WO1995031695A1

WO1995031695A1 - Aiming or pointing means

Info

Publication number: WO1995031695A1
Application number: PCT/GB1995/001082
Authority: WO
Inventors: Barry James Gorham; James Richard Dudley
Original assignee: British Technology Group Limited
Priority date: 1994-05-12
Filing date: 1995-05-12
Publication date: 1995-11-23
Also published as: AU2415795A

Abstract

A piece of apparatus such as a theodolite telescope which is orientationally adjustable by rotation about first and second axes (e.g. an altitude axis and an azimuthal axis), by means of first and second drive means respectively, is provided, for detecting a remote target and controlling the said drive means to align the apparatus on such target, with aiming or pointing means which comprises: a mounting secured to the said apparatus for movement therewith about one of the said axes, a rotor mounted on the mouting for rotation about the other of the said axes, third drive means for so rotating the rotor, means defining mutually-intersecting first and second planes relative to the rotor, which planes during rotation of the rotor also rotate therewith about the said other axis so as to sweep over a remote target, with at least one of said planes being skewed relative to the said other axis, target-sensing means for detecting pulses of radiation from such remote target as the said planes sweep thereover and thereby indicating the corresponding instantaneous rotational positions of the rotor, apparatus-sensing means for generating pulses indicating the instantaneous rotational setting of the apparatus in relation to the rotor and the said first and second planes defined relative thereto, and comparator means arranged to detect lack of coincidence between pulses from the target-sensing means and pulses from the apparatus-sensing means and to render operative the first and second drive means to bring such pulses into coincidence and align the apparatus upon the remote target.

Description

AIMING OR POINTING MEANS This invention relates to means for establishing the direction of a remote object or target, and orienting a piece of apparatus in predetermined relationship to such direction. It may be employed, for example, in aiming or pointing one optical or other communications unit at another such unit, remotely situated, with which it is to communicate, and in maintaining the required alignment even if one or both units is in movement; or it may be used for automatically aligning a telescope of a theodolite or "total station" on a surveying target.

It is, therefore, an object of the invention to provide a piece of apparatus, such as a theodolite or other telescope, or a directional communications unit, with aiming or pointing means by means of which such apparatus can be automatically aligned with respect to the direction of a remote co-operating target. It is a further object of the invention to provide a novel method of operation of such aiming or pointing means. According to the invention there is provided the combination of: a piece of apparatus orientationally adjustable by rotation about first and second axes (e.g. an altitude axis and an azimuthal axis); first and second drive means operable to rotate the apparatus about the its first and second axes respectively; and aiming or pointing means for detecting a remote target and controlling the said drive means to align the apparatus on such target, characterised in that the aiming or pointing means comprises: a mounting secured to the said apparatus for movement therewith about one of the said axes, a rotor mounted on the mounting for rotation about the other of the said axes, third drive means for so rotating the rotor, means defining mutually-intersecting first and second planes relative to the rotor, which planes during rotation of the rotor also rotate therewith about the said other axis so as to sweep over a remote target, with at least one of said planes being skewed relative to the said other axis, target-sensing means for detecting pulses of radiation from such remote target as the said planes sweep thereover and thereby indicating the corresponding instantaneous rotational positions of the rotor, apparatus-sensing means for generating pulses indicating the instantaneous rotational setting of the apparatus in relation to the rotor and the said first and second planes defined relative thereto, and comparator means arranged to detect lack of coincidence between pulses from the target-sensing means and pulses from the apparatus-sensing means and to render operative the first and second drive means to bring such pulses into coincidence and align the apparatus upon the remote target.

In one embodiment of the invention, the rotor of the aiming or pointing means comprises radiation beam projector means arranged to project first and second fan-shaped beams of radiation defining the said first and second planes. The plane defined by one of the fan-shaped beams may include the said other axis, or both the planes defined by the fan- shaped beams may be skewed relative to it.

In another embodiment of the invention the rotor of the aiming or pointing means is provided with first and second linear image forming means which, together with the said target-sensing means, defines the said first and second planes as those planes in which, if a remote target is located, it will be imaged on the target-sensing means by the image forming means. The first and second linear image forming means may be first and second slots, or they may be first and second elongate cylindrical lens elements. One of the said first and second linear image forming elements may extend, and form an image, parallel to the said other axis, or both the first and second image forming means, as well as the images which they form, may be skewed relative to the said other axis. The target-sensing means may be a detector cell located on the said other axis and receiving images formed by both

' the first and second linear image forming means. An embodiment of the invention may comprise: a piece of apparatus orientationally adjustable by rotation about first and second axes (e.g. an altitude axis and an azimuthal axis); first and second drive means operable to rotate the apparatus about the its first and second axes respectively; aiming or pointing means secured to the said apparatus for movement therewith and comprising a rotor unit rotatable about an axis coaxial with the first axis of the apparatus and comprising first and second light beam projectors and first and second light detectors, the light beam projectors being arranged to emit respective first and second fan-shaped light beams disposed substantially in a first plane which includes the axis of rotation of the projector unit and, respectively, a second plane which is skewed relative to that axis, and the first and second light detectors being arranged to receive a pulse of light from a remote target on illumination thereof by the first or second fan-shaped beam respectively; third drive means operable to rotate the projector unit about its axis; means for indicating the particular moments during a revolution of the projector unit when the first and second projectors thereof and the said apparatus are in particular angular relationships about the said first axis; means for comparing the moment indicated in respect of the first projector with the moment of reception of a pulse of light by the first receiver and, in case of non-coincidence, rendering the second drive means inoperative and the first drive means operative to rotate the said apparatus about its first axis in the direction to produce co-incidence and then terminating such rotation; and means for comparing thereafter the moment indicated in respect of the second projector with the moment of reception of a pulse of light by the second receiver and, in case of non-coincidence, rendering the second drive means operative to rotate the said apparatus about its second axis in the direction to produce co-incidence and then terminating such rotation.

Another embodiment of the invention may comprise: a piece of apparatus orientationally adjustable by rotation about first and second axes; first and second drive means operable to rotate the apparatus about the its first and second axes respectively; aiming or pointing means secured to the said apparatus for movement therewith about the said first axis and comprising a rotor unit rotatable about an axis coaxial with the said second axis and comprising light detector means and first and second linear image forming means which, with the light detector means, define mutually-intersecting first and second planes which, during rotation of the rotor unit, rotate therewith such that as either such plane sweeps over a remote target a line image thereof sweeps over the light detector means to produce a pulse indicating the instantaneous rotational position of the rotor unit, third drive means operable to rotate the rotor unit about its axis; means for indicating the particular moments during a revolution of the rotor unit when the said first and second planes thereof and the said apparatus are in particular angular relationships about the said second axis; means for comparing the moment indicated in respect of the first plane with the moment of reception of a pulse of light from a remote target instantaneously in that plane and, in case of non-coincidence, rendering the first drive means inoperative and the second drive means operative to rotate the said apparatus about its second axis in the direction to produce co-incidence and then terminating such rotation; and means for comparing thereafter the moment indicated in respect of the second plane with the moment of reception of a pulse of light from a remote target instantaneously in that plane and, in case of non-coincidence, rendering the first drive means operative to rotate the said apparatus about its first axis in the direction to produce co-incidence and then terminating such rotation.

The apparatus for which the aiming or pointing means is provided may be a telescope of a theodolite or a "total station", and its first and second axes may (probably most conveniently) be its altitude and azimuthal axes respectively, though they could be its azimuthal and altitude axes respectively.

The invention will be further disclosed and understood in greater detail from the following description of embodiments thereof with reference to the accompanying

' drawings, in which :- Figure 1 is a schematic rear view of a theodolite telescope and its mountings, fitted with a first embodiment of aiming or pointing means according to the invention;

Figure 2 is a side view of the telescope, as viewed in the direction 2-2 shown in Figure 1 ; Figure 3 is a side view of the aiming or pointing means, shown in Figure 1, as viewed in the direction 3-3 shown in that Figure; Figures 4 and 5 illustrate fan-shaped light beams emitted by the aiming or pointing means shown in Figures 1 and 3;

Figure 6 shows a target suitable for use with the apparatus shown in Figures 1 to 3; Figure 7 is a schematic rear view of a theodolite telescope and its mountings similar to that shown in Figure 1, but fitted with a second embodiment of aiming or pointing means according to the invention;

Figure 8 is a side view of the telescope, as viewed in the direction 8-8 shown in Figure 7; Figure 9 is a schematic side view of the aiming or pointing means shown in

Figure 7, as viewed in the direction 9-9 shown ^"in that Figure;

Figure 10 is a diagrammatic perspective view of a rotor unit shown in Figure 9, viewed in the direction indicated by an arrow A in that Figure;

Figure 11 shows a target suitable for use with the apparatus shown in Figures 7 to 10; and

Figure 12 shows a modification of the rotor unit shown in Figure 10.

As shown diagrammatically in Figures 1 and 2, a theodolite 11 comprises a telescope 12 mounted on shafts 13 in an alidade 14 which in turn is mounted on a fixed base 15 of the theodolite. The telescope 12 is pivotable about the horizontal axis of the shafts 13 by means of an altitude drive motor 16, and the alidade 14 is rotatable relative to the fixed base 15 about a vertical axis by means of an azimuth drive motor 17.

As shown in Figures 1 and 3, the theodolite 11 is combined according to the invention with aiming or pointing means 18 mounted on the theodolite alidade 14 by means of a mounting bracket 19 rigidly secured on the alidade. The means 18 comprises a rotor unit 20, mounted on the bracket 19 to be rotatable about an axis 20' coaxial with that of the telescope mounting shafts 13, and a motor 21 connected to rotate the rotor unit 20 at a steady rate of, say, one cycle per second in, say, the clockwise direction as seen in Figure 3. The rotor unit 20 contains two light-beam projectors 22 and 23, with each of which is associated a respective light receiver 24 or 25. Also associated with the projectors 22 and 23 are projecting pegs PI and P2. respectively, which, during rotation of the rotor unit 20, interact with an opto-switch 26 mounted by means of an arm 27 which projects laterally from the rear of the telescope 12. The opto-switch 26 has a central slot 28 between a light source 29, which projects a narrow light beam across the slot, and a photodetector 30 which detects the beam. As the rotor unit 20 rotates, the pegs PI and P2 pass through the slot 28 and momentarily interrupt the light beam so that their passage is detected by the photodetector 30.

As represented in Figure 4, the projector 22 is arranged to project a fan-shaped beam 31 which extends in a plane which contains the rotational axis 20' of the projector unit. Within that plane the beam spreads up to, say, 5° in each direction from its central line 32, which is perpendicular to the rotational axis 20'. It will be understood that, if the beam 31 were incident on a plane surface perpendicular to its central line 32, it would illuminate the surface in a line 33 parallel to the rotational axis 20'.

The projector 23 is similarly arranged to produce a fan-shaped beam, but in a plane which is skewed relative to the rotational axis 20' such that, as illustrated in Figure 5, a plane surface on which the projector 22 would illuminate a line 33 parallel to the rotational axis 20' would be illuminated by the projector 23 on an oblique line 34.

In use of the apparatus to find a target and align the telescope 12 upon it, the motor 21 is operated continuously to provide a steady rotation of the rotor unit 20 at, say, one revolution per second, with the fan-shaped beams from the projectors 22 and 23 sweeping downwardly, in front of the telescope 12, during each revolution. It will be understood that, in the absence of any movement of the alidade 14, this would cause the light beams from the projectors 22 and 23 to sweep out repeatedly an annular zone in space, with a 10° angular width. However, the motor 17 is also energised, to rotate the alidade 14 about a vertical axis, at a rate not greater than 10° per second, so that successive annular-zone

• sweeps are angularly displaced from one another but by an amount which leaves them overlapping so that no direction is left unswept between successive sweeps and in due course all directions are swept.

In due course, the beam 31 will sweep over a co-operative target, namely a target which, on being illuminated, returns a signal to the apparatus, where it is detected by the receiver 24. The target may be a glass corner-cube prism which reflects incident light back along its own path, or a panel of retroreflective material (which may suitably be disc-shaped) or a photocell with a transponding beacon coupled to it so that momentary illumination of the photocell by the beam 31 triggers the transponding beacon to emit a light pulse which is detected by the detector 24. In this latter case, the emitted light pulse is preferably very short, say of about 50 ns duration, and the detector 24 and associated circuitry are arranged to utilise that fact in discriminating between such pulses and illumination from other, random, sources.

Receipt by the receiver 24 of a signal from a target indicates that at that moment the projector 22 and receiver 24 are (momentarily) pointing at the correct elevation of the target. It is desired to set the telescope 12 at the same elevation, and to that end the receipt of pulses by the receiver 24 is caused to switch off the azimuth motor 17 of the alidade 14, and the timing of the light pulse received by the receiver 24 during each revolution of the rotor unit 18 is compared with the timing of the no-signal pulse in the receiver 30 which indicates passage of the peg PI through the slot 28 of the opto-switch 26. In general, the two pulses will not be coincident: the pulse at the receiver 24 will precede or follow that at the receiver 30 according as the altitude angle of the telescope is too low or too high, respectively. The pulses are fed to comparator circuitry (not shown) which detects the order of the pulses and switches on the telescope altitude motor 16 to raise or lower the telescope as appropriate, until the pulses are in coincidence, when the motor 16 is automatically switched off with the telescope altitude now adjusted to that of the target.

The comparator circuitry is then used to compare the timing (and, in particular, the relative order) of pulses received by the receiver 25 and no-signal pulses from the receiver

30 due to passage of the peg P2 through the slot 28. Each pulse at the receiver 25 indicates the moment when the beam 34 sweeps over the target, but the obliquity of the beam 34 means that it will precede or follow the pulses from the receiver 30 depending on whether the target is to the left or the right of the azimuth angle at which the telescope is set. The comparator circuitry is made to switch on the telescope azimuth motor 17 to alter the telescope azimuth angle in one direction or the other until coincidence of the pulses from the receivers 25 and 30 is achieved, and the motor 17 is again switched off. The telescope 12 is now aligned on the target, correct in both azimuth and elevation. The angular values can then be read off, manually or electronically, in known manner utilising conventional components of the theodolite 11 provided for that purpose. The theodolite may be part of a "total station" which, in known manner, also incorporates an electronic distance meter. which enables the target distance to be measured and recorded along with the altitude and azimuth data, and radio telemetry means for transmitting the measured data to a desired location.

It will be appreciated that the above-described alignment of the telescope on a target does not require the provision of angle-encoders or circle scales (which are only needed for providing a final read-out, if that is required). The actual pointing operation relies entirely on timing, to obtain coincidence of pairs of pulses, and uses the central (and best defined) parts of the fan-shaped beams. It is not sensitive to any lack of flatness of these beams, and the degree of precision of pointing which may be obtained is about one second of arc. Although the fan-shaped beam 31 is described above as being in a plane which includes the rotational axis 20' about which it rotates, this is not essential. Indeed, it may be difficult to practice to achieve the required accuracy in mounting the projector unit 22. However, the beam 31 (like the beam 34) may allowably be skewed relative to the axis 20', so long as the two beams 31 and 34 are in mutually intersecting planes. The desired alignment of the telescope on a target may then still be achieved, by an iterative process in which the altitude and azimuth are adjusted alternatively, repeated as often as necessary.

Figure 6 shows a surveyor's target which may be provided for use with the apparatus shown in Figures 1 to 3. It comprises a staff 35 with a pointed lower end and, at its upper end at a known distance from the lower point, a head 36 containing, centrally, a corner-cube prism 37 which may be used to reflect light from the fan-shaped beams from the projectors

22 and 23. The head also contains a transponder unit which includes a pair of photodetectors 38 and 39, disposed one above the other, and a holographic lens 40 for transmitting a beam of light from a laser source (not shown) housed within the head. With

' the photodetectors 38 and 39 disposed as shown, each fan-shaped beam will sweep first over the detector 38 then over the detector 39. The outputs of these two detectors are preferably connected differentially, i.e. in opposition, to give a combined output signal which first rises, then falls through zero and beyond, and finally returns to zero; and the occurrence of the zero at mid-signal is preferably used to trigger the laser source into emitting a very short pulse through the lens 40 to be received by the receiver 24 or 25, as the case may be. These receivers would then be tuned to receive the very short pulses from the laser source rather than the much longer pulses reflected from the prism 37. If the surveyor carrying the target shown in Figure 6 keeps it turned towards the theodolite equipped in accordance with the invention as he moves around the field, the theodolite in search mode will track the target in real time and be ready for almost instant measurement when initiated by the surveyor via a radio link which, preferably, enables him to command the apparatus from a distance.

It will be understood that the opto-switch 26 and the co-operating pegs PI and P2 as described above are but one means (which could, within the scope of the invention be replaced by equivalent other means) of establishing and indicating the particular moments during a revolution of the projector unit 20 when its projectors 22 and 23 are in a particular - altitude-angle relationship (co-alignment) with the telescope 12, enabling a determination of whether those moments are before, after or coincident with the detection of light pulses by the detectors 24 and 25 respectively.

It will also be understood that although the above-described embodiment of the invention is arranged to provide for correct setting first of the altitude of the telescope and then of its azimuth, it would be equally within the scope of the invention to arrange for the projector unit to rotate about an axis coaxial with the telescope azimuthal axis and thus to set first the azimuthal and then the altitude angle of the telescope. It will also be understood that in a case in which, as described above, the planes of the two fan-shaped light beams 31 and 34 are both skewed relative to the rotational axis 20', the iterative process of repeatedly adjusting the azimuthal and altitude settings alternatively which is then required is not necessarily a lengthy process. The successive corrections may be arranged to effect rapid convergence of the telescope alignment on the remote target, so that only a small number of iterative steps are required.

The theodolite 11 shown in Figure 7 and 8 comprises a telescope 12 which, like that shown in Figures 1 and 2, is mounted on shafts 13 in an alidade 14 which in turn is mounted on a fixed base 15 of the theodolite with an altitude drive motor 16 for pivoting the telescope 12 about the horizontal axis of the shafts 13, and an azimuth drive motor 17 for rotating the alidade 14 relative to the fixed base 15 about a vertical axis.

As shown in Figures 7 and 8, the theodolite 12 is combined with a second embodiment of aiming or pointing means 118 according to the invention, mounted on the theodolite alidade 14, as before, by means of a mounting bracket 19 rigidly secured on the alidade. The means 118 comprises a cylindrical rotor unit 120, mounted on the bracket 19 to be rotatable about an axis 120' coaxial with that of the telescope mounting shafts 13, and a motor 121 connected to rotate the rotor unit 120 at a steady rate of, say, one revolution per second in, say, the clockwise direction as seen in Figure 9. The rotor unit 120 contains at its cylindrical surface two cylindrical lens elements 122 and 123, as also shown in Figure 10. The lens element 122 extends parallel with the axis 120' of the rotor unit 120, so as to form along that axis a line image of a distant light source located in the common plane of the axis 120' and the lens element 122. As the rotor unit 120 rotates, clockwise as viewed in Figure 9, that plane sweeps down over a light source in front of the apparatus and the line image of the source sweeps down through the position in which it coincides with the axis 120'. As it does so, it sweeps across a photocell 1^'24 mounted at the axis 120', which generates a signal pulse as further referred to below. The lens element 123 is skewed relative to the axis 120', so that the line image which it forms of a distant light source is also skewed relative to that axis. As the rotor unit 120 rotates, clockwise as viewed in Figure 9, the lens element 123 has a leading end 123a and a trailing end 123b. During such rotation, the line image of a distant source which comes into alignment with the photocell 124 and a part of the element 123 towards its leading end 123a or its trailing end 123b will sweep over the photocell respectively earlier or later than the line image of a source which comes into alignment with the photocell and the centre or mid point of the lens element 123.

Associated with the lens elements 122 and 123 are projecting pegs PI and P2, respectively, which, during rotation of the rotor unit 120, interact with an opto-switch 26 mounted by means of an arm 27 which projects laterally from the rear of the telescope 12.

• The opto-switch 26 has a central slot 28 between a light source 29, which projects a narrow light beam across the slot, and a photodetector 30 which detects the beam. As the projector unit 20 rotates, the pegs PI and P2 pass through the slot 28 and momentarily interrupt the light beam so that their passage is detected by the photodetector 30.

A suitable target for use with this apparatus may comprise, as shown in Figure 11 , a staff 135 having a pointed lower end and. at a known distance therefrom, a head 136 containing an effectively point light source comprising a laser source (contained within the head but not shown) which illuminates a holographic lens 140 which emits a conically divergent beam of light and which therefore does not require to be accurately lined up on the apparatus which is to observe it. The centre of the lens 140 may be laterally offset from the axis of the staff 135 by a distance equal to that by which the photocell 124 is offset from the optical axis of the telescope 12. The target may be a multi-use target, which also includes a conventional corner-cube prism 137 for retroreflecting light from a source associated with other aiming or pointing means for a theodolite telescope, for instance as described above with reference to Figures 1 to 5.

In use of the apparatus shown in Figures 7 to 10 to find such a target and align the telescope 12 upon it, the motor 121 is operated continuously to provide a steady rotation of the rotor unit 120 at, say, one revolution per second. It will be understood that, in the absence of any movement of the alidade 14, this would cause the photocell 124 to scan repeatedly an annular zone in space, with an angular width of, say, 10° as determined by the angle which is subtended at the photocell by the lens element 122 and the corresponding component of the angle subtended by the skewed lens element 123. However, the motor 17 is also energised, to rotate the alidade 14 about a vertical axis, at a rate not greater than 10° per second, so that successive annular-zone scans are angularly displaced from one another but by an amount which leaves them overlapping so that no direction is left unscanned between successive scans and in due course all directions are scanned.

In due course, the target will be scanned and light from it, imaged by the lens element 122, will be detected by the photocell 124 which thereupon emits an electrical output signal pulse.

Emission by the photocell 124 of such an output signal pulse indicates that at that moment the angular position of the rotor unit 120 is such that the plane containing the axis 120', photocell 124 and lens element 122 also (momentarily) contains the target, and thus defines the elevation or altitude angle at which the telescope must be set to align it on the target. In order to set the telescope 12 at this desired elevation, the receipt of pulses from the photocell 124 due to its being swept by light from the lens element 122 is caused to switch off the azimuth motor 17 of the alidade 14, and the timing of the light pulse received by the photocell 124 via the lens element 122 during each revolution of the rotor unit 1 18 is compared with the timing of the no-signal pulse in the receiver 30 which indicates passage of the peg PI through the slot 28 of the opto-switch 26. In general, the two pulses will not be coincident: the signal pulse from the photocell 124 will precede or follow that from the receiver 30 according as the altitude angle of the telescope is too low or too high, respectively. The pulses are fed to comparator circuitry (not shown) which detects the order of the pulses and switches on the telescope altitude motor 16 to raise or lower the telescope as appropriate, until the pulses are in coincidence, when the motor 16 is switched off with the telescope altitude now adjusted to that of the target.

The comparator circuitry is then used to compare the timing (and, in particular, the relative order) of pulses received from the photocell 124 due to its being swept by light from the lens element 123 and no-signal pulses from the receiver 30 due to passage of the peg P2 through the slot 28. Each such pulse from the photocell 124 indicates the moment when the skewed plane containing the photocell 124 and the lens element 123 sweeps over the target, but the obliquity of that plane means that the pulses from the photocell 124 will precede or follow the pulses from the receiver 30 depending on whether the target is to the left or the right of the azimuth angle at which the telescope is set. The comparator circuitry is made to switch on the telescope azimuth motor 17 to alter the telescope azimuth angle in one direction or the other as appropriate until coincidence of the pulses from the photocell 124 and the receiver 30 is achieved, and the motor 17 is again switched off. The telescope 12 is now aligned on the target, correct in both azimuth and elevation. The angular values can then be read off, manually or electronically, in known manner utilising conventional components of the theodolite 11 provided for that purpose. The theodolite may be part of a "total station" which, in known manner, also incorporates an electronic distance meter which enables the target distance to be measured and recorded along with the altitude and azimuth data, as well as radio telemetry means for transmitting the measured data to a desired location. It will be understood that, within the scope of the invention, the lens elements 122 and 123 might be replaced by simple slots in the cylindrical surface of the rotor unit 120. However, the lens elements are preferred because, for a given light source target and a given width of slot or lens element, the focusing effect of lens elements produces narrower images, of greater intensity, than slots would do. Although, as illustrated in Figures 9 and 10. and as above described, the lens element 122 is disposed parallel to the rotational axis 120' of the rotor unit 120, it may within the scope of the invention, and as illustrated in Figure 12, also be skewed relative to that axis so long as the lens elements 122 and 123, and the respective planes containing them and the photocell 124, are at an angle to one another. Preferably, in that case, the two lens elements (or, more precisely, the respective planes containing them and the photocell 124) are oppositely skewed relative to the axis 120'. If neither lens element is arranged parallel to the rotational axis 120', it will not be possible, in only two sequential operations, to set first one and then the other of the two variables, azimuth and altitude, of the telescope or other apparatus which is to be aligned on a target, since setting the second will in general show that the first was set incorrectly. It is therefore necessary to use an iterative process in which first one, then the other, then the first again, and so on, is set until resetting one no longer requires subsequent resetting of the other. It will be appreciated that if one of the lens elements is nominally parallel to the rotational axis as in Figure 10 but its degree of parallelism is found to be less than adequately perfect, the imperfection may be overcome by iterative setting as just described. Although such iterative setting may appear laborious, it can in fact be easily programmed into the control circuitry and in practice, even for the case in which neither lens element is even approximately parallel, only a small number of iterations will usually be found necessary. It will be appreciated that the variant shown in Figure 12 is analogous to the above-described variant of the version of the first-described embodiment in which the planes of both the fan-shaped beams 31 and 34 are skewed relative to the axis 20'.

It will be appreciated that the above-described alignment of the telescope on a target does not require the provision of angle-encoders or circle scales (which are only needed for providing a final read-out, if that is required). The actual pointing operation relies entirely

' on timing, to obtain coincidence of pairs of pulses; and the degree of precision of pointing which may be obtained is about one second of arc.

Although the photocell 124 has been described as a single photocell emitting single output pulses, it may be replaced by a pair of photocells which a line image from either lens element sweeps over one after the other. With the outputs of these two photocells connected differentially, i.e. in opposition, they give a combined output signal which first rises, then falls tlirough zero and beyond, and finally returns to zero; and the occurrence of the zero at mid-signal then gives a very accurately defined instant against which the corresponding signal pulse from the receiver 30 may be compared.

If the surveyor carrying the target shown in Figure 11 keeps it turned towards the theodolite equipped with aiming or pointing means in accordance with the invention as he moves around the field, the theodolite in search mode will track the target in real time and be ready for almost instant measurement when initiated by the surveyor via a radio link which, preferably, enables him to command the apparatus from a distance.

It will be understood that the opto-switch 26 and the co-operating pegs PI and P2 as described above with reference to Figures 7 to 9 are but one means (which could, within the scope of the invention be replaced by equivalent other means) of establishing and indicating the particular moments during a revolution of the rotor unit 20 when its lens elements 122 and 123 are in a particular altitude-angle^* relationship with the telescope 12, enabling a determination of whether those moments are before, after or coincident with the detection of light pulses by the photocell 124.

It will also be understood that although the above-described embodiments of the invention are arranged to provide for setting first the altitude of the telescope and then its azimuth, it would be equally within the scope of the invention to arrange for the rotor unit to rotate about an axis coaxial with the telescope azimuthal axis and thus to set first the azimuthal and then the altitude angle of the telescope.

Claims

1. The combination of: a piece of apparatus orientationally adjustable by rotation about first and second axes (e.g. an altitude axis and an azimuthal axis); first and second drive means operable to rotate the apparatus about the its first and second axes respectively; and aiming or pointing means for detecting a remote target and controlling the said drive means to align the apparatus on such target, characterised in that the aiming or pointing means comprises: a mounting secured to the said apparatus for movement therewith about one of the said axes, a rotor mounted on the mounting for rotation about the other of the said axes, third drive means for so rotating the rotor, means defining mutually-intersecting first and second planes relative to the rotor, which planes during rotation of the rotor also rotate therewith about the said other axis so as to sweep over a remote target, with at least one of said planes being skewed relative to the said other axis, target-sensing means for detecting pulses of radiation from such remote target as the said planes sweep thereover and thereby indicating the corresponding instantaneous rotational positions of the rotor, apparatus-sensing means for generating pulses indicating the instantaneous rotational setting of the apparatus in relation to the rotor and the said first and second planes defined relative thereto, and comparator means arranged to detect lack of coincidence between pulses from the target-sensing means and pulses from the apparatus-sensing means and to render operative the first and second drive means to bring such pulses into coincidence and align the apparatus upon the remote target.

2. A combination as claimed in Claim 1 , wherein the rotor of the aiming or pointing means comprises radiation beam projector means arranged to project first and second fan- shaped beams of radiation defining the said first and second planes.

3. A combination as claimed in Claim 2, wherein the plane defined by one of the fan- shaped beams includes the said other axis.

4. A combination as claimed in Claim 2, wherein both the planes defined by the fan- shaped beams are skewed relative to the said other axis.

5. A combination as claimed in any of Claims 1 to 4, and having associated therewith a remote target adapted, on being swept by the rotating fan-shaped beams, to direct radiation back to the combination for detection by the said target-sensing means.

6. A combination as claimed in Claim 5, wherein the associated remote target includes retroreflecting means which, on being swept over by the fan-shaped beams, reflects radiation therefrom back to the combination.

7. A combination as claimed in Claim 5, wherein the associated remote target includes radiation emission means and radiation detector means which, on being swept over by the fan-shaped beams, triggers the emission of radiation pulses from the radiation emission means for detection by the target-sensing means of the aiming or pointing means of the combination.

8. A combination a claimed in Claim 1, wherein the rotor of the aiming or pointing means is provided with first and second linear image forming means which, together with the said target-sensing means, defines the said first and second planes as those planes in which, if a remote target is located, it will be imaged on the target-sensing means by the image forming means.

9. A combination as claimed in Claim 8, wherein the first and second linear image forming means are first and second slots.

10. A combination as claimed in Claim 8, wherein the first and second linear image forming means are first and second elongate cylindrical lens elements.

11. A combination as claimed in Claim 8, wherein one of the said first and second linear image forming elements extends, and forms an image, parallel to the said other axis.

12. A combination as claimed in Claim 8, wherein both the first and second image forming means, as well as the images which they form, are skewed relative to the said other axis.

13. A combination as claimed in any of Claims 8 to 12, wherein the target-sensing means is a detector cell located on the said other axis and receiving images formed by both the first and second linear image forming means.

14. A combination as claimed in any of Claims 8 to 13, and having associated therewith a remote target adapted to act as a steady source of radiation to be imaged by the linear image forming means.

15. A piece of apparatus orientation ally adjustable by rotation about first and second axes; first and second drive means operable to rotate the apparatus about the its first and second axes respectively; aiming or pointing means secured to the said apparatus for movement therewith about the said first axis and comprising a rotor unit rotatable about an axis coaxial with the said second axis and comprising first and second light beam projectors and first and second light detectors, the light beam projectors being arranged to emit respective first and second fan- shaped light beams disposed substantially in a first plane which includes the axis of rotation of the projector unit and, respectively, a second plane which is skewed relative to that axis, and the first and second light detectors being arranged to receive a pulse of light from a remote target on illumination thereof by the first or second fan-shaped beam respectively; third drive means operable to rotate the projector unit about its axis; means for indicating the particular moments during a revolution of the projector unit when the first and second projectors thereof and the said apparatus are in particular angular relationships about the said second axis; means for comparing the moment indicated in respect of the first projector with the moment of reception of a pulse of light by the first receiver and, in case of non-coincidence, rendering the first drive means inoperative and the second drive means operative to rotate

' the said apparatus about its second axis in the direction to produce co-incidence and then terminating such rotation; and means for comparing thereafter the moment indicated in respect of the second projector with the moment of reception of a pulse of light by the second receiver and, in case of non-coincidence, rendering the first drive means operative to rotate the said apparatus about its first axis in the direction to produce co-incidence and then terminating such rotation.

16. A piece of apparatus orientationally adjustable by rotation about first and second axes; first and second drive means operable to rotate the apparatus about the its first and second axes respectively; aiming or pointing means secured to the said apparatus for movement therewith about the said first axis and comprising a rotor unit rotatable about an axis coaxial with the said second axis and comprising light detector means and first and second linear image forming means which, with the light detector means, define mutually-intersecting first and second planes which, during rotation of the rotor unit, rotate therewith such that as either such plane sweeps over a remote target a line image thereof sweeps over the light detector means to produce a pulse indicating the instantaneous rotational position of the rotor unit, third drive means operable to rotate the rotor unit^' about its axis; means for indicating the particular moments during a revolution of the rotor unit when the said first and second planes thereof and the said apparatus are in particular angular relationships about the said second axis; means for comparing the moment indicated in respect of the first plane with the moment of reception of a pulse of light from a remote target instantaneously in that plane and, in case of non-coincidence, rendering the first drive means inoperative and the second drive means operative to rotate the said apparatus about its second axis in the direction to produce co-incidence and then terminating such rotation; and means for comparing thereafter the moment indicated in respect of the second plane with the moment of reception of a pulse of light from a remote target instantaneously in that plane and, in case of non-coincidence, rendering the first drive means operative to rotate the said apparatus about its first axis in the direction to produce co-incidence and then ' terminating such rotation.

17. A combination of orientationally adjustable apparatus and aiming or pointing means for aiming or pointing such apparatus at a remote target, substantially as described herein with reference to Figures 1 to 6 or Figures 7 to 12 of the accompanying drawings.