GB2445015A

GB2445015A - Electromagnetic below ice communications

Info

Publication number: GB2445015A
Application number: GB0625269A
Authority: GB
Inventors: Mark Rhodes; Brendan Hyland
Original assignee: WFS Technologies Ltd
Current assignee: WFS Technologies Ltd
Priority date: 2006-12-19
Filing date: 2006-12-19
Publication date: 2008-06-25
Anticipated expiration: 2026-12-19
Also published as: GB2445015B; GB0625269D0

Abstract

A system adapted to allow communication through water and ice, the system comprising a transmitter having an electrically insulated, magnetic coupled antenna for transmitting an electromagnetic signal, and a receiver having an electrically insulated magnetic coupled antenna for receiving an electromagnetic signal from the transmitter, wherein one of the transmitter and receiver is below the ice and one is in or above it.

Description

1, 2445015 Electromagnetic Below Ice Communications

Introduction

The present invention relates to the use of electromagnetic or magnetic signals to provide communications and/or navigation links to a station situated within a body of water below ice from above or within the ice.

Background

Equipment or divers deployed in water beneath an ice sheet currently face great challenges in signalling with the surface for communication or navigation purposes.

Wired links present practical mechanical challenges restricting operational range and manoeuvrability due to the weight of the long towed cable. Wireless communications are sometimes attempted using acoustic modem technology. However, acoustic signals face a number of issues in this environment. Horizontal acoustic propagation is difficult due to the pressure gradient within the water. This tends to refract an acoustic wave downward away from the ice/water boundary resulting in attenuation and multi-path effects. Additionally, an acoustic link can only be maintained through a continuous water path so a transducer must be wired back to the surface, usually from the diver's point of entry. Furthermore, acoustic links require a through ice cable penetration and do not effectively accomplish a "wireless" link across the water to ice boundary.

An object of this invention to provide a method and system for providing a means of communication and/or navigation to a station submerged in a body of water below ice using electromagnetic waves.

Summary of the Invention

According to one aspect of the present invention, there is provided a communication system comprising at least one station partially or wholly submerged in water below ice and a transmitter for transmitting a signal from the submerged station to a remote station above the ice, wherein the transmitter includes at least one magnetically coupled antenna. a a

Conventionally, electromagnetic transmission has not been considered an effective method when communication is partially or wholly underwater. Our pending patent application PCT/GB2006/002123, the contents of which are incorporated herein by reference, describes how this may be accomplished over useful distances. One potential difficulty to be considered in communicating data underwater is the higher attenuation encountered by an electromagnetic signal when transmitted through this partially conductive medium. Ice, however, has a much lower conductivity than water and hence a lower attenuation of electromagnetic signals. This means that low frequency radio signals can pass from air, through ice and into water with acceptable loss. This has not been appreciated previously.

Conductivity is the main mechanism for attenuation of electromagnetic signals in water and ice through transfer of energy from the electric field component to heat in the ice or water. A magnetically coupled antenna achieves lower transmission loss for IS a signal launched in water compared to conventional electric field coupled electromagnetic antennas of the types commonly used in free space. This is because the through-water path is in a medium of significant conductivity that, while immediately attenuating an electrical field, leaves a magnetic field largely unaffected.

Preferably, the signal includes data. Ideally, the transmitter is operable to transmit the data in a digitally modulated electromagnetic or magnetic signal. Means may be provided for compressing the data that has to be transmitted. This is advantageous, because, through water electromagnetic attenuation increases rapidly with frequency.

Compressing the data prior to transmission causes the occupied transmission bandwidth to be reduced. This allows use of a lower carrier frequency, which leads to lower attenuation and in turn allows communication over greater transmission distances. This significantly alleviates the difficulty of communication through water.

Digital representation of audio and/or video, data compression and transmission at the lowest practicable frequency are particularly advantageous in the sub-sea environment. The requirement for compressed digital transmission is very different in terrestrial electromagnetic communications applications. Here there is very little variation of attenuation with frequency over most of the radio spectrum. The requirement for digital communication with data compression is driven by the

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restricted spectrum resources and related licensing issues not to address concerns with attenuation.

Pure water has a very low conductivity but the presence of dissolved ions greatly increases conductivity Sea water has many times the conductivity of nominally fresh water. Conductivity is greatly reduced when water transitions to the solid state as ionic charge carriers are immobilised. In winter, multi-year sea ice can have a conductivity of 3 mS/rn. During the summer low salinity brine or melt water fills extensive pore space increasing average bulk conductivity to 23 mS/rn. This compares to fresh water conductivity around the 10 mS/rn range while sea water has much higher conductivity at around 4,000 mS/rn. The bulk conductivity of sea ice lies in the range usually seen in fresh water. For this reason, magnetically coupled antenna systems, optimised for low frequency operation will be beneficial compared to standard, terrestrial radio systems.

According to another aspect of the present invention, there is provided a radio navigation system comprising at least one receiving station partially or wholly submerged in water below an ice sheet, and a beacon transmitter above or within the ice, wherein the transmitter and receiver stations include at least one magnetically coupled antenna.

Preferably, the submerged station is able deduce its position relative to the transmitter based on received signal strength. The maximum signal strength will occur at a position directly below the surface transmitting station. Signal strength increases as the submerged station travels toward the point directly below the transmitter. in this way a submerged station is able to navigate toward a beacon situated within or above the ice sheet.

Several transmitter beacons may be provided allowing the submerged station to home in on consecutive beacons in turn to allow point to point navigation. This allows navigation of a submerged station by following the signal strength of beacons arranged on the surface of the ice or embedded within the ice sheet. Each beacon will require a unique identifying property in its electromagnetic signal. This can be implemented through amplitude or phase modulation or by assigning an identifying

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frequency to individual beacons. This system can be enhanced to allow bi-directional communications, so that transmissions from the submerged station can be used to monitor the submerged station's position from above the ice based on the signal strength received at known beacon locations above the ice.

A surface deployed distributed radiating cable can provide a continuous signal for tracking from beneath the ice sheet for submerged vehicle navigation. The submerged station operates using a very simple tracking algorithm to follow the maximum received signal strength. h this way a route can be defined above the surface of the ice where conventional navigation techniques such as the Global Positioning System (GPS) are more readily available and effectively used to define operations below the ice.

According to yet another aspect of the present invention there is provided a radio navigation system comprising at least one transmitter station partially or wholly submerged in water below an ice sheet, and a receiver above or within the ice, wherein the transmitter and receiver stations include at least one magnetically coupled antenna.

By using the received signal strength, the surface station is able deduce its position relative to the submerged transmitter. Signal strength increases as the surface station travels toward the point directly above the transmitter. Maximum signal strength occurs at a position directly above the submerged transmitting station. By monitoring the signal strength, a surface station is able to navigate towards a beacon situated within a body of water below the ice sheet. This is particularly useful for recovery of submerged objects or divers.

Brief Description of the Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: Figure 1 is a block diagram of an above ice station communicating with a second station submerged in water below ice; Figure 2 is a block diagram of a station within a body of ice communicating with a second station submerged in water below ice; Figure 3 is a block diagram of a transceiver for use in the systems of figures 1 and 2; Figure 4a represents a loop or coil antenna; Figure 4b represents another loop antenna in which a core of high magnetic permeability material is introduced into the coil to form a solenoid of more compact form; Figure 5 represents an omni-direction antenna arrangement; Figure 6 is schematic diagram of a navigation beacon system providing a signal to a submerged navigating station; Figure 7 is schematic diagram of a multiple navigation beacon system providing a signal to a submerged navigating station; Figure 8 is a block diagram of a transmitter as employed in Figures 6 and 7; Figure 9 is a block diagram of a receiver as employed in Figures 6 and 7; Figure 10 is schematic diagram of a leaky radiating cable navigation system; Figure 11 is schematic diagram of a leaky radiating cable navigation system shown in cross section; Figure 12 is a block diagram of a transmitting beacon as employed in Figure 10, and Figure 13 is an illustration of a radiating cable construction.

Detailed Description of the Invention

The present invention relates to various communications and navigation systems that use electromagnetic waves as the propagation means for signalling into water below a layer of ice. Because the transmission path includes an underwater and through ice portion, the communication method is different from that conventionally applicable to air propagation systems such as radio. This is primarily because ice and water, especially saline water, exhibit much higher attenuation of the signal over distance.

To alleviate the problem of high attenuation, magnetic coupled antennas are used.

Magnetic antennas, formed by a wire loop, coil or similar arrangement, create both magnetic and electromagnetic fields. Close to a transmit antenna there is a predominantly magnetic field that transitions, over an area conventionally known as

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the near field, to an electromagnetic field with an intrinsic impedance relationship between E and H (electric and magnetic) components characteristic of the medium.

The magnetic or magneto-inductive field is generally considered to comprise two components of different magnitude that, along with other factors, attenuate with distance (r) at rates including a factor proportional to /r2 and hr3 respectively.

Together these form the predominant near field components. The electromagnetic field has a still different magnitude and, along with other factors, attenuates with distance at a rate including a factor proportional to hr. It is often termed the far field or propagating component. The far field is the area in which the electromagnetic transmission signal has transitioned to the characteristic impedance relationship between E and 1-I components. The near field dominates at short distances, whereas the far field is relatively stronger at greater distances. Dependent on distance between the transmit antenna and the receive antenna, either or both near and far field components may be used. Using a magnetic coupled antenna rather than an 1 5 electrically coupled antenna reduces signal dissipation in the near field, but allows

data transmission in the near and far fields.

In conductive medium, electrically insulated magnetically coupled antennas provide various advantages over the alternative of electrically coupled antennas. In far field electromagnetic propagation, the relationship between the electric and magnetic field is determined by the characteristic or intrinsic impedance of the transmission medium.

An electrically coupled antenna launches a predominantly electric field that transitions to the characteristic impedance over the near field. Underwater attenuation is largely due to the effect of conduction on the electric field. Since electrically coupled antennas produce a higher E-field component in the near field the radiated signal experiences higher attenuation. The same performance issues apply to a receive antenna. Magnetically coupled antennas do not suffer from these problems and so are more efficient underwater than electrically coupled antennas.

Figure 1 is a block diagram of a first station 20 above ice communicating with a second station 21 that submerged in water below the ice. In an alternative arrangement, the first station 25 may be located within a body of ice, as shown in Figure 2. In both cases communication is achieved through electromagnetic signalling and launched using magnetically coupled loop or solenoid antennas. Both

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arrangements can be used for communicating data andlor for use in a navigation system.

Figure 3 is a functional block diagram of a specific implementation of a through ice station for communicating data for half duplex communications. Data is generated by a peripheral 110 device 44, for example an unmanned vehicle or autonomous underwater vehicle (AUV), and passed to the transceiver through the external data interface 31 to a processor 32. The processor 32 encodes the data to create a bit stream, which is passed to a modulator 33. The modulator 33 synthesises a digital representation of a modulated waveform, which is passed to the Digital to Analogue Converter (DAC) 34. The DAC 34 generates an analogue modulated waveform which is amplified by a transmit driver 35 and passed to a transmit loop antenna 36.

Preferably, the transmitter is operable to transmit signals having a frequency in the range of 10 Hzto 100MHz.

Incoming signals from the submerged station are received at a receive antenna 37 and amplified by a receive amplifier 38. An Analogue to Digital Converter 39 creates a digital representation of the signal and a demodulator 40 extracts a digital data stream.

This data is interpreted by the processor 32, which presents the received data at the external data interface 31. The external data interface passes received data for utilisation by the peripheral 110 device 44. The equipment is housed in a housing 30 and powered from a battery 42 conditioned by the power supply regulator 43. This specific illustration shows separate antennas for transmit and receive functions, but an alternative implementation may use a single antenna diplexed to perform both transmit and receive functions.

The nature of the peripheral inputloutput (110) device 44 may vary depending on the applications. In the description of Figure 3, the AUV is the peripheral 110 data source 44 and data destination of the transceiver. A second transceiver on the surface of the ice would allow communications between the A1JV and the surface. The peripheral at the surface would be typically a personal computer (PC) type device with display and data input terminal. In a further example the submerged peripheral 110 data source could be a submerged sensor relaying data back to the surface through the ice and receiving control commands from the PC at the surface. In many cases the peripheral 110 can interface with a modem, microphone and speaker to implement voice communications. This type of system can provide voice communications from the surface with a diver submerged below ice. The peripheral 110 could alternatively interface with text entry and display at both terminals to implement texting communications with divers. This would have a lower bandwidth requirement compared to voice. A lower carrier frequency is possible resulting in greater range.

The shape of the coil antennas at both ends of the link is not critical but the received signal will be maximised if they are nearly circular and each arranged to have a large area, because this creates the greatest magnetic signal flux for transmitting and intercepts maximum flux for receiving, both of which increase the received signal andlor enable performance over longer distance. For example, a coil antenna may 1 5 have a typical cross sectional area of 0.2 square metres but may be increased or decreased dependent on distance and signal strength required. Multiple turns of one or both coils are usually beneficial because this also increases the signal flux created by a transmit antenna and the voltage induced in a receive antenna.

Figure 4a shows a simple coil antenna suitable for communication. This has multiple turns of wire 6, typically 100, insulated from the water and which may be connected directly to the transmitter and/or receiver so that signal current flows in it. Figure 4b depicts a similar coil modified to be more compact but with comparable performance.

The coil 8 usualy takes a solenoid shape, and has a highly permeable core 7, typically of known ferrite material; introduced within it. The permeable core multiplies the magnetic flux created by a coil by around 50 to 100 times, and therefore increases its effective size and ability to create and intercept a magnetic field. Another possible antenna arrangement is shown in Figure 5. This has three orthogonal solenoids, which produce in approximation to omni-direction operation for use in situations where antenna alignment is difficult to maintain. In this case, each antenna is aligned with one of the three Cartesian axes.

Figure 6 is schematic diagram of a transmitting beacon 101 above or within the ice radiating an electromagnetic signal as a navigational aid to a submerged station 102 below the ice. The transmitting beacon antenna 100 and submerged station antenna 101 use magnetically coupled ioop or solenoid antennas. The submerged station 102 can follow a path of increasing received signal strength to navigate toward the radiating beacon 100. For example a mobile station 102 following cross section A-B will measure increasing received signal strength until it reaches a point directly below the radiating beacon 100. This technique will prove particularly relevant for recovery of mobile equipment or divers. Electromagnetic signal strength decreases rapidly with distance in the near field region of a magnetically coupled antenna. Attenuation is increased still further as the signal passes through water. Far field electromagnetic components are also rapidly attenuated with distance as they pass through a conductive media.

A mobile station on one side of the ice sheet equipped with a receiver will measure maximum received signal strength when it is at the nearest vertical point to a transmitter on the other side. This provides a simple means for location of a submerged transmitter. The high attenuation through water and ice makes this method particularly effective since rapid variation of received power with distance allows accurate location. This location method is described in more detail in our pending patent application PCT/G82006/0021 11, the contents of which are incorporated herein by reference. In contrast, conventional far field radio transmission through air experiences very low attenuation and signal strength location methods are rarely used. In a similar way a submerged mobile station will experience maximum signal strength when directly under a radiating surface station so enabling navigation to a known position.

Figure 7 is schematic diagram of a navigation system employing multiple radiating beacons 50, 51 and 52 which radiate an electromagnetic signal as a navigational aid for a submerged station 54. Transmitting beacons 50, 51 and 52 and mobile station 54 employ magnetically coupled loop or solenoid antennas. Figure 7 illustrates a system where navigation station 54 may follow a route defined by multiple beacons 50, 51 and 52 by homing in on increasing signal strength of successive beacons. In this way beacons 50, 51 and 52 provide contiguous navigational coverage along a pre-determined route. Each of beacons 50, 51 and 52 will radiate an identifying property, for example different frequency of operation or encoded identifying modulation. By way of illustration beacon 50 could radiate at 3 kl-Iz, 51 at 3.5 kHz and 52 at 4 kl-Iz. Each beacon must be placed close enough to its neighbour that navigation station 54 can receive both beacons at the mid point to achieve effective contiguous coverage.

Figure 8 is a block diagram of an example transmitting beacon as employed in Figures 6 and 7. Crystal oscillator 70 provides a stabilised reference frequency for frequency synthesiser 71. Frequency synthesiser 71 generates a frequency unique to that individual beacon based on the settings of beacon identifier switch 72, for example 4 kl-lz. Frequency synthesiser 71 passes its output signal to power amplifier 73 which generates a signal of the required amplitude for the required range coverage of the beacon. Load match 74 achieves efficient power coupling to 100 turn 0.5 m diameter radiating loop antenna 75. The transmitter is enclosed in housing 76.

Figure 9 is a block diagram of an example receiver as employed in Figures 6 and 7.

The signal is received by a 100 turn 0.5 m diameter loop antenna 60 and passed to a tuned band pass filter 61 which receives only the signal from the corresponding transmit beacon, for example 4 kHz from the beacon of figure 8. Filter 61 interfaces to receive amplifier 62 which increases amplitude to within the detector range of signal amplitude measurement device 63. The signal amplitude measurement circuit passes a digital representation of signal amplitude to data interface unit 64 which passes data to the vehicle navigation unit (not illustrated). The receiver is enclosed in housing 65.

Figure 10 is schematic diagram of a leaky radiating cable navigation system. In this example the radiating cable 80 is deployed on the surface of the ice, although it could equally be located within the ice sheet. Cable 80 is described as leaky because it is designed to radiate a fraction of the signal as it passes down the cable. Radiating cable 80 is powered from a transmitter 84 and terminated in load 83. The magnetic component of the radiating field passes through the ice and is received by a loop antenna 81, which passes the signal to submerged navigating station 82. As shown in Figure 11, the strength of the radiated field varies as a function of the distance from the cable 80. Submerged station 82 contains a field strength measuring receiver and is able to navigate under water by tracking the received signal strength. A relatively simple control algorithm allows the navigating station to follow a path of locally constant field strength thus ensuring it effectively tracks the cable 80. In this way cable 80 is used to define a route for the submerged vehicle.

Figure 12 is a block diagram of a transmitting beacon as employed in Figures 10 and 11. Crystal oscillator 110 provides a stabilised reference frequency for frequency synthesiser Ill. Frequency synthesiser 111 generates a frequency unique to that individual beacon based on the settings of beacon identifier switch 112, for example 4 kHz. Frequency synthesiser Ill passes its output signal to power amplifier 113 which generates a signal of the required amplitude for the required range coverage of the beacon. Load match 114 achieves efficient power coupling to controlled impedance radiating cable 115. Radiating cable 115 is terminated in load impedance 116. The transmitter is enclosed in housing 117.

Figure 13 illustrates an example implementation of a leaky radiating cable 80 construction for use in navigation and communication applications. The cable is of co-axial construction with central conductor 120 separated from conducting shield 122 by insulating dielectric 121. The dielectric permittivity and diameter are designed to form a controlled impedance wave guide between conductor 120 and shield 122.

The shielding is covered by a protective insulating sleeve 123. In a conventional co-axial cable the outer shield 122 is continuous. In this radiating design the shield is formed by winding a conductive tape in a helical arrangement with gaps between turns to allow some leakage of the signal as it propagates along the cable. Figure 13 shows a side view showing insulating sleeve 1 23 cut away to expose the helical shielding 122.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the essence of the invention. For example, the reciprocal nature of any propagation path is fundamental in radio transmission theory and although some systems are described herein as having a single directional link, all the radio links described are capable of supporting bi-directional communications.

Also, it is readily apparent that the invention has greater generality and can be applied to many other systems requiring conveyance of measurements or other data from/to underwater sites. Also, the invention is applicable to fresh water or seawater, fresh water ice or sea ice. Deployment is envisaged in rivers, estuaries, lakes and sea.

Accordingly, the above descriptions of specific embodiments are made by way of examples only and not for the purposes of limitation. It wifl be clear to the skilled person that minor modifications may be made without significant changes to the operation and features described.

Claims

Claims I. A signalling system comprising a transmitter including at

least one electrically insulated magnetically coupled antenna for transmitting an electromagnetic or magnetic signal, the transmitter being positioned to transmit through ice.
2. A system as claimed in claim 1 wherein the transmitter is located above, below or within the ice.
3. A system as claimed in claim I or claim 2 comprising a receiver for receiving an electromagnetic or magnetic signal from the transmitter through ice.
4. A system as claimed in claim 3 wherein the receiver is located above, below or within the ice.
5. A system as claimed in any of the preceding claims wherein the transmitter andlor receiver is/are located in or on an underwater vehicle.
6. A system as claimed in claim 5 wherein the underwater vehicle is an unmanned underwater vehicle.
7. A system as claimed in any of the preceding claims comprising an input for receiving data that is to be transmitted.
8 A system as claimed in any of the preceding claims that is operable to transmit and/or receive data.
9. A system as claimed in any of the preceding claims comprising a data input connected to the transmitter.
10. A system as claimed in claim 9 wherein the data input is at least one of a text input; an audio input; a sensor; a video camera; and a still image camera.
11. A system as claimed in any of the preceding claims comprising a data output connected to the receiver.
12. A system as claimed in claim 11 wherein the data output is at least one of a text display; an audio output; a display, for example a still image or video display.
13. A system as claimed in any of the preceding claims wherein at least one of the transmitter/receiver is an elongate transducer.
14. A system as claimed in any of the preceding claims wherein the transmitter and/or receiver are provided in or at a mobile station.
15. A system as claimed in any of the preceding claims, wherein the transmitter 1 5 and/or receiver are provided in or at a station that is located at a fixed position.
16. A system as claimed in any of the preceding claims having two or more stations, both including a transmitter andlor receiver.
1 7. A system as claimed in claim 16 wherein at least one of the stations is mobile.
1 8. A communication system for communicating through water and ice, the system comprising a transmitter having an electrically insulated, magnetic coupled antenna for transmitting an electromagnetic signal, and a receiver having an electrically insulated magnetic coupled antenna for receiving an electromagnetic signal from the transmitter, wherein one of the transmitter and receiver is below the ice and one is in or above it.
19. A signalling method for communicating through water and ice, the method comprising using an electrically insulated, magnetic coupled antenna for transmitting an electromagnetic signal to a remote receiver through an interposing layer of ice andlor receiving an electromagnetic signal from a remote transmitter through the said interposing layer of ice.
20. A method as claimed in claim 19 comprising including data in the electromagnetic signal.
21. A method as claimed in claim 20 comprising digitising the data carrying signal and compressing a bandwidth of the digitised signal to lower the carrier frequency and increase the signal range.
22. A method as claimed in any of claims 19 to 21, wherein the magnetic coupled antennas include loops and/or solenoids
23. A method as claimed in claim 23, wherein the solenoid is formed around a high magnetic permeability material.
24. A method as claimed in any claims 19 to 23, wherein the transmitter is operable to transmit signals having a frequency in the range of 10 Hz to 100 Ml-lz.
25. A method as claimed in any of claims 19 to 24 comprising a data link for communication with an unmanned underwater vehicle.
26. A method as claimed in any of claims 19 to 25 including displaying data/text/video signals received andlor generating sound.
27. A method as claimed in any of claims 19 to 26 comprising positioning the transmitter below the ice and the receiver above or in the ice.
28. A method as claimed in any of claims 19 to 27 comprising positioning a receiver below the ice and the transmitter above or in the ice.
29. A method as claimed in any of claims 19 to 28 comprising communicating with one or more submerged sensors.
30. A method as claimed in any of claims 19 to 29 comprising communicating with divers over a voice channel.
31. A navigation system for a submerged station in water below ice comprising a transmitter having an electrically insulated, magnetic coupled antenna for transmitting an electromagnetic signal, a receiver having an electrically insulated magnetic coupled antenna for receiving an electromagnetic signal from the transmitter, one of the transmitter and receiver being below the ice and one being in or above it, and determining means for determining the position of the receiver relative to the transmitter using the received electromagnetic signal.
32. A navigation system as claimed in claim 31 wherein the determining means are operable to determine the receiver position using a measurement of electromagnetic signal strength radiated from above the ice
33. A navigation system as claimed in claim 31 or claim 32 wherein a plurality of electromagnetic transmitters are provided, each being deployed on the surface or embedded within the ice sheet.
34. A navigation system as claimed in any of claims 31 to 33 wherein the transmitter comprises an elongate, leaky transducer, for example a radiating cable.