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16.1.1 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.
16.1.2 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.
16.1.3 LORAN is also supported in the Canadian airspace system. Currently, LORAN receivers are only certified for en route navigation.
16.1.4 Additional information can be found in the “LORAN.C User Handbook,” COMDT PUBP 16562.6, or the website: http://www.navcen.uscg.gov.
16.2 LORAN Chain
16.2.1 The locations of the U.S. and Canadian LORAN transmitters and monitor sites are illustrated in FIG ENR 4.1.6. 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 ENR 4.1.7.

FIG ENR 4.1.6
U.S. and Canadian LORAN System Architecture

FIG ENR 4.1.7
LORAN Chain Based Coverage

16.2.2 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 ENR 4.1.8.) 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.
16.2.3 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 ENR 4.1.9) 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.
16.2.4 The line between the Master and each secondary station is the “baseline” for a pair of stations. Typical baselines are from 600 to 1000 nautical miles in length. The continuation of the baseline in either direction is a “baseline extension.”
16.2.5 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.

Federal Aviation Administration Twentieth Edition
16.2.6 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.
　
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