Radio waves transmit information underwater

Written by: Tom Shelley | Published:
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Tom Shelley reports on a way of transmitting information by radio beneath the waves



A system under development permits the transmission of electronic data underwater using electromagnetic radio waves.

While it is not suitable for voice transmission over long distances, it does allow voice transmission over short distances, and more importantly, the transmission of data at high rates for control and telemetry.

Intended applications are mainly in the offshore oil and gas industries, but it also has great potential for use in oceanographic research and by the military.

Conventional radio does not work under water because of the conducting nature of the medium especially in the case of sea water. Nonetheless attempts to send messages through water pre-date attempts to send them through the air. Notably, in 1842, Samuel Morse laid wires along each bank of the Susquehanna River, connecting the transmitting key and a battery to wires to a pair of copper plates upstream and downstream, with a galvanometer on the opposite bank connected to wires to a second set of plates opposite the first set. This allowed signals to be sent through water over nearly a mile. About the same time, James Lindsay was experimenting with sending telegraph signals through ponds in and around Dundee, after which he succeeded in signalling across the River Tay over a distance of more than two miles. These developments led to Sir William Preece, chief engineer for the General Post Office, performing his own experiments on wireless by conduction and also on wireless by induction, after which he instead turned to backing the efforts of a clever Italian named Guglielmo Marconi.

Germany pioneered radio communications to submarines underwater during World War II, building their "Goliath," antenna North East of Calbe by the river Elbe in 1941. The antenna was capable of outputting up to 1 to 2MW of power, strong enough to send signals to submarines submerged in the Indian Ocean. Naval submarines today communicate globally from under water using an extremely low frequency signal, typically around 80Hz signal at much lower powers, but this is only possible because most of the transmission path is through the atmosphere. Voice transmission is generally considered impractical, apart from the magnetic field based diver to diver system described in Eureka's November 1970 cover feature story, because the attenuation of radio waves passing through water increases greatly with frequency.

For this reason, the currently favoured technology for underwater communications is acoustic. This performs well in thermally stable, deep water applications, but nearer the surface, is adversely affected by temperature gradients, ambient noise from engines, and by waves and shallow water. It is also limited to low data transmission rates.

Wireless Fibre Systems, however, has been developing a system that enables high speed data or voice transmission over distances of 10m. Project manager Mark Rhodes told Eureka that it would work over much greater distances, perhaps up to several km, but 10m is the maximum depth difference he can achieve between transmitter and receiver antennae mounted beneath his development harbour pontoon beside the Forth Bridge in Scotland. Longer distances are possible horizontally, but he explained that these do not constitute a fair test, because in such cases, the transmissions would be likely to be using an air path over much of the distance.

The breakthrough enabling technologies are in the form of the antennae, which we cannot describe, but can reveal are very different from antennae used for conventional service in the atmosphere, and in the sensitivity and signal processing capabilities of the receivers.

100m data paths are more than long enough for diver to diver communications and for control and telemetry. If the distance is shorter, around 100mm, it is possible to transmit 10 Mbit/s, the main purpose of which would be to do away with connectors, always a problem under water with solutions that cost significant amounts of money, especially if they have to work at depth.

Other applications could include signalling under water between autonomous under water vehicles (AUVs) and base stations. It is envisioned that after completing a mission, perhaps downloading data from sub sea surveillance data loggers, the AUV would move to within range of a base station, unload its data to it, receive new instructions, and then proceed on a new mission. In a military situation, one the advantages of using radio waves of limited range is that they would be unlikely to be detected by opposition forces.

In addition to the systems being developed by Wireless Fibre Systems, which already appear to work, readers may be interested to know that there is in addition, a European Community project aimed at the same goals. The project is named EMCOMMS which unlike most Euro projects, is not an acronym for the title which is, "Underwater Communications using Electromagnetic Waves". The project is being co-ordinated by Professor J Lucas at the University of Liverpool, although the web pages, all hosted at Liverpool, state that, "The coordinator of this project is the University of Limerick." The web site declares that it is to investigate electromagnetic wave propagation through sea water for carrier frequencies in the rage 1 to 5MHz over distances of up to 1km. The EC preamble states, "It has been shown theoretically and by preliminary experiments that em waves within a frequency range 1 to 10MHz will be able to propagate over distances of up to 1000m. These properties of em waves allow data to be propagated with high bit rates up to 1Mbits/sec. This high rate will allow compressed video images to be propagated at standard camera frame rates (25Hz)." Interestingly, the vertical distance tests are limited to 0 to 30m.

Other participants are: Bonn Elektronik in Ottobrunn, Norwegian Underwater Intervention in Bergen, The Underwater Centre in Fort William (a diver training school) and FS Antennentechnik in Unterschleissheim. The project began in January 2002 and is scheduled to run for 42 months as part of the Energy, Environment and Sustainable Development programme. Stated potential applications are: sensor systems, imaging, position fixing, measurement of speed, obstacle detection and avoidance, guidance, communication or data and voice and remote control. The emphasis is on civilian commercial and scientific. DARPA, the US Defense Advanced Research Projects Agency, on the other hand, is also seeking such technologies, with the stated goal of maintaining US naval supremacy but with fewer ships and submarines.

Wireless Fibre Systems
EMCOMMS project

Eureka says: Underwater radio has long been sought but looks to have at last become a practicable tool. It should save not a few lives both by allowing more tasks to be undertaken robotically and improving communication with divers

Pointers

* Practicable data rates are achievable using electromagnetic wave transmissions underwater without incurring unacceptable expense or working at very high power levels

* The higher the frequency, the shorter the range, but even short ranges are useful when they do away with the need to make physical connections between cables


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