LORAN
LORAN, in its original form, was an expensive system to implement, requiring a cathode ray tube (CRT) display.
The U.S. 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 U.S. Air Force worked on a different concept, Cyclan, which the Navy took over as Loran-C, which offered longer range than LORAN and accuracy of hundreds of feet.
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.
[3] The "National Timing Resilience and Security Act" of 2017, proposed repurposing LORAN real estate and radio spectrum for a new terrestrial navigation system as a backup for the United States in case of a GPS outage caused by space weather or attack.
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".
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.
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.
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.
The decision was made to abandon the current efforts,[11] use Gee on their own aircraft, and re-develop their system for the long-range role instead.
By the end of that year additional stations had been installed in Greenland, Iceland, the Faroe Islands and the Hebrides, offering continuous coverage across the North Atlantic.
RAF Coastal Command had another station installed in Shetland, offering coverage over Norway, a major staging ground for German U-boats and capital ships.
In particular, the accuracy offered by LORAN allowed aircraft to reduce the amount of extra fuel they would otherwise have to carry to ensure they could find their base after a long mission.
Chains were also installed in China, prior to the ultimate end of the Chinese Communist Revolution, and these stations remained on the air at least into the 1990s.
[3] During early experiments with LORAN's skywaves, Jack Pierce noticed that at night the reflective layer in the ionosphere was quite stable.
This led to the possibility that two LORAN stations could be synchronized using skywave signals, at least at night, allowing them to be separated over much greater distances.
This led to a second round of tests in late 1943, this time using four stations, Montauk, East Brewster, Massachusetts, Gooseberry Falls, Minnesota,[17] and Key West, Florida.
Ideally, perfectly formed rectangular blips would be displayed on the CRT, whose leading edge could be compared with a high degree of accuracy.
In practice, the transmitters cannot turn on and off instantly, and due to a variety of factors the resulting blips are spread out in time, forming an envelope.
The trick to such a system is to ensure the primary and secondary stations are phase-coherent, a complex proposition during World War II.
The experiments demonstrated that the inaccuracy inherent to the design while working at such low frequencies was simply too great to be useful; operational factors introduced errors that overwhelmed the capabilities.
Nevertheless, the three transmitters were re-installed in northern Canada and Alaska for experiments in polar navigation, and ran for three years until shutting down again in March 1950.
Raytheon won a contract to develop a system called "Cytac", which used the same basic techniques as LF LORAN, but included considerable automation to handle the timing internally without operator intervention.
As the mission changed from short-range tactical bombing to over-the-pole nuclear delivery, the (newly formed) U.S. Air Force lost interest in the concept.
[24] In 1953 the Navy took over the Cyclan system and began a wide series of studies ranging as far away as Brazil, demonstrating accuracy to about 100 meters (330 ft).
At night, as many as thirty different skywaves might be received from a single transmitter, often overlapped in time, creating a complex return pattern.
Accordingly, the LORAN operator set the delays so the primary signal appeared on one trace and the secondary on the second, allowing the complex patterns to be compared.
Turning to sweep speed 4 did not change the timing, but instead superimposed the signals on a single trace so final tuning could take place, using the gain and amplifier balance controls.
[3] During the day the ionosphere only weakly reflects shortwave signals, and LORAN was usable at 500–700 nautical miles (930–1,300 km) using the groundwaves.
When two such signals from a single chain are considered, the resulting pattern of lines becomes increasingly parallel as the baseline distance becomes smaller in comparison to the range.
