LORAN-C HISTORY
The Pacific war showed the need for a Loran-like system that could be operated over much greater distances in daylight than Loran-A could provide. There were few islands on which transmitters could be located and they were great distances apart. The only potential solution was to try Loran techniques at low frequencies so an experimental set of three Loran transmitters operating at 180 kHz was set up on the US East Coast during 1945 using balloon supported aerials. The main result of these tests was to show that pulse envelope matching, as used in Loran-A, was too inaccurate with the long pulses necessary at these low frequencies, and that phase comparison would he required.
The experiments were not followed up and the system, called LF Loran at that time, was dismantled following the end of the war. The MIT Radiation Laboratory, which had sponsored the work, was also closed and responsibility for further work given to the new US Air Force. The transmitters that had been used for LF Loran were re-installed in Alaska for trials of LF propagation in Arctic areas and the experimentation yielded much useful data. According to the book "Sixty Years: the RCAF and CF Air Command 1924-84 edited by Larry Milberry, three station sites were established. Kittigazuit, N.W.T was the master. Slaves were close to Barrow, Alaska and Cambridge Bay, N.W.T. The chain was built in 1947, but was shut down in March of 1950. It operated at 180 kHz but "it became evident that attenuation of the ground wave over the permafrost and certain sea ice conditions was much more severe than predicted. ... interaction between the groundwave and the first hop skywave created severe pulse matching problems''. The station at Kittigazuit, NWT was called "Yellow Beetle".

In 1946, the Sperry company proposed a navigation system called Cyclan which would use phase comparison and operate at two frequencies of 180 and 200 kHz, the difference between them being used to resolve ambiguities. It was tested by the USAF in 1948 using 160 and 180 kHz and later reduced to one frequency and renamed Cytac for possible use as a military tactical navaid.

After further tests in 1951, the U.S. Air Force decided to concentrate on inertial and Doppler systems for tactical use and stopped development. Parallel development of a system known as Navarho had also been proceeding, a system that had been derived from the British POPI system. Navarho was a long range system providing both range and bearing, obtaining range by measuring the change of phase between the transmitted signal and a local very high stability reference oscillator - the first time this had been attempted. The state of the art for portable oscillators left too much to be desired so the project was abandoned. Also scrubbed, was a further development known as Navaglobe - a system intended to give wide area coverage.

The US Navy began to take an interest a few years later, and recommissioned the three original Cytac transmitters at Forestport, NY (later used for experimental Omega transmissions), Carolina Beach, NC, and Carabelle, Florida, for a trial abroad the USCG cutter Androscoggin in April 1956. The transmissions were phase coherent 100 kHz pulses 100 microseconds long at peak powers of 60 kW except from Forestport, which had a 1,280 foot tower and radiated an estimated 200 kW. These pulses were less than half the duration later adopted for the Loran-C system (240 microseconds) and rose to a maximum amplitude in only 25 microseconds as compared with the later 60 microseconds. The results showed a daytime ground wave range of 2,250 miles, nighttime ground wave of 1,650 miles and skywaves out to 3,000 miles. Time difference accuracy was estimated at 0.15 microseconds.

Thus encouraged, the US Navy established transmitters in the north-eastern Atlantic and the Mediterranean during 1957, followed by many others in the Pacific and elsewhere, naming the system Loran-C. Initial contracts for receivers were subcontracted  to, amongst others, Decca Navigator, who produced the AN/SPN, probably the most successful of the early Loran-C receivers. It had 52 controls and weighed over 100 pounds. Although initiated as a marine system, Loran-C came into use for aerial navigation quite widely and during trials in 1963, was flown at over Mach 1 in a British Vulcan aircraft. A chain was installed in Vietnam in the 1960's specifically for the use of USAF aircraft. Loran-C was installed in many long range civil aircraft while inertial systems were being proven and is still used in long range military aircraft. The Decca company sued the US Navy in 1969, alleging that Loran-C infringed patents it held concerning a 100 kHz pulsed navigation system. As early as 1944, Decca had deliberately not used the 7F (98 kHz) frequency of the Navigator system because of its possible use for this pulse system. The claim was upheld at first but reversed on appeal, with the US Navy pleading military necessity.



Contributor: Gil McElroy <gmcelroy(at)eagle.ca>
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Mar 21/04