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Explains the Omega global radio navigation system, which was used by the US Navy and six other nations from 1971 to 1997, when it was shut down because it had been replaced by Global Positioning Satellites (GPS).
US Navy Training Film MN-10782A.
NEW VERSION with improved video & sound:
Public domain film from the US Navy, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).
OMEGA was the first truly global-range radio navigation system, operated by the United States in cooperation with six partner nations. It enabled ships and aircraft to determine their position by receiving very low frequency (VLF) radio signals in the range 10 to 14 kHz, transmitted by a network of fixed terrestrial radio beacons, using a receiver unit. It became operational around 1971 and was shut down in 1997 in favour of the Global Positioning Satellite system.
History
Previous systems
Taking a “fix” in any navigation system requires the determination of two measurements. Typically these are taken in relation to fixed objects like prominent landmarks or the known location of radio transmission towers…
The introduction of radio systems during the 20th century dramatically increased the distances over which measurements could be taken…
The first of these hyperbolic navigation systems was the UK’S Gee and Decca, followed by the US LORAN and LORAN-C systems. LORAN-C offered accurate navigation at distances over 1,000 kilometres, and by locating “chains” of stations around the world, they offered moderately widespread coverage.
Atomic clocks
Key to the operation of the hyperbolic system was the use of one transmitter to broadcast the “master” signal, which was used by the “secondaries” as their trigger. This limited the maximum range over which the system could operate…
The problem of synchronizing very distant stations was solved with the introduction of the atomic clock in the 1950s, which became commercially available in portable form by the 1960s…
Omega
By this time the Loran-C and Decca Navigator systems were dominant in the medium-range roles, and short-range was well served by VOR and DME. The expense of the clocks, lack of need, and the limited accuracy of a long wave system eliminated the need for such a system for many roles.
However, the United States Navy had a distinct need for just such a system, as they were in the process of introducing the TRANSIT satellite navigation system. TRANSIT was designed to allow measurements of location at any point on the planet, with enough accuracy to act as a reference for an inertial navigation system (INS). Periodic fixes re-set the INS, which could then be used for navigation over longer periods of time and distances.
TRANSIT had the distinct disadvantage that it generated two possible locations for any given measurements… Loran offered enough accuracy to resolve the fix, but did not have global scope of TRANSIT. This produced the need for a new system with global coverage and accuracy on the order of a few kilometres. The combination of TRANSIT and the new OMEGA produced a highly accurate global navigation system.
OMEGA
Omega was approved for development in 1968 with eight transmitters and the ability to achieve a four mile (6 km) accuracy when fixing a position. Each Omega station transmitted a sequence of three very low frequency (VLF) signals (10.2 kHz, 13.6 kHz, 11.333… kHz in that order) plus a fourth frequency which was unique to each of the eight stations. The duration of each pulse (ranging from 0.9 to 1.2 seconds, with 0.2 second blank intervals between each pulse) differed in a fixed pattern, and repeated every ten seconds; the 10-second pattern was common to all 8 stations and synchronized with the carrier phase angle, which itself was synchronized with the local master atomic clock. The pulses within each 10-second group were identified by the first 8 letters of the alphabet within Omega publications of the time.
The envelope of the individual pulses could be used to establish a receiver’s internal timing within the 10-second pattern. However, it was the phase of the received signals within each pulse that was used to determine the transit time from transmitter to receiver. Using hyperbolic geometry and radionavigation principles, a position fix with an accuracy on the order of 5–10 kilometres (3.1–6.2 mi) was realizable over the entire globe at any time of the day…
