Loran-C

Loran-C is a hyperbolic radio navigation system that allows a receiver to determine its position by listening to low frequency radio signals that are transmitted by fixed land-based radio beacons. Loran-C combined two different techniques to provide a signal that was both long-range and highly accurate, features that had been incompatible. Its disadvantage was the expense of the equipment needed to interpret the signals, which meant that Loran-C was used primarily by militaries after it was introduced in 1957.
By the 1970s, the cost, weight and size of electronics needed to implement Loran-C had been dramatically reduced because of the introduction of solid-state electronics and, from the mid-1970s, early microcontrollers to process the signal. Low-cost and easy-to-use Loran-C units became common from the late 1970s, especially in the early 1980s, and the earlier LORAN system was discontinued in favor of installing more Loran-C stations around the world. Loran-C became one of the most common and widely-used navigation systems for large areas of North America, Europe, Japan and the entire Atlantic and Pacific areas. The Soviet Union operated a nearly identical system, CHAYKA.
The introduction of civilian satellite navigation in the 1990s led to a rapid drop-off in Loran-C use. Discussions about the future of Loran-C began in the 1990s; several turn-off dates were announced and then cancelled. In 2010, the US and Canadian systems were shut down, along with Loran-C/CHAYKA stations that were shared with Russia. Several other chains remained active; some were upgraded for continued use. At the end of 2015, navigation chains in most of Europe were turned off.
In December 2015 there was also renewed discussion of funding an eLoran system, and NIST offered to fund development of a microchip-sized eLoran receiver for distribution of timing signals. The National Timing Resilience and Security Act of 2017, proposed resurrecting Loran as a backup for the United States in case of a GPS outage caused by space weather or attack.

History

Loran-A

The original LORAN was proposed in 1940 by Alfred Lee Loomis at a meeting of the U.S. Army's Microwave Committee. The Army Air Corps were interested in the concept for aircraft navigation, and after some discussion they returned a requirement for a system offering accuracy of about at a range of, and a maximum range as great as for high-flying aircraft. The Microwave Committee, by this time organized into what would become the MIT Radiation Laboratory, took up development as Project 3. During the initial meetings, a member of the UK liaison team, Taffy Bowen, mentioned that he was aware the British were also working on a similar concept, but had no information on its performance.
The development team, led by Loomis, made rapid progress on the transmitter design and tested several systems during 1940 before settling on a 3 MHz design. Extensive signal-strength measurements were made by mounting a conventional radio receiver in a station wagon and driving around the eastern states. However, the custom receiver design and its associated cathode-ray tube displays proved to be a bigger problem. In spite of several efforts to design around the problem, instability in the display prevented accurate measurements as the output shifted back and forth on the face of the oscilloscope.
By this time the team had become much more familiar with the British Gee system, and were aware of their related work on "strobes", a time base generator that produced well-positioned "pips" on the display that could be used for accurate measurement. This meant that inaccuracy of the positioning on the display had no effect: any inaccuracy in the position of the signal was also in the strobe, so the two remained aligned. The Project 3 team met with the Gee team in 1941, and immediately adopted this solution. This meeting also revealed that Project 3 and Gee called for almost identical systems, with similar performance, range and accuracy, but Gee had already completed basic development and was entering into initial production, making Project 3 superfluous.
In response, the Project 3 team told the Army Air Force to adopt Gee, and instead, at the behest of the British team, realigned their efforts to provide long-range navigation on the oceans where Gee was not useful. This led to United States Navy interest, and a series of experiments quickly demonstrated that systems using the basic Gee concept, but operating at a lower frequency around 2 MHz would offer reasonable accuracy on the order of a few miles over distances on the order of, at least at night when signals of this frequency range were able to skip off the ionosphere. Rapid development followed, and a system covering the western Atlantic was operational in 1943. Additional stations followed, first covering the European side of the Atlantic, and then a large expansion in the Pacific. By the end of the war, there were 72 operational LORAN stations and as many as 75,000 receivers.
In 1958 the operation of the LORAN system was handed over to the United States Coast Guard, which renamed the system "Loran-A", the lower-case name being introduced at that time.

LF LORAN

There are two ways to implement the timing measurements needed for a hyperbolic navigation system, pulse timing systems like Gee and LORAN, and phase-timing systems like the Decca Navigator System.
The former requires sharp pulses of signal, and their accuracy is generally limited to how rapidly the pulses can be turned on and off, which is a function of the carrier frequency. There is an ambiguity in the signal; the same measurements can be valid at two locations relative to the broadcasters, but in normal operation, they are hundreds of kilometres apart, so one possibility can be eliminated.
The second system uses constant signals and takes measurements by comparing the phase of two signals. This system is easy to use even at very low frequencies. However, its signal is ambiguous over the distance of a wavelength, meaning there are hundreds of locations that will return the same signal. Decca referred to these ambiguous locations as cells. This demands some other navigation method to be used in conjunction to pick which cell the receiver is within, and then using the phase measurements to place the receiver accurately within the cell.
Numerous efforts were made to provide some sort of secondary low-accuracy system that could be used with a phase-comparison system like Decca in order to resolve the ambiguity. Among the many methods was a directional broadcast system known as POPI, and a variety of systems combining pulse-timing for low-accuracy navigation and then using phase-comparison for fine adjustment. Decca themselves had set aside one frequency, "9f", for testing this combined-signal concept, but did not have the chance to do so until much later. Similar concepts were also used in the experimental Navarho system in the United States.
It was known from the start of the LORAN project that the same CRT displays that showed the LORAN pulses could, when suitably magnified, also show the individual waves of the intermediate frequency. This meant that pulse-matching could be used to get a rough fix, and then the operator could gain additional timing accuracy by lining up the individual waves within the pulse, like Decca. This could either be used to greatly increase the accuracy of LORAN, or alternately, offer similar accuracy using much lower carrier frequencies, and thus greatly extend the effective range. This would require the transmitter stations to be synchronized both in time and phase, but much of this problem had already been solved by Decca engineers.
The long-range option was of considerable interest to the Coast Guard, who set up an experimental system known as LF LORAN in 1945. This operated at much lower frequencies than the original LORAN, at 180 kHz, and required very long balloon-borne antennas. Testing was carried out throughout the year, including several long-distance flights as far as Brazil. The experimental system was then sent to Canada where it was used during Operation Muskox in the Arctic. Accuracy was found to be at, a significant advance over LORAN. With the ending of Muskox, it was decided to keep the system running under what became known as "Operation Musk Calf", run by a group consisting of the United States Air Force, Royal Canadian Air Force, Royal Canadian Navy and the UK Royal Corps of Signals. The system ran until September 1947.
This led to another major test series, this time by the newly-formed United States Air Force, known as Operation Beetle. Beetle was located in the far north, on the Canada-Alaska border, and used new guy-stayed steel towers, replacing the earlier system's balloon-lofted cable antennas. The system became operational in 1948 and ran for two years until February 1950. Unfortunately, the stations proved poorly sited, as the radio transmission over the permafrost was much shorter than expected and synchronization of the signals between the stations using ground waves proved impossible. The tests also showed that the system was extremely difficult to use in practice; it was easy for the operator to select the wrong sections of the waveforms on their display, leading to significant real-world inaccuracy.

CYCLAN and Whyn

In 1946 the Rome Air Development Center sent out contracts for longer-ranged and more-accurate navigation systems that would be used for long-range bombing navigation. As the United States Army Air Forces were moving towards smaller crews, only three in the Boeing B-47 Stratojet for instance, a high degree of automation was desired. Two contracts were accepted; Sperry Gyroscope proposed the CYCLAN system which was broadly similar to LF LORAN but with additional automation, and Sylvania proposed Whyn using continuous wave navigation like Decca, but with additional coding using frequency modulation. In spite of great efforts, Whyn could never be made to work, and was abandoned.
CYCLAN operated by sending the same LF LORAN-like signals on two frequencies, LF LORAN's 180 kHz and again on 200 kHz. The associated equipment would look for a rising amplitude that indicated the start of the signal pulse, and then use sampling gates to extract the carrier phase. Using two receivers solved the problem of mis-aligning the pulses, because the phases would only align properly between the two copies of the signal when the same pulses were being compared. None of this was trivial; using the era's tube-based electronics, the experimental CYCLAN system filled much of a semi-trailer.
CYCLAN proved highly successful, so much so that it became increasingly clear that the problems that led the engineers to use two frequencies were simply not as bad as expected. It appeared that a system using a single frequency would work just as well, given the right electronics. This was especially good news, as the 200 kHz frequency was interfering with existing broadcasts, and had to be moved to 160 kHz during testing.
Through this period the issue of radio spectrum use was becoming a major concern, and had led to international efforts to decide on a frequency band suitable for long-range navigation. This process eventually settled on the band from 90 to 100 kHz. CYCLAN appeared to suggest that accuracy at even lower frequencies was not a problem, and the only real concern was the expense of the equipment involved.