WO2001095529A1

WO2001095529A1 - Underwater communications system using electromagnetic signal transmission

Info

Publication number: WO2001095529A1
Application number: PCT/GB2001/002409
Authority: WO
Inventors: Stephen Ralph Takel; James Lucas; Ahamed Al-Shamma'a
Original assignee: Qinetiq Limited
Priority date: 2000-06-08
Filing date: 2001-06-01
Publication date: 2001-12-13
Also published as: AU2001262500A1; GB0013910D0

Abstract

An underwater communications system comprises a first and second communication station (1, 7) each of which is provided with an aerial (5, 8) which is surrounded by a waterproof, electrically insulating material (15). When the aerials are disposed at a distance from each other and immersed in a body of water (6) an electromagnetic signal (4) can be transmitted between the communication stations via the water.

Description

UNDERWATER COMMUNICATIONS SYSTEM USING ELECTROMAGNETIC SIGNAL TRANSMISSION

This invention relates to an underwater communications system. Traditionally communication between two sub-surface vessels or between a 5 sub-surface vessel and a surface vessel or land based station has been achieved using optical or acoustic methods. Both of these methods have limitations. Optical systems generally have a limited range, particularly in marine environments where suspended matter in the water can reduce visibility to zero or at best a few metres, or they rely on the use of fibre optics, necessitating a physical linkage between

10 transmitter and receiver. Acoustic systems, such as sonar, although widely used, suffer from surface reflection and refraction in shallow waters and the relatively slow propagation of sound waves in water, ca. 1500ms^"1, puts a limitation on potential data transfer rates. Additionally, neither optical nor acoustic methods can penetrate objects and thus suffer from shadow zones. Underwater communication using

15 electromagnetic (EM) waves is potentially an effective way of resolving the limitations encountered using optical and acoustic methods. Furthermore, as the carrier frequency for an EM wave with a wavelength comparable to those used for ultrasonic acoustics is some 200,000 times greater, the potential data transmission rates are extremely high (speed of transmission in water is 3.3 x 10⁷ms^"1).

20 A major factor influencing the use of electromagnetic waves in sea water is that the launching of the wave into the water using conventional metal aerials occurs within the conduction band. This results in high losses, severely limiting the transmission range of any signal. Attenuation of high frequency electromagnetic waves by sea water is also high, 100dB/m at 1MHz in seawater.

25 US 4207568 discloses a method of improving the through water transmission efficiency of an electromagnetic signal by placing one electrode each of a dipole transmitter and receiver in a sea water filled flexible hose. This requires a physical connection to be maintained between the transmitting and receiving stations, putting severe limitations on the use of such a system. Communication between two

30 submerged submarines, for example, would be impractical. Furthermore, the disclosed method uses a low frequency, 1000Hz, restricting potential data transmission rates.

US 5434584 discloses the use of a submarine hull as a core for an electromagnetic antenna. The stated aim of this is to provide transmission at

35 frequencies of less than 1000Hz, which would again, only allow for limited data transmission rates. A further practical problem associated with this method is the extremely high magnetic field generated during use. The effect of this on onboard magnetic storage media and electronics would be catastrophic.

Successful transmission of electromagnetic signals at frequencies up to 7MHz has been claimed by Page Communications Engineers, Wireless World, February 1966, page 80 in which they found that the range of transmission was directly related to the length of aerial used. Attempts were made to use radio communications between submerged submarines by the US Navy in 1919, Jn. Amer. Phys. Soc, vol.14, p.193, 1919. Despite these studies, modern underwater communications are still dominated by acoustic and optical methods and there is no practical usage of electromagnetic waves.

In accordance with a first aspect of the present invention, an underwater communications system comprises a first and second communication station; wherein the first communication station comprises a first aerial surrounded by a waterproof, electrically insulating material; wherein the second communication station comprises a second aerial surrounded by a waterproof, electrically insulating material; and wherein, in use, the first and second aerials are immersed in a body of water and an electromagnetic signal is transmitted between the first and second aerials through the water. Preferably, the body of water is a body of seawater or a body of brackish water. Brackish water is water which has a salinity partway between fresh water and seawater, such as that commonly found in estuarine regions.

The invention enables underwater electromagnetic (EM) communications, by electrically insulating the aerial from the water, so that an EM wave can be launched from a transmitter into a body of water, propagate through the body of water and be received by a receiver with acceptable total attenuation losses.

Preferably, the first and second aerials are disposed at a distance from each other. This allows EM communications to be conducted between remote communication stations. Alternatively, the first and second aerials are substantially co-located. In this arrangement the system can be used as a bulk detection system. An EM wave launched from a transmitter may be reflected from a bulk object, such as the sea floor or other large object, and received by a receiver. Information concerning the location and size of the bulk object can thus be determined. Preferably, at least one of the communication stations is completely immersed in the body of water, and more preferably, both the first and second communication stations are completely immersed in the body of water. There is no requirement for any part of the system to protrude from the surface of the body of water allowing it to be used to communicate between two or more completely submerged bodies.

The first and second aerials may be constructed from a metal, such as steel, or copper; or a non metallic conductor such as carbon and be of any size and shape depending on the frequency of transmission, but preferably the first and second aerials are dipole antennas, loops or aperture antennas. The waterproof, electrically insulating material may be any electrically insulating material which will also prevent direct contact between the aerials and the water, thereby preventing large conduction losses, but preferably the material is a dielectric material comprising a non-absorbent polymer such as PVC, Perspex, polythene or polyethylene.

Preferably, the system further comprises a matched medium intermediate the aerial and the waterproof, electrically insulating material of each of the first and second aerials. Improved transmission efficiency is achieved by launching the electromagnetic wave in this fashion. Preferably the matched medium comprises a low conductivity fluid such as distilled water or oil.

In accordance with a second aspect of the present invention, a method of electromagnetic communication underwater between two communication stations comprises immersing a first aerial of a first communication station in water, the first aerial being surrounded by a waterproof, electrically insulating material; immersing a second aerial of a second communication station in the body of water, the second aerial being surrounded by a waterproof, electrically insulating material; and transmitting an electromagnetic signal between the first and second communication stations through the water.

Preferably, the communication stations communicate at frequencies in the range 3 x 10³ Hz to 10¹⁰ Hz.

More preferably, the communication stations communicate at frequencies in the range 10⁴ Hz to 2 x 10⁹ Hz, although other frequencies may be effective.

Preferably, the first and second communication stations are adapted to both transmit and receive.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic representation of one embodiment of an underwater communication system of the present invention;

Figure 2 shows a representation of an experiment to transmit and receive an electromagnetic signal through a sea water filled, cylindrical waveguide; Figure 3 shows a detailed view of a metal aerial coated with a waterproof insulating material;

Figure 4 shows a diagrammatic representation of an experiment using loop aerials; and,

Figure 5 shows how the strength of reception of a signal transmitted at three different frequencies varies with aerial separation during the experiment shown in Fig. 4.

Fig. 1 illustrates an example of an underwater communication system according to the present invention in this case for use as a means of transmitting an electromagnetic signal from a submarine to an unmanned underwater vehicle (UUV). The system comprises a first communication station 1 , with a first power supply 2 which is used to drive a signal generator 3. A signal 4 produced by the signal generator 3 is input to a first metallic aerial 5, surrounded by a waterproof, electrically insulating material, disposed in a body of sea water 6. The signal 4 propagates through the body of sea water 6 and is received by a second communication station 7. The second communication station 7 comprises a second metallic aerial 8, surrounded by a waterproof, electrically insulating material, disposed at a distance from the first metallic aerial 5 within the same body of sea water 6; and a signal receiver 9 powered by a second power supply 10. The system of Fig.1 allows the transmission of an electromagnetic signal from the first communication station 1 to the second communication station 7. However the siting of an additional signal receiver at the first communication station 1 and an additional signal generator at the second communication station 7 or a transceiver at each communication station will allow two way communication between the communication stations.

The first and second communication stations may be any suitable site or structure with which it is desirable to communicate, for example, submarines, submersibles, unmanned underwater vehicles (UUV) or fixed undersea structures. The stations may be attached to undersea pipelines or sited directly on the sea bed. Alternatively, one communication station may be sited on a floating platform, surface vessel or on the land and the other station may be on a sub-surface platform. The transmitted signal may originate from an audio source, for example a microphone (voice source), an analogue or digital data source, a video source or any other source suitable for modulated EM carrier transmission.

Some specific examples of the system will now be described. Example 1.

By way of comparison, a non-insulated, metal transmitting aerial and a non- insulated, metal receiving aerial were placed in a body of a dielectric fluid, distilled water and separated by a distance of 300mm. The metal aerials were in direct contact with the water. Transmission of a signal over a frequency range of 1 MHz to 200 MHz was clearly demonstrated. When the distilled water was replaced with a conducting fluid, weak sea water simulant equivalent to 0.1 mol I^"1 of salt, no signal transmission was possible. This result confirms that very high conduction and attenuation losses are encountered when there is direct contact between the metal aerials and a conducting fluid. Nor was signal transmission possible using full strength sea water simulant, equivalent to 0.4 mol I^"1 of salt. Example 2.

A transmitting aerial 11 and a receiving aerial 12, as shown in Fig. 2, were placed in a hollow cylindrical PVC waveguide 13 at a distance apart of 2.8m. The waveguide 13 was filled with full strength sea water simulant 14, equivalent to 0.4 mol I^"1 of salt, and both aerials 11,12 were coated with a thin layer of epoxy resin 15

(shown in more detail in Fig. 3) to prevent direct contact with the salt water. Low loss transmission of an EM signal was clearly demonstrated, with a frequency of 147MHz the best match for the aerial and solution used. From this result it can be calculated that for a 3dB loss the transmission distance would be 27m and for a 100dB loss the signal could be transmitted for a distance of 1143m due to the absence of conduction losses. Modern communication systems can operate with total losses of up to 200 dB, thus the magnitude of the losses demonstrated by the system of the present invention is tolerable. Transmission of electromagnetic signals through a body of sea water with acceptable levels of attenuation is possible, whereas putting non-coated aerials in direct contact with the sea water causes large conduction losses making communication impractical. Example 3.

A through water data transmission experiment was undertaken using the equipment described in Example 2. Data was transmitted from a first PC, via the serial port, in the form of an RS232 signal. This signal was converted to TTL levels and used to amplitude modulate (AM) the output of a 10kHz - 1 GHz signal generator. The carrier frequency ranged from 110 - 220MHz which satisfied the resonant frequencies through the full strength sea water simulant, equivalent to 0.4 mol I^"1 of salt, in the 2.8m long, 10cm diameter, PVC waveguide 13. The carrier frequency was coupled into the waveguide by a magnetic loop through an N-connector, and an identical arrangement was used to couple out this modulated signal. A +20dB amplifier was used prior to the demodulation stage. The AM detector consisted of a simple envelope detector and low pass filter circuits. The demodulated signal was then sent to a comparator where the TTL voltage levels (0 - 5V) were restored. This was then converted back to RS232 standard format (± 12V) and received by a second PC. Numerous tests were performed to investigate the transmission of serial data through sea water. The data, in the form of characters, was successfully received at baud rates ranging from 9600 to 115000 bps. Example 4. Fig. 4 shows a further experiment using two loop aerials 16,17 surrounded by an insulating material (heat shrink) similar to those of Fig. 3. The aerials were set at a depth of 2m and a separation of 3.5m in the sea water of Weymouth Bay, Dorset, UK. The first aerial 17 was coupled to a spectrum analyser 18 and a power supply (not shown) which were also completely immersed in the water. The second aerial 16 was connected to a transmitter 19 and a power supply (not shown), mounted on a test frame (not shown) and placed in the water such that the aerial, transmitter, power supply and test frame were completely immersed in the water. Both power supplies were shielded by Faraday cages. A carrier was transmitted from the transmitter 19 at three different frequencies (a) to (c) in the range between 10kHz and 100 kHz and the received signal was displayed on the spectrum analyser 18. The strength of the received signals are shown in Fig. 5, which illustrates variation in received signal strength in dBv with the aerial separation in metres. Fig. 5 shows that outside very near field effects the loss in dB/m reduces with distance, i.e. the gradient of the curves becomes less. If measurements were taken until the received signal disappeared into the noise floor, the effective range would extend to ca. 7m. This suggests that at these attenuation rates improvements in aerial matching and transmit power would yield significantly longer effective transmission ranges. The transmit powers used in this example are relatively low, so there is ample scope for a large increase in power which would translate to a large increase in transmission range. This example shows that transmission of the EM signal is entirely through the water as all components of the system were completely immersed in water. This is in contrast to other systems in which sections of one or more of the communication stations may protrude from the surface of the water. For example, this could be a connection to a surface vessel or a land based station. This introduces the possibility of a through air path to enable communication, rather than the through water path of the present invention.

Claims

1. An underwater communications system, the system comprising a first and second communication station; wherein the first communication station comprises a first aerial surrounded by a waterproof, electrically insulating material; wherein the second communication station comprises a second aerial surrounded by a waterproof, electrically insulating material; and wherein, in use, the first and second aerials are immersed in a body of water and an electromagnetic signal is transmitted between the first and second aerials through the water.

2. An underwater communications system according to claim 1 , wherein the body of water is a body of seawater or brackish water.

3. An underwater communications system according to claim 1 or claim 2, wherein the first and second aerials are disposed at a distance from each other.

4. An underwater communications system according to claim 1 or claim 2, wherein the first and second aerials are substantially co-located.

5. An underwater communications system according to any preceding claim, wherein at least one of the communication stations is completely immersed in the body of water.

6. An underwater communications system according to any preceding claim, wherein both the first and second communication stations are completely immersed in the body of water.

7. An underwater communications system according to any preceding claim, wherein the first and second aerials comprise dipole antennas, loops or aperture antennas.

8. An underwater communications system according to any preceding claim, wherein the waterproof, electrically insulating material is a dielectric material comprising a non-absorbent polymer such as PVC, Perspex, polythene or polyethylene.

9. An underwater communications system according to any preceding claim further comprising a matched medium intermediate the aerial and the waterproof, electrically insulating material of each of the first and second aerials.

10. An underwater communications system according to claim 9, wherein the matched medium comprises a low conductivity fluid such as distilled water or oil.

11. A method of electromagnetic communication underwater between two communication stations, the method comprising immersing a first aerial of a first communication station in a body of water, the first aerial being surrounded by a waterproof, electrically insulating material; immersing a second aerial of a second communication station in the body of water, the second aerial being surrounded by a waterproof, electrically insulating material; and transmitting an electromagnetic signal between the first and second communication stations through the body of water.

12. A method according to claim 11 , wherein at least one communication station is completely immersed in the body of water.

13. A method according to claim 11 or claim 12, wherein the body of water is a body of seawater or brackish water.

14. A method according to any of claims 11 to 13, wherein the communication stations communicate at frequencies in the range 3 x10³ Hz to 10¹⁰ Hz.

15. A method according to claim 14, wherein the communication stations communicate at frequencies in the range 10⁴ Hz to 2 x 10⁹ Hz.

16. A method according to claim 6 and claim 7, wherein the communication stations are adapted to both transmit and receive.