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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 Searchlight,
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.
2/14/08 AIM
Navigation Aids 1-1-19
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
2/14/08 AIM
Navigation Aids 1-1-21
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 transmitting
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, 0.1microsecond is equal to about 100 feet.
3. An aircraft's LORAN receiver must recognize
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
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