GB2204401A - Direction finding - Google Patents

Abstract

A direction finder comprises a first sensor for monitoring the earth's magnetic field in order to indicate a first direction, means for storing an indication of the direction and elevation of a subject e.g. satellite whose position is to be found, and a sighting device movable to a position whereby a user may look at the object. A first embodiment utilises a magnetic compass 6 to monitor the earth's magnetic field and marks on an optical instrument 7, 8 to indicate the direction and elevation of the object. A second embodiment utilizes electro-magnetic sensors in order to detect the disposition of the sighting device in 3-dimension, a processor and a memory which stores the coordinates of the object whereby to permit a user to look at the object through the sighting device. The device is used for line-of-sight alignment of an aerial with a broadcasting satellite. <IMAGE>

Description

Direction Finding The present invention relates to the field of direction finding and the associated subject of direction measurement. In particular, the invention concerns the use of the Earth's magnetic field to find and/or measure directions.

Hereinafter the term "direction finder" shall be used to indicate both true direction finders (i.e.

devices with the principal function of indicating a desired direction in space when specified in terms of coordinates in a known co-ordinate system), and also direction measuring devices (i.e. devices adapted to classify a spatial direction in terms of co-ordinates in a known co-ordinate system). The use of a common term to encompass both types of device is appropriate because the types of device are to some extent interchangeable, for example, a direction measuring device may be used to locate a desired direction in space by repeatedly altering the direction being measured until the desired direction is found.

Conventional direction finders, e.g. those used in navigation, consist of devices operating in two dimensions, i.e. devices adapted to use co-ordinate systems which are effectively two-dimensional, such as latitude/longitude. However there are other uses, e.g.

aligning a telescope with a planet in the sky, where a device using a three-dimensional co-ordinate system would be preferred. The prior art solution to this problem has been to successively use a combination of devices to find a desired "three-dimensional" direction. Such a procedure is often far from straightforward and errors can arise if the succession of instruments used are not correctly aligned with each other.

The present invention provides direction finders adapted to use the Earth's magnetic field to locate a desired "three-dimensional" direction in space.

The invention will be described below in terms of methods and apparatus for use in the siting and alignment of aerials/antennae for reception of direct broadcasts from satellites. This is but one application of the present invention which, as mentioned above, relates to direction finding (and measuring) in general.

Proposals have been made for direct broadcasting by satellite (D.B.S.) using geostationary satellites.

The transmission from these satellites is at frequencies such that the radiation behaves similarly to light in many respects. Accordingly, for the best direct reception from such satellites it is necessary for a receiving aerial to be directed along the line of sight to the satellite with no intervening obstacles. Since the satellite is not generally visible the use of alignment instruments is required.

Conventionally the line-of-sight to a satellite is found by the use of a magnetic compass followed by a clinometer, using the known values of the satellites a zimuth and elevation. The instruments are used successively and a rough indication of the desired direction is obtained. To obtain a more accurate measurement of direction it is necessary to fix the receiving aerial in place and to vary its position whilst monitoring the received signal strength.

One of the disadvantages of the known method is that the aerial must be physically positioned at a potential reception site before an accurate assessment of the suitability of the site can be made (by viewing along the pointing direction of the aerial to see if there are any obstacles obscuring the line-of-sight. Also, if the receiving aerial is of the type which uses an offset feed dish it is difficult to tell its pointing direction.

It would be desirable to have a single instrument which would allow a potential aerial site to be assessed for suitability before physically positioning the aerial at the potential location. Also it would be useful, once a suitable site is located, to have advice to assist in the alignment of the aerial with the satellite direction.

The present invention provides a device comprising means for sensing the Earth's magnetic field, and means for viewing along a desired direction relative to the direction of the Earth's magnetic field.

Further features and advantages of the invention will become apparent from the following description of embodiments thereof, given by way of example, with reference to the accompanying drawings, in which: Fig. 1 illustrates the directional nature of the Earth's magnetic field at a given location relative to a set of orthogonal co-ordinate axes; Fig. 2 shows a diagrammatic form a first embodiment of the invention.

Fig. 3. shows a perspective view of a further embodiment of the invention; and Fig. 4 shows a block diagram of the circuitory used in the further embodiment.

Embodiments of the present invention enable desired directions to be found at a locality by sensing the Earth's magnetic field at that locality and using the directional properties of the sensed field, in combination with knowledge of the expected field direction at the locality, to obtain an indication of the desired direction.

The directional nature of the Earth's magnetic field is well-known.

The Earth's magnetic field has a horizontal and a vertical component for locations on the Earth's surface other than the magnetic poles (where the field direction is vertical with respect to a local observer) or the magnetic equator (where the field direction is horizontal with respect to a local observer). Between the magnetic equator and magnetic poles the dip or inclination of the lines of force with respect to the local horizontal varies from place to place.

The direction of the horizontal component of the Earth's magnetic field also varies from place to place.

There are a limited range of locations where the measured "magnetic north" direction coincides with "true" or geographical north. In other places the direction of pointing may be to the east or to the west of "true north" depending on location. The departure of measured magnetic north from true north at a particular location is known as variation.

Figure 1 illustrates the directional nature of the Earth's magnetic field, BE, at a locality with reference to a set of orthogonal axes x, y and z (axes x and y lie in the local horizontal plane and axis z points vertically downward). The arrowhead indicates the direction the north seeking pole of a bar magnet would take if placed in field BE The strength and direction of the Earth's magnetic field at a particular location are, to a first approximation, constant. Variations do occur with time but these are generally of a relatively small size, or take place over a very long period of time, and are thus primarily of interest to geologists or astronomers. For reference the main variations of field direction with time are as follows: a)a daily variation, larger near the poles and smaller near the equator.

b) an annual variation, occurring in opposite directions in opposite hemispheres; c) a secular variation, occurring over a period of centuries and apparently cyclic; d) an irregular variation apparently due to sunspot activity and solar flares (radio fade-out and other such phenomena will also occur).

Further information on the Earth's magnetic field may be found from standard texts such as "Magnetic Compasses and Magnetometers" by A. Hine (1968).

The relative stability of the Earth's magnetic field at a particular location has enabled magnetic maps to be made. Thus, for a given location, any desired direction may be considered to be defined relative to the known direction of the local magnetic field (due to the Earth). In order to find a desired direction at a given location it is only necessary to find the magnetic field direction and to know the "offset" of the desired direction from the local field direction.A first embodiment of the invention for use in inspection of a line-of sight to a satellite is known in Fig. 2. This device may be hand held (as shown), mounted on a base, or mounted on an aerial to facilitate alignment of the aerial with a desired direction.

The device 1 comprises a hollow transparent sphere 33 mounted on handle 2. The sphere 3 contains a transparent liquid 5 in which an inner sphere 4 is located so as to be free to rotate within the inner sphere a bar magnet 6 and a pair of lenses 7,8 are located. The bar magnet 6 is positioned low down in the inner sphere 4 in order to maintain the sphere's general orientation with respect to gravity. The lenses 7 and 8 are positioned in the sphere 4 to form a telescopic arrangement and the orientation of the lenses relative to the axis of the magnet (and thus relative to the local direction of the Earth's magnetic field), is such that it corresponds to the satellites direction relative to the local direction of the Earth's magnetical field.

Accordingly an observer using the device 1 simply has to look through lenses 7 and 8 to be looking along the line of sight to a particular satellite.

The components within the sphere 4 should be positioned so as to take into account the effects of refraction at the boundaries in device 1.

When viewing the line of sight to a satellite from a range of geographical locations the apparent satellite direction will change with each location. When viewing a geostationary satellite, which is situated over the equator, for example, view points at high latitude will have lower apparent satellite viewing angles. The device 1 may be provided with the facility to view a satellite from a range of locations by marking the apparent satellite position, corresponding to a number of viewing locations, on the instrument field of view. The apparent satellite position is found in use by using the viewfinder mark that corresponds most closely to the observers geographical position. The marks in the viewfinder will correspond to a geometrically distorted map.

Similarly the device 1 may be provided with the facility to view a number of satellites from a particular location by the inclusion of a greater number of lenses in sphere 4 in appropriate orientations relative to magnet 6. The positions of the eyepieces and the disposition of the corresponding viewing targets in such a version of device 1 could be arranged also, as described above, to enable the number of satellite to be viewed from any one of a number of different locations.

In addition to enabling a desired line of sight to be inspected the device 1 may be used to align an object in this case an aerial, with a desired direction.

One method for aerial alignment according to to the invention involves the attachment of a adaptor to the aerial to indicate the aerial boresight direction. Such an adapter may be a pair of sights for forward viewing, or a reflecting mirror for reverse viewing. A device 1 can then be used in conjunction with the adapter to align the aerial with a particular satellite.

Another method for aerial alignment involves the attachment of a device 1 directly to the aerial, with a known relationship existing between the aerial boresight direction and the fixed parts of the device 1.

Adjustment of the aerial pointing direction would then be made to line up the moving and fixed parts of the device, which process would automatically line up the aerial with the wanted satellite.

In the first embodiment described above the direction finder operates to locate certain predetermined directions by sensing the local direction of the Earth's magnetic field and automatically aligning the sensing element with the sensed field, prearranged elements then indicate the desired direction (by virtue of their orientation relative to the sensing element).

Embodiments of the invention offer a greater degree of flexibility in the directions which they may be used to find by having, in addition to elements for sensing the local magnitude and direction of the Earth's magnetic field, means for calculating the offset between a desired direction and the known direction of the Earth's magnetic field at that location.

In a presently preferred embodiment of the invention sensing elements are used which sense the Earth's magnetic field at a given location in three dimensions.

It can be seen from Figure 1 that the magnetic field BE can be completely expressed in terms of the magnitude and sign of the components Bx, By and Bz (where Bx is the magnetic field in the x direction, By is the magnetic field in the y direction, and Bz is the magnetic field in the z direction). Thus, if a set of orthogonal measuring axes x, y and z are used (i.e. if the Earth's magnetic field is sensed in the-direction of each axis of the orthogonal set) then the magnitude and direction of the sensed field relative to the measuring axes is found.

Since the direction and magnitude of the Earth's magnetic field at a location is generally already known it is possible to compare the sensed direction with the known direction and thus to evaluate the orientation in space of the measuring axes (and thus of the measuring device).

If a reference direction is defined on the measuring device (e.g. using a pair of sights the reference direction being the sighting direction) and the relative position of the measuring axes and the reference direction are known, it then becomes a simple matter to calculate the direction in which the "reference direction" is pointed by comparing the sensed magnetic field direction with the known magnetic field direction.

Figure 3 shows a perspective view of a second embodiment of the invention, using elements for sensing the Earth's magnetic field in three dimensions. The direction finder 10 of fig. 3 has a display 11, a key section 12, and sights 14 defining a reference direction.

Figure 4 illustrate diagrammatically the contents of direction finder 10 of Fig. 3. The direction finder 10 includes a sensor section 15 containing elements for sensing the Earth's magnetic field in three dimensions and their associated circuitry. Leads 16 convey the output of the senser section to a processor 18. The processor is connected to a memory 19, an input section 20, the display 11, and if desired to an indicator device 22.

The sensor section 15 comprises elements for sensing the Earth's magnetic field in three dimensions.

Such elements may be made up of e.g. appropriately configured Hall elements, an arrangement of saturable inductors, three fluxgate magnetometer elements (arranged orthogonally) on three magneto-resistive sensors (arranged orthogonally) i.e. with mutually perpendicular sensing axes). More information as to the constructon and/or arrangement of these devices may be found in the article "Earth's Field Magnetometry" by W. F. Stuart, Reports on Progress in Physics Vol. 35, 1972 P.803-881 in the book "Magnetic Compasses and Magnetometers" referred to above, and in "Design Abstracts" by Elektor Electronics, March 1988 issue P.60-61.

The processor 18 is arranged to receive the sensor section 15 and compute output signals which are fed to the display 11 and/or the indicator serial 22. It is preferred that the location of one or more satellites be stored in the memory 19, the keys 12 being used to select the data relating to a desired satellite and to enter the location of the aerial.

The processors 18 can them evaulate the signals from the sensor section 15 in the light of the stored data and then indicate when the sights 14 on the aerial 10 are pointed on the satellite. The indication can taken any desired form e.g. au audible indication, a visual indication can be given in the line of sight of the user so that as the user looks through the sights 14 an indication in the form of lights or a moving needle can show the user when he is approaching the correct orientation as well as when he is actually at the correct orientation.

The sights 14 may be included in a telescope or other viewing device. Also, it may be preferable to include a separate device indicating a horizontal level.

This may be arranged to produce a signal to be fed to the processor '8' in which case the amount of data stored in the memory 19 and/or the amount of computations time required may be reduced.

The memory may contain more information than that outlined above. For example, the locations of various major combinations may be stored to avoid the need for users in those location to know the exact location. Also correction factor for change in the Earth's magnetic field could be included.

Both the above arrangement can indicate to a user when an antennae has a clear line-of-sight- to a satellite in a simple but accurate manner.

Claims (14)

1. A direction finder device comprises a first sensor for monitoring the earth's magnetic field in order to indicate a first direction, means for storing an indication of the direction and elevation of an object whose position is to be found, and a sighting device movable to a position whereby a user may look at the object.

2. A device according to claim 1, and comprising means for indicating when the device is horizontal.

3. A device according to claim 1 or 2, wherein the first sensor is a magnetic compass.

4. A device according to claim 1, 2 or 3, wherein the storing means comprise at least one mark on an optical element in the line-of-sight of the sighting device.

5. A device according to claim 3 or 4, wherein the sighting device and magnetic compass are provided in a first member movable with respect to a second member which is provided with said mark.

6. A device according to claim 1, wherein the first sensor comprises a plurality of electro-magnetic devices for producing output signals indicative of the direction of the direction finding device.

7. A device according to claim 6, and including a second sensor comprising a further electro-magnetic device for producing further output signals indicative of the elevation of the direction finding device.

8. A device according to claim 7, and comprising a store for storing data relating to the position of the object to be viewed, a processor for comparing the output signals from the first and second sensors with the stored data, and indicating means for indicating when the sighting device is pointing at the object to be viewed.

9. A device according to claim 8, wherein the indicating means comprises a visual indication.

10. A device according to claim 9, and comprising means for causing the visual indication to be given in the sighting device.

11. A device according to claim 9 or 10, wherein the indicating device comprises an audible indication.

12. A device according to any one of claims 6 to 11 wherein the electro-magnetic devices are flux-gate devices.

13. A device according to any one of claims 6 to 11 wherein the electro-magnetic devices are magnetoresistive devices.

14. A direction finding device substantially as hereinbefore described with reference to the accompanying drawings.