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International NOTAMs are issued under the KNMH
series. Pilots may obtain these NOTAMs from FSS
briefers upon request.
f. LORAN status information. To find out more
information on the LORAN system and its
operational status you can visit http://www.navcen.
uscg.gov/loran/default.htm or contact NAVCEN’s
Navigation Information Service (NIS)
watchstander, phone (703) 313−5900, fax (703)
313−5920.
g. LORAN’s future. The U.S. will continue to
operate the LORAN system in the short term. During
this time, the FAA LORAN evaluation program,
being conducted with the support of a team
AIM 2/19/04
1−1−24 Air Navigation Aids
comprising government, academia, and industry, will
identify and assess LORAN’s potential contributions
to required navigation services for the National
Airspace System (NAS), and support decisions
regarding continued operation of the system. If the
government concludes LORAN should not be kept as
part of the mix of federally provided radio navigation
systems, it will give the users of LORAN reasonable
notice so that they will have the opportunity to
transition to alternative navigation aids.
1−1−16. VHF Direction Finder
a. The VHF Direction Finder (VHF/DF) is one of
the common systems that helps pilots without their
being aware of its operation. It is a ground−based
radio receiver used by the operator of the ground
station. FAA facilities that provide VHF/DF service
are identified in the A/FD.
b. The equipment consists of a directional antenna
system and a VHF radio receiver.
c. The VHF/DF receiver display indicates the
magnetic direction of the aircraft from the ground
station each time the aircraft transmits.
d. DF equipment is of particular value in locating
lost aircraft and in helping to identify aircraft on
radar.
REFERENCE−
AIM, Direction Finding Instrument Approach Procedure,
Paragraph 6−2−3.
1−1−17. 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.
1−1−18. 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.
1−1−19. Global Positioning System (GPS)
a. System Overview
1. GPS is a U.S. satellite−based radio navigational,
positioning, and time transfer system operated
by the Department of Defense (DOD). The system
provides highly accurate position and velocity
information and precise time on a continuous global
basis to an unlimited number of properly−equipped
users. The system is unaffected by weather and
provides a worldwide common grid reference system
based on the earth−fixed coordinate system. For its
earth model, GPS uses the World Geodetic System of
1984 (WGS−84) datum.
2. GPS provides two levels of service: Standard
Positioning Service (SPS) and Precise Positioning
Service (PPS). SPS provides, to all users, horizontal
positioning accuracy of 100 meters, or less, with a
probability of 95 percent and 300 meters with a
probability of 99.99 percent. PPS is more accurate
than SPS; however, this is limited to authorized U.S.
and allied military, federal government, and civil
users who can satisfy specific U.S. requirements.
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