GB2177511A - Measuring ship's magnetic signature - Google Patents

Abstract

A portable magnetic self-ranging system, for making measurements of a ship's magnetic signature to enable degaussing compensation to be made, includes a seabed unit 2 containing a 3-axis flux gate magnetometer 3, a depth Sensor 7, and an acoustic signal generator 4; information from the seabed unit 2 being transmitted to the ship 1 via a buoy 11 releasably deployed from the seabed unit. The ship includes acoustic sensors 5 and 6 respectively at bow and stern to give tracking information to a ship's computer 14 during passage of the ship near the seabed unit. <IMAGE>

Description

SPECIFICATION A magnetic self-ranging system for use in the degaussing of ships The invention relates to a self-ranging system to enable a vessel to make adequate measurements of its magnetic signature and then to compute any compensation required.

Most naval vessels are fitted with degaussing systems which afford a degree of protection against sea mines with magnetic sensors. These degaussing systems initially have to be calibrated and then periodically checked and adjusted. The calibration and checking measurements are conventionally carried out at degaussing ranges. The degaussing system may be carried onboard as a permanent arrangement or there may be coils and magnets for temporary off-board use as disclosed in UK patent application No 8318111. There are a number of different types of degaussing ranges, the most common being the open degaussiny range. This comprises a number of magnetometer units (usually about 18) on a framework on the sea bed which are connected by cable to shore-based instrumentation. A vessel is required to sail over one or two of these underwater arrays on reciprocal headings.From the information gained, the amount of adjustment required can be calculated so as to give minimum residual fields and enable a forecast of the system settings in other latitudes to be made.

This forecast makes two assumptions, both associated with the permanent magnetism content of the vessel. The amount of permanent vertical magnetism present cannot be determined trom measurements in one latitude - a substantial variation in the vertical component of the Earth's magnetic intensity is required, leading to a need for degaussing ranges at widely different latitudes. The available maximum variation of magnetic intensity in the UK, between Scotland and the South Coast, is insufficient for an assessment of the permanent vertical magnetism of a ship. Therefore, an empirical ratio of permanent to total vertical magnetization of the vessel, based upon modelwork, is assumed.If the vessel sails to another latitude and stays there for some time, say six months or more, the permanent vertical magnetism will change towards the equilibrium value for the new area.

The permanent horizontal magnetization comprises athwartships and longituinal components.

On a long voyage which is basically on one heading there will be variation in both of these components, but prirnarily in the longitudinal component.

This is due to the continous vibration of the Ship's hull in the Earth's magnetic field. This effect causes the degaussing compensation applied at the degaussing range at the home port to become incorrect.

Thus, there is a need for a degaussing range to be available at the end of a voyage to another latitude. This second range measurement will check the compensation of the vertical and horizontal magnetization and allow the compensation to be adjusted accordingly.

As permanent magnetic measurement facilities are not available worldwide, a mobile degaussing facility is required. In the past, transportable degaussing ranges have been used for some of these purposes. The transportable ranges have been similar to the fixed open degaussing ranges. The sensor framework, of about 18 sensors, is placed on the seabed by divers and is connected to a land-based, moveable instrumentation unit. Thus these ranges have major disadvantages in the support and time required for deployment and recovery.

It is an object of the present invention to provide a magnetic self-ranging system for a ship, the system being portable so that it may be transported aboard the ship.

According to the present invention a magnetic ranging system for determining the magnetic signature of a ship comprises: a) a magnetic sensor for placement on the seabed; b) an acoustic tracking system to determine the position of the ship relative to the magnetic sensor; c) a telemetry link, to transfer the magnetic signature and any positional information from the seabed to a surface receiver; d) a means for obtaining ship heading and speed information from onboard equipment; and e) computer facilities to store the measurement data and to perform the necessary calculations.

Advantageously the invention provides for the surface receiver to be onboard the ship so that the ship carries its own degaussing ranging facility. An individual ship would then be capable of measuring and applying the appropriate compensation to its magnetic signature and the operation may be carried out in a significantly shorter time and without calling upon external support. Thus, the selfranging system can offer greater flexibility and speed of response.

The fact that the equipment is to be carried by a ship means that it must be reasonably small and transportable. The facility could also be held for possible use in any port, or it could be air transported to a particular area where shipping is trapped by a minefield. In a Task Force situation one vessel could deploy the equipment and the other vessels use the underwater equipment, providing they have the necessary complementary onboard equipment.

The port breakout situation is of particular interest. Normally the degaussing coils on board a ship are set to compensate the permanent and induced components of magnetization independently. In a breakout situation this may not be necessary, as only the magnetic signature for the exit heading needs to be considered. If there is more than one heading associated with the exit then the need to change the compensation will depend upon the required changes in heading, for example changes of less than 10 degrees are probably insignificant.

In this situation the magnetic signature minimisation can be achieved by external means such as electrical coils or magnets floating alongside the ship. If such external means are used there will be no need to make changes in the onboard degaussing system. The resulting signature of the vessel can then be checked by the self-ranging system to make sure that it is as required at typical mining distances.

It is considered that the self-ranging system will overcome the problems of the transportable degaussing ranges and yet fulfil the same requirements. The system may be designed to be portable and, in certain circumstances, to be cheap enough so that the underwater equipment may be abandoned if circumstances prevent its recovery.

The magnetic sensor is preferably enclosed in an underwater container which can be laid and recovered by the parent vessel. Preferably the underwater container contains a magnetometer, a depth sensor and a data collection system.

Use of a single unit increases the reliability of the system and assists in its deployment and recovery by the parent vessel.

In order to transfer the measured data to the parent vessel, the information is preferably digitised for transmission by means of the telemetry link.

Preferably the telemetry link includes a surface buoy. The buoy may be permanently on the surface or it may rise to the surface on receipt of an acoustic command, depending on which system is the most suitable.

The measurement data is preferably transmitted via the telemetry link to a ship receiver and, together with information regarding the ship's heading and speed and the ship's tracking information, is connected to a computer onboard the ship to develop the magnetic model of the ship.

The acoustic tracking system advantageously includes an acoustic transmitter in the underwater container and an acoustic transponder located on the ship. Advantageously respective transponders may be located at the bow and the stern of the ship which will give additional information on the relative heading of the ship.

The computer has information stored concerning the effects of the onboard degaussing system or offboard temporary coils and magnets, and this information is used together with the measurement data to complete changes necessary for the required final magnetic signature.

It will be appreciated that the use of an onboard computer and a telemetry link to transfer data is not the only solution. In the port breakout situation, the information recorded by the underwater unit can be transferred to the shore or to another ship by an airborne signal telemetry link or by cable. In this case, temporary equipment will need to be installed on the vessel being measured to receive the acoustic emissions at the bow and stern transponders in the correct time sequence and to telemeter this information to the base station..Details of the ship's heading and speed can be transmitted by voice if required.

There are a number of methods that can be used to obtain the magnetic field measurements. The preferred system is the three axis fluxgate vector magnetometer which is the most readily available and reliable. Vector outputs are easily analysed and can conveniently be used with existing computer software programs. Alternatively three axis Hall effect magnetometers may be used but these are not so readily available. Other possibilities are the three axis cryogenic vector magnetometer and the total field scalor magnetometer, both of which are unnecessarily sensitive for the range of magnetic measurements required, or the three axis fluxgate vector gradiometer which measures the magnetic field gradients.This though would create technical problems in the measuring of the lowest possible magnetic field gradients and also the inter-relationship between magnetic field and magnetic field gradient of a degaussed ship is complicated. Advantageously the underwater unit should be deployable on the seabed without the use of divers and thus the orientation of the magnetometer will not be controllable. Preferably a suitable gimballing system is used to ensure that the vertical fluxgate sensor is vertical. The underwater unit is preferably designed so that it will sit on the seabed in a stable manner. Preferably it is capable of falling from the surface in an upright manner so that the gimballed fluxgate sensor can attain the correct vertical attitude.The two horizon- tal fluxgate sensors may then have any orientation and therefore, in the intital stage after deployment, this orientation will need to be determined by measuring the Earth's magnetic field horizontal component along the two axis.

The maximum value of the horizontal component of the Earth's magnetic field is 45,000 nT (nano tesla) and for the vertical component is 55,000 nT. The range of magnetic measurements required for vessels is approximately 1 nT to 125,000 nT. Thus the dynamic range of the magnetometer required to measure the largest ship field is sufficient to measure the Earth's field components. The Earth's magnetic field components as seen by the sensors should preferably be compensated before ship magnetic measurements begin.

In order to operate at these sensitivies the equipment in the underwater unit is advantageously manufactured in non-magnetic or low magnetic content materials.

Preferably the seabed is unit is arranged so that it does not vibrate or resonate in underwater tidal streams. These vibrations are likely to occur when a buoy is attached directly to the underwater body.

If a buoy is necessary it is preferably arranged with its own sinker, which should not be rigidly attached to the underwater unit. An alternative system would be for the unit to contain a small winch and buoy system. During measurements the buoy is withdrawn into the unit so that it cannot cause vibration and on receiving an acoustic command the buoy can be released to the surface. The surface buoy can be used to house the antenna for transmitting the magnetic field data to the ship. It is also useful as an aid to recovery of the system and as a visual guide to direct the vessel to the measurement location.

The acoustic system preferably has to meet four requirements which are, in order of priority; a) to provide information on the distance between the magnetic sensors and a position (or positions) on the vessel being measured; b) to provide information to the ship's helmsman to enable him to steer the required course; c) to enable the underwater unit to be relocated when the ship is not in the immediate area; and d) to give commands to the underwater unit if required.

To enable the underwater unit to be re-located, the acoustic device in the seabed unit is advantageously the active one.

Preferably the system is able to track both the bow and the stern of the ship so that the ship track can be compared with the heading and the appropriate corrections allowed for in the computer software.

Preferably the degaussing system can be adjusted to a similar accuracy to that achieved on a normal open degaussing range, so the acoustic system should be capable of establishing the distance to the ship to within 058 1 metre. This is most essential at and around the closest point of approach.

For a particular type of magnetic field sensor there is a minimum field strength that can be measured with acceptable accuracy. The closest point of approach should be selected therefore such that the general level of the field components remains significantly higher than the minimum detectable field strength.

The magnitude of the components of the field gradient are important because they influence the error in the components of the ship's magnetic field that are associated with a particular position with respect to the ship. If the position of the ship relative to the magnetic sensor unit is incorrectly determined then the difference between the measured field components and the field components which should be associated with the measured ship position will be greater for the higher field gradients.

Thus, the requirements for field strength and field gradient are opposing and hence an optimum range of closest crossing distances should exist.

The maximum magitude of field strength likely to be encountered at 30m depth is about 12,000 nT. Values of over 100,000nT are likely to be present at 10m depth or less. The maximum valves of field strength for a small ship with compensated signature could be as low as about 150nT at 30m, and potentially under 50nT at 50m depth.

For a large ship with an uncompensated signature and an acoustic tracking accuracy of 1m, in or der to limit the errors in the field values to the region of 5%, the ship should not pass the mag netic sensor unit with a plan range (crossing distance) of less than 25m. An adequate field strength is maintained in a plan range of at least 100m. The magnetic field components are at least 50% of the maximum component values as the ship passes the magnetic sensors with a crossing distance of 20-25m.

The results for the ship with the compensated (low) magnetic signature are not so easy to interpret due to the asymmetry of the signature. For these ships it appears that for a water depth of 30m, a plan crossing distance of 20-25m is most appropriate.

Hence, in general, for a water depth of 30m and a tracking accuracy of 1m, the most appropriate plan crossing distance for a ship using the selfranging facility is 20-25m. For ships with large magnetic signatures a crossing distance of up to 70m is acceptable.

The acoustic tracking system provides information to the operator to enable the ship to be manoeuvred to sail past the seabed sensor unit with an appropriate crossing distance for ship signature readings to be taken. The tracking system provides; a) the distance from the ship to the seabed unit; b) the predicted crossing distance; c) the ship velocity; d) the angle of sway of the ship (angle between ship heading and ship track).

It also enables the position of the ship with respect to the seabed sensor unit to be known at regular intervals as the ship sails past the unit.

Specifically the point of intersection on the magnetic axis of the ship of the perpendicular from the magnetometer to the magnetic axis is required.

The length of the perpendicular and the angle between the perpendicular and the ship's vertical axis are derived from the tracking system data.

Two acoustic sensor or transponder are preferably sited on the ship as part of the acoustic tracking system in order to achieve the requirement for a maximum uncertainty of 1 metre in the position of any point on the magnetic axis of the ship.

However, it is possible to use the self-ranging system with only a single sensor fitted to the ship under particular circumstances.

With a single sensor on the ship other positions along the magnetic axis of the ship may be determined by projecting from the known position of the sensor using the ship heading information. In general, however, where only a gyro-compass may be available, this method does not locate all points along the ship's axis with adequate accuracy. Thus it is considered that two acoustic sensors should preferably be fitted, near to the bow and stern of the ship, in order to achieve the required positional accuracy. With only one sensor fitted the effectiveness of the self-ranging system, and hence the accuracy of the evaluated signature compensation would be reduced.

One possible tracking system is the ranging positional fixing system which measures distances of different sensors to the ships. An alternative is the range-bearing positional fixing system.

The ranging positional fixing system can be one of two forms; i) an acoustic transmitter/receiver attached to the bow and stern of the ship, and a transducer fitter to the seabed sensor unit; ii) a transmitter fitted to the seabed unit and hydrophones attached to the bow and stern of the ship.

Because of the requirement for the seabed unit to contain an active acoustic device for relocation purposes, the second form is preferred.

The geometry of the ranging positional range system is such that there is ambiguity as to which side of the seabed sensor unit the ship is located.

This ambiguity must be resolved for the measured field components to be converted into a dipole representation of the ship.

The ambiguity in position in the athwartships direction which is inherent in this acoustic tracking system is preferably overcome by placing an array of acoustic sensors on the seabed unit containing the magnetic sensors. Such an array of sensors would determine uniquely any position with respect to the seabed sensor unit. In addition, only one sensor would need to be mounted on certain ships if three sensors were placed on the seabed.

The ambiguity can then be resolved by considering the relative times of arrival of the acoustic signal at each of the three sensors. The simplification in the ship fittings achieved in this system is at the expense of an increase in data which would have to be measured at the sensor unit.

There are at least two alternative methods of implementing such a system: a) by placing at least three acoustic sensors on the underwater unit, or b) by placing at least one acoustic sensor on the seabed unit and including at least one additional acoustic device on the seabed, separate from the sensor unit.

If at least three sensors are placed on the underwater unit, as in the first option, the maximum sensor separation which can be achieved is about 0.5m, therefore an acoustic frequency of greater than 100KHz is required. The transmission loss at this frequency (typically 30dB/km) is not a problem when the ship is at or near its closest point of approach to the seabed unit. However, problems might be experienced in locating the seabed unit after a run, in which case an acoustic beacon operating at a lower frequency could be fitted.

It would be possible to increase the separation and hence reduce the frequency if the sensors were mounted on booms. However this would complicate the deployment of the seabed sensor unit and hence lead to reliability problems.

The system in the first option has the advantages that: i) only one unit need be deployed into the sea; ii) the angle of sway can be determined; iii) under certain conditions only one sensor need be mounted on the ship; iv) any orientation of the ship trade with respect to the seabed unit can be accommodated; and v) all three-co-ordinates (x, y, z) of a sensor on the ship can be measured by the tracking system.

This is important in retaining the performance of the tracking system as sea-state increases.

However, it also has disadvantages, including: i) the effective sensor separation is affected by the attitude of the underwater unit after settling on the seabed.

ii) real-time data transmission from the three sensors to the ship is needed to allow- corrections in the ship's course; iii) the high frequency (greater than 100kHz) reduces the availability of existing commercial hardware; and iv) an additional acoustic beacon, operating at a lower frequency, may be needed to locate the seabed unit and to guide a ship toward the seabed unit during a measurement run.

A preferred acoustic tracking system includes an additional acoustic device, as in the second option, laid in a line with the seabed sensor unit, approximately at right angles to the intended course of the ship during self-ranging. This system has the following advantages; i) the angle of sway can be determined; ii) only one range need be recorded at the seabed sensor unit (this is the range between the seabed unit and the additional acoustic device which is constant and hence need only be sampled at the start and end of a run).Hence there is a lower range data rate at the seabed sensor unit and lower real-time data transmission to the ship than for the system without the additional acoustic device for corrections to the ship's course; iii) a synchronised acoustic transmitter can be used as the additional acoustic device with existing equipment; iv) this acoustic tracking system is less sensitive than the first option to the attitude of the seabed unit; v) a wide range of existing hardware is available due to the lack of restrictions on the operating frequency (typically 20kHz); and vi) a marker buoy could be attached to the additional acoustic beacon to provide a visual reference. If an RF link was used to relay data (magnetic information etc) to the ship then the antenna could be attached to the buoy.

The disadvantages of the system include; i) the system requires two acoustic sensors to be fitted to the ship; ii) the tracking system can only be used over a limited range of angles between the ship track and the line between the seabed unit and the additional acoustic beacon; and iii) only the x and y co-ordinates of a sensor on the ship are measured by the tracking system. The depth of the ship sensor is required as an input to the tracking geometry.

The first system has some operational advantages over the second in that only a single unit seeds to be deployed and there is the potential to operate the self-ranging system with only a single sensor fitted to the ship. As this sensor could be an existing sensor this mode of operation could be of particular advantage to Naval vessels. Operationally it could be considered that the deployment of the extended version of the second system would be more complicated than that of the first because of the additional acoustic beacons. However, the seabed sensor unit and the main syn chronised- acoustic transmitter unit would be deployed as a single package and the requirement on the spacing between the acoustic beacons is such that the auxilliary beacon could be deployed from the opposite side of a ship at the same time.

A benefit of the second system from the data security standpoint is that the ranging information is carried accoustically to the ship while the magnetic signature data passes over an RF data link. This is less vulnerable than the first system where both ranging and signature data are passed by RF link.

An alternative positional fixing system is the range-bearing tracking system. Such a system utilises three acoustic sensors (or possibly four) on the seabed sensor unit. In this case the sensor separation is less than or equal to half a wavelength and hence, at an appropriate acoustic frequency, the sensor separation could be achieved easily on the body of the seabed sensor unit.

The performance of the range-bearing system is affected by the attitude of the seabed sensor unit after it has settled on the seabed. Any deviation of the sensor unit from its intended attitude would reduce the effective separation of the sensors and hence broaden the tracking beam. Another problem associated with this system is the effect of the angular extent of the ship, which is not compatible with the narrow beam widths required for accurate position measurement. As such the range bearing system is not particularly suitable for use with the magnetic self ranging system.

In order that the invention may be more fully understood, it will be described in more detail, by way of example only, with reference to the accompanying drawings, wherein; Figure 1 shows a block diagram of a magnetic self-ranging system; Figures 2a and 2b show two alternative configurations of the underwater unit and the telemetry buoy; Figure 3 is a schematic diagram of a method of implementing the acoustic tracking system; Figure 4 is a schematic diagram of an alternative method of implementing the acoustic tracking system.

Referring to Figure 1, a system of measuring the magnetic signature of a ship 1 by means of a 3axis fluxgate vector magnetometer in an underwater unit 2 is shown. The position of the slip 1 is determined by means of an acoustic system 4 which transmits acoustic signals to transponders Sand 6 at the bow and stern of the ship 1. A trigger pulse from the acoustic system 4, together with the magnetic signature information from the magnetometer 3 and the information about the depth of the underwater unit from depth sensors 7, is passed to a data collection system 8 where the information is digitised and passed via a cable link 9 to a telemetry transmitter 10 housed in abuoy 11. The informaiton is then transmitted via a telemetry link 12 to a receiver 13 on the ship 1.The information from the telemetry receiver 13 is passed to a computer 14 together with information on the ship heading and speed from a ship data system 15 and information on the ship track from a tracking system 16. The tracking system 16 derives the ship track from information received from the bow and stern transponders 5 and 6.

The computer 14 stores the measurement data received and performs the necessary calculations.

Referring now to Figures 2a and 2b, two alternatives configurations of the underwater unit and the telemetry buoy are shown. In Figure 2a the buoy 11 is shown connected to a sinker 20 by a wire 21, the sinker 20 is connected to the underwater body 2 by a non-rigid connection 22. The data from the underwater unit 2 is passed to the telemetry transmitter 10, housed in the buoy 11 via the cable link 9.

Figure 2b shows a system wherein the buoy 11 is withdrawn into a niche 23 in the underwater unit 2 by a small winch system (not shown) while the acoustic and magnetic measurements are being made. After the measurement data has been stored by the data collection system 8 the body is released to the surface by an acoustic command from the ship 1 and the measurement data is then transmitted to the ship 1 via the telemetry transmitter 10 housed in the buoy 11, and the link 9.

Figure 3 is a schematic diagram of a methodsof implementing the acoustic tracking system. Three acoustic sensors, 4a, 4b and 4c, are included in the underwater unit. The three sensors are placed as far apart as is possible in the underwater unit.

Measurements from each sensor, 4a, b, and c are then used to measure the distance to the acoustic transmitters 5 and 6 on the ship 1. Thus there are three distances measured for each acoustic transmitter 5 and 6, on the ship 1 to the underwater unit 2 so that the position of the ship relative to the underwater unit can be accurately fixed. The acoustic sensor information is passed to the data collection system 8 along with the information from the 3axis magnetometer 3 and the depth sensor 7. Attitude sensors 30 also are included in the underwater unit 2 as the effective separation of the acoustic sensors, 4a b and c, is affected by the attitudes of the underwater unit on the seabed. The information is then passed to the ship 1 via the telemetry transmitter 10 in the buoy 11.

Figure 4 shows an alternative method implementing the acoustic tracking system. In this case there is only one acoustic sensor 4d on the underwater unit 2 and there is an additional acoustic sensor 40 separate from the underwater unit 2. The acoustic sensor 4d measures the distance r, and r,' to the transponders 5 and 6 on the ship 1 and the additional acoustic sensor 40 measures the distance r2 and r2, to the transponders 5 and 6. The acoustic sensor 4d also measures the distance r3 from the underwater unit 2 to the additional acoustic sensor 40. The measurement data collected by the additional sensor 40 is stored in a data collection system 41 and transmitted via a radio frequency link 40 to the data collection system 8 in the underwater unit 2.The positional measurement data from the acoustic sensor 4d and the additional acoustic sensor 40 are collected, together with the information from the magnetic sensor 3 and the depth sensor 7, in the data collection system 8 are transmitted to the ship 1 as before via the telemetry transmitter 10 housed in the buoy 11.

It is possible to represent the magnetic field of a ship as that due to a three-axis matrix configuration of a number of magnetic dipoles. To obtain this matrix, the invention makes use of the fact that it is only necessary to make three-axis measurements at a single position as the ship sails past.

There are three stages in the operation of the magnetic self-ranging system; i) the measurement by a magnetic sensor deployed on the seabed of the three component of a ships magnetic signature as the ship sails past the seabed sensor. The position of the ship relative to the magnetic sensor is monitored using an acoustic tracking system; ii) the use of a computer program to represent the ship by a three-dimensioned array of magnetic dipoles using the signature and positional measurements made in (i); and iii) the computation of changes to the degaussing coil settings to improve the compensation of the ship's signature. The same process can be implemented for a ship which is not fitted with internal degaussing coils. The system then computes the location of external coils or magnets for signature compensation.

Once the system has been deployed, during the subsequent measurement phase, the self-ranging system must perform two functions: i) the system provides information in real time to an operator to enable the ship to be manoeuvred to sail past the seabed unit with an appropriate plan crossing distance for ship signature readings to be taken. The information is distance to the seabed unit predicted plan crossing distance ship velocity along its track angle of sway of the ship.

Normally a ship is required to sail past the seabed unit on reciprocal intercardinal headings (headings other than N.S.E or W). in certain operational circumstances, such as port breakout situations, where a particular heading is of specific importance, a ship will only need to make a single measurement run past the seabed unit, on that heading; ii) information is collected and stored during a measurement run both at the ship and at the seabed unit. At the end of the run information from the seabed unit is transmitted back to the ship using an RF (or possibly acoustic) link. Data collected is then processed to determine any changes that are needed in the compensation of the ship's signature.

The vertical sensor of the three axis fluxgate vector magnetometer 3 is set in a suitable gimballing system so that it is in a vertical position when the underwater unit 2 settles on the seabed. The underwater unit 2 is weighted so that it sinks to the seabed in an approximately upright position so that the gimballing system is able to position the vertical sensor correctly.

It will be apparent to those skilled in the art that any suitable magnetic, acoustic, depth and attitude sensors may be used and there will be many mod if location possible within the scope of the invention.

Claims (16)

1. A magnetic ranging system for determining the magnetic signature of a ship comprises; a. a magnetic sensor for placement on the seabed; b. an acoustic tracking system to determine the position of the ship relative to the magnetic sensor; c. a telemetry link, to transfer the magnetic signature and any positional information from the seabed to a surface receiver; d. a means for obtaining ship heading and speed information from onboard equipment; and e. computer facilities to store the measurement data and to perform the necessary calculations.

2. A magnetic ranging system as claimed in claim 1 or 2 wherein the surface receiver is adopted to be located on board the ship.

3. A magnetic ranging system as claimed in claim 1 or 2 wherein there is provided a depth sensor and a data collection system for the magnetic sensor; and magnetic sensor, the depth sensor and the data collection system are enclosed in a watertight container.

4. A magnetic ranging system as claimed in claim 3 wherein the information is digitised for transmission by means of the telemetry link.

5. A magnetic ranging system as claimed in any one preceding claim wherein the telemetry link includes a surface buoy.

6. A magnetic ranging system as claimed in claim 5 wherein the buoy is attached to the watertight container and is releasable therefrom for development on the surface of the water on receipt of an acoustic command.

7. A magnetic ranging system as claimed in any one preceding claim wherein the acoustic tracking system includes an acoustic transmitter in the underwater container and an acoustic transponder located on the ship.

8. A magnetic ranging system as claimed in claim 7 wherein respective transponders are located at the bow and the sterm of the ship to give additional information on the relative heading of the ship.

9. A magnetic ranging ranging system as claimed in any one preceding claim wherein the coumputer has information stored concerning the effects of the onboard degaussing system or offboard temporary coils and magnets, and this information is used together with the measurement data to compute changes necessary for the required final magnetic signature.

10. A magnetic ranging system as claimed in any one preceding claim wherein the magnetic sensor is a 3-axis fluxgate magnetometer.

11. A magnetic ranging system as claimed in any one preceding claim wherein the fluxgate magnetometer is gimal mounted to ensure that one sensor is vertical.

13. A magnetic ranging system as claimed in any one preceding claim wherein the watertight container is so designed that it will remain stably located on the seabed once deployed.

12. A magnetic ranging system as claimed in claim 11 wherein there is included means to determine the orientation of the non-vertical sensors by reference to measurements of the Earth's magnetic field.

13. A magnetic ranging system as claimed in any one preceding claim wherein the acoustic tracking system inlcudes an array of sensors on the seabed.

14. A magnetic ranging system as claimed in claim 13 wherein at least one acoustic sensor is located in a first seabed unit including the magnetic sensor and at least one additional acoustic sensor is provided in a further seabed unit for deployment on the seabed in spaced relationship to the first unit.

15. A magnetic ranging system substantially as described with reference to Figures 1, 2 and 3 of the accompanying Drawings.

16. A magnetic ranging system substantially as described with reference to Figures 1, 2 and 4 of the accompanying Drawings.