Inertial Reference Unit (IRU),
Inertial Navigation System (INS), and
Attitude Heading Reference System (AHRS)
a. IRUs are self-contained systems comprised of
gyros and accelerometers that provide aircraft
attitude (pitch, roll, and heading), position, and
velocity information in response to signals resulting
from inertial effects on system components. Once
aligned with a known position, IRUs continuously
calculate position and velocity. IRU position
accuracy decays with time. This degradation is
known as "drift."

b. 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.
c. AHRSs are electronic devices that provide
attitude information to aircraft systems such as
weather radar and autopilot, but do not directly
compute position information.

d. Aircraft equipped with slaved compass systems
may be susceptible to heading errors caused by
exposure to magnetic field disturbances (flux fields)

found in materials that are commonly located on the
surface or buried under taxiways and ramps. These
materials generate a magnetic flux field that can be
sensed by the aircraft's compass system flux detector
or "gate", which can cause the aircraft's system to
align with the material's magnetic field rather than
the earth's natural magnetic field. The system's
erroneous heading may not self-correct. Prior to take
off pilots should be aware that a heading
misalignment may have occurred during taxi. Pilots

are encouraged to follow the manufacturer's or other

appropriate procedures to correct possible heading

misalignment before take off is commenced.

1-1-16. 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.

1-1-17. Global Positioning System (GPS)
a. System Overview

1. System Description. The Global Positioning
System is a space-based radio navigation system
used to determine precise position anywhere in the
world. The 24 satellite constellation is designed to
ensure at least five satellites are always visible to a
user worldwide. A minimum of four satellites is
necessary for receivers to establish an accurate
three-dimensional position. The receiver uses data
from satellites above the mask angle (the lowest
angle above the horizon at which a receiver can use
a satellite). The Department of Defense (DOD) is
responsible for operating the GPS satellite constella-
tion and monitors the GPS satellites to ensure proper
operation. Each satellite's orbital parameters (eph-
emeris 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).

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.

Navigation Aids 1-1-15

Page 69 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   68 -- Page 69 -- 70