WO2006134329A2

WO2006134329A2 - Underwater remote sensing

Info

Publication number: WO2006134329A2
Application number: PCT/GB2006/002114
Authority: WO
Inventors: Mark Rhodes; Brendan Hyland; Derek Wolfe
Original assignee: Wireless Fibre Systems Ltd
Priority date: 2005-06-13
Filing date: 2006-06-09
Publication date: 2006-12-21
Also published as: WO2006134329A3; GB0724359D0; GB2441269B; GB2441269A

Abstract

An underwater remote sensing system comprising a transmitter for transmitting an electromagnetic signal, a receiver for receiving an electromagnetic signal reflected from an object and determining means for determining the location of the object, wherein at least one of the transmitter and receiver is underwater. The determining means may be operable to determine the location of the object using signals received at three or more receiver positions. To do this, three or more receiver antennas may be provided. Alternatively, a single receiver antenna may be provided and moved between three or more different measurement locations.

Description

Underwater Remote Sensing

Introduction

The present invention relates to system that makes use of electromagnetic energy to detect the presence or otherwise of objects in water and to provide information about their position, the systems being operated submerged in the water.

Background

Underwater detection and location has typically been accomplished using sonar techniques. Acoustic systems are degraded by noise and interference from a number of sources. They are also subject to multi-path effects and in some environments are virtually unusable.

Summary of invention

According to one aspect of the present invention, there is provided an underwater remote sensing system comprising a transmitter for transmitting an electromagnetic signal, a receiver for receiving an electromagnetic signal reflected from an object and determining means for determining the location of the object using the received signal, wherein at least one of the transmitter and receiver is underwater.

In use, a probe or exploratory signal is transmitted from the transmitter through the water. In the event that there is an object in the transmitted signal propagation path, part of the transmitted signal is reflected. The reflections are picked up by one or more receivers, processed electronically, and used to determine the relative position of the object.

At least one of the receiver(s) and transmitter may have a magnetic coupled antenna. At least one of the receiver(s) and transmitter has an electrically insulated antenna. In this case, the insulated antenna may be surrounded by an impedance-matched low conductivity medium, for example distilled water.

In the underwater environment, using electrically insulated magnetic coupled antennas provides 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 transmission media's characteristic impedance. An electrically coupled antenna launches a predominantly electric field that transitions to the characteristic impedance over an area known as 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. Magnetic coupled antennas do not suffer from these problems and so are more efficient under water than electrically coupled antennas. Using an electrically insulated antenna provides further advantages. This is because for a non-insulated antenna, there is a direct conduction path between it and the dissipative water. This leads to dissipation as the signal propagates along the antenna even before the electromagnetic signal is launched. Providing an electrically insulated antenna reduces this effect.

An electromagnetic wave can be characterised by magnitude (averaged over a cycle) and phase variation over time and space. Non-propagating near field components can be characterised in the same way. In air or free space, electromagnetic attenuation is low and phase varies with distance much more rapidly than magnitude. In air and free space direction finding techniques for remote sensing are based almost exclusively on phase information. Under water electromagnetic waves are rapidly attenuated and loss increases with frequency, hi this environment the signal magnitude varies more rapidly than phase. The inventors of the invention described herein are the first to recognise this fundamental difference and to describe remote sensing techniques that exploit this property.

hi water, an electromagnetic signal decreases in magnitude as it travels away from a source or a secondary source, which reflects an incident signal. This change of magnitude over distance can be fully described by a loss vector. Measurement of this loss vector gives an indication of the bearing to the reflecting object. The loss vector has a magnitude, which quantifies the rate of change of field strength, and a direction.

To establish a three dimensional loss vector, four field strength measurements are necessary, one to define an origin or point of reference and three others to allow the direction to be determined relative to the origin or point of reference. This can be done using conventional geometry to determine the direction of maximum loss. Ih some cases, the loss vector could be restricted to two dimensions, for example, when the location of an object is known to be on the sea floor. In this case, only three measurements would be needed.

In addition or as an alternative to using field strength, phase based direction finding techniques may be used to determine the direction of arrival.

In any case, the determining means may be operable to determine the location of the object using three or more receiver antennas at different locations or alternatively a single receiver antenna that is moved between three or more different locations.

The receiver antenna may be a directional antenna and the means for determining may be operable to use the directionality of the receiver to determine the relative directional position of the reflecting target. More specifically, the direction of reflected signal propagation may be determined by aligning a highly directional antenna.

Radiated wavelength is one of the aspects of remote sensing that determines the positional resolution of the system. In air propagating radar systems high frequencies with short wavelengths are used to achieve good positional resolution. In the underwater environment, attenuation increases with frequency. This limits the use of high frequencies over a useful range. The relative permittivity of water helps to some extent since this results in a substantial decrease in wavelength in water compared to air propagation at the same frequency. At 100 kHz air wavelength is 3 km while it is about 100 m in typical fresh water and around 5 m in sea water. To overcome the problem of attenuation, in one implementation of the invention continuous wave techniques are used to vary the frequency of the probe signal by, for example, starting at a high frequency and reducing it gradually until a return signal is detected. In the event that a return signal is detected the receiver is then moved closer to the target and the frequency increased. At this higher frequency spatial resolution is improved. In an alternative implementation, a reflected signal with a frequency offset proportional to range can be produced. In this implementation, an oscillator is typically used to ramp the frequency of transmitted signal over a period of time. This same oscillator signal is used to demodulate the received signal resulting in the frequency offset proportional to the range. As an alternative to ramping the frequency, the signal may be stepped in frequency between two or more values. The range may then be determined either through comparison of the resulting demodulated signal or through measuring the relative phase of the returned signal from each of the frequency values. The magnitude of the relative frequency in this case is much greater than it would be in air for the same distance because the velocity of signal propagation is very much less in water than in air.

Without using transmit frequency ramping, but instead a fixed frequency, relative distance can also be determined by comparing the relative delays of the reflected signals as received by the antennas. Signal delay or phase shift (its equivalent at a known frequency) is proportional to distance from the object, and therefore the differences can be used to calculate coordinates in a similar manner to signal strength differences. For delay to be used unambiguously, it will usually be necessary that the wavelength of the signal be less than the expected distance to and from the object.

The receiver may be remotely located from the transmitter or may be co-located. For example, the transmitter and receiver may both be located on a mobile, remote sensing station that is operable to move around in an underwater environment.

The location of the object may be determined relative to at least one of the receiver, the transmitter or a pre-determined reference.

The transmitted signal may have a frequency of below 3 MHz. The transmitted signal may have a frequency in the range of 100 Hz to 3 MHz, dependent on the distance and accuracy required.

According to another aspect of the present invention, there is provided a system for determining underwater electromagnetic signal propagation direction comprising at least one receiver for receiving the electromagnetic signal and determining means for determining the direction of propagation using field strength or phase or delay at three or more receiver locations.

Three or more receiver antennas may be provided and the determining means may be operable to determine the location of the object by comparison of the field strength or phase or delay of the signal received at each antenna.

Alternatively, a single receiver antenna may be provided and the determining means may be operable to determine the location of the object by comparison of the field strength or phase or delay received at three or more different receiver locations.

The system may be extended to detect objects buried under the seabed, hi this case, the remote sensing station may be located in the water or on the seabed. Having the sensing station on the seabed helps to maximise the energy coupled into the seabed.

Brief Description of 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 shows an operational illustration of an electromagnetic remote sensing underwater system;

Figure 2 is a block diagram of an underwater navigation system;

Figure 3 is a block diagram of an underwater transmitter for use in the underwater remote sensing system of Figure 2; Figure 4 is a block diagram of an underwater receiver for use in the remote sensing system of Figure 2;

Figure 5 is a diagrammatic representation of a magnetically coupled solenoid antenna in a waterproof enclosure for use in the transmitter of Figure 3 and the receiver of Figure 4; Figure 6 is a diagrammatic representation of a direction finding technique;

Figure 7 is a block diagram of a receiver arrangement for use in the technique of Figure 6; Figure 8 is a diagrammatic representation of the field pattern produced by a magnetically coupled solenoid antenna;

Figure 9 is a diagram of E-field polarisation alignment to maximise reflections from a linear target, and Figure 10 is a flow diagram representation of a remote sensing target seeking sequence.

The present invention relates to an underwater remote sensing system. The underwater environment is very different from air and requires completely new detection techniques from those applicable for air propagation systems. This is primarily because water exhibits a high dielectric permittivity and conductivity that leads to high attenuation. In practice this means that the water operational range is generally less than one wavelength and so sub-wavelength resolution techniques may have to be used.

Figure 1 shows a remote sensing system 10 that is operable to transmit an electromagnetic signal using an electrically insulated magnetic coupled antenna 12, and subsequently receive any radiation reflected from an object 14 that is on the propagation path of the transmitted signal. Figure 2 shows the sensing system 10 in more detail. This includes a transmitter 16 and a receiver 18 both of which have an electrically insulated magnetic coupled antenna 20 and 22 respectively. Connected to both of the transmitter 16 and the receiver 18 is a processor 24 that carries out signal processing of the received signal to calculate target properties. The processor 24 has a data interface (not shown) for providing remote sense data to related sub-systems. Although the transmitter and receiver 16 and 18 are shown as being co-located on the remote sensing station 10, this is not essential, and instead the transmitter and receiver 16 and 18 could be provided at different locations.

Figure 3 shows an example of a transmitter 16 for use in the remote sensing station 10 of Figure 2. This has a data interface 26 that is connected to each of a processor 28 and a waveform generator 30. The waveform generator 30 provides the amplitude or frequency modulation of a carrier wave that is required for various remote sensing techniques. At an output of the wave generator 30 are a frequency synthesiser 31 that provides a local oscillator signal for up-conversion of the carrier signal and a transmit amplifier 32, which is connected to the underwater, electrically insulated magnetic coupled antenna 20. In use, the processor 28 is operable to cause electromagnetic waveforms to be transmitted as required by the particular remote sensing technique in use. Typically, the electromagnetic signals have a frequency that is less than 3 MHz.

Figure 4 shows an example of a receiver 18 for use in the remote station 10 of Figure 2. This is operable to amplify the returned signal and minimise any interference such as from noise, the transmitted signal or external sources. As noted previously, the receiver 18 has an electrically insulated magnetic coupled antenna 22 adapted for underwater usage. This antenna 22 is operable to receive electromagnetic field signals from the transmitter antenna 20 that have been reflected from a target. Connected to the antenna 22 is a tuned filter 34 that is in turn connected to a receive amplifier 36. At the output of the amplifier 36 are a signal amplitude measurement module 38 that is coupled to a signal processor 40 and a frequency synthesiser 42 that provides a local oscillator signal for down conversion of the carrier. Connected to the signal processor 40 is a data processor 44 that is in turn connected to a data interface 46. The data interface 46 is provided for transferring data from the receiver 16 to a control or monitoring means (not shown), which may be located in the sensing station 10 or at another remote location. This is operable to take the raw information provided by the receiver and extract the information relating to the presence and/or location of objects. Techniques for doing this will be described later.

The frequency synthesisers of Figures 3 and 4 may be the same unit or linked such that a coherent system is provided. This allows phase processing of the received signals to be performed such as the comparison of the transmitted and received signal phase to allow range estimation. In another implementation, the units are not connected to allow them to be spatially separated, so that non-coherent processing can be performed.

Figure 5 shows an example of an electrically insulated, magnetic coupled antenna 20, 22 that can be used in the transmitter and receiver of Figures 3 and 4. This has a high permeability ferrite core 48. Wound round the core 48 are multiple loops 50 of an insulated wire. The number of turns of the wire and length to diameter ratio of the core 48 can be selected depending on the application. However, for operation at 125IcHz, one thousand turns and a 10:1 length to diameter ratio are suitable. The antenna 20, 22 is connected to the relevant transmitter 16 or receiver 18 and is included in a waterproof housing 52. Of course, whilst the transmitter and receiver 16 and 18 respectively are shown as having separate antennas, it will be appreciated that a single antenna could be used. Within the housing 52 the antenna may be surrounded by air or some other suitable insulator, for example, an impedance-matched low conductivity medium such as distilled water.

To determine a relative bearing of a reflecting target, direction-finding techniques are employed. Because the field strength is attenuated in the direction of propagation, the bearing of a target relative to the receiver can be determined by calculating a local loss gradient vector using a comparison of the field strength at three or more receiver positions, as shown in Figure 6. This can be done either by measuring the reflected signal at three or more different locations using a single receiver antenna, as shown in Figure 2, or by using three or more receiver antennas on the remote sensing station. The relative position of the target can be determined using standard triangulation techniques based on the measurement of three or more bearings distributed over a larger area.

To determine a two-dimensional position, three measurements can be used, thereby to determine to a two-dimensional loss vector. This can be useful when, for example, it is known that the transmitter is on the seabed. However, in many circumstances it is necessary to know the three dimensional position. To determine this, four field strength measurements would be necessary, this time to establish a three-dimensional loss vector.

For the sake of simplicity, a Cartesian co-ordinate system will be used to describe the principle of operation of the three dimensional loss vector, although any other co-ordinate systems would be equally applicable. In this case, the first measurement forms the origin of a standard 3 -axis Cartesian system. A further three field strength measurements are made at an equal distance along each of the three axes. This set of measurements gives three orthogonal loss vector components, which allows the direction of arrival and magnitude of the loss vector to be established through standard geometry.

For the sake of clarity, a system that is operable to determine a two dimensional loss vector is shown in Figure 7. This is a simplified representation of a remote sensing station 54. It has three electrically insulated magnetic coupled receiver antennas 56. Connected to each antenna 56 is a field magnitude measurement module 58 for measuring the field strength of the reflected signal that is received at each antenna 56. The measured magnitude data is made available to a processor 58 in the station 10. Because of the relatively high signal attenuation in the underwater environment, differences in signal magnitude will be measurable within the dimensions of a typical mobile, remote sensing station 54 that might, for example, accommodate three antennas at the vertices of an equilateral triangle with, for example, a separation of two metres. The two dimensional loss vector can be calculated by simple geometry using an algorithm executed by the remote station processor.

In addition or as an alternative to using signal strength, direction finding can also be done by measuring the relative delays or phase differences of three or more signals that are received at different locations. Where one antenna is further from the object than another, the phase of its received signal will be relatively retarded largely in proportion to the increased distance. Equally, signal delay may be measured, since phase and delay are directly related when the frequency is known. Knowing the velocity of propagation of the electromagnetic signal in water, itself dependent on frequency in partially conductive media, and the signal frequency in use, relative distances may be calculated readily from the signal phase differences, taking into account the doubled path length of the transmitted and reflected signal. A partially conductive medium such as water causes much higher signal attenuation than air and the attenuation reduces with frequency for the same launched signal power. Consequently, many applications will benefit from use of a low frequency provided that the signal phase can be measured with sufficient accuracy. Low frequency also avoids phase ambiguity when delay measurement is used because wavelength can then be longer than the distances to and from the object. To improve accuracy, especially where this otherwise might be degraded by noise interference when using a signal of low frequency, it usually will be beneficial before phase measurement to subject the received signals to narrowband filtering in some manner, for example by averaging many cycles of the received signal. Alternatively, with close functional equivalence, the filtering may be incorporated in the phase measurement process.

Another direction finding technique that may be used involves using a directional antenna. An example of this is a solenoid type antenna, as illustrated in Figure 8. This produces polarised propagating electromagnetic radiation. This is particularly useful when searching for a linear target, because the antenna could be rotated to produce a maximised reflected signal when the propagated E-field is aligned with the axis of the linear target as illustrated in Figure 9. Rotation of the antenna may be effected in any suitable manner, for example by movement of the remote sensing station itself or by provision of a mechanism for rotation of the antenna independently of the remote sensing station. The antennas may also be rotated electrically but employing multiple elements and time or phase shifting the feed signals to each antenna.

In practice, the remote sensing system in which the invention is embodied uses a range of different target seeking frequencies in order to focus in on targets and optimise sensing process. Low frequency offers greater range due to lower attenuation but may also achieve lower positional resolution, hi order to accurately locate an object by remote sensing, the process of Figure 10 may be used. In this case, the target seek sequence is initiated 60 and the remote sense system starts a scan at its highest frequency of operation 62, for example 3 MHz. A signal at this frequency is then transmitted 64, and the receiver receives signals that have potentially been reflected by a target 66. These received signals are analysed 68 to determine whether a target has been detected 70. In the event that a target is not detected, the transmission signal frequency is decreased 72, and the process of transmitting and receiving signals repeated. This is continued until the system receives a return signal of interest 70. In the event that a signal of interest is detected, the remote sense station moves toward the detected target 74 and increases the radiated frequency 76. In this, way the positional resolution improves as the remote sense station approaches the target. This procedure could be used, for example, to find and track a buried pipeline.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, although position- finding techniques are described, it will be appreciated that analysis of reflected signals could be used to determine the surface properties of the detected object. Information regarding an object's shape can be deduced by looking at reflections from several incident angles since strong reflections are produced from objects presenting flat normally incident surfaces to a co-located transmitter receiver system. Furthermore, some objects such as a linear metallic cable may reflect a signal with efficiency, which differs markedly with its orientation relative to signal polarisation. Also, as well as providing an indication of relative position, the remote sensing station may be operable to communicate the absolute position in space of an object found with reference to a standard co-ordinate system, for example latitude, longitude and altitude.

Whilst the primary function of the systems in which the invention is embodied is to find underwater objects, they could be used to implement underwater electromagnetic communications links. Also, whilst the systems and methods described are generally applicable to seawater, fresh water and any brackish composition in between, because relatively pure fresh water environments exhibit different electromagnetic propagation properties from saline seawater, different operating conditions may be needed in different environments. Any optimisation required for specific saline constitutions will be obvious to any practitioner skilled in this area. Accordingly the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. An underwater remote sensing system comprising a transmitter for transmitting an electromagnetic signal, a receiver for receiving a reflection of the electromagnetic signal from an object and determining means for using the received signal to determine the location of the object, wherein at least one of the transmitter and receiver is underwater.

2. An underwater remote sensing system as claimed in claim 1 wherein the determining means are operable to determine the location of the object using signals received at three or more receiver positions.

3. An underwater remote sensing system as claimed in claim 2 wherein three or more receiver antennas are provided and the determining means are operable to determine the location of the object using a signal received at each receiver.

4. An underwater remote sensing system as claimed in claim 2 wherein a single receiver antenna is provided and the determining means are operable to determine the location of the object using the signal received at three or more different receiver locations.

5. An underwater remote sensing system as claimed in any of the preceding claims wherein the determining means are operable to determine the location of the object using the magnitude and/or phase and/or delay of the signals received at the three or more receiver positions.

6. An underwater remote sensing system as claimed in any of the preceding claims wherein at least one of the receiver and transmitter has a magnetic coupled antenna.

7. An underwater remote sensing system as claimed in any of the preceding claims wherein at least one of the receiver and transmitter has an electrically insulated antenna.

8. An underwater remote sensing system as claimed in claim 7 wherein each insulated antenna is surrounded by an impedance-matched low conductivity medium, for example distilled water.

9. An underwater remote sensing system as claimed in any of the preceding claims wherein the receiver antenna is a directional antenna and the means for determining are operable to use the directionality of the receiver to determine the relative directional position of the reflecting object.

10. An underwater remote sensing system as claimed in any of the preceding claims wherein means are provided for varying the frequency of the transmitted signal.

11. An underwater remote sensing system as claimed in claim 10 comprising means for monitoring the reflected signal as a function of the frequency and selecting a new frequency or range of frequencies for the transmitted signal as a function of the monitored signal.

12. An underwater remote sensing system as claimed in any of the preceding claims wherein the transmitter and receiver are operable to implement a communications link.

13. An underwater remote sensing system as claimed in any of the preceding claims wherein the receiver shares an antenna with the transmitter.

14. An underwater remote sensing system as claimed in any of the preceding claims wherein the receiver is remotely located from the transmitter.

15. An underwater remote sensing system as claimed in any of claims 1 to 14 wherein the transmitter and receiver are substantially co-located.

16. An underwater remote sensing system as claimed in any of the preceding claims wherein the location of the object is determined relative to at least one of the receiver, the transmitter or a pre-determined reference.

17. An underwater remote sensing system as claimed in any of the preceding claims wherein the transmitted signal has a frequency of below 3 MHz.

18. An underwater remote sensing system as claimed in claim 17 wherein the transmitted signal has a frequency in the range of 100 Hz to 3 MHz.

19. A system for determining underwater electromagnetic signal propagation direction comprising at least one receiver for receiving the electromagnetic signal and determining means for determining the direction of propagation using field strength at three or more receiver locations.

20. A system as claimed in claim 18 wherein three or more receiver antennas are provided and the determining means are operable to determine the location of the object using the signal received at each antenna.

21. A system as claimed in claim 18 wherein a single receiver antenna is provided and the determining means are operable to determine the location of the object using the signal received at three or more different receiver locations.

22. A system as claimed in any of the preceding claims wherein at least one of the object being detected, a transmit antenna and a receive antenna is not submerged in water.

23. A system as claimed in any of the preceding claims wherein the object that is to be detected is under or in contact with the seabed.