Omega: A Navigator's Dream

Abstract

Transoceanic navigators have at their disposal today a variety of sophisticated electronic and inertial aids for accurate position determination-if their needs are urgent and their purses are full. Polaris-missile submarines, for example, are provided with three inertial navigation systems, regularly updated by inputs from a periscope-installed automatic star-fixer, a Navy Navigational Satellite System receiver and other radio-directional aids. New commercial air transports, such as the Boeing 747, carry three inertial systems for long-range navigation. But for a Polaris sub the cost is in the millions and for a 747 the investment exceeds $300,000. Commercial receivers are available for ocean-crossing vessels to use with the Navy Navsat System; these too are expensive. Such ships do carry other navigational aids-radar, gyro and electronic compasses and Loran-but for accurate navigation the limit for electronics is about 1,000 nautical miles from shore. Reliance thereafter is on the compass, the sextant and dead reckoning for up to another 1,000 nautical miles before electronic aids can again be used. To fleet owners, even small errors in course are costly. A different approach that significantly lowers user costs, the Omega very low-frequency (VLF) radio navigation system, will come into full global use sometime in 1973. Under development since the early 1950's, the system will use eight transmitters to girdle the world with low-frequency navigation signals that can be used to help guide all aircraft and ships, civilian and military, of all nations. The effort is directed by the Omega Project Office, Naval Electronic Systems Command, in Alexandria, Va. Although the Omega concept seems simple, implementing the system has required much supporting study and experimentation; many consider it one of the most thoroughly researched projects ever carried out. At present, military and commercial marine receivers are operational, but only a part of the transmitter-station network is built and it is operating at below design power on what the Navy terms an interim basis. Receivers for aircraft use are only now at the technical evaluation stage and will not complete operational testing until next January. The Omega concept can be traced back to 1947 when Dr. J. A. Pierce of Harvard University's Cruft Laboratory recommended changes to extend the range and reliability of Loran, a radio navigation system developed by the Massachusetts Institute of Technology during World War II and still in use. Loran A, the first, operated in the frequency range of two megahertz and had a 100-nautical-mile base line; its stuccessors, Loran C and D operate at about 100 kilohertz and reach out from 150 to 1,000 nautical miles. By very precisely controlling and synchronizing the frequency and time of shore transmissions, a series of electromagnetic bands or lanes are formed every half wavelength between stations by signal cancellation. Loran receivers measure the time differences between the arrival of two signals from stations of known coordinates and thus fix a position on a hyperbolic grid. Dr. Pierce suggested a study of VLF propagation and measurement of a phase-difference rather than time-difference for possible use in a new Lorantype system. Low-frequency signals, he reasoned, offer inherently longer ranges and are relatively insensitive to electromagnetic interference. Also, VLF Signals penetrate seawater from 40 to 50 feet and thus could be received by submerged submarines on station. Research continued for 10 years, first using a single-frequency system called Radux at 50 kilohertz and below from stations on Hawaii and the West Coast and then a two-frequency system called Radux-Omega at 10 and 40 kilohertz. Optimum frequencies were narrowed to the 10-14 kilohertz band by 1957 and the program became Omega. Today's system calls for the installation of eight transmitter stations roughly 5,000 nautical miles apart around the world. Each transmits three frequencies (10.2, 11.333 and 13.6 kilohertz) sequentially for 10 seconds at precise times, pulse lengths and intervals for identification. The operational stations will radiate a 10-kilowatt signal to 'a range of 8,000 nautical miles. (Higher power would increase the range but the system might become self-interfering.) The four interim-operation transmitting stations are located at Bratland, Norway; Port of Spain, Trinidad; Haiku, Hawaii, and Forestport, N.Y. Westinghouse Electric performed much of the early site surveys, selection and testing for the Navy on the three American-operated stations and also provided their large AN/ FRA-3 1 transmitters. The New York site is temporary and soon will be moved to one now under construction at La Moure, N.D., due to be operational in 1971, according to Omega project manager Capt. James A. Burke. The Norwegian station was built and is operated by that country. Similarly, the other four stations, it is hoped, will be built and operated by cooperating governments. And therein lies the reason for some of the delay Westinghouse