2. The signal processor acquires LORAN
signals and measures the difference between the
time-of-arrival of each secondary station pulse
group and the Master station pulse group. The
measured TDs depend on the location of the receiver
in relation to the three or more transmitters.
AIM 2/14/08
1-1-23
Navigation Aids
FIG 1-1-16
First Line-of-Position
(a) The first TD will locate an aircraft
somewhere on a line-of-position (LOP) on which the
receiver will measure the same TD value.
(b) A second LOP is defined by a TD
measurement between the Master station signal and
the signal from another secondary station.
FIG 1-1-17
Second Line-of-Position
(c) The intersection of the measured LOPs is
the position of the aircraft.
FIG 1-1-18
Intersection of Lines-of-Position
3. The navigation computer converts TD values
to corresponding latitude and longitude. Once the
time and position of the aircraft are established at
two_points, distance to destination, cross track error,
ground speed, estimated time of arrival, etc., can be
determined. Cross track error can be displayed as the
vertical needle of a course deviation indicator, or
digitally, as decimal parts of a mile left or right of
course.
e. Notices to Airmen (NOTAMs) are issued for
LORAN chain or station outages. Domestic
NOTAM_(D)s are issued under the identifier “LRN.”
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 2/14/08
1-1-24 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. 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).
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.
AIM 2/14/08
1-1-25
Navigation Aids
(b) The operational status of GNSS opera-
tions depends upon the type of equipment being used.
For GPS-only equipment TSO-C129(a), 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.
3. 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.
4. 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. 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.
5. RAIM 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.
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 capa-
bility (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 proce-
dures (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 capabili-
ty 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
AIM 2/14/08
1-1-26 Navigation Aids
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
source, such as the Airport/Facility Directory,
Sectional Chart, or En Route Chart.
(c) Antenna Location
(1) In many VFR installations of GPS
receivers, antenna location is more a matter of
convenience than performance. In IFR installations,
care is exercised to ensure that an adequate clear view
is provided for the antenna to see satellites. If an
alternate location is used, some portion of the aircraft
may block the view of the antenna, causing a greater
opportunity to lose navigation signal.
(2) This is especially true in the case of
hand-helds. The use of hand-held receivers for VFR
operations is a growing trend, especially among
rental pilots. Typically, suction cups are used to place
the GPS antennas on the inside of cockpit windows.
While this method has great utility, the antenna
location is limited to the cockpit or cabin only and is
rarely optimized to provide a clear view of available
satellites. Consequently, signal losses may occur in
certain situations of aircraft-satellite geometry,
causing a loss of navigation signal. These losses,
coupled with a lack of RAIM capability, could
present erroneous position and navigation informa-
tion with no warning to the pilot.
(3) While the use of a hand-held GPS for
VFR operations is not limited by regulation,
modification of the aircraft, such as installing a
panel- or yoke-mounted holder, is governed by
14_CFR Part 43. Consult with your mechanic to
ensure compliance with the regulation, and a safe
installation.
4. As a result of these and other concerns, here
are some tips for using GPS for VFR operations:
(a) Always check to see if your unit has
RAIM capability. If no RAIM capability exists, be
suspicious of your GPS position when any
disagreement exists with the position derived from
other radio navigation systems, pilotage, or dead
reckoning.
(b) Check the currency of the database, if any.
If expired, update the database using the current
revision. If an update of an expired database is not
possible, disregard any moving map display of
airspace for critical navigation decisions. Be aware
that named waypoints may no longer exist or may
have been relocated since the database expired. At a
minimum, the waypoints planned to be used should
be checked against a current official source, such as
the Airport/Facility Directory, or a Sectional
Aeronautical Chart.
AIM 2/14/08
1-1-27
Navigation Aids
(c) While hand-helds can provide excellent
navigation capability to VFR pilots, be prepared for
intermittent loss of navigation signal, possibly with
no RAIM warning to the pilot. If mounting the
receiver in the aircraft, be sure to comply with
14_CFR Part 43.
(d) Plan flights carefully before taking off. If
you wish to navigate to user-defined waypoints,
enter them before flight, not on-the-fly. Verify your
planned flight against a current source, such as a
current sectional chart. There have been cases in
which one pilot used waypoints created by another
pilot that were not where the pilot flying was
expecting. This generally resulted in a navigation
error. Minimize head-down time in the aircraft and
keep a sharp lookout for traffic, terrain, and obstacles.
Just a few minutes of preparation and planning on the
ground will make a great difference in the air.
(e) Another way to minimize head-down
time is to become very familiar with your receiver's
operation. Most receivers are not intuitive. The pilot
must take the time to learn the various keystrokes,
knob functions, and displays that are used in the
operation of the receiver. Some manufacturers
provide computer-based tutorials or simulations of
their receivers. Take the time to learn about your
particular unit before you try to use it in flight.
5. In summary, be careful not to rely on GPS to
solve all your VFR navigational problems. Unless an
IFR receiver is installed in accordance with IFR
requirements, no standard of accuracy or integrity has
been assured. While the practicality of GPS is
compelling, the fact remains that only the pilot can
navigate the aircraft, and GPS is just one of the pilot's
tools to do the job.
c. VFR Waypoints
1. VFR waypoints provide VFR pilots with a
supplementary tool to assist with position awareness
while navigating visually in aircraft equipped with
area navigation receivers. VFR waypoints should be
used as a tool to supplement current navigation
procedures. The uses of VFR waypoints include
providing navigational aids for pilots unfamiliar with
an area, waypoint definition of existing reporting
points, enhanced navigation in and around Class B
and Class C airspace, and enhanced navigation
around Special Use Airspace. VFR pilots should rely
on appropriate and current aeronautical charts
published specifically for visual navigation. If
operating in a terminal area, pilots should take
advantage of the Terminal Area Chart available for
that area, if published. The use of VFR waypoints
does not relieve the pilot of any responsibility to
comply with the operational requirements of 14 CFR
Part 91.
2. VFR waypoint names (for computer-entry
and flight plans) consist of five letters beginning with
the letters “VP” and are retrievable from navigation
databases. The VFR waypoint names are not intended
to be pronounceable, and they are not for use in ATC
communications. On VFR charts, stand-alone VFR
waypoints will be portrayed using the same
four-point star symbol used for IFR waypoints. VFR
waypoints collocated with visual check points on the
chart will be identified by small magenta flag
symbols. VFR waypoints collocated with visual
check points will be pronounceable based on the
name of the visual check point and may be used for
ATC communications. Each VFR waypoint name
will appear in parentheses adjacent to the geographic
location on the chart. Latitude/longitude data for all
established VFR waypoints may be found in the
appropriate regional Airport/Facility Directory
(A/FD).
3. VFR waypoints shall not be used to plan
flights under IFR. VFR waypoints will not be
recognized by the IFR system and will be rejected for
IFR routing purposes.
4. When filing VFR flight plans, pilots may use
the five letter identifier as a waypoint in the route of
flight section if there is an intended course change at
that point or if used to describe the planned route of
flight. This VFR filing would be similar to how a
VOR would be used in a route of flight. Pilots must
use the VFR waypoints only when operating under
VFR conditions.
5. Any VFR waypoints intended for use during
a flight should be loaded into the receiver while on the
ground and prior to departure. Once airborne, pilots
should avoid programming routes or VFR waypoint
chains into their receivers.
6. Pilots should be especially vigilant for other
traffic while operating near VFR waypoints. The
same effort to see and avoid other aircraft near VFR
waypoints will be necessary, as was the case with
VORs and NDBs in the past. In fact, the increased
accuracy of navigation through the use of GPS will
AIM 2/14/08
1-1-28 Navigation Aids
demand even greater vigilance, as off-course
deviations among different pilots and receivers will
be less. When operating near a VFR waypoint, use
whatever ATC services are available, even if outside
a class of airspace where communications are
required. Regardless of the class of airspace, monitor
the available ATC frequency closely for information
on other aircraft operating in the vicinity. It is also a
good idea to turn on your landing light(s) when
operating near a VFR waypoint to make your aircraft
more conspicuous to other pilots, especially when
visibility is reduced. See paragraph 7-5-2, VFR in
Congested Areas, for more information.
d. General Requirements
1. Authorization to conduct any GPS operation
under IFR requires that:
(a) GPS navigation equipment used must be
approved in accordance with the requirements
specified in Technical Standard Order (TSO)
TSO-C129, or equivalent, and the installation must
be done in accordance with Advisory Circular
AC_20-138, Airworthiness Approval of Global
Positioning System (GPS) Navigation Equipment for
Use as a VFR and IFR Supplemental Navigation
System, or Advisory Circular AC_20-130A, Airwor-
thiness Approval of Navigation or Flight
Management Systems Integrating Multiple Naviga-
tion Sensors, or equivalent. Equipment approved in
accordance with TSO-C115a does not meet the
requirements of TSO-C129. Visual flight rules
(VFR) and hand-held GPS systems are not
authorized for IFR navigation, instrument ap-
proaches, or as a principal instrument flight
reference. During IFR operations they may be
considered only an aid to situational awareness.
(b) Aircraft using GPS navigation equipment
under IFR must be equipped with an approved and
operational alternate means of navigation appropriate
to the flight. Active monitoring of alternative
navigation equipment is not required if the GPS
receiver uses RAIM for integrity monitoring. Active
monitoring of an alternate means of navigation is
required when the RAIM capability of the GPS
equipment is lost.
(c) Procedures must be established for use in
the event that the loss of RAIM capability is predicted
to occur. In situations where this is encountered, the
flight must rely on other approved equipment, delay
departure, or cancel the flight.
(d) The GPS operation must be conducted in
accordance with the FAA-approved aircraft flight
manual (AFM) or flight manual supplement. Flight
crew members must be thoroughly familiar with the
particular GPS equipment installed in the aircraft, the
receiver operation manual, and the AFM or flight
manual supplement. Unlike ILS and VOR, the basic
operation, receiver presentation to the pilot, and some
capabilities of the equipment can vary greatly. Due to
these differences, operation of different brands, or
even models of the same brand, of GPS receiver
under IFR should not be attempted without thorough
study of the operation of that particular receiver and
installation. Most receivers have a built-in simulator
mode which will allow the pilot to become familiar
with operation prior to attempting operation in the
aircraft. Using the equipment in flight under VFR
conditions prior to attempting IFR operation will
allow further familiarization.
(e) Aircraft navigating by IFR approved GPS
are considered to be area navigation (RNAV) aircraft
and have special equipment suffixes. File the
appropriate equipment suffix in accordance with
TBL 5-1-2, on the ATC flight plan. If GPS avionics
become inoperative, the pilot should advise ATC and
amend the equipment suffix.
(f) Prior to any GPS IFR operation, the pilot
must review appropriate NOTAMs and aeronautical
information. (See GPS NOTAMs/Aeronautical
Information.)
(g) Air carrier and commercial operators
must meet the appropriate provisions of their
approved operations specifications.
e. Use of GPS for IFR Oceanic, Domestic
En_Route, and Terminal Area Operations
1. GPS IFR operations in oceanic areas can be
conducted as soon as the proper avionics systems are
installed, provided all general requirements are met.
A GPS installation with TSO-C129 authorization in
class A1, A2, B1, B2, C1, or C2 may be used to
replace one of the other approved means of
long-range navigation, such as dual INS. (See
TBL 1-1-5 and TBL 1-1-6.) A single GPS installa-
tion with these classes of equipment which provide
RAIM for integrity monitoring may also be used on
short oceanic routes which have only required one
means of long-range navigation.
AIM 2/14/08
1-1-29
Navigation Aids
2. GPS domestic en route and terminal IFR
operations can be conducted as soon as proper
avionics systems are installed, provided all general
requirements are met. The avionics necessary to
receive all of the ground-based facilities appropriate
for the route to the destination airport and any
required alternate airport must be installed and
operational. Ground-based facilities necessary for
these routes must also be operational.
(a) GPS en route IFR RNAV operations may
be conducted in Alaska outside the operational
service volume of ground-based navigation aids
when a TSO-C145a or TSO-C146a GPS/WAAS
system is installed and operating. Ground-based
navigation equipment is not required to be installed
and operating for en route IFR RNAV operations
when using GPS WAAS navigation systems. All
operators should ensure that an alternate means of
navigation is available in the unlikely event the GPS
WAAS navigation system becomes inoperative.
TBL 1-1-5
GPS IFR Equipment Classes/Categories
TSO-C129
Equipment
Class
RAIM
Int. Nav. Sys. to
Prov. RAIM
Equiv.
Oceanic En Route Terminal
Nonprecision
Approach
Capable
Class A - GPS sensor and navigation capability.
A1 yes yes yes yes yes
A2 yes yes yes yes no
Class B - GPS sensor data to an integrated navigation system (i.e., FMS, multi-sensor navigation system, etc.).
B1 yes yes yes yes yes
B2 yes yes yes yes no
B3 yes yes yes yes yes
B4 yes yes yes yes no
Class C - GPS sensor data to an integrated navigation system (as in Class B) which provides enhanced guidance to an autopilot, or
flight director, to reduce flight tech. errors. Limited to 14 CFR Part 121 or equivalent criteria.
C1 yes yes yes yes yes
C2 yes yes yes yes no
C3 yes yes yes yes yes
C4 yes yes yes yes no
AIM 2/14/08
1-1-30 Navigation Aids
TBL 1-1-6
GPS Approval Required/Authorized Use
Equipment
Type1
Installation
Approval
Required
Operational
Approval
Required
IFR
En Route2
IFR
Terminal2
IFR
Approach3
Oceanic
Remote
In Lieu of
ADF and/or
DME3
Hand held4 X5
VFR Panel Mount4 X
IFR En Route
and Terminal
X X X X X
IFR Oceanic/
Remote
X X X X X X
IFR En Route,
Terminal, and
Approach
X X X X X X
