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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.
2/14/08 AIM
Navigation Aids 1-1-17
4. Additional information can be
foundinthe“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
Stratum1 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.
EXAMPLETransmitters
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
1,000nautical 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.,commercial
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
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