7. Pilots must be aware of the vertical height
between the aircraft's glide slope antenna and the
main gear in the landing configuration and, at the DH,
plan to adjust the descent angle accordingly if the
published TCH indicates the wheel crossing height
over the runway threshold may not be satisfactory.
Tests indicate a comfortable wheel crossing height is
approximately 20 to 30 feet, depending on the type of
aircraft.
NOTE-
The TCH for a runway is established based on several
factors including the largest aircraft category that
normally uses the runway, how airport layout effects the
glide slope antenna placement, and terrain. A higher than
optimum TCH, with the same glide path angle, may cause
the aircraft to touch down further from the threshold if the
trajectory of the approach is maintained until the flare.
Pilots should consider the effect of a high TCH on the
runway available for stopping the aircraft.
e. Distance Measuring Equipment (DME)
1. When installed with the ILS and specified in
the approach procedure, DME may be used:
(a) In lieu of the OM;
(b) As a back course (BC) final approach fix
(FAF); and
(c) To establish other fixes on the localizer
course.
2. In some cases, DME from a separate facility
may be used within Terminal Instrument Procedures
(TERPS) limitations:
(a) To provide ARC initial approach seg-
ments;
(b) As a FAF for BC approaches; and
(c) As a substitute for the OM.
f. Marker Beacon
1. ILS marker beacons have a rated power
output of 3 watts or less and an antenna array
designed to produce an elliptical pattern with
dimensions, at 1,000 feet above the antenna, of
approximately 2,400_feet in width and 4,200 feet in
length. Airborne marker beacon receivers with a
selective sensitivity feature should always be
operated in the “low” sensitivity position for proper
reception of ILS marker beacons.
2. Ordinarily, there are two marker beacons
associated with an ILS, the OM and MM. Locations
with a Category II ILS also have an Inner
Marker_(IM). When an aircraft passes over a marker,
the pilot will receive the indications shown in
TBL 1-1-3.
(a) The OM normally indicates a position at
which an aircraft at the appropriate altitude on the
localizer course will intercept the ILS glide path.
(b) The MM indicates a position approxi-
mately 3,500 feet from the landing threshold. This is
also the position where an aircraft on the glide path
will be at an altitude of approximately 200 feet above
the elevation of the touchdown zone.
(c) The IM will indicate a point at which an
aircraft is at a designated decision height (DH) on the
glide path between the MM and landing threshold.
TBL 1-1-3
Marker Passage Indications
Marker Code Light
OM _ _ _ BLUE
MM _ _ _ _ AMBER
IM _ _ _ _ WHITE
BC _ _ _ _ WHITE
AIM 2/14/08
1-1-10 Navigation Aids
3. A back course marker normally indicates the
ILS back course final approach fix where approach
descent is commenced.
g. Compass Locator
1. Compass locator transmitters are often
situated at the MM and OM sites. The transmitters
have a power of less than 25 watts, a range of at least
15_miles and operate between 190 and 535 kHz. At
some locations, higher powered radio beacons, up to
400 watts, are used as OM compass locators. These
generally carry Transcribed Weather Broadcast
(TWEB) information.
2. Compass locators transmit two letter identifi-
cation groups. The outer locator transmits the first
two letters of the localizer identification group, and
the middle locator transmits the last two letters of the
localizer identification group.
h. ILS Frequency (See TBL 1-1-4.)
TBL 1-1-4
Frequency Pairs Allocated for ILS
Localizer MHz Glide Slope
108.10 334.70
108.15 334.55
108.3 334.10
108.35 333.95
108.5 329.90
108.55 329.75
108.7 330.50
108.75 330.35
108.9 329.30
108.95 329.15
109.1 331.40
109.15 331.25
109.3 332.00
109.35 331.85
109.50 332.60
109.55 332.45
109.70 333.20
109.75 333.05
109.90 333.80
109.95 333.65
110.1 334.40
110.15 334.25
110.3 335.00
110.35 334.85
110.5 329.60
110.55 329.45
Localizer MHz Glide Slope
110.70 330.20
110.75 330.05
110.90 330.80
110.95 330.65
111.10 331.70
111.15 331.55
111.30 332.30
111.35 332.15
111.50 332.9
111.55 332.75
111.70 333.5
111.75 333.35
111.90 331.1
111.95 330.95
i. ILS Minimums
1. The lowest authorized ILS minimums, with
all required ground and airborne systems components
operative, are:
(a) Category I. Decision Height (DH)
200_feet and Runway Visual Range (RVR) 2,400 feet
(with touchdown zone and centerline lighting, RVR
1,800 feet);
(b) Category II. DH 100 feet and RVR
1,200_feet;
(c) Category IIIa. No DH or DH below
100_feet and RVR not less than 700 feet;
(d) Category IIIb. No DH or DH below
50_feet and RVR less than 700 feet but not less than
150_feet; and
(e) Category IIIc. No DH and no RVR
limitation.
NOTE-
Special authorization and equipment required for
Categories II and III.
j. Inoperative ILS Components
1. Inoperative localizer. When the localizer
fails, an ILS approach is not authorized.
2. Inoperative glide slope. When the glide
slope fails, the ILS reverts to a nonprecision localizer
approach.
REFERENCE-
See the inoperative component table in the U.S. Government Terminal
Procedures Publication (TPP), for adjustments to minimums due to
inoperative airborne or ground system equipment.
AIM 2/14/08
1-1-11
Navigation Aids
k. ILS Course Distortion
1. All pilots should be aware that disturbances to
ILS localizer and glide slope courses may occur when
surface vehicles or aircraft are operated near the
localizer or glide slope antennas. Most ILS
installations are subject to signal interference by
either surface vehicles, aircraft or both. ILS
CRITICAL AREAS are established near each
localizer and glide slope antenna.
2. ATC issues control instructions to avoid
interfering operations within ILS critical areas at
controlled airports during the hours the Airport
Traffic Control Tower (ATCT) is in operation as
follows:
(a) Weather Conditions. Less than ceiling
800 feet and/or visibility 2 miles.
(1) Localizer Critical Area. Except for
aircraft that land, exit a runway, depart or miss
approach, vehicles and aircraft are not authorized in
or over the critical area when an arriving aircraft is
between the ILS final approach fix and the airport.
Additionally, when the ceiling is less than 200 feet
and/or the visibility is RVR 2,000 or less, vehicle and
aircraft operations in or over the area are not
authorized when an arriving aircraft is inside the ILS
MM.
(2) Glide Slope Critical Area. Vehicles
and aircraft are not authorized in the area when an
arriving aircraft is between the ILS final approach fix
and the airport unless the aircraft has reported the
airport in sight and is circling or side stepping to land
on a runway other than the ILS runway.
(b) Weather Conditions. At or above ceil-
ing 800 feet and/or visibility 2 miles.
(1) No critical area protective action is
provided under these conditions.
(2) A flight crew, under these conditions,
should advise the tower that it will conduct an
AUTOLAND or COUPLED approach to ensure that
the ILS critical areas are protected when the aircraft
is inside the ILS MM.
EXAMPLE-
Glide slope signal not protected.
3. Aircraft holding below 5,000 feet between
the outer marker and the airport may cause localizer
signal variations for aircraft conducting the ILS
approach. Accordingly, such holding is not
authorized when weather or visibility conditions are
less than ceiling 800 feet and/or visibility 2 miles.
4. Pilots are cautioned that vehicular traffic not
subject to ATC may cause momentary deviation to
ILS course or glide slope signals. Also, critical areas
are not protected at uncontrolled airports or at airports
with an operating control tower when weather or
visibility conditions are above those requiring
protective measures. Aircraft conducting coupled or
autoland operations should be especially alert in
monitoring automatic flight control systems.
(See FIG 1-1-7.)
NOTE-
Unless otherwise coordinated through Flight Standards,
ILS signals to Category I runways are not flight inspected
below 100 feet AGL. Guidance signal anomalies may be
encountered below this altitude.
1-1-10. Simplified Directional Facility
(SDF)
a. The SDF provides a final approach course
similar to that of the ILS localizer. It does not provide
glide slope information. A clear understanding of the
ILS localizer and the additional factors listed below
completely describe the operational characteristics
and use of the SDF.
b. The SDF transmits signals within the range of
108.10 to 111.95 MHz.
c. The approach techniques and procedures used
in an SDF instrument approach are essentially the
same as those employed in executing a standard
localizer approach except the SDF course may not be
aligned with the runway and the course may be wider,
resulting in less precision.
d. Usable off-course indications are limited to
35_degrees either side of the course centerline.
Instrument indications received beyond 35 degrees
should be disregarded.
e. The SDF antenna may be offset from the runway
centerline. Because of this, the angle of convergence
between the final approach course and the runway
bearing should be determined by reference to the
instrument approach procedure chart. This angle is
generally not more than 3 degrees. However, it should
be noted that inasmuch as the approach course
originates at the antenna site, an approach which is
continued beyond the runway threshold will lead the
aircraft to the SDF offset position rather than along
the runway centerline.
AIM 2/14/08
1-1-12 Navigation Aids
FIG 1-1-7
FAA Instrument Landing Systems
AIM 2/14/08
1-1-13
Navigation Aids
f. The SDF signal is fixed at either 6 degrees or
12_degrees as necessary to provide maximum
flyability and optimum course quality.
g. Identification consists of a three-letter identifi-
er transmitted in Morse Code on the SDF frequency.
The appropriate instrument approach chart will
indicate the identifier used at a particular airport.
1-1-11. Microwave Landing System (MLS)
a. General
1. The MLS provides precision navigation
guidance for exact alignment and descent of aircraft
on approach to a runway. It provides azimuth,
elevation, and distance.
2. Both lateral and vertical guidance may be
displayed on conventional course deviation indica-
tors or incorporated into multipurpose cockpit
displays. Range information can be displayed by
conventional DME indicators and also incorporated
into multipurpose displays.
3. The MLS supplements the ILS as the standard
landing system in the U.S. for civil, military, and
international civil aviation. At international airports,
ILS service is protected to 2010.
4. The system may be divided into five
functions:
(a) Approach azimuth;
(b) Back azimuth;
(c) Approach elevation;
(d) Range; and
(e) Data communications.
5. The standard configuration of MLS ground
equipment includes:
(a) An azimuth station to perform functions
(a) and (e) above. In addition to providing azimuth
navigation guidance, the station transmits basic data
which consists of information associated directly
with the operation of the landing system, as well as
advisory data on the performance of the ground
equipment.
(b) An elevation station to perform
function_(c).
(c) Distance Measuring Equipment (DME) to
perform range guidance, both standard DME
(DME/N) and precision DME (DME/P).
6. MLS Expansion Capabilities. The stan-
dard configuration can be expanded by adding one or
more of the following functions or characteristics.
(a) Back azimuth. Provides lateral guidance
for missed approach and departure navigation.
(b) Auxiliary data transmissions. Provides
additional data, including refined airborne position-
ing, meteorological information, runway status, and
other supplementary information.
(c) Expanded Service Volume (ESV) propor-
tional guidance to 60 degrees.
7. MLS identification is a four-letter designa-
tion starting with the letter M. It is transmitted in
International Morse Code at least six times per
minute by the approach azimuth (and back azimuth)
ground equipment.
b. Approach Azimuth Guidance
1. The azimuth station transmits MLS angle and
data on one of 200 channels within the frequency
range of 5031 to 5091 MHz.
2. The equipment is normally located about
1,000 feet beyond the stop end of the runway, but
there is considerable flexibility in selecting sites. For
example, for heliport operations the azimuth
transmitter can be collocated with the elevation
transmitter.
3. The azimuth coverage extends:
(See FIG 1-1-8.)
(a) Laterally, at least 40 degrees on either side
of the runway centerline in a standard configuration,
(b) In elevation, up to an angle of 15 degrees
and to at least 20,000 feet, and
(c) In range, to at least 20 NM.
AIM 2/14/08
1-1-14 Navigation Aids
FIG 1-1-8
Coverage Volume
Azimuth
APPROACH
AZIMUTH
AZIMUTH
-40
40
20 NM
ESV
ESV
14 NM
60
MAXIMUM LIMIT
14 NM
-60
c. Elevation Guidance
1. The elevation station transmits signals on the
same frequency as the azimuth station. A single
frequency is time-shared between angle and data
functions.
2. The elevation transmitter is normally located
about 400 feet from the side of the runway between
runway threshold and the touchdown zone.
3. Elevation coverage is provided in the same
airspace as the azimuth guidance signals:
(a) In elevation, to at least +15 degrees;
(b) Laterally, to fill the Azimuth lateral
coverage; and
(c) In range, to at least 20 NM.
(See FIG 1-1-9.)
FIG 1-1-9
Coverage Volumes
Elevation
ELEVATION
GLIDE NORMAL
PATH
MAXIMUM LIMIT 20,000’
20 NM
30
3
15
o
o
o
o
d. Range Guidance
1. The MLS Precision Distance Measuring
Equipment (DME/P) functions the same as the
navigation DME described in paragraph 1-1-7,
Distance Measuring Equipment (DME), but there are
some technical differences. The beacon transponder
operates in the frequency band 962 to 1105 MHz and
responds to an aircraft interrogator. The MLS DME/P
accuracy is improved to be consistent with the
accuracy provided by the MLS azimuth and elevation
stations.
2. A DME/P channel is paired with the azimuth
and elevation channel. A complete listing of the
200_paired channels of the DME/P with the angle
functions is contained in FAA Standard 022 (MLS
Interoperability and Performance Requirements).
3. The DME/N or DME/P is an integral part of
the MLS and is installed at all MLS facilities unless
a waiver is obtained. This occurs infrequently and
only at outlying, low density airports where marker
beacons or compass locators are already in place.
e. Data Communications
1. The data transmission can include both the
basic and auxiliary data words. All MLS facilities
transmit basic data. Where needed, auxiliary data can
be transmitted.
2. Coverage limits. MLS data are transmitted
throughout the azimuth (and back azimuth when
provided) coverage sectors.
3. Basic data content. Representative data
include:
(a) Station identification;
AIM 2/14/08
1-1-15
Navigation Aids
(b) Exact locations of azimuth, elevation and
DME/P stations (for MLS receiver processing
functions);
(c) Ground equipment performance level;
and
(d) DME/P channel and status.
4. Auxiliary data content: Representative
data include:
(a) 3-D locations of MLS equipment;
(b) Waypoint coordinates;
(c) Runway conditions; and
(d) Weather (e.g., RVR, ceiling, altimeter
setting, wind, wake vortex, wind shear).
f. Operational Flexibility
1. The MLS has the capability to fulfill a variety
of needs in the approach, landing, missed approach
and departure phases of flight. For example:
(a) Curved and segmented approaches;
(b) Selectable glide path angles;
(c) Accurate 3-D positioning of the aircraft in
space; and
(d) The establishment of boundaries to ensure
clearance from obstructions in the terminal area.
2. While many of these capabilities are
available to any MLS-equipped aircraft, the more
sophisticated capabilities (such as curved and
segmented approaches) are dependent upon the
particular capabilities of the airborne equipment.
g. Summary
1. Accuracy. The MLS provides precision
three-dimensional navigation guidance accurate
enough for all approach and landing maneuvers.
2. Coverage. Accuracy is consistent through-
out the coverage volumes. (See FIG 1-1-10.)
FIG 1-1-10
Coverage Volumes
3-D Representation
3. Environment. The system has low suscepti-
bility to interference from weather conditions and
airport ground traffic.
4. Channels. MLS has 200 channels- enough
for any foreseeable need.
5. Data. The MLS transmits ground-air data
messages associated with the systems operation.
6. Range information. Continuous range in-
formation is provided with an accuracy of about
100_feet.
1-1-12. NAVAID Identifier Removal During
Maintenance
During periods of routine or emergency maintenance,
coded identification (or code and voice, where
applicable) is removed from certain FAA NAVAIDs.
Removal of identification serves as a warning to
pilots that the facility is officially off the air for
tune-up or repair and may be unreliable even though
intermittent or constant signals are received.
NOTE-
During periods of maintenance VHF ranges may radiate
a T-E-S-T code (-_______ⴀ⤀⸀
AIM 2/14/08
1-1-16 Navigation Aids
NOTE-
DO NOT attempt to fly a procedure that is NOTAMed out
of service even if the identification is present. In certain
cases, the identification may be transmitted for short
periods as part of the testing.
1-1-13. NAVAIDs with Voice
a. Voice equipped en route radio navigational aids
are under the operational control of either an FAA
Automated Flight Service Station (AFSS) or an
approach control facility. The voice communication
is available on some facilities. Hazardous Inflight
Weather Advisory Service (HIWAS) broadcast
capability is available on selected VOR sites
throughout the conterminous U.S. and does not
provide two-way voice communication. The avail-
ability of two-way voice communication and HIWAS
is indicated in the A/FD and aeronautical charts.
b. Unless otherwise noted on the chart, all radio
navigation aids operate continuously except during
shutdowns for maintenance. Hours of operation of
facilities not operating continuously are annotated on
charts and in the A/FD.
1-1-14. User Reports on NAVAID
Performance
a. Users of the National Airspace System (NAS)
can render valuable assistance in the early correction
of NAVAID malfunctions by reporting their
observations of undesirable NAVAID performance.
Although NAVAIDs are monitored by electronic
detectors, adverse effects of electronic interference,
new obstructions or changes in terrain near the
NAVAID can exist without detection by the ground
monitors. Some of the characteristics of malfunction
or deteriorating performance which should be
reported are: erratic course or bearing indications;
intermittent, or full, flag alarm; garbled, missing or
obviously improper coded identification; poor
quality communications reception; or, in the case of
frequency interference, an audible hum or tone
accompanying radio communications or NAVAID
identification.
b. Reporters should identify the NAVAID, loca-
tion of the aircraft, time of the observation, type of
aircraft and describe the condition observed; the type
of receivers in use is also useful information. Reports
can be made in any of the following ways:
1. Immediate report by direct radio communica-
tion to the controlling Air Route Traffic Control
Center (ARTCC), Control Tower, or FSS. This
method provides the quickest result.
2. By telephone to the nearest FAA facility.
3. By FAA Form 8000-7, Safety Improvement
Report, a postage-paid card designed for this
purpose. These cards may be obtained at FAA FSSs,
Flight Standards District Offices, and General
Aviation Fixed Base Operations.
c. In aircraft that have more than one receiver,
there are many combinations of possible interference
between units. This can cause either erroneous
navigation indications or, complete or partial
blanking out of the communications. Pilots should be
familiar enough with the radio installation of the
particular airplanes they fly to recognize this type of
interference.
1-1-15. LORAN
a. Introduction
1. The LOng RAnge Navigation-C (LORAN)
system is a hyperbolic, terrestrial-based navigation
system operating in the 90-110 kHz frequency band.
LORAN, operated by the U.S. Coast Guard (USCG),
has been in service for over 50 years and is used for
navigation by the various transportation modes, as
well as, for precise time and frequency applications.
The system is configured to provide reliable, all
weather navigation for marine users along the
U.S._coasts and in the Great Lakes.
2. In the 1980's, responding to aviation user and
industry requests, the USCG and FAA expanded
LORAN coverage to include the entire continental
U.S. This work was completed in late 1990, but the
LORAN system failed to gain significant user
acceptance and primarily due to transmitter and user
equipment performance limitations, attempts to
obtain FAA certification of nonprecision approach
capable receivers were unsuccessful. More recently,
concern regarding the vulnerability of Global
Positioning System (GPS) and the consequences of
losing GPS on the critical U.S. infrastructure
(e.g.,_NAS) has renewed and refocused attention on
LORAN.
3. LORAN is also supported in the Canadian
airspace system. Currently, LORAN receivers are
only certified for en route navigation.
AIM 2/14/08
1-1-17
Navigation Aids
4. Additional information can be
found_in_the_“LORAN-C User Handbook,”
COMDT PUB-P16562.6, or the website
http://www.navcen.uscg.gov.
b. LORAN Chain
1. The locations of the U.S. and Canadian
LORAN transmitters and monitor sites are illustrated
in FIG 1-1-11. Station operations are organized into
subgroups of four to six stations called “chains.” One
station in the chain is designated the “Master” and the
others are “secondary” stations. The resulting chain
based coverage is seen in FIG 1-1-12.
FIG 1-1-11
U.S. and Canadian LORAN System Architecture
FIG 1-1-12
LORAN Chain Based Coverage
AIM 2/14/08
1-1-18 Navigation Aids
2. The LORAN navigation signal is a carefully
structured sequence of brief radio frequency pulses
centered at 100 kHz. The sequence of signal
transmissions consists of a pulse group from the
Master (M) station followed at precise time intervals
by groups from the secondary stations, which are
designated by the U.S. Coast Guard with the letters V,
W, X, Y and Z. All secondary stations radiate pulses
in groups of eight, but for identification the Master
signal has an additional ninth pulse. (See
FIG 1-1-13.) The timing of the LORAN system is
tightly controlled and synchronized to Coordinated
Universal Time (UTC). Like the GPS, this is a
Stratum_1 timing standard.
3. The time interval between the reoccurrence
of the Master pulse group is called the Group
Repetition Interval (GRI). The GRI is the same for all
stations in a chain and each LORAN chain has a
unique GRI. Since all stations in a particular chain
operate on the same radio frequency, the GRI is the
key by which a LORAN receiver can identify and
isolate signal groups from a specific chain.
EXAMPLE-
Transmitters in the Northeast U.S. chain (FIG 1-1-14)
operate with a GRI of 99,600 microseconds which is
shortened to 9960 for convenience. The master station (M)
at Seneca, New York, controls secondary stations (W) at
Caribou, Maine; (X) at Nantucket, Massachusetts; (Y) at
Carolina Beach, North Carolina, and (Z) at Dana, Indiana.
In order to keep chain operations precise, monitor
receivers are located at Cape Elizabeth, ME; Sandy Hook,
NJ; Dunbar Forest, MI, and Plumbrook, OH. Monitor
receivers continuously measure various aspects of the
quality (e.g., pulse shape) and accuracy (e.g., timing) of
LORAN signals and report system status to a control
station.
4. The line between the Master and each
secondary station is the “baseline” for a pair of
stations. Typical baselines are from 600 to
1,000_nautical miles in length. The continuation of
the baseline in either direction is a “baseline
extension.”
5. At the LORAN transmitter stations there are
cesium oscillators, transmitter time and control
equipment, a transmitter, primary power (e.g.,_com-
mercial or generator) and auxiliary power equipment
(e.g., uninterruptible power supplies and generators),
and a transmitting antenna (configurations may either
have 1 or 4 towers) with the tower heights ranging
from 700 to 1350 feet tall. Depending on the coverage
area requirements a LORAN station transmits from
400 to 1,600 kilowatts of peak signal power.
6. The USCG operates the LORAN transmitter
stations under a reduced staffing structure that is
made possible by the remote control and monitoring
of the critical station and signal parameters. The
actual control of the transmitting station is
accomplished remotely at Coast Guard Navigation
Center (NAVCEN) located in Alexandria, Virginia.
East Coast and Midwest stations are controlled by the
NAVCEN. Stations on the West Coast and in Alaska
are controlled by the NAVCEN Detachment (Det),
located in Petaluma, California. In the event of a
problem at one of these two 24 hour-a-day staffed
sites, monitoring and control of the entire LORAN
system can be done at either location. If both NACEN
and NAVCEN Det are down or if there is an
equipment problem at a specific station, local station
personnel are available to operate and perform repairs
at each LORAN station.
7. The transmitted signal is also monitored in
the service areas (i.e., area of published LORAN
coverage) and its status provided to NAVCEN and
NAVCEN Det. The System Area Monitor (SAM) is
a single site used to observe the transmitted signal
(signal strength, time difference, and pulse shape). If
an out-of-tolerance situation that could affect
navigation accuracy is detected, an alert signal called
“Blink” is activated. Blink is a distinctive change in
the group of eight pulses that can be recognized
automatically by a receiver so the user is notified
instantly that the LORAN system should not be used
for navigation. Out-of-tolerance situations which
only the local station can detect are also monitored.
These situations when detected cause signal
transmissions from a station to be halted.
8. Each individual LORAN chain provides
navigation-quality signal coverage over an identified
area as shown in FIG 1-1-15 for the West Coast
chain, GRI 9940. The chain Master station is at
Fallon, Nevada, and secondary stations are at George,
Washington; Middletown, California, and Search-
light, Nevada. In a signal coverage area the signal
strength relative to the normal ambient radio noise
must be adequate to assure successful reception.
Similar coverage area charts are available for all
chains.
AIM 2/14/08
1-1-19
Navigation Aids
FIG 1-1-13
The LORAN Pulse and Pulse Group
AIM 2/14/08
1-1-20 Navigation Aids
FIG 1-1-14
Northeast U.S. LORAN Chain
AIM 2/14/08
1-1-21
Navigation Aids
FIG 1-1-15
West Coast U.S. LORAN Chain
AIM 2/14/08
1-1-22 Navigation Aids
c. The LORAN Receiver
1. For a currently certified LORAN aviation
receiver to provide navigation information for a pilot,
it must successfully receive, or “acquire,” signals
from three or more stations in a chain. Acquisition
involves the time synchronization of the receiver with
the chain GRI, identification of the Master station
signals from among those checked, identification of
secondary station signals, and the proper selection of
the tracking point on each signal at which
measurements are made. However, a new generation
of receivers has been developed that use pulses from
all stations that can be received at the pilot's location.
Use of “all-in-view” stations by a receiver is made
possible due to the synchronization of LORAN
stations signals to UTC. This new generation of
receivers, along with improvements at the transmit-
ting stations and changes in system policy and
operations doctrine may allow for LORAN's use in
nonprecision approaches. At this time these receivers
are available for purchase, but none have been
certified for aviation use.
2. The basic measurements made by certified
LORAN receivers are the differences in time-ofarrival between the Master signal and the signals
from each of the secondary stations of a chain. Each
“time difference” (TD) value is measured to a
precision of about 0.1 microseconds. As a rule of
thumb, 0.1_microsecond is equal to about 100 feet.
3. An aircraft's LORAN receiver must recog-
nize three signal conditions:
(a) Usable signals;
(b) Absence of signals, and
(c) Signal blink.
4. The most critical phase of flight is during the
approach to landing at an airport. During the
approach phase the receiver must detect a lost signal,
or a signal Blink, within 10 seconds of the occurrence
and warn the pilot of the event. At this time there are
no receivers that are certified for nonprecision
approaches.
5. Most certified receivers have various internal
tests for estimating the probable accuracy of the
current TD values and consequent navigation
solutions. Tests may include verification of the timing
alignment of the receiver clock with the LORAN
pulse, or a continuous measurement of the
signal-to-noise ratio (SNR). SNR is the relative
strength of the LORAN signals compared to the local
ambient noise level. If any of the tests fail, or if the
quantities measured are out of the limits set for
reliable navigation, then an alarm will be activated to
alert the pilot.
6. LORAN signals operate in the low frequency
band (90-110 kHz) that has been reserved for marine
navigation signals. Adjacent to the band, however,
are numerous low frequency communications
transmitters. Nearby signals can distort the LORAN
signals and must be eliminated by the receiver to
assure proper operation. To eliminate interfering
signals, LORAN receivers have selective internal
filters. These filters, commonly known as “notch
filters,” reduce the effect of interfering signals.
7. Careful installation of antennas, good metalto-metal electrical bonding, and provisions for
precipitation noise discharge on the aircraft are
essential for the successful operation of LORAN
receivers. A LORAN antenna should be installed on
an aircraft in accordance with the manufacturer's
instructions. Corroded bonding straps should be
replaced, and static discharge devices installed at
points indicated by the aircraft manufacturer.
d. LORAN Navigation
1. An airborne LORAN receiver has four major
parts:
(a) Signal processor;
(b) Navigation computer;
(c) Control/display, and
(d) Antenna.
