GB2094981A

GB2094981A - Magnetic velocity measuring systems

Info

Publication number: GB2094981A
Application number: GB8108283A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-03-17
Filing date: 1981-03-17
Publication date: 1982-09-22
Also published as: DE3209235A1; GB2094981B

Abstract

To measure the speed of a vehicle, e.g. a low-flying aircraft (such as a missile or an unmanned reconnaissance aircraft) or a submarine, the vehicle has magnetic sensors (2, 3) spaced apart on its underside. These separately monitor the magnetic field in which the vehicle moves, i.e. the earth's magnetic field, at somewhat different times. The outputs of these sensors are converted into electrical form and applied to cross-correlate (6,7), and the outputs from these cross- correlators are applied to computing means to determine vehicle speeds. The sensors used are highly sensitive, e.g. Hall effect sensors or fibre-optic magnetic sensors. <IMAGE>

Description

SPECIFICATION Magnetic detection systems The present invention relates to apparatus for the determination of the velocity of manned or unmanned vehicles, which vehicles can be airborne, on land, or under water.

According to the invention there is provided apparatus for measuring the speed of a vehicle, which includes at least two magnetic sensors spaced apart in the direction of movement of the vehicle, which sensors separately monitor the magnetic field within which they are moving, and correlation circuitry to which signals representing the magnetic field as monitored by the sensors are applied, which circuitry derives from said signals an output indicative of the vehicle's speed.

The above invention exploits the fact that solid state sensors are now available which have extremely high bandwidth and fast response times, and which are highly sensitive to variations in magnetic field. Such sensors are superior to flux-gate magnetometers, which have a considerable inductance, while Hall effect devices are purely resistive. A Hall effect sensor as used in the present arrangement is, in effect, a Hall relay minus its coils, although it will be appreciated that the Hall effect sensors used in the present arrangement are "custom-designed" for this purpose.

A number of magnetic sensors are located on the underside of a vehicle, e.g. under the fuselage of an aerolplane or on booms extended from the vehicle. At any instant in time the various magnetic sensors detect different values of magnetic field, and as the vehicle moves, the sensors respond to the same magnetic field conditions at slighly different times. Thus each of the peaks in the magnetic field which is detected by the front-most sensor is detected by the rearmost sensor at a slightly later time.

The variations in magnetic field detected by the sensors are therefore detected and subjected to correlation analysis to determine the vector of movement. From this can be determined the speed of the vehicle. The accuracy of such a method of measurement increases with closeness to the ground, which is the reverse of what applies with certain other measurement techniques. The correlation techniques can also be used to determine the distance of the vehicle from the surface. Thus low-fiying missiles or reconnaissance vehicles can pick up land variations due to anomalies, while at high altitudes the lines of force which influence the sensors flatten and merge, so accuracy decreases.

To improve accuracy, especially in the case of a small unmanned aircraft, e.g. used as a reconnaissance vehicle, one or more sensors may be mounted on booms extending from the nose of the aircraft forwardly, and/or from its tail backwardly. Where the technique is applied to a submarine, the submarine is comparatively close to the anomalies, so accuracy may be good, depending on the speed range.

We now refer to the drawing in which Fig. 1 shows schematically an aeroplane fitted with two sensors, Fig. 2 shows a typical magndhic field pattern to which the sensors respond, and Fig. 3 is a highly simplified block diagram of the "hardware" implementation of an embodiment of the invention.

In Fig. 1 we show the aeroplane 1 which may be an unmanned reconnaissance vehicle or a missile, fitted with two sensors 2, 3, each carried on a boom extending from one of the extremities of the aeroplane. Fig. 2 is an example of the sort of magnetic field to which the sensors each respond. Sensor 3 responds to such a pattern somewhat later than does sensor 2, the time difference being dependent, inter alia, on the distance between the sensors and on the aeroplane's speed. Thus two patterns are developed, which are in general similar. These are applied, see Fig. 3, from the sensors 2, 3 via amplifier 4, 5 to cross-correlators 6 and 7 the outputs from which are applied to a computer.

This can be expanded to incorporate correlation information from further detectors oriented at different angles and relative positions on the vehicle. Depending on the requirements, the computer outputs can provide information on aeroplane speed over the ground, climb angle and distance from the surface.

We now consider the underlying theory on which the present arrangements are based. For any correlation or phase angle technique, ideally the dimensional characteristic which is moving should be comparable with the spacing of the detectors, whose sensitivity should be such that they can respond within that distance.

To consider a specific example, we assume that the aircraft is flying at 30 metres and the sensors are spaced at 3 metres. Results of geomagnetic surveys taken at 1000 feet (about 330 metres) over land indicate a varition, due to magnetic anomalies and other factors, of typically 600 nT/km, i.e. nanoteslas per kilometre.

ly=l 0-5, G=1 0-9T, so this gives a variation of 0.6y. Naturally if the measurement is done at 30 metres the variation is considerably greater.

Hall effect sensors, and also fibre optic magnetic sensors (see "Fibre Optics Magnetic Sensors, Yariv and Winsor, Optics Letters, Vol. 5, March 1980, pp 87-89) have sensitivities better than 0.6y, and in fact Hall sensors should measure to at least 0.01y. Both types of sensors have a wide-band width, which increases the ease of correlation, and improves accuracy.

However, the specification of these sensors here should not be prohibitive in the use of other magnetic sensors which may be suitable.

Thus it will be seen that the use of magnetic sensing and correlation techniques enables dead reckoning measurement of low-flying aircraft, which gives velocity relative to the ground, and possibly also angles relative to 00, i.e vertical movement. Note also that such magnetic detectors are low power devices which are essentially covert as they do not emit radiation.

We have mentioned fibre-optic magnetic sensors: such a sensor uses a magnetostrictively coated optical fibre supported non-magnetically.

The magnetic field being monitored causes the magnetostrictive coating to influence the light transmissive characteristics of the fibre, and hence the detection involves monitoring light in the fibre. The use of oriented spaced optical fibre magnetic sensors on the aircraft could provide the required information. Microphonic noise can be filtered using parallei (parallel with the magnetic fibres) non-magnetic detecting fibres, and subtracting-out the effect. Since the magnetic correlation noise is being used for velocity measurement, multimode fibre can be used as well as monomode (where phase difference effects can be measured) fibre. The speckle pattern movement across the optical detector surface from the multimode fibre will provide a frequency distribution consistent with the perturbations to the magnetic sensor.

The correlation system can be coupled with other sensors (e.g. the barometric accelerometer which provides data on vertical velocity) to provide simplified navigational.aid.

Claims

1. Apparatus for measuring the speed of a vehicle, which includes at least two magnetic sensors spaced apart in the direction of movement of the vehicle, which sensors separately monitor the magnetic field within which they are moving, and correlation circuitry to which signals representing the magnetic field as monitored by the sensors are applied, which circuitry derives said signals an output indicative of the vehicle's speed.

2. Apparatus as claimed in claim 1, and in which the sensors are fibre optic magnetic sensors.

3. Apparatus as claimed in claim 1, and in which the sensors are Hall effect magnetic sensors.

4. Apparatus for measuring the speed of a vehicle, substantially as described with reference to the accompanying drawings.

New Claim filed on 6 October 1981.

New Claim:

5. Apparatus for measuring the speed of a vehicle, in which at least two magnetic sensors are mounted on the vehicle and are spaced apart in the direction of movement of the vehicle, in which the sensors separately monitor the magnetic field within which the vehicle and hence the sensors mounted thereon move, in which the said magnetic field varies in amplitude so that signals representing the sensed magnetic field are produced by all of the sensors but at different 'times, the times between the signals from the sensors being dependent on the speed of the vehicle, and in which the signals representing the magnetic field as monitored by the sensors are applied to correlation circuitry in the vehicle, which circuitry derives from the signals an output indicative of the vehicle's speed.