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accuracy decays with time. This degradation is
known as drift."
17.2INSs 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.
17.3AHRSs are electronic devices that provide
attitude information to aircraft systems such as
weather radar and autopilot, but do not directly
compute position information.
18. Global Positioning System (GPS)
18.1System Overview
18.1.1System 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).
18.1.2System Availability and Reliability
18.1.2.1The 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, satellite
status is available through the Notice to Airmen
(NOTAM) system.
18.1.2.2The operational status of GNSS operations
depends upon the type of equipment being used. For
GPS−only equipment TSO−C129a, the operational
status of nonprecision approach capability for flight
planning purposes is provided through a prediction
program that is embedded in the receiver or provided
separately.
18.1.3Receiver 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.
18.1.4The 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. For
receivers capable of doing so, RAIM needs
6satellites 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
AIP ENR 4.1−23
United States of America 15 MAR 07
Federal Aviation Administration Nineteenth Edition
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.
18.1.5RAIM messages vary somewhat between
receivers; however, generally there are two types.
One type indicates that there are not enough satellites
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.
18.1.6Selective Availability. Selective Availability
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