LORAN (Long Range Navigation) was a hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee system, but operated at lower frequencies in order to provide an improved range up to 1,500 miles (2,400 km) with an accuracy of tens of miles. It was first used for ship convoys crossing the Atlantic Ocean, and then by long-range patrol aircraft, but found its main use on the ships and aircraft operating in the Pacific theater during World War II. LORAN, in its original form, was an expensive system to implement, requiring a cathode ray tube (CRT) display and a well-trained operator. This limited use to the military and large commercial users. Automated receivers became available in the 1950s, but the same improved electronics also opened the possibility of new systems with higher accuracy. The United States Navy began development of Loran-B, which offered accuracy on the order of a few tens of feet, but ran into significant technical problems. The United States Air Force worked on a different concept, Cyclan, which offered longer range than LORAN and accuracy of hundreds of feet. When the Air Force turned their attention to inertial navigation systems, the Navy took over Cyclan and renamed it Loran-C. The United States Coast Guard took over operations of both LORAN systems in 1958. Despite the dramatically improved performance of Loran-C, LORAN, now known as Loran-A (or "Standard LORAN"), would become much more popular during this period. This was due largely to the large numbers of surplus Loran-A units released from the Navy as ships and aircraft replaced their sets with Loran-C. The widespread introduction of inexpensive microelectronics during the 1960s caused Loran-C receivers to drop in price dramatically, and Loran-A use began to rapidly decline. The Loran-A transmitter network was slowly dismantled starting in the 1970s; it remained active in North America until 1980 and the rest of the world until 1985. A Japanese chain remained on the air until May 9, 1997, and a Chinese chain was still listed as active as of 2000. Loran-A used two frequency bands, at 1.85 and 1.95 MHz. These same frequencies in the 160-meter band were used by amateur radio operators, and they were under strict rules to operate at reduced power levels to avoid interference; depending on their location and distance to the shore, U.S. operators were limited to maximums of 200 to 500 watts during the day and 50 to 200 watts at night.
History
Project 3 At a 1 October 1940 meeting of the U.S. Army Signal Corps' Technical Committee, Alfred Loomis, chair of the Microwave Committee of the National Defense Research Committee, proposed building a hyperbolic navigation system. He predicted that such a system could provide an accuracy of 1,000 feet (300 m) or better at a range of 200 miles (320 km), and a maximum range of 300–500 miles (480–800 km) for high-flying aircraft. This led to the "Precision Navigational Equipment for Guiding Airplanes" specification, which was sent back to the Microwave Committee and formed up as "Project 3". Orders for initial systems were sent out at a follow-up meeting on 20 December 1940. Edward George Bowen, developer of the first airborne radar systems, was also at the 20 December meeting. He stated that he was aware of similar work in the UK, but didn't know enough about it to offer any suggestions. Project 3 moved to the newly-formed MIT Radiation Laboratory's Navigation Group in 1941. Early systems operated around 30 MHz, but it was later decided to try experiments with different equipment that could be tuned from 3 to 8 MHz. These lower frequency systems were found to be much more stable electronically. After first considering setting up transmitters on mountain peaks, the team instead settled on two abandoned Coast Guard stations at Montauk Point, New York, and Fenwick Island, Delaware. On the receiving end, a station wagon was fitted with a simple receiver and sent around the country looking for solid signals, which were found as far away as Springfield, Missouri. For a production system, the team began working with a system using a circular J-scope display for improved accuracy. The more common A-scope represents distances across the diameter of the tube, while the J-scope presents this as the angle around the cathode ray tube's face. This increases the amount of room on the scale by a factor of π for any given display size, improving accuracy. In spite of using the J-scope, and adopting the lower frequency change for more stability, the team found accurate measurements of range quite difficult. At the time, the procedure for generating sharp pulses of signals was in its infancy, and their signals were considerably spread out in time, making measurements difficult. By this time the team had become aware of the UK's Gee efforts, and were aware that Gee used a system of electronically generated strobes that produced pips on the display that were accurately aligned with system timing. They sent a team to the UK to learn about the strobe concept, and immediately adopted it for their work. As part of this exchange, the Project 3 team also found that Gee was almost identical to their own system in concept and desired performance. Unlike their system, Gee had largely completed development and was proceeding to production. The decision was made to abandon the current efforts, use Gee on their own aircraft, and re-develop their system for the long-range role instead.
LORAN
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