View source for Loran-C

===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]].{{sfn|Blanchard|1991|pp=302-303}}

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 [[Center frequency|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.{{sfn|Blanchard|1991|pp=302-303}}

The second system uses constant signals ("continuous wave") 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.{{sfn|Blanchard|1991|pp=302-303}}

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 [[Post Office Position Indicator|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.{{sfn|Blanchard|1991|p=302}}

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.{{sfn|Blanchard|1991|pp=302-303}}

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&nbsp;[[Hertz|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 {{convert|150|feet}} at {{convert|750|mi}}, 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.{{sfn|Hefley|1972|p=16}}

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 {{convert|625|ft|m}} 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 wave]]s 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.{{sfn|Hefley|1972|p=16}}