OMEGA
By Jerry Proc VE3FAB

INTRODUCTION

The OMEGA radionavigation system, developed by the United States Navy for military aviation users, was approved for full implementation in 1968 and promised a true worldwide oceanic coverage capability and the ability to achieve a four mile accuracy when fixing a position. Initially, the system was to be used for navigating nuclear bombers across the North Pole to Russia.  Later, it was found useful for submarines.

When the eight station chain became operational, day to day operations were managed by the United States Coast Guard  in partnership with Argentina, Norway, Liberia, France, Japan and Australia. Coast Guard personnel operated two U.S. stations - one in LaMoure, North Dakota and the other in Haiku, Hawaii. OMEGA employed hyperbolic radionavigation techniques and the chain operated in the VLF portion of the spectrum between 10 to 14 kHz. Near its end,  it evolved into a system used primarily by the civil community. By receiving signals from three stations, am Omega receiver could locate a position to within 4 nm using the principle of phase comparison of signals. In the Royal Canadian Navy, the OMEGA system was used in the AOR, 280 and Halifax class ships.

HISTORY

John Alvin Pierce, the "Father of Omega," first proposed the use of continuous wave modulation of VLF signals for navigation purposes in the 1940's. Working at the Radiation Laboratory at the Massachusetts Institute of Technology, he proved the viability of measuring the phase difference of radio signals to compute a location solution. Pierce originally called this system RADUX. After experimenting with various frequencies, he  settled on a phase stable, 10 kHz transmission in the 1950's. Thinking this frequency was the far end of the radio spectrum   Pierce dubbed the transmission "Omega," for the last letter of the Greek alphabet.

Radux-Omega showed the possibilities of very-low-frequency propagation, but there were fears about ambiguity errors if a single low frequency were used on its own. In the 1950's two new factors appeared - the inertial navigation system (INS) and the great increase in electronic system reliability following the introduction of the transistor. INS was not all that accurate, particularly in ships, where it had to run for days on end without correction, but it could certainly carry over short losses of signal and resolve any cycle slippage that might have occurred, while better reliability meant that such outages were far less likely anyway.

Thus, ambiguities might be much less of a problem than thought, and the development of a single frequency system began to seem feasible. The 40 kHz of Radux was dropped and a new system using transmitters in California and Hawaii was set up, transmitting at 12.5 kHz. They provided good results and two further transmitters were added in Panama and the Post Office station at Criggion, North Wales. All these stations ran on their own time standards, the development by Dr L. Essen of the National Physical Laboratory. This new type of extremely stable crystal oscillator, named after him, made this progress possible. Later, Dr Essen also built the first cesium beam atomic resonator.

These experiments continued throughout the 1950's and provided a great deal of data on propagation characteristics. Nothing that was found discouraged the idea of a navaid operating at low frequencies. In 1963, an Omega Implementation Committee (OIC) was formed chaired by Prof. Pierce and consisting of most of those who had been concerned with the earlier experiments. They were charged with designing the new navaid and, on the basis of their experiments, took the decisions about how Omega would work - the choice of frequencies, location of transmitters, power levels, etc. Originally it was calculated that a 10 KW power level from each transmitter would prove more than sufficient for reliable reception. Due to the high cost of constructing VLF antennas (Omega antenna towers were more than 1,200 feet in height), the first experimental transmissions were actually existing VLF communications stations that were modified for Omega transmissions. This committee always denied later that the Decca work on Delrac, disclosed 9 years earlier, had had any effect on their deliberations, but it was interesting that they chose identical frequencies and other characteristics.

Over 31 possible transmitting sites were considered. Eventually, eight locations were established as permanent transmitting stations. The Bratland, Norway station (near the Arctic Circle) and the Haiku Valley station on Oahu, Hawaii, originally experimental stations, were among the first in the system. In 1968, the U.S. Navy authorized full scale implementation of the Omega System based on the OIC report. Responsibility for the operation was transferred from the U.S.Navy to the U.S. Coast Guard in 1971, under the terms of title 14, USC 82. The Coast Guard created a new command, the Omega Navigation System Operations Detail (ONSOD) to operate the system. ONSOD control of the synchronization of the system was perfected while the Navy Project Office finished the task of constructing the stations. As construction of the final six stations proceeded through the 1970's, ONSOD assumed the duties of engineering maintenance for those stations as they were declared operational. Eventually, eight permanent stations located in Bratland, Norway; Paynesville, Liberia; Kaneohe, Hawaii, US; La Moure, North Dakota, US; Plaine Chabrier, La Reunion, France (Indian Ocean); Golfo Nuevo, Chubut, Argentina; Woodside, Victoria, Australia; and Shushi-Wan, Tsushima Island, Japan were completed.

Separate bilateral agreements were negotiated between the U.S. and the six partner nations. ONSOD, later the Omega Navigation System Center (ONCEN), was named the Operational Commander (OPCON) with each partner nation maintaining responsibility for administrative control (ADCON). The U.S. owned and maintained all the Omega related equipment at each station. The host nation provided personnel, funding and non-Omega support for the station. Partner nation crews came from military and civilian sources. The Argentine and French stations were crewed by both military and civilian members of their respective Navies; the Japanese station was crewed by uniformed members of the Japanese Maritime Safety Agency, while the Australian station were staffed by civilian employees of the Maritime Safety Agency (equivalents of the U.S. Coast Guard); and the Liberian and Norwegian stations were crewed by civilian government employees. It took a tremendous effort, on the part of Team Coast Guard, to provide the system with world class support. The organizations involved in this unique international system included Commandant (G-OPN-3); CG Navigation Center (NAVCEN), the current  OPCON; Engineering Logistics Center (ELC) Baltimore; Electronics Engineering Center (EECEN); Civil Engineering Unit (CEU) Cleveland; Civil Engineering Unit (CEU) Honolulu; CG Finance Center (FMCEM, Chesapeake, VA; the Eighth Coast Guard District, New Orleans, LA; and the Fourteenth Coast Guard District, Honolulu, Hawaii.

Before OMEGA could even be inaugurated, it invoked litigation against the United States government as the Decca Navigator Company of London, England had proposed a very similar system many years earlier and called it DELRAC. In 1962, what eventually became the OMEGA system appeared in a U.S. proposal to International Civil Aviation Organization using the title "DELRAC/OMEGA" although it later defaulted to plain OMEGA. The technical similarity between OMEGA and DELRAC was obvious and there was considerable bad feeling at Decca that they had not received due recognition of their much earlier efforts. Decca eventually sued the U.S. Government in 1976 for infringement of DELRAC patents and were awarded $44,000,000 damages. The U.S. never claimed OMEGA was a military navaid in the court case. By then, they didn't really need it for either aircraft or submarines,  having developed inertial navigation systems. It had only implemented OMEGA world-wide by claiming it was a civilian navaid.

It was not the first time Decca had sued the U.S. Government over a navaid - they had done so in 1967 over Loran-C, and won the case there as well. Unfortunately for Decca, the Americans claimed Loran-C was a military system necessary for "national security" and did not have to pay up even though found guilty by a court of law. It's strange that the same argument was not raised in the case of OMEGA.

Omega achieved full eight station implementation in 1983 and was used by several airlines flying long range routes over water as well as by military forces. Towards the end of it's service life, the Omega system was upgraded with new timing and control equipment; Paynesville, Liberia being the last station to be upgraded in the Spring of 1996. Since the original equipment had been designed in the 1960's, certain critical components had become obsolete and could no longer be procured for replacement purposes. With an initial termination date set for the year 2005 or longer, this upgrade program had to be executed to ensure that the system continued full and reliable operation in the short term.

omega_john_ pierce.jpg John (Jack) A. Pierce, who retired from a position as a senior research fellow at Harvard University, Cambridge, Mass. was awarded the Medal For Engineering Excellence in 1990 for the "design , teaching and advocacy of radio propagation, navigation and timing which led to the development of Loran,  Loran C and Omega." In 1941, Pierce began working at the Massachusetts Institute of Technology's Radiation Laboratory which was testing the United States' first hyperbolic radio aid to navigation called Loran. It inaugurated in October 1942. Later work produced Loran C which operated at a lower frequency of 100 kHz. After WWII, he was appointed senior research fellow in applied physics at Harvard and from 1950 to 1974 did work on low frequency navigation aids that lead to Omega.

Among his many awards are a 1948 Presidential Certificate of Merit and the 1953 Morris Liebmann Prize of the Institute of Radio Engineers. He earned a BA in physics from Harvard while an assistant at the University's Cruft Laboratory. (Photo and copy courtesy IEEE Spectrum, August 1990) 

Herbert Rideout, an engineer who worked on the development of long range radionavigation and communications at Pickard & Burns, recalls some of the early research.  "Jack Pierce worked at Cruft Laboratory, Harvard University. Working for the university was considered prestigious , however the wages paid were low, so engineers associated with the radionavigation program usually worked for commercial companies who paid prevailing wages . One of those companies was Pickard & Burns, Needham MA which was under contract with the US Navy. We were closely associated with Jack and were in constant daily contact.  We were able to accomplish whatever Cruft Laboratory could not do such as designing and fabricating prototype equipment. (Draco equipment, described further in this passage, fell into this class).  My direct boss at Pickard & Burns was Dr. Richard H. Woodward, a graduate of Harvard and during WWII,  worked  along side Jack Pierce at the M.I.T. Radiation Laboratory developing Loran. Richard was one of the authors of "Loran" Volume 4 of the Radiation Laboratory Series.  Pickard & Burns was a small company, having about 20 engineers on staff but we did other work besides radionavigation. As an engineer, I occasionally skipped around to other jobs, however Jack liked me and when I went on my trip aboard the vessel USS Compass Island to the Mediterranean,  he said I brought back the best and most accurate data he had ever received, so from then on I belonged to Jack. The Compass Island was a US Navy research vessel stationed at the Navy Yard in New York City.

When the Compass Island departed New York, she was packed several different navigation systems which were being evaluated by the US Navy.  At that time, the Navy was interested in testing out any navigation system that might be suitable for submarines. One of them, from Cornell University, measured gravity. Since the force of gravity is never the same in any two places on earth,  measuring it would permit position to be determined.  A second system, SINS (Ships Inertial Navigation System) was North American Aviation's inertial navigation system. The third system from Reeves Kodak used some type of celestial based system to fix position. Lastly, there was Draco, which was intended to be a worldwide VLF hyperbolic radionavigation system.  It was the brain child of John Pierce with Pickard & Burns supporting him. That system was named Draco after the constellation Draco but I do not know who gave it that name.

During the voyage, a formal Draco test program was followed which would investigate these specific areas:

* Field intensity and noise in the VLF spectrum.
* Draco navigation capability.
* Reception of communication signals with a Draco receiver having a 100 cycle bandwidth1.
* Reception of communication signals with a Draco receiver having a special 20 cycle filter1..
* Reception of special phase shift keyed signals with a Draco receiver.

Two AN/URM-6 (14 kHz - 250 kHz)  field intensity meters, one narrow bandwidth filter for the Draco receiver-indicator. and three magnetic tape recorders had been installed on the Compass Island during the first week in March 1958. An electronic antenna coupler was also installed so two URM-6 units could be attached to the ship's VLF whip antenna. Both of the field intensity meters were calibrated by the Dinger shield injection method and the effective height of the antenna was determined. Once  all the equipment was installed and pretested,  a preliminary cruise was scheduled from March 8 to 11, to check the full operation of the gear. During this trip it was found that the noise in the antenna coupler was too high so the URM-6 equipment was connected directly to the whip antenna thus bypassing the antenna coupler.

Once everything was operating to expectations, the ship departed New York City on 13 March and reached the Mediterranean on 23 March taking up position at 17 degrees East longitude. After cruising for 18 days (including a 3 day stopover at Palma, Spain) and taking measurements,  it was time to depart. On the 9th of April the Compass Island left the Med returning to New York on April 17. The tests were very promising. Field intensity and Draco measurements were recorded using three VLF transmitter stations; NSS (15.5 kHz) at Annapolis, Maryland; NLK (18.6 kHz) at Jim Creek. Washington; and GBZ (19.6 kHz) at Criggion, Wales.

Besides being designed for hyperbolic navigation, Draco was being proposed for use as a secret, one-way communications system for submarines.  It worked like this: VLF transmitters NLK at Jim Creek, Washington State, and NSS Annapolis, MD had their individual frequencies stabilized to very accurate levels -  below  that of one cycle. To an astute observer it seemed they drifted at a random rate. The drift was introduced by using mechanical cams which drove servo motors  which in turn introduced a precise known drift rate of less than one cycle.  At the receiving end, Draco consisted of rack mounted equipment comprising of two receivers, a phase comparator and a stabilized frequency reference all designed by Pickard & Burns. Received VLF signals from NLK and NSS were then fed  into the phase comparator and in turn compared to a highly accurate oscillator. The difference or output representing the drift rate of the VLF transmitted signals was represented by a voltage - the faster the drift the larger the voltage. The output voltage drove servo motors in a mechanical device that in turn drove other servos which gave a DC output voltage corresponding to the drift rate. This voltage was used to drive Esterline Angus chart recorders.

Uncorrected, the line on the chart would go from left to right representing the drift but when mechanical cams were installed which were the reverse of those in the transmitter we would see a  straight line down the center representing zero phase shift in the transmitted signal.  At predetermined  times we would have our engineers at the transmitting sites introduce different drift rates and these would show up at our receive end as a lower or higher chart reading. It was these deviations that were proposed for communications since submarines could receive these signals without surfacing.  In one test, the phase of the signals from Jim Creek were shifted 3 times during a period of 7 minutes to produce the letter 'S' in Morse code. These special transmissions were repeated once an hour for several days during the tests in the Med. Since the Draco equipment responds to shifts of phase, it was easy to read the strip recordings produced by the special transmissions.

Before leaving on the trip I asked Pierce how often he wanted the equipment calibrated and he said every 4 hours around the clock for the whole six week trip.  This became somewhat complicated since it took two hours to calibrate everything.  Aboard ship, I shared a cabin with one of the officers and my getting out of bed every two hours in the night did upset him a bit but even worse for me. Because I was a civilian, I was considered the junior officer and had to sleep in the upper bunk which had a ventilating duct four inches above my head.  It was then that I discovered whenever I turned over during the night I automatically lifted my head five inches!

After the trip, the Cornell and Reeves-Kodak systems were never to be seen again. In August 1958,  Jack Pierce and Dick Woodward prepared a technical report on the operation of the Draco equipment . The measurements showed that the average field intensity of the signals from station NLK at Jim Creek in Washington varied from roughly 30 microvolts per meter during the day to 100 to 200 microvolts per meter at night as observed in the Western Mediterranean Sea at a range of about 6,000 nautical miles.

The corresponding signal from station NSS at Annapolis, Maryland, varied from 300 microvolts per meter during the day to nearly 1.000 microvolts per meter at night in the same area at a range of about 5,000 nautical miles. These observations are in reasonable agreement with predictions based on the Pierce empirical formula for VLF propagation. But the observed signals from Jim Creek were several decibels weaker than the predictions. Presumably the losses were at caused at reflection points where the ground had poor conductivity.

The average noise level in the Western Mediterranean Sea varied from about 30 microvolts per meter at 0800 hours GMT to about 90 microvolts per meter at 1500 hours GMT. These observations were made in the springtime and, of course, higher values would be expected during the winter. It was difficult to analyze some of the results obtained from the Draco navigation equipment because station NSS was out of sync most of the time. However, the errors in the navigator's fixes were comparable with the errors in the Draco system. The consistency of the Draco observations indicate that the Draco errors rarely exceeded a mile or two in the Mediterranean area. Comparison of the qualities of signals from NLK and NSS as received with the Draco and the Model AN/SRR11 receivers indicated that the Draco receiver was equivalent in performance to the latter.

It was therefore concluded that the Draco equipment could be used for communication as well as navigation. No significant improvement in performance was obtained by the introduction of a 20-cycle filter in the Draco receiver. The Draco strip chart recordings of special phase-shift keyed transmissions from Jim Creek demonstrated that simple messages could be transmitted reliably at a range of at least 6,000 nautical miles. Presumably such simple messages could be recorded and read at the same range (6,000 nautical miles) and depth (20 feet) under sea water as can Draco signals.

After I completed my work on the Draco project,  I began to realize my interests weren't in the field of radionavigation so in 1959 I came west  to work for North American Aviation. Soon I designed some equipment for the  McDonald F-4 aircraft which made the company a great deal of money so I remained with them as a Project Manager. Jack Pierce retired to Weare, NH and died there in 1996 at the age of 88. As of 2005, Richard Woodward is living in Cape Cod, MA.".

Although Draco never became a radionavigation system in its own right, measurements made during its
development may have been applied to Omega or into other submarine communication systems.

(Click to enlarge)
omega_pandb_logo_s.jpg Cover of P& B Inc. product brochure from 1957. The company was founded in 1945 and later became a subsidiary of the Gorham Corp. in 1960  In 1964, P&B was auctioned off and purchased by LTV Corp . By 1970, Cardwell Condenser Co. of New York purchased P&B from LTV. After that P&B slipped into oblivion. (Brochure provided by Herbert Rideout)
omega_pandb_products1_s.jpg This is just some of the equipment produced by P & B Inc around 1957. (Brochure provided by Herbert Rideout)
omega_compass_island1_s.jpg This is the certificate presented to personnel who participated in the Draco and SINS evaluations aboard the USS Compass Island.  (Provided by Herbert Rideout)

SINS was the "Ships Inertial Navigation System" made by Autonetics a 
division of North American Aviation in California.  SINS was used on the 
submarines and other programs such as the Minuteman Intercontinental 
Ballistic Missile and North American's Cruise Missile (GAM-77 [AGM-28] HOUND DOG) that was launched from a B-52. 
 

omega_burns_s.jpgHarold S. Burns - Co-Founder and President P & B Inc

Harold Burns, W1KVX, received engineering training at the Eastern Radio Institute and the University of Hampshire and is a member of the Institute of Radio Engineers, Institute of Navigation and the  American Management Association. He has a broad background of experience in the installation, testing and operation of Navy shipboard radio and electronic equipment and high power international short-wave transmitting installations. During WWII,  he was chief engineer and production manager  directing projects of both applied research and the production of precision quartz frequency elements, frequency measuring apparatus and radar components. (Photo and copy courtesy P & B Inc) 

Harold left P&B in 1962 to start a new company Electro Marine Corp. on Cape Cod, MA. He  died 8 Sept.1999 at the Cape Heritage Nursing Home in Sandwich, Mass., after a brief period of hospitalization. He was 81. 

omega_woodward_s.jpgDr. Richard H. Woodward - Vice President Engineering, Chief Navigation Section of  P& B Inc.

Dr. Woodward, B.S., M.S., D.Sc. in Electrical Communications from Harvard University, is a member of the Institute of Radio Engineers, the American Physical Society, and the American Association for the Advancement of Science.  He helped to develop the Loran system of navigation and was of the technical advisors assigned to the Telecommunications Research  Establishment in England. There he helped to introduce Loran into the Royal Air Force and had close contact with the anti-jamming  problems associated with Gee and Loran. His work at Pickard & Burns included studies of radio propagation and navigation systems for the  Air Force, development of a high precision short range navigation system for the US Navy, and consulting on a long range navigation system at the Navy  Electronics Laboratory in San Diego along with the design and construction of equipment for this system. (Photo and copy courtesy P & B Inc) 

SIGNAL CHARACTERISTICS

Omega utilized CW (continuous wave) phase comparison of signal transmission from pairs of stations. The stations transmitted time-shared signals on four frequencies, in the following order: 10.2 kHz, 11.33 kHz, 13.6 kHz, and 11.05 kHz. During its life cycle, the system used quite a lot of  frequencies at different times. For instance, 12.1, 12.0, 11.55, 13.1, 12.3, 12.9, 13.0 and 12.8 kHz were employed. 11.05 kHz was introduced in an attempt to enlarge the area of non-ambiguity. The difference frequency between this and 11.33333 kHz produces a lane width of no less than 328 miles. In addition to these common frequencies, each station transmitted a unique frequency to aid station identification.

The inherent accuracy of the OMEGA system was limited by the accuracy of the propagation corrections that were applied to the individual receiver readings. These corrections were in the form of predictions from tables which were applied to manual receivers or stored in memory and applied automatically in computerized receivers. The system was designed to provide a predictable accuracy of 2 to 4 nm which depended on location, station pairs used, time of day, and validity of the propagation corrections.
 

TRANSMISSION INTERVAL > 0.9 1.0 1.1 1.2 1.1 0.9 1.2 1.0 0.9
A. Norway 10.2 13.6 11.33
f 1
------ ------ ------ ------ ------
B. Liberia
f 2
10.2 13.6 11.33
f 2
------ ------ ------ ------
C. Haiku, Hawaii ------
f 3
10.2 13.6 11.33
f 3
------ ------ ------
D. LaMoure, ND. U.S.A. ------ ------
f 4
10.2 13.6 11.33
f 4
------ ------
E. Reunion Island ------ ------ ------
f 5
10.2 13.6 11.33
f 5
------
F. Argentina ------ ------ ------ ------
f 6
10.2 13.6 11.33
f 6
G. Australia (Trinidad was a temporary site) 11.33 ------ ------ ------ ------
f 7
10.2 13.6 11.33
H. Japan 13.66 11.33 ------ ------ ------ ------
f 8
10.2 13.6
The Omega signals consisted of a sequence of C.W. pulses  transmitted from each station on three frequencies, 10.2 kHz, 11.33 kHz and 13.6 kHz. The sequence was non-ambiguous and synchronized to universal time. Each interval was separated by 0.2 seconds. The eight stations provided world wide coverage on approximately a 10 second period. Three Omega transmissions were needed to determine a position fix. Later, 11.03 kHz was introduced to enlarge the area of non-ambiguity.  (Table courtesy of Decca Navigator News, June 1973)
All OMEGA signal patterns are transmitted starting at zero time (OMEGA Time) and are maintained at the exact starting time through atomic clocks at each transmitting site. All frequencies are phase locked to zero time. All frequencies cross zero phase with a positive slope at exactly 0000 OMEGA Time.

Initially OMEGA station transmissions were started at universal time. However, universal time is corrected for changes in the earth's rate of rotation; these conditions, called leap seconds, are made periodically. Corrections to OMEGA Time to account for leap seconds are difficult because of complex interrelationships between stations. Additionally, signals used during the time change present a synchronization problem. Consequently, OMEGA Time is maintained at a steady rate and is not updated. All OMEGA stations are timed and controlled by a cesium beam atomic clock which is accurate to 1 second in 3000 years. The overall accuracy is on the order of a few parts in 10^12.

STATION TABLE
STATION 
LETTER
STATION LOCATION
ANTENNA TYPE
LAT/LONG
TRANSMISSION STATUS
A Bratland, Norway Valley span 66° 25'N 
13° 08'E
Normal 10 kw transmission.
B Paynesville, Liberia Grounded tower 06° 18'N
10° 40'W
10 kw transmission . Modal interference at night.
C Kaneoke, Hawaii Valley span 21°   24'N 
157° 50'W
Normal 10 kw transmission.
D LaMoure, ND Insulated tower 46° 21'N 
98° 20'W
Normal 10 kw transmission.
E Plaine Chabrier, 
LaReunion (Indian Ocean)
Grounded tower 20° 58'S 
55° 17'E
Normal 10 kw transmission.
F Golfo Nuevo, Chalut
Argentina
Insulated tower  43° 03'S 
65° 11'W
Normal 10 kw transmission.
G Woodside,   Victoria, Australia. Temporarily assumed by Trindad In planning 38° 29'S
146° 56'E
Came on line around 1980. Trinidad power level was 1 kw
H Shushi-Wan, 
Tsushima Island, Japan
Insulated tower 34° 37'N 
129° 27'E
Normal 10 kw transmission.
STATION TABLE. Omega summary information was broadast from WWV, Boulder Colorado at 16 minutes after each hour in a 40 second duration.
The VLF range of 10-14 KHz was selected as the best range for OMEGA primarily because of:

1. Presence of a wave guided mode to VLF signals which follows the earth's curvature and provides signal detection over great distances with a relatively low (10KW) power output.

2. Excellent stability of VLF signals.

3. Relatively wide distances between points where phase measurements would be the same (distances between points of equal phase measurements).

VLF WAVE GUIDED MODE

VLF propagation contains several different transmission modes: ground wave. sky waves and wave guided wave.
The wave guide effect occurs when a wave passes through a cavity which reflects the wave and confines it to the enclosed space within the cavity. An effect similar to the wave guide effect occurs when very low frequency transmissions travel over the earth's surface. Signals in the 10-14 KHz range behave as though propagated through a waveguide of concentric spheres.  In this case, the spheres are the earth and the ionosphere.

The stability of an OMEGA signal is the primary reason these waves are desirable for navigation. Stability of a VLF OMEGA signal indicates the wave propagates with similar characteristics, without distortion. at almost any distance from the transmitting station as long as it is receivable. This stability is confirmed through monitoring of OMEGA VLF signals at various earth locations. Monitoring has also shown changes in exact phase measurement of VLF signals. Actual measurement at a given time can be predicted with great accuracy even though exact phase measurements differ greatly day to night. season to season.

Wave guided signals travel great distances from the station with almost unlimited range over water; over land they attenuate at a greater rate. The greatest loss of signal occurs over the ice cap. A wave propagated in one direction over water (the long way around the world) could be received while a direct signal from the station might not be received due to signal attenuation over an ice cap. When this occurs the predicted position fix accuracy becomes extremely low and the signals should be considered invalid.

Also, when the receiver is a great distance from the transmitters, signals may be received from both directions, resulting in a combined signal phase shifted an unknown amount. Therefore, use of OMEGA signals is not recommended when the receiver is more than 8000 NM  (great circle distance) from the transmitting stations.

SYSTEM ERRORS

The earth is not a perfect waveguide. The imperfect walls of the earth ionosphere waveguide affect signals in many ways. Phase velocities in the VLF range are primarily dependent upon the condition of this waveguide through which they are propagated. The earth's waveguide condition is a function of the shape and height of the ionosphere which is in turn a function of the position of the sun and the season of the year. As a consequence of these, and other factors, there are eight (8) basic error sources which contribute in varying degrees to the overall OMEGA system accuracy. Two out of 8 error sources are described below. The other six were missing in the source material.

DIURNAL EFFECT

The first error source of concern is called the diurnal effect. It is principally associated with the sun's position since its radiation adjusts the height and shape of the ionosphere. During daylight hours, the ionization layer will lower to about 70 KM, thereby increasing the phase velocity. At night, the layer moves up to about 90 KM, thus decreasing the phase velocity. This effect will also be seasonal and, of course, nonlinear during transition.

A long propagation path may be either entirely sunlit (day), entirely dark (night), or experiencing mixed illumination (transition). For long paths, night may be only a few hours; for Arctic paths during the summer months there may be no night at all. Propagation tends to be most stable during the day although conditions do vary slowly. At night conditions tend to be constant but less stable than during the day. Transition periods are of intermediate stability and present additional complications in prediction and application.

GROUND CONDUCTIVITY

The second source of error is ground conductivity. Extreme variations in phase velocities are detectable between sea water, representing  low attenuation, and ice which is high in attenuation and hence slows the phase velocities. Water is a near perfect conductor in the VLF spectrum and does not greatly affect the signal.

Propagation correction tables and formulas were based on theoretical models calibrated from worldwide monitor data taken over long periods. A number of permanent monitors were maintained to assess the system accuracy on a long term basis. The specific accuracy attained depended on the type of equipment used as well as the time of day and the location of the user. In most cases, the accuracies attained were consistent with the 2 to 4 nm stated in the system design goal. There were a few cases where much better accuracy was reported. A validation program conducted by the USCG indicated that the OMEGA system met its design goal.

OMEGA had an availability of greater than 99 percent per year for each station and 95 percent for three stations. The annual system availability was greater than 97 percent which included scheduled off air time. Scheduled off air periods were announced up to 30 days before the off air activity was to occur.

The system provided independent position fixes once every ten seconds and was capable of two or more lines of position (LOP's) fix . Due to the fact that Omega antennas were towers around 1,200 feet tall,  that made the system very expensive to install.

AMBIGUITY

In this CW system, ambiguous LOP's occur since there is no means to identify particular points of constant phase difference which reoccur throughout the coverage area. The area between lines of zero phase difference were called lanes. Single frequency receivers use the 10.2 kHz signals whose lane width is about eight nautical miles on the baseline between stations.

Multiple frequency receivers extended the lane width, for the purpose of resolving lane ambiguity. Lane widths of approximately 288 nm along the baseline could be generated with a four-frequency receiver. Because of the lane ambiguity, a receiver had  to be preset to a known location at the start of a voyage. The accuracy of that position had be known with sufficient accuracy to be within the lane that the receiver was capable of generating (i.e., 4 nm for a single frequency receiver or approximately 144 nm for a four-frequency receiver).

Once set to a known location, the Omega receiver counted the number of lanes it crossed in the course of a voyage. This lane count was subject to errors which could be  introduced by an interruption of power to the receiver, changes in propagation conditions near local sunset and sunrise, and other factors. To use the single frequency Omega receiver effectively for navigation, it was essential that a dead reckoning plot or similar means be carefully maintained and the Omega positions compared to it periodically so that any lane ambiguities could be detected and corrected.

The accuracy of an Omega phase difference measurement was independent of the elapsed time or distance since the last update. Unless the Omega position was verified occasionally by comparison to a fix obtained with another navigation system or by periodic comparison to a carefully maintained plot, the chance of an error in the Omega lane count increased with time and distance. These errors were reduced in multiple frequency receivers since they were capable of developing larger lane widths to resolve ambiguity problems.

Omega receivers were used in the Number 3 position on some 747ís as backup for the two Inertial Navigation Systems.

AN/SRN-12
omega_srn12_rx.jpg
The AN/SRN-12 is one example of an OMEGA  receiving set.  It's a solid state, single frequency, phase-locked, superheterodyne navigation receiver designed for use in surface ships. The AN/SRN-12 received the 10.2 kHz transmissions, phase-locks and tracks any four selected stations' signals, measured the phase of each tracked signal with respect to a highly stable internal oscillator and computed and displayed three selected phase differences (LOPs). Lines of position were displayed on nixie tube indicators and a permanent record was stored on a graphic recorder. A built in oscilloscope was used for visual monitoring of received OMEGA signals and troubleshooting. An built-in emergency battery power supply maintained synchronization during brief (up to 5 min) power outages. (Photo courtesy RCN)

 
omega_marconi_cma771.jpg
This is an example of a  processor receiver unit removed from an unknown commercial aircraft. It has a serviceable tag from Avionics & Aircraft Systems, Inc. dated 1/30/92. The nameplate reads: CANADIAN MARCONI COMPANY CMA-771, RECEIVER PROCESSOR UNIT, OMEGA NAVIGATION SYSTEM, PN 473-157-023.

 
omega_cma740_control_box.jpg
Canadian Marconi CMA740 control head. (e-Bay photo)

 
CLOSURE

With the Global Positioning System (GPS) being declared fully operational, the use of OMEGA had dwindled to a point where continued operation was not economically justified. The 1994 edition of the United States Federal Radionavigation Plan (FRP), which delineates policies and plans for federally provided radionavigation services, stated "the U.S. expects to continue OMEGA operations until September 30, 1997, to accommodate the transition of  civil aviation users to GPS. Continued operation after that date will depend upon validating requirements for OMEGA that cannot be met by GPS or another system." The Federal Aviation Administration (FAA) completed its review of Omega navigation requirements for the U.S. aviation industry and notified the U.S. Coast Guard that most users will complete their conversion to GPS technology by September 1997. OMEGA was shut down precisely at 0300Z on September 30, 1997 - the end of another era. To VLF experimenters, the very high power OMEGA signals were both a blessing and a curse; a blessing in that they provided convenient test signals in the 9.5 to 14 kHz range, and a curse in that they tended to interfere with the reception of natural radio phenomena such as "whistlers" and "dawn chorus".

Besides affecting users, the closure of Omega had a small impact on tourism. Because of their prominent antennas and interesting mission, many Omega stations were recognized in their local areas as major tourist attractions, including official listings and pictures in area tourist brochures. Omega station North Dakota was located in the town of LaMoure, with a population of less than 1,000. In this small town is located the Omega Motel, the Omega Plaza, and the Omega Room at one of the restaurants. After Omega ceased, the USN took over the site from the US Coast Guard and continued VLF communications under the name of Naval Computer and Telecommunications Area Master Station Atlantic (NCTAMSLANT). The mission statement of the new station is: " To manage, operate, and maintain those facilities, systems, equipment, and devices necessary to provide requisite  communications and information system support for the command, operational control and administration of the naval establishment, and the fixed submarine broadcast system; to test and evaluate new Very Low Frequency (VLF) broadcast technology and minimize downtime of operational sites during VLF system upgrades and major transmitter and antenna maintenance". The people of LaMoure will not soon forget the Omega system as the 1200 foot  tower still looms in the western horizon and a number of retired Coast Guardsmen now reside in LaMoure.

Omega Station Norway had a prominent sign along the road near the helix building proclaiming their antenna as the longest antenna span in Europe. The Japan tower was the highest structure in Japan, and the Argentina and Liberia towers were the tallest structures in their entire continents. Australia registered over 10,000 visitors per year to its station.

THE MESSAGE WHICH ANNOUNCED THE CLOSURE OF THE SYSTEM
The following message was sent by the United States Coast Guard to all OMEGA users advising of the system shutdown.

P 011416Z OCT 97
FM COGARD NAVCEN ALEXANDRIA VA//NIS//
SUBJ: OMEGA STATUS AS OF 01 OCT 97
1. THE OMEGA NAVIGATION SYSTEM TERMINATED AND ALL STATIONS CEASED OMEGA TRANSMISSIONS AT 0300Z 30 SEPTEMBER 1997 IN ACCORDANCE WITH NAVCEN OPERATIONS ORDER DATED 141026Z AUG 97.
2. OFF AIR PERIODS 221000Z SEP 97 THROUGH 300300Z SEP 97:
A. NONE
B. NONE
C. NONE
D. NONE
E. NONE
F  NONE
G. NONE
H. NONE
3.  REDUCED POWER PERIODS 221000Z SEP 97 THROUGH 300300Z SEP 97:
A. NONE
B. DOWN 3.7 DB 221000Z  TO 300300Z
C. NONE
D. NONE
E.  DOWN 1.7 DB 221000Z TO 230250Z
     DOWN 1.3 DB 230715Z TO 231215Z
     DOWN 4.2 DB 231415Z TO 232355Z
     DOWN 1.7 DB 241000Z TO 242225Z
     DOWN 1.4 DB 251440Z TO 251951Z
     DOWN 1.2 DB 260101Z TO 260230Z
     DOWN 5.0 DB 260740Z TO 270608Z
     DOWN 2.5 DB 271602Z TO 280335Z
F.  DOWN 1.9 DB 221700Z TO 230731Z
     DOWN 1.2 DB 241839Z TO 242023Z
     DOWN 2.4 DB 272100Z TO 280358Z
G  NONE
H  DOWN 1.6 DB 231230Z TO 231810Z
     DOWN 1.6 DB 250305Z TO 251800Z
     DOWN 1.6 DB 261750Z TO 262220Z
4.  QUESTIONS REGARDING OMEGA STATUS/OPERATION MAY BE DIRECTED TO: PHONE NUMBER (703)313-5900.

A facsimile transmission received by the Navigation Center (NAVCEN) from the Japanese Maritime Safety Agency truly summarized the 25 year relationship between the U.S. and Japan. The FAX, received just days before the signal was terminated stated:

..."(The final status message) will shine brilliantly, foot marking the world wide radio navigation history cooperatively linking OMEGA with six partner nations. We can't say enough in praise of your excellent duties. In Japan, both the Station and the Analysis Office employees amount to nearly three hundred persons since opening time. They have a favorable impression of the system. Your friendship and kind support with us over the years has been deeply appreciated. It will stay with me as a rewarding memory of the valuable experience received from OMEGA. I hope that the OMEGA community members will continue to have a successful and enjoyable life."

These kind words were expressed by Toshiichiro Kawamura, Director of JMSA.


AN OMEGA POEM

This Omega Poem was published in the USCG Radionavigation  Bulletin Fall/Winter 1997 after the system closed.


Omega ... Omega in the sky.
Ships and planes use you as their eyes.
You've run so long, Your waves held high.
You cover the world in the blink of an eye.
You sing a song like no other. 
Your cycle covers places that no other could cover.
Your lattice ... out-stretched like the arms of a mother,
 helping so many, no matter what weather.
We bid you farewell, so long and good bye...
and from all the men, past and present,
we salute you and all who have served under your majestic tower, 
here at OMSTA LaMoure, North  Dakota... 
And Just like your song,
We are gone ... In the Blink of an eye 

Fireman Adam Powers, 
OMSTA LaMoure, North Dakota 

FOOTNOTES:

1. The use of an RF filter on any receiver effectively extends the range.



REFERENCES and CREDITS:

1) The Journal Of Navigation - Chapter 4. W.F. Blanchard, Royal Institute of
    Navigation; Vol 44, No. 3; Sept 1991. Used with permission.
2) USGC Web sites: www.uscg.mil/lantarea/bjc/aton.htm
                                 www.navcen.uscg.mil/omega/ogeninfo/omstat.txt
3) DGNSS web page: www.dgnss.com/descriptions/omega.htm
4) USCG Radionavigation Bulletin - Spring/Summer 1996.
    Omega Navigaton To Be Terminated; Lt. Kenneth Pierro, NAVCEN,
    New Timing and Control Equipment for Omega; R.C. Hoyler, NAVCEN

    USCG Radionavigation Bulletin - Fall/Winter 1997.
    Omega Silent After 25 Years Int's Service; LCDR Lori Mathieu, Lt Kyle Smith, Mr. Vinicio Vannicola.

5) Herbert Rideout e-mail:  <wa6ipd(at)dslextreme.com>
6) Excerpts from "Field Intensity Measurements and Draco Performance In the Mediterranean Sea" Aug 6, 1958. Publication 458, prepard by Pickard & Burns , Needham, Mass.
7) Pickard & Burns information brochures 1957.
8)  Stuart A Wolf  e-mail:  <stuart.wolf(at)nats.co.uk>
9) James Churchill    <jchurchill(at)milcom-systems.com>
10) Enabling Network Centric Warefare   http://www.nctamslant.navy.mil/index.php?id=392&secid=14
11) RCN's N.E. Tech TQ6B Common Equipment Manual.

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May 5/12