Learn about Loran-C, a key marine radionavigation system. Explore its principles, components, and historical significance, and see how it compares to GPS and other navigation technologies.
- The New Handbook Edition: More Than Just the Cover Has Changed
- Introduction
- What is Loran?
- Comparison of and Relationship of Loran-C to Other Marine Radionavigation Systems
- Omega
- Global Positioning System (GPS)
- Marine Radiobeacons
- Transit
- Summary
- Simplified Principle of Loran-C Operation
- Components of the Loran System
- A Brief History of Loran
This new edition retains many of the useful tables, figures, and charts of the original edition (updated as necessary), but has been considerably expanded in scope to cover the major developments of the past decade.
The New Handbook Edition: More Than Just the Cover Has Changed
In particular, much more material has been added on how to use loran for navigation to complement the systems information presented in this and the earlier Green Book. Although this handbook is not intended to be an academic treatise on lorm navigation, parts of this text, particularly The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems” and Loran-C Position Determination and Accuracy“LORAN-C System: Accuracy and Position Determination”, are quite technical. Most of the chapters, however, do not presume any extensive technical background on the part of the reader.
To facilitate quick reading and to simplify some of the more technical sections of this handbook, capsule summaries are found throughout the text, set apart in shaded insets. Readers lacking interest in the technical details of these specialized sections can skim these capsule summaries and skip ahead to more interesting topics.
The focus of this articles is on marine applications of Loran-C. However, aviators may find this handbook useful as well-mentally replace the word “mariner” with “aviator” and the vessel icons with aircraft. Lastly, terrestrial users may also find this handbook of interest – particularly the discussions of the system and the technical material in the appendices.
Introduction
This introductory article provides a brief overview of the loran system and shows how this system compares with other radionavigation systems used in the United States. A simplified discussion of the principle of operation is presented, along with an identification of the components of the loran system. Article concludes with a brief history of loran.
Subsequent articles build upon this basic treatment, detailing the Loran-C system in greater depth (The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems” and Loran-C Position Determination and Accuracy“LORAN-C System: Accuracy and Position Determination”), Loran-C receivers (Loran-C Receiver Features and Their Use“Understanding Loran Receivers: Features and Functionality”), practical aspects of Loran-C navigation (Practical Aspects of Loran Navigation“Mastering Loran-C Navigation: Techniques, Accuracy, Best Practices”), relevant charts (Loran-C Charts and Related Information“Loran-C Charts Key Components and Navigation Techniques”), and installation and related matters (Installation and Related Matters of Loran-C“Complete Guide to Loran-C Installation and Related Matters”). Numerous appendices provide additional material of a more technical nature.
Readers without any background in loran are advised to read this article, then skip ahead to Loran-C Receiver Features and Their Use“Understanding Loran Receivers: Features and Functionality” through Installation and Related Matters of Loran-C“Complete Guide to Loran-C Installation and Related Matters”. The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems” and Loran-C Position Determination and Accuracy“LORAN-C System: Accuracy and Position Determination” can be deferred for later study and/or skimmed. Readers more familiar with loran and wishing to learn the technical details of this system should read the various chapters in numerical sequence.
What is Loran?
The name, “loran”, is an acronym for long-range navigation When the word loran is used in a generic sense, it is not capitalized. Specific loran systems, such as Loran-C are capitalized in this handbook.x. It is aradionavigation system using land-based radio transmitters (operated in the United States by the USCG) and receivers to allow mariners, aviators, and (more recently) those interested in terrestrial navigation to determine their position. Loran-C is the federally provided radionavigation system for the US Coastal Confluence Zone (CCZ). The CCZ is defined as the area seaward of a harbor entrance to 50 nautical miles offshore or the edge of the Continental Shelf – 100 fathom curve – whichever is greater. The CCZ does not include the harbor, however. Loran-C is also approved as a supplemental air navigation system.
The Federal Aviation Administration (FAA) is presently in the process of certifying Loran-C for non-precision approaches (NPA) conducted under Instrument Flight Rules (IFR). As of this writing only a few such approaches have been established and certified, but the pace of certification is expected to increase substantially in the next few years.

A discussion of the details of the Loran-C system is presented later in this article and elsewhere in this category. In general terms, however, Loran-C can be characterized as a highly accurate (better than 0,25 nautical mile (NM) absolute accuracy in the defined coverage area), available (99,7 % availability), 24-hour-a-day, all-weather Weather,in particular thunderstorms, does degrade the performance of Loran-C. However, usable navigation information can often be obtained during these circumstances.x radionavigation system. Loran-C (the present version of this system) coverage extends over the conterminous United States, portions of Alaska, and many other areas of the world.
Loran is also used extensively to establish a precise time reference. Power companies, telephone companies, and many others use Loran-C as a source of timing information for such purposes as controlling and monitoring cesium clocks.
Loran-C is the federally provided radionavigation system for the US Coastal Confluence Zone. It is a highly accurate, highly available, 24-hour-a-day, all-weather, radionavigation system.
From the perspective of the mariner, Loran-C is designed to be used in several phases of marine navigation, including ocean navigation and coastal navigation. Loran-C is also a useful supplemental system for harbor and harbor approach navigation. It can also be a valuable supplemental navigation system for inland navigation of recreational vessels. Table 1 provides brief definitions of these phases of navigation and identifies the navigation techniques and systems commonly in use for each phase.
Table 1. Phases of marine navigation and systems or techniques in use in each phase | ||
---|---|---|
Phase | Brief Description | Techniques and Systems in Use |
Ocean Navigation | Vessel beyond Continental Shelf, and more than 50 nm from land, where position fixing by pilotage impractical. | Loran-C, Omega, Transit, RDF, GPS, Inertial Navigation, Dead Reckoning and Celestial Fixes. |
Coastal Navigation | Vessel within 50 nm from shore or the limit of the Continental Shelf, whichever is more distant. Also applies to other waters where traffic separation schemes have been established. | Loran-C, RDF, and GPS. |
Harbor and Harbor Approach Navigation | Vessel generally inland from coastal waters. The harbor approach phase begins with a transition zone between the relatively unrestricted coastal waters and narrowly restricted waters and/or within the entrance to a bay, river, or harbor where the harbor phase begins. | RDF, DGPS Under study.x fixed and floating ATONs, audible warning signals, and radar. Loran-C may also be useful, but as a supplemental navigation system. |
Inland Navigation | Conducted in restricted waters similar to harbors or harbor approaches. For inland navigation, however, the focus is on nonseagoing vessels, including most recreational craft. | Fixed and floating ATONs, and radar. Loran-C may also be useful, depending upon the waters, but not as a primary navigation system. |
According to estimates given in the 1990 Federal Radionavigation Plan (FW), in 1991 there are expected to be more than 572 000 users of the Loran-C system, the second largest “user community” to employ a single radionavigation system. According to other estimates, the number of loran users is much larger, perhaps one million or more. The majority (82 %) of Loran-C users are marine users (both domestic and international). Other Loran-C users include US civil aviation users (14 %), US civil land users (3,8 %), and a small number of Department of Defense (DOD) users.

With the exception of DOD applications,which are scheduled to cease as of 31 December 1994, these numbers of Loran-C users are projected to continue to grow in number. Aviation uses of Loran-C, inparticular, are expected to increase substantially in the years ahead. Accurate projections of the future number of users depend upon several factors, such as the upcoming (1994) decision by the US Department of Transportation (DOT) whether to continue the Loran-C system, or to begin to phase this system out in favor of other alternatives, such as the Global Positioning System (GPS). Although the outcome of DOT’S deliberations cannot be forecast with any certainty, many believe that the Loran-C system will remain in operation well into the next century.
Comparison of and Relationship of Loran-C to Other Marine Radionavigation Systems
Before discussing the details of loran, it is useful to understand the role, limitations, and capabilities of the Loran-C systemin the context of the overall US radionavigation systems mix. That is, loran should be comp,xed with other competing and complementary radionavigation systems. Table 2 provides relevant data on the Loran-C system and several other marine radionavigation systems in use throughout the United States and elsewhere.
Table 2. Relevant characteristics of several navigation systems | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
System | Accuracy | Unit Availability | Coverage | System Availability | Fix Rate | Fix Dimension | System Capacity | Ambiguilty Potential | ||
Predictable | Repeatable | Relative | ||||||||
Loran-C | At least 0,25 nm (460 m) 1:3 SNR | 60-300 ft. (18-90 m) | 99+ % Transmitting station signal availability greater than 99,9 % | Coastal continental US and selected overseas areas | 99,7 % Triad reliability; individual station availability normally exceeds 99,9 %. Note also that many areas of the United States are served by more than one loran chain, increasing the availability.x | 10-20 fixes/minute | Two dimensions | Unlimited number of simultaneous uses | Yes, easily resolved | |
Omega | 2-4 nm (3,7-7,4 km) | 0,25-0,5 nm (463-926 m) | 99+ % | Worldwide continuous | 97 % Three station joint signal availability.x | 1 fix every 10 seconds | Two dimensions | Unlimited | Requires knowledge to ±36 nm Three frequency receiver (10,2, 11,33, 13,6 kHz).x | |
GPS | PPS Precise position service for US and Allied military, US government, and selected civil users specifically approved by the US government. PPS is specified as spherical error probable. Multiply this quantity by 2,5 to ensure comparability with other entries in this column. SPS numbers do not need to be adjusted.x Horz – 17,8 mVert – 27,7 mTime – 90 ns | Horz – 17,8 mVert – 27,7 m | Horz – 7,6 mVert – 11,7 m | Expected to approach 100 % | Worldwide continuous | 98 % probability that a 21-satellite constellation will be operating | Essentially continuous | Three dimensions plus, velocity and time | Unlimited | None |
SPS Standard positioning service for other uses.x Horz – 100 mVert – 156 mTime 175 ns | Horz – 100 mVert – 156 m | Horz – 28,4 mVert – 44,5 m | ||||||||
Marine RDF | Marine ±3° | NA | 99 % | Out to 50 nm or 100 fathom curve | 99 % | Function of the type of beacon continuous or sequenced | One LOP per beacon | Unlimited | Potential is high for reciprocal bearing without sense antenna | |
Transit | Dual frequency 25 m 2 sigma, position accuracy is highly dependent on the user’s knowledge of his velocity.x | 15 m | Under 10 m with translocation techniques | 99 % when satellite is in view | Worldwide non-continuous | 99 % | Every Maximum satellite waiting time varies with latitude. (30 minutes at 80°, 110 minutes at equator).x 30 seconds | Two dimensions | Unlimited | None |
Single frequency 500 m | 50 m |
Radionavigation systems included in this comparison include Omega, GPS, marine radiobeacons, and Transit, together accounting for the principal radionavigation systems in use by US mariners. Several key system characteristics of each system, including accuracy, availability, coverage, reliability, fix rate, fix dimension, system capacity and ambiguity potential are summarized in this table.
Omega
The Omega system was originally developed and implemented by the Department of the Navy, and now operated by seven nations under the operational control of the USCG. Omega is a very low frequency (VLF 10,2-13,6 kHz) hyperbolic Hyperbolic systems, including loran, are discussed later in this and subsequent chapters and defined in the Glossarx radionavigation system used chiefly for ocean navigation. Table 3 summarizes the various radio frequency bands, provides a capsule description of therelevant characteristics of each, and identifies past and present radionavigation systems using each band.
Table 3. Frequency bands and radionavigation systems | |||||
---|---|---|---|---|---|
Frequency Range | Name | Abbreviation | Brief Description | Line of Sight Limitation | Navigation and Other Systems Operating in This Range |
3 kHz – 30 kHz | Very low frequency | VLF | VLF signals propagate between the bounds of the ionosphere and the earth. Signals follow the curvature of the earth to great distances with excellent stability. | No | Omega Delrac |
30 kHz – 300 kHz | Low frequency | LF | Compared to VLF there is greater signal attenuation with distance, and range for a given power output decreases substantially. Some skywave interference possible. | No | Loran-C Decca Consol Radiobeacons |
300 kHz – 3 000 kHz | Medium frequency | MF | Groundwaves provide reliable service, but the range for a given power output falls off rapidly. Skywaves begin to penetrate the ionosphere at the upper end of the frequency range. | No | Radiobeacons Consol Standard radio broadcast band Loran-A |
3 MHz – 30 MHz | High frequency | HF | Groundwave range is limited to a few miles, but long-range communications still possible. | Variable | Ship-to-ship and ship-to shore communications |
30 MHz – 300 MHz | Very high frequency | VHF | Transmission limited by direct wave and/or ground-reflected wave. | Yes | Short- and medium-range communications* VOR*, ILS* Transit Hi-Fix* Gee |
300 MHz – 3 000 MHz | Ultra high frequency | UHF | Skywaves cannot be used in this band. Reception of signals is virtually free of fading and interference by atmospheric noise. | Yes | Some communications DME* TACAN* Transit GPS |
3 000 MHz – 30 000 MHz | Super high frequency | SHF | Also called microwave band. No skywave reception possible. Interference virtually nonexistent. | Yes | Radar |
Position information is obtained by measuring relative phase differences of received Omega signals. There are now eight Omega transmitters. These are located in:
- Norway (at the arctic circle);
- Monrovia, Liberia;
- La Reunion Island (in the Indian Ocean);
- Golfo Nuevo, Argentina;
- Victoria, Australia;
- Tsshima, Japan;
- and in the United States at La Moure, North Dakota, and Oahu, Hawaii.
The Omega user community was estimated to number approximately 26 500 in 1991. Under present plans, Omega will remain in operation past the year 2000.
In broad terms (see Table 2), Loran-C offers superior fix accuracy compared to Omega, but lacks Omega’s worldwide coverage. Fix accuracy (more on this in Loran-C Position Determination and Accuracy“LORAN-C System: Accuracy and Position Determination”) for the Loran-C system within the designated coverage area is no worse than 0,25 NM compared to 2-4 NM for Omega. Omega’s accuracy constraints limit its use to ocean navigation. Approximate areas of Loran-C coverage can be found below.
Data Sheets and Coverage Diagrams
This appendix contains the latest and best information available as of the publication date of this Loran-C User Handbook. Users should consult the Radionavigation Bulletin, Local Nofice to Mariners, Notices to Airmen (NOTAMs), and other sources for updated information. Recommended secondaries (shown in the figures of this appendix) and limits of coverage are only approximate, and may be revised as a result of ongoing studies. Finally, users are again cautioned not to rely on any one system of navigation, but rather to use all available information.



Transmitter:
- M – Iwo Jima, Japan;
- W – Marcus Island, Japan;
- X – Hokkaido, Japan;
- Y – Gesashi, Japan;
- Z – Barrigada, Guam.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 58,1 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – St. Paul, AK;
- X – Attu, AK;
- Y – Port Clarence, AK;
- Z – Kodiak, AK.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 48,2 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Tok, AK;
- X – Kodiak, AK;
- Y – Shoal Cove, AK;
- Z – Port Clarence, AK.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 49,0 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Williams Lake, Canada;
- X – Shoal Cove, AK;
- Y – George, WA;
- Z – Port Hardy, Canada.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 46,4 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Fallon, NV;
- W – George, WA;
- X – Middletown, CA;
- Y – Searchlight, NV.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 52,4 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Havre, MT;
- W – Baudette, MN;
- X – Gillette, WY;
- Y – Williams Lake, Canada.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 57,8 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Boise City, OK;
- V – Gillette, WY;
- W – Searchlight, NV;
- X – Las Cruces, NM;
- Y – Raymondville, TX;
- Z – Grangeville, LA.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 57,8 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Dana, IN;
- W – Malone, FL;
- X – Seneca, NY;
- Y – Baudette, MN;
- Z – Boise City, OK.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 58,1 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Malone, FL;
- W – Grangeville, LA;
- X – Raymondville, TX;
- Y – Jupiter, FL;
- Z – Carolina Beach, NC.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 60,5 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Seneca, NY;
- W – Caribou, ME;
- X – Nantucket, MA;
- Y – Carolina Beach, NC;
- Z – Dana, IN.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 58,1 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Caribou, ME;
- X – Nantucket, MA;
- Y – Cape Race, Canada;
- Z – Fox Harbor, Canada.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 47,6 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Fox Harbor, Canada;
- W – Cape Race, Canada;
- X – Angissoq, Greenland.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 43,1 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Sandur, Iceland;
- W – Angissoq, Greenland;
- X – Ejde, Denmark.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 50,0 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Ejde, Denmark;
- X – Bo, Norway;
- W – Sylt, Germany;
- Y – Sandur, Iceland.
- Z – Jan Mayen, Norway.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 43,1 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.

Transmitter:
- M – Sellia Marina, Italy;
- X – Lampedusa, Italy;
- Y – Kargabarun, Turkey.
- Z – Estartit, Spain.
SNR | 1:3 |
Fix Accuracy | 1/4 NM (95 % 2dRMS) |
Atmospheric Noise | 51,2 dB above 1 uV/m |
NOTE: Estimated Groundwave coverage, actual coverage will vary.
Although Loran-C coverage exists for many areas of the world, there are also broad expanses of ocean (such as the South Pacific and South Atlantic Oceans) where Loran-C coverage is not available. In contrast, the Omega system offers virtually worldwide coverage. Although not listed among the characteristics given in Table 2, Loran-C receivers are substantially less expensive than corresponding equipment for Omega – and likely to remain so in view of the relative size of the two user communities.
Global Positioning System (GPS)
GPS is a space-based military and civilian radio positioning system operated by DOD that will provide three-dimensional position, velocity, and time information to users on or near the surface of the earth. The space component consists of 21 satellites plus three operational spares operating in high altitude (10 900 NM) orbits, and transmitting navigational signals on 1575,42 and 1227,6 MHz. There were an estimated 15 000 GPS users in 1991, a figure projected to grow substantially in the coming years.
GPS is an emerging system that offers improved coverage and accuracy compared to Loran-C, and is the likely successor to the loran (and Omega) system. However, as of this writing, the entire constellation of satellites necessary for continuous worldwide GPS coverage has not been deployed. According to present plans, the GPS will be fully operation as of 1993.
However this schedule may slip. Additionally, GPS receivers are substantially more expensive than Loran-C receivers, although this price differential will undoubtedly narrow in the future as the market expands for GPS receivers.
Marine Radiobeacons
Marine radiobeacons are nondirectional low power radio transmitting stations which operate in the low- and medium-frequency bands (285-325 kHz) to provide ground wave signals to a shipboard receiver equipped with a directional antenna. The receiver, termed a radiodirection finder (RDF) or (typically in aircraft installations) an automatic direction finder (ADF), is used to measure the relative bearing of the transmitter with respect to the user. The line of position (LOP) so determined can be crossed with another derived from a second radiobeacon to determine a fix. As well an RDF LOP can be advanced or retired and crossed with an earlier or later LOP from the same or another station to determine a running fix. Currently, there are approximately 200 marine radiobeacons (operated by USCG), located on or near the coasts of the United states AdditionaIly, there are many more aeronautical radiobeacons that are located at or near to major airports in the United States.x. The area of reliable signal reception from these radiobeacons varies with location, but generally includes coastal waters within 200 NM from the shore.
Marine radiobeacons and RDFs provide a redundant or backup system to more sophisticated radionavigation systems. RDF is a popular low-cost, medium-accuracy system for vessels equipped with only minimal radionavigation equipment. Some RDF receivers are powered with self-contained batteries, and can be used in applications where electrical power is at a premium (e. g., sailboats) and/or an independently powered backup navigation system is desired.
According to some estimates, the size of the present RDF user community is the largest among US radionavigation systems. It was estimated to number 675 000 users in 199 1, but this figure is projected to decrease in the coming years. Additionally, the present network of RDF stations is being rationalized, and some reductions in their number are being planned. Under present plans, marine radiobeacons will remain in operation past the year 2000.
Marine radiobeacons are presently under consideration as a component of a differential global positioning system (DGPS). Using this concept, the DGPS signal would be transmitted in concert with a digital GPS correction to increase the accuracy of the GPS. A prototype system at Montauk Point, Long Island, has enabled a position-fixing accuracy of 30 ft (10 meters) to be achieved.
In contrast to Loran-C, marine radiobeacons do not provide sufficient accuracy or coverage to be used as a primary aid to navigation for large vessels in US coastal waters. Although RDF receivers are still being manufactured, there are far fewer makes and models to choose from, compared to the wide variety of commercial Loran-C receivers. The price differential between RDF and Loran-C receivers, once substantially in favor of RDF, has now become almost nonexistent. Moreover, most Loran-C receivers are integrated with special-purpose computers that provide the user with a wealth of additional information of navigationalrelevance (e. g., ground speed, estimated time enroute, etc.). In contrast, marine radiobeacon receivers offer only the capability to fix the vessel’s position, and track or home towards or away from the transmitter.
Transit
As with GPS, the Transit system is another DOD operated military and civilian satellite-based system consisting of satellites in approximately 600 NM polar orbits. These satellites transmit information continuously on 150 and 400 MHz. Only one frequency is required to determine a position. However, accuracy is increased by using two frequencies.
Transit offers slightly improved fix accuracy compared to Loran-C, and offers worldwide, but noncontinuous coverage. Fix rates range from an average of once every 30 minutes at 80 degrees latitude to an average of once every 100 minutes near the equator. Under realistic worst-case conditions (5 % of the time) a user must wait as many as six hours between fixes. Dead reckoning is used in the periods between fixes. Transit receivers are presently much more expensive than corresponding Loran-C receivers and likely to remain so. There were an estimated 95 599 users of the Transit system in 1991. It is anticipated that the Transit system will be phased out in favor of GPS. Under present schedules, operation of the Transit system will be discontinued in 1996.
Summary
The foregoing discussion, coupled with the material in Tables 1 and 2 shows the role and utility of the various radionavigation systems. Loran-C fills an important place in the mix of radionavigation systems and, moreover, has found wide acceptance; Loran-C has at least the second largest number of users of the major radionavigation systems, a point highlighted in Figure 1. From the perspective of the user, Loran-C offers a proven, easy-to-use, accurate, all-weather radionavigation system applicable (as either a primary or complementary system) to nearly all phases of navigation within designated areas of coverage.
From the perspective of the user, Loran-C offers a proven, easy-to-use, accurate, all-weather radionavigation system applicable (as either a primary or complementary system) to nearly all phases of navigation within designated areas of coverage.
Simplified Principle of Loran-C Operation
This explanation is confined to use of loran as a hyperbolic system. Specially equipped receivers, costing much more than conventional receivers, can measure the range to the master or secondary stations. This range-range or RHO-RHO mode of operation is not discussed in this article.
A more comprehensive technical discussion of the Loran-C system can be found in The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems” and Loran-C Position Determination and Accuracy“LORAN-C System: Accuracy and Position Determination”. But briefly, the basic Loran-C system consists of a chain of three or more land-based transmitting stations, each separated by several hundred miles. Within the loran chain, one station is designated as amaster station (M), and the other transmitters as secondary stations, conventionally designated Victor (V), Whiskey (W), Xray (X), Yankee (Y), and Zulu (Z).

For example, the loran chain that serves the north-east United States (NEUS), consists of a master station located in Seneca, New York, with a Whiskey secondary located in Caribou, Maine, an Xray secondary in Nantucket, Massachusetts, a Yankee secondary in Carolina Beach, North Carolina, and a Zulu secondary in Dana, Indiana.
Figure 2 illustrates the simplest possible loran chain, called a triad, with a master (denoted M) and two secondary transmitters; Xray (X) and Yankee (Y).

Themaster station and the secondaries transmit radio pulses at precise time intervals. An onboard Loran-C receiver (depicted by the vessel and aircraft icons in Figure 2) measures the slight difference in the time that it takes for these pulsed signals to reach the ship or aircraft from both master-secondary pairs. These time differences (TDs) are quite small, and are measured in millionths of a second, microseconds (usec or us). Time differences for each master-secondary pair, denoted (TDX and TDY in Figure 2) are displayed by the mobile loran receiver.
The difference in the time of arrival of signals from a given master-secondary pair, observed at a point in the coverage area, is a measure of the difference in distance from the vessel to each of the two stations. The locus of points having the same TD from a specific master-secondary pair is a curved line of position (LOP). Mathematically,these curved LOPS are hyperbolas – or, more accurately, spherical or spheroidal hyperbolas on the curved surface of the earth. This is why Loran-C and related systems are termed hyperbolic systems. The intersection of two or more LOPS from the TDs (shown as TDX-LOP and TDY-LOP in Figure 2) determine the position of the user (a hyperbolic fix). This is shown as a circle in Figure 2, but one course charting convention specifies plotting all electronically determined fixes with a triangular symbol. Using this convention the loran fix would be plotted as a triangle, with the fix time and the word “loran” written next to the fix parallel to one of the chart axes.
In practice, the operator simply reads the observed time differences from the Loran-C receiver display, and converts these TD readings to more commonly-used coordinates, such as latitude and longitude, using special charts (termed loran overprinted charts) that display the lattice of possible loran LOPS spaced in convenient units (e. g., every 5 or 10 usec for large-scale charts and at greater intervals for small-scale charts, see Loran-C Charts and Related Information“Loran-C Charts Key Components and Navigation Techniques” for details). Alternatively, most modern loran receivers employ computer algorithms for this coordinate conversion process, and when this feature is selected, an estimate of the user’s latitude and longitude can be read directly from the loran receiver. Aviation users deal exclusively in latitudebongitude coordinates.
Basic marine Loran-C receivers merely displayed measured TDs, so that the navigator was required to fix the vessel’s position from these TDs and suitable loran charts. Other necessary or useful navigational tasks (e. g., estimating current set and drift, determining course to steer, estimating speeds and times of arrival, etc.) had to be done manua ly using the fix information supplied by the loran receiver. However, in the past decade, there have been major advances in the state-of-the-art of loran receivers. Most loran receivers now have the ability to determine the vessel’s (or aircraft’s) speed and course over the ground, to define waypoints (points of specified position, such as entrance buoys, turnpoints, wrecks, prime fishing locations, shoals or other hazards to navigation, etc.) and monitor the progress of the vessel or aircraft towards these waypoints,providing such useful information as course corrections, estimated times of arrival, etc.
Many loran receivers can interface with other shipboard electronic systems, including:
- radar,
- autopilots,
- gyroscopes,
- fluxgate compasses,
- speed sensors,
- electronic charting systems.
These and other useful features of Loran-C receivers are discussed in Loran-C Receiver Features and Their Use“Understanding Loran Receivers: Features and Functionality”.
Components of the Loran System
Simply put, the components land-based facilities, receiver (and associated equipment), and appropriate loran overprinted charts.
Land-based facilities are highlighted in Figure 3. These include amaster transmitter, at least two secondary transmitters, a control station, amonitor site and time reference. The function of the transmitters are to transmit the loran signals at precise instants in time. The control station and associated Loran Monitor Sites (LORMONSITES) Simply put, a “station” is manned, a “site” is not. For aviation users, Loran Aviation Monitors (LAM) are presently installed at 192 facilities.x continually measure the characteristics of the loran signal as received, detect any anomalies or out-of-tolerance conditions (see The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems”), and relay this information so that any necessary corrective action can be taken (e. g., to maintain TDs within specified tolerances). Although Loran-C transmitters incorporate extremely accurate cesium clocks as standard equipment, these signals need to be synchronized with standard time references. The US Naval Observatory (USNO) supplies this time reference for the various loran chains.

In practice, the user simply reads the observed time differences from the Loran-C receiver display, and converts these TD readings to more commonly-used coordinates,such as latitude and longitude, using special charts or the coordinate converter of the receiver (if so equipped).
The second basic component of the loran system is the receiver (and associated antenna, antenna coupler, and ground). This receives the loran signals and converts these into useful navigational information. Receivers are discussed in Loran-C Receiver Features and Their Use“Understanding Loran Receivers: Features and Functionality”.
The third basic component of the system consists of a set of loran overprinted nautical charts that enable the mariner to convert the time differences into latitude and longitude. Loran-C charts are discussed in Loran-C Charts and Related Information“Loran-C Charts Key Components and Navigation Techniques”. As noted, aviation users work in latitude and longitude terms, so aeronautical charts are not overprinted with loran TDs.
A Brief History of Loran
The first “loranlike” hyperbolic radionavigation system was proposed by R. J. Dippy in 1937, and later implemented as the British Gee system in early 1942 (Pierce and Woodward 1971, Pierce 1989, Watson-Watt 1957, Johnson 1978, Webster and Frankland 1961). Gee was a hyperbolic system operated at frequencies from 30 MHz to 80 MHz consisting of master and “slave” transmitters located approximately 100 miles apart. The choice of the frequency simplified the problem of dealing with the irregular variation of radio signal propagation, but limited the system to nearly a “line-of-sight” basis.
There is some bending of radio waves, so the distance to the radio horizon is slightly greater than the distance to the visual horizon. This limitation was of lesser consequence to Gee, because Gee was intended as a system to assist bomber navigation in World War II. Obviously a line-of-sight constraint would severely limit the range of a marine navigation system.

The same principles of hyperbolic radionavigation systems were also recognized in the, United States, where a far-reaching project was initiated at the MIT Radiation Laboratory. John Alvin Pierce is generally credited as being the “father of loran” (and, for that matter, Omega) in the United States. Credit for the name “loran” apparently goes to LCDR L. M. Harding, USCG, who coined the name in response to security concerns that the name of the so-called “LRN Project” (as it was known at the time) was too obvious. Development of loran in the United States proceeded rapidly, spurred by urgent wartime needs, and soon (1943) a chain of transmitters (later called the standard loran or the Loran-A system) was being operated by the USCG. By the close of the war, at least 75 000 receivers had been distributed, as well as 2,5 million loran charts. Some 70 transmitters were in operation offering nighttime coverage over 30 % of the earth’s surface.
Loran-A operated at frequencies between 1 850 and 1 950 kHz, and featured groundwave coverage (see The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems”) of some 400 to 800 miles from the transmitting stations during the daytime, and skywave coverage at greater distances, as much as 1 400 NM at night. The accuracy of Loran-A fixes was approximately 1 NM for groundwave reception (explained in The Loran-C System: A More Detailed View“Understanding Loran Transmitters and Hyperbolic Systems”) and as poor as 6 NM for skywave reception. Loran-A continued to operate after the war, serving both military and civilian users, as researchers sought to develop more reliable and accurate systems. Indeed, Loran-A continued to be operated in the United States until 1980, when this system was finally phasedout in favor of Loran-C. Some stations in a Canadian Loran-A chain continued operation until 1983 and the system is still in operation in Japan as of this writing.
Although the accuracy figures presented above can reasonably be thought of as typical for the Loran-A system, it is interesting to note that, with care, more accurate results could be obtained (Pierceand Woodward 1971,Pierce 1989). As an historical aside, it is interesting to mention that during the closing days of World War II, Pierce was in Bermuda taking field measurements. He noted (Pierce and Woodward 1971) that “…the measurements showed a probable error in the position of the receiver of only 130 feet, although it must be admitted that the indicated position was about 1 200 feet from the [charted] position.” Pierce later continued (1989):
It was disappointing that the average error was about a quarter of a mile, and that there was no time to determine what might have caused so big an error. I was, however, somewhat relieved a year or two later, when the Hydrographic Office decided to “move” Bermuda about 1 200 ft on its charts. I never cared to inquire in what direction they moved it.
Postwar research sponsored by DOD and other agencies was directed at developing a more accurate and longer range version of loran. Various improvements, with names such as Loran-B, Cyclan, Cytac, ultimately culminated in the creation of the Loran-C system, which was made operational in 1957 and placed under USCG control in 1958. Loran-C offered greater accuracy and longer range than Loran-A. Nonetheless, both loran systems were operated in parallel for many years to ease the financial burdens on mariners equipped with Loran-A receivers. By 1974, however, the decision was made to phase out the Loran-A system, and to designate Loran-C the primary navigation system for Alaska and the Coastal Confluence Zone of the United States.
As of this writing, the USCG operates 49 Loran-C stations worldwide (including those in Italy, Japan, Spain, and Turkey) and yet other loran chains are operated by several other countries of the world, including China (Guoqiang, 1991), the Soviet Union (Funtikov, 1991), South Korea, Germany, Egypt, France, Denmark, Norway, Iceland, Canada, and Saudi Arabia. Additional loran chains are being considered to extend coverage to other areas in Europe, India, South Africa, and off the northerncoast of South America.