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How OMEGA Ruled The World Before GPS By Dr DAVID MADDISON 12 Silicon Chip Question: What was the main global real-time radio navigation system before GPS took over in 1997? If you answered “Omega”, you are correct. And where is the tallest man-made structure in the Southern Hemisphere? The answer is the nowobsolete 427m high Omega tower near Woodside in Victoria. Here’s how the Omega system worked – and was used for more than 20 years. siliconchip.com.au O The US Navy had mega is now obsolete a very specific and but the huge antenna specialised need for mast still stands as Omega, apart from it an imposing landmark in being a general purthe Gippsland area in southpose, global, real-time eastern Victoria. navigation system. The tower is 427.45 metres In 1964 it was introhigh (1,402.4 feet) and when ducing a fleet of naviit commenced operation in gation satellites called 1982 it was the fourth highTRANSIT (also known est structure in the world. It as NAVSAT – Navy is also said to be the world’s Navigation Satellite tallest Very Low Frequency System). These would (VLF) radio antenna. allow global high acSince the tower is close to curacy position fixes the sea, it is used by fishermen of around 120 metres as a visual guide to their loca(decreased to 3m by tion. Assuming a 472 metre 1980 with software height (the base of the tower improvements and betis about 45m above seal level) To get to the top, you have to climb that ladder seen in the foreand a fisherman with his eyes ground – there is no lift or hoist. The person you can (perhaps!) see ter geodesy models) which would origi2 metres above the water, the is an abseiler, probably illegally descending from inside the top of top (bright red during the day the Woodside Omega Tower. There has also been at least one death nally be used to reset the inertial navigation and illuminated at night) can amongst daredevils using the tower for illegal BASE jumping. be calculated to be visible Attribution: “Woodside Omega Transmitter” by Nickinator – Own work. systems on submarines (later TRANSIT from about 83km away. became widely used elsewhere). The tower was originally intended to be built in a more Ground stations used the Doppler effect from signals optimal location in New Zealand but this did not happen, transmitted from these satellites to determine their position because of opposition by protest groups against military much like (but in reverse of) how the Argos satellites (see infrastructure, even though it could be used by civilians. SILICON CHIP, July 2014) determine the position of some There were seven other stations around the world, used models of the Argo floats. in conjunction with the tower in Victoria. TRANSIT satellites were in use for more than 30 years, The Omega Navigational System preceded GPS as a from 1964 to 1996, for both civilian and military use and global navigation system and operated between 1971 and these seemingly indestructible satellites remain in use 1997, transmitting VLF radio signals between 10kHz and today, as part of the US Navy’s Ionospheric Monitoring 14kHz to provide navigational fixes. System. The system was developed by the United States, although A problem with this Doppler navigational technique is a US court subsequently determined that it was based that it results in two position on work by the British Decca fixes, one real and one Navigator company. It refalse and it is necesceived US Governsary to distinment approval guish befor go-ahead NORWAY tween in 1968. NORTH DAKOTA FRANCE JAPAN HAWAII LIBERIA ARGENTINA siliconchip.com.au AUSTRALIA Omega stations were located in Bratland, Norway (Station A), Paynesville, Liberia (Station B), Kaneohe, Hawaii (Station C), La Moure, North Dakota (Station D), Chabrier, France (Station E), Trelew, Argentina (Station F), September September 2014 13 2014 13 Woodside, Victoria, Australia (Station G) and Tsushima, Japan (Station H). After the Omega System shut down, some of the equipment was transferred to the Maritime Museum in nearby Port Albert (http://yarrampa.customer.netspace.net.au/pamm.html). This is just one of the items on display: an Omega receiver (although there were many types). the two. Traditional navigation techniques were not always accurate enough to do this but Omega could provide the correct one. Omega’s design accuracy was 2 to 4 nautical miles (3.7 to 7.4km) but this was largely dependent upon the accuracy of tabulated radio propagation predictions. TRANSIT positions were more accurate than Omega’s so they were used but Omega’s fix was good enough to distinguish between the real reading and the false one. And although TRANSIT position fixes were more accurate than Omega’s, another problem with TRANSIT was that it took several hours before a fix could be obtained at the equator and one or two hours at mid latitudes. Thus it was not a real-time system, unlike Omega. So despite the better positional accuracy available with TRANSIT, Omega soon became an accepted navigation system in its own right. And before GPS, Omega was the state-of-the-art realtime global navigation system. Other navigation systems were available and more accurate but were not global in scope, having only limited range or regions where they could be used. Very low frequencies Omega antennas were so large because they operated at very low radio frequencies; from 10 to 14kHz. The wavelength of these frequencies is 21km to 30km so the antennas have to be very large to be even moderately efficient. Even so, they are only a fraction of the wavelength in height. One quarter wavelength would be ideal but a This Omega Aerial Tuning Relay was one of five units and is shown with cutaway sections. Each of the five relays might switch twice in each ten second cycle to change the frequency of the signal (see table overleaf) meaning that each relay would have to operate around 6,000,000 times per year. The noise of the switching relays could be heard up to 400 metres away, even though housed in a heavily built concrete building. The white objects inside the relay are vacuum switches designed to prevent arcing as voltages as high as 200,000 volts may be present. The vertical copper tubes were designed to prevent electrical losses as AC currents tend to be conducted on the surface (skin effect) rather than deep inside a solid conductor. The donut-shaped rings were designed to prevent corona discharge and arcing as they offered a smooth surface and the black spheres on the right were spark gaps to allow a harmless discharge of any electrical arcing that was to occur. 14 Silicon Chip siliconchip.com.au Woodside Omega Tower Facts The Australian Department of Transport built the station for a cost of $12 million (1982 dollars) not including land. The tower itself was built by the Sydney-based Electric Power Transmission company for a cost of $1.1 million. The tower weighs around 500 tonnes and the base of the tower is in the form of a ball joint which allows the tower to move under wind load. The tower is designed to withstand wind loads of 228km/h at the top and 145km/h at the bottom. Beneath the tower is embedded an extensive copper wire earth system or “counterpoise”. Woodside Station G was the eighth and last station in the Omega System but was subject to an eight month delay because of union industrial action. quarter wavelength of 10kHz is 7500 metres so the size cannot be anything like that. Because of this the efficiency is very low, perhaps only a few percent . Due to the low efficiency of VLF antennas a very high input power and drive voltage is required for a much smaller output power, however the output power of this Omega antenna was 10kW, as it was for all other Omega installations around the world. VLF frequencies were chosen because (a) the VLF signals would be propagated globally with little attenuation in the natural wave-guide created between the earth and the bottom of the ionosphere, which would also minimise the number of stations required, (b) the stability of VLF signals as they are not subject to much fading and (c) the relatively great distances between lines of zero phase difference within which vessels would locate themselves, provided by the long wavelength of the radio waves. Coast Guard in partnership with Argentina, Norway, Liberia, France, Japan and Australia. Conspiracy theories abound As with most technical subjects that have any military connection, conspiracy theories abound regarding the actual use of the Woodside Omega Tower (just as they still abound regarding North West Cape). And who could forget the crackpot claims which came Top guy wires driven elements In the case of the Victorian installation, the tower itself is not an active antenna element. Instead, the uppermost guy wires are the electrically active elements. These radiating elements are electrically insulated from both ground and the mast and are arranged in a pattern akin to the ribs of an umbrella. “Umbrella” antennas radiate vertically polarised waves which is appropriate for the atmospheric wave-guide mode of propagation with wave-fronts oriented vertically between the ground and the bottom of the ionosphere. Other towers may have had different designs depending upon local circumstances. The eight Omega stations were installed around the world, distributed in such a manner that a receiver would be able to receive signals from at least five transmitters. In addition there was a testing station in Forestport, New York. The Omega system was operated by the United States siliconchip.com.au This enormous helix coil is part of the matching circuitry in the antenna feed, required because the wavelength of the antenna is very much shorter than even a quarter-wave of the signal wavelength. The gauge of the coil “wire” and the size give some idea of the powers involved. (Photo courtesy Catherine McAloon, ABC Gippsland). September 2014 15 Omega’s “Hyperbolic” Navigation System LOP-2 A hyperbolic navigation system involves measuring the timing or phase difference for the reception of two X OBSERVER radio signals from different places to establish the rela(Hyperbolic Fix) tive distance from each station. One way these timing differences can be determined is through looking at the phase difference between the received signals. If the transmitted signals were perfectly synchronised with each other and if the receiver was equidistant between two stations, there would be zero phase difP-1 LO ference between the two signals. If the receiver was closer to one station than the other, there would be a phase difference, indicative of the relative distance to each station. Plotting on a map the location for all possible positions corresponding to the observed timing or phase Y M differences results in a hyperbolic line called the Line of Position (LOP). The receiver could be anywhere along that line so another measurement is made with Hyperbolic Lines of Position (solid lines). The dashed green lines a different (third) transmitter. This results in another show zero phase contour lines between station M and X and the Line of Position. The point at which these two lines dashed red lines show zero phase contour lines between station intersect corresponds to the position of the receiver. M and Y. These dashed lines correspond to “lanes”. The Lines of The reality is a little more complicated though, as Position (LOP) for the observed timing or phase differences have there are multiple points where zero phase difference been plotted by the observer for both stations X and Y and the can occur. As one travels away from a point of zero intersection of these lines gives the position of the vessel. phase difference going closer to one transmitter and away from the other, the phase difference increases from At the lowest frequency of 10.2kHz used by Omega, the zero to 360°. A phase difference of 360° is the same as a lane width was around 14.7km or 8 nautical miles (correspondzero phase difference. The distance between these points ing to half the wavelength). Different frequencies resulted in of zero phase difference (or zero phase contour lines) cor- different lane widths and different frequencies could also be respond to one half of the wavelength of the radio signal. combined to generate very wide lane widths for purposes Since the distance between stations is much greater we shall see later. than half the wavelength of the transmitted signal, there The position of these lanes was plotted on reference maps. are multiple zero phase contour lines. These zero phase For reference purposes the lane midway between stations contour lines are known as “lanes” which are shown as the was numbered 900 and with the lane numbers decreasing dashed lines on the diagram above toward the station with the lower letter designation and increasing toward the station with TRANSMISSION DURATION (s) 0.9 1.0 1.1 1.2 1.1 0.9 1.2 1.0 the higher letter designation (eg, decreasing toward StaStation A: Bratland, Norway 10.20 13.60 11.33 ------ ------ ------ ------ -----tion A and increasing toward Station B: Paynesville, Liberia ------ 10.20 13.60 11.33 ------ ------ ------ -----Station H). In addition, it was possible to Station C: Kaneohe, Hawaii ------ ------ 10.20 13.60 11.33 ------ ------ -----determine the relative position Station D: LaMoure, ND, USA ------ ------ ------ 10.20 13.60 11.33 ------ -----within these lanes by examinStation E: Chabrier, France ------ ------ ------ ------ 10.20 13.60 11.33 -----ing the proportion of phase difference corresponding to the Station F: Trelew, Argentina ------ ------ ------ ------ ------ 10.20 13.60 11.33 relative position within the lane Station G: Woodside, Vic, Australia 11.33 ------ ------ ------ ------ ------ 10.20 13.60 (from zero to 360° representing Station H: Tsushima, Japan 13.60 11.33 ------ ------ ------ ------ ------ 10.20 from zero to 100 percent of the lane width), so the lanes Each Omega station transmitted three different frequencies in a unique order and could be further divided into duration and could be identified on that basis. For example, Station A transmits 100 equal parts for greater 10.2kHz for 0.9 seconds, then 13.6kHz for 1.0 seconds, then 11.33kHz for 1.1 seconds. accuracy. Between each transmission there is a 0.2 second delay. Note that for any point in the The big question was, which transmission sequence there were always three different frequencies being received, lane was one within? As it 10.20, 11.33 and 13.66kHz from three different stations. For example, when Station A turned out, it was not possible transmitted its 0.9 second 10.20kHz signal, Station G was transmitting on 11.33kHz to directly determine what lane and Station H was transmitting on 13.6kHz with a total transmission cycle takes of 10 one was within. It was necesseconds, endlessly repeated. All stations were synchronised via Caesium beam atomic clocks. Note that throughout Omega’s service life some frequencies were changed and sary to count the lanes a vessel also each Omega station transmitted an additional frequency as an aid to identifying crossed from the time it began the station; not for navigational purposes. 16 Silicon Chip siliconchip.com.au its voyage from the port whose position was accurately known. This lane count was related to a map showing the lanes for a particular geographic area. A problem arose if the lane count was lost for any reason, such as due to power or equipment failure, adverse radio propagation conditions or transmitter failure. In the event of a loss of lane count it was easy enough to re-determine the position within lanes but the actual lane numbers were not known and had to be determined. This inability to directly determine a lane number if the initial lane count was lost was known as “lane ambiguity”. It was therefore necessary to know one’s approximate position using navigation techniques such as maintaining a vessel track with dead reckoning. But the accuracy of the vessel track determined by dead reckoning or other traditional means was less than the spacing of the lanes so it was not possible to accurately locate the vessel within any particular lanes with certainty. In reality, a proper location fix within the lanes required that the other navigation method (eg, dead reckoning) be accurate to half a lane width. Lane width could be increased by changing the transmitting frequency but 10.2kHz was already the longest wavelength in use and therefore the widest lane width. But as shown in the table at left, there were three transmission frequencies, at 10.20kHz, 11.33kHz and 13.60kHz. The 13.60kHz signal gave a lane width of 11.0km or around 6 nautical miles; unfortunately an even narrower lane width. But here is the clever part. These frequencies were carefully selected so that four 13.6kHz lanes (4 x 11.0 = 44km) equal the same width as three 10.2kHz lanes (3 x 14.7 = 44km ignoring rounding errors). When these frequencies are electronically subtracted in the Omega Navigation equipment, 13.60 – 10.20 = 3.4kHz, we get a much lower frequency with a correspondingly greater lane width, the halfwavelength of this frequency being 44km, ignoring rounding errors. This is exactly triple the width of the 10.2kHz lanes. Then to establish lane count it was only necessary to establish one’s position by dead reckoning or other means to within half the lane width or 22km instead of around 7.35km, as with the 10.2kHz signal. This is within the capability of dead reckoning and other means. Having established one’s position within the 44km wide lane the position within that lane is determined (on a scale of zero to 100 percent of width). Recalling that this 44km wide lane corresponds to exactly three 10.2kHz lanes it is then possible to establish which of those lanes the vessel is located within by reference to Omega navigational charts. The frequency is reset to 10.2kHz and lane counting can then resume. The 11.33kHz frequency can be utilised in a similar manner and wider lane widths of 133km can be established. Of course, in matters maritime, the original measurements were in nautical miles. All Omega readings had to be adjusted according to correction tables which related the propagation of the VLF radio signal in relation to changes in the ionosphere related to time of year, location on the globe, time of day etc. The corrections may change the percent of lane readings and may even change the lane count number. siliconchip.com.au Above is a Google Earth image of the tower with the ground plane clearly visible, along with the many guy wires. out about the HAARP research station (see SILICON CHIP, October 2012). One story about the Tower in particular and Omega in general, which was published back in 1994 in the “Green Left Weekly”, had all sorts of claims and ‘statistics’ (see www.greenleft.org.au/node/7434). Now, we’re not saying we subscribe to ANY of the conspiracy theories in that story but “reading between the lines” does give a few more insights, despite its sensationalism and outright bias. For example, they query the “10kW” radiated power of Omega, maintaining there is also a half-million watt “ground wave” transmitter also associated with the installation. (We believe that the very high input power – perhaps even approaching 500kW – is required because of the very low efficiency of the antenna. But why ruin a good story with facts?). Omega shutdown The Omega Navigation System was shut down worldwide at precisely 0300Z on September 30, 1997. All navigation users, having been given appropriate warning, were as- Visiting the Tower The Woodside Omega Tower is well worth a visit, particularly if you’re in the Gippsland area. To get there, go a short distance to the east of Woodside on the South Gippsland Highway, A440. You can’t miss it! It can be seen from the side of the road (make sure you park safely). You can also see it on Google Earth: its specific location is at 38° 28’52” S, 146° 56’ 7” E or -38.481111, 146.935278. There is also a YouTube video by the author showing the installation at http://youtu.be/S_T7hd0oXUE September 2014 17 Warsaw Radio Mast While the Woodside Omega Tower remains one of the largest radio masts in existence, it is a pygmy compared to the long-wave AM radio mast which once stood in Warsaw, Poland. This was 646 metres (2,120 feet) tall and was the world’s tallest structure until its collapse in August 1991. Its two megawatt, 227kHz broadcasts could be heard around the world. This equipment rack contains the following instruments: 1) a synchronometer, which kept the signal correctly timed with others around the world, deriving its timing pulses from a caesium beam atomic clock; 2) an Omega monitor, receiving signals from a remote site at Blackwarry for self-monitoring purposes; 3) Omega Format Display showing which stations were transmitting and when; 4) Omega Signal Controller – provided computer control of the signal and also monitoring capabilities; 5) Power supply, one of several required to provide voltages of 12, 28 and 11,000 volts DC and 115, 240, 415, 480 and 9,000 volts AC; 6) Frequency Counter to check the signal or do other system checks; 7) Relay Driver to switch high voltage relays; 8) a modem to communicate with the remote monitoring site mentioned in (2); 9) a high voltage relay to switch 100,000 volts for aerial tuning purposes. sumed to have migrated to GPS by that time. While GPS has replaced most radio-navigation systems, they were much more difficult to jam by enemy or terrorist activity than is GPS, which is relatively easy to jam. They were also likely to be more resistant to the effects of electromagnetic pulses from nuclear explosions than are satellites. Today, we take GPS for granted and it is built into numerous devices such as phones, tablet computers, cameras, cars, collars to track pets and other animals and there are even dedicated GPS units! It is highly accurate (typically within 3 metres) and simple to use. In fact, the typical consumer experience goes no further than seeing one’s location on a map or planning a trip route. Navigation technology has come a long way in a very short time. Use by the Royal Australian Navy After the Woodside Omega station was shut down it was acquired by the Royal Australian Navy (RAN) for use in submarine communications. VLF signals are useful for submarine communications because unlike other radio frequencies, VLF waves penetrate between 10 and 40 me18 Silicon Chip tres into seawater. For this purpose the station had to be converted from operation at 13.0kHz to 10-14kHz and other significant equipment changes had to be made to support the 2-channel constant envelope minimum-shift keying (MSK) required for unidirectional submarine communications. California-based company Hi Q Engineering, who specialise in LF and VLF communications, was contracted to make these changes. Changes included: adding a phase shift network and converting some existing tuning variometers to T-network coils plus adding a capacitor bank, removal of the existing matching transformer and adding another tuning variometer, reducing the inductance of remaining tuning variometers, modifying the main helix and tapping into it with fewer turns, major changes to the helix network; and adding digital antenna current metering and an automatic antenna tuner. After these modifications the station supported 100 baud two channel MSK transmission with 150kW antenna input power and a radiated power of 36.5kW. Its station designation was VL3DEF. It was shut down for Navy and all other use on December 31st 2008, as it was no longer considered viable for use. Incidentally, there is another large VLF transmitter still in use in Australia. The Naval Communication Station Harold E. Holt at North West Cape is a joint facility of the US Navy and RAN. With 13 towers ranging in height from 304 to 387 metres it operates at 19.8kHz, with an input power of around 1MW. A counterpoise comprising a mat of 386 kilometres of copper wire is buried underneath the antenna array. The Omega tower’s future? The future of the Woodside tower is uncertain, depending on who you ask. Some say it should be removed as even ongoing maintenance is no longer justifiable (there are numerous reports of its aviation hazard lights failing and not being replaced as quickly as they might). But many others say it represents an important part of Australian and international navigational history. Let’s hope an appropriate alternate use is found for this tower, such as for amateur radio groups or scientific organisations (or even legal BASE jumpers or tourist operators) and it is not demolished just because some bureaucrat deems it convenient, economic or politically correct to do so. SC Want to know more? There’s a wealth of information on the ’net – including a US Navy movie on Youtube showing how ships and planes can work out their position and much more. Enter “Omega Navigation” in YouTube’s search panel. siliconchip.com.au