GB2113939A - Angular position determination - Google Patents

Abstract

Apparatus for determining the position of an object within the field- of-view of a remote projector by reference to a radiation beam emitted by the projector is primarily, but not exclusively, intended for use in an optical beam rider missile guidance system. In known such systems, the field is scanned by a laser beam and a reference laser pulse is also emitted to the field. A missile within the field can determine its position by reference to the time elapsing between each reference pulse and its subsequent glimpse of the scanned beam or vice- versa. Herein, no reference pulse is necessary because the beam traces out a scanning path in which it twice becomes incident on any point within the field, the time elapsing between the two incidences being dependent on the position of the point within the field. For example, the beam can trace out one or more scan patterns in first one direction and then in the other. Preferably the sequence includes orthogonal patterns to allow position determinations with respect to two adjacent edges of the field. <IMAGE>

Description

SPECIFICATION Position determining apparatus This invention relates to position determining apparatus for determining the position of an object within the field-of-view of a system which projects a beam of radiation such as a laser beam in a controllable direction into said field.

Such apparatus may be used for surveillance and/or guidance of objects such as aircraft, missiles, spacecraft and satellites.

For example, in a so-called optical beam rider (O.B.R.) missile guidance system, a continuous wave (C.W.) laser beam may be repetitively scanned over a field-of-view containing a target by say a mechanical scanning arrangement incorporating a rotating polygonal mirror which, as it rotates, deflects the beam to trace out the lines of the scan pattern and a nodding plane mirror for indexing the scan lines across or down the scanned area. At predetermined times, say at the start of each scan, a reference radiation pulse from a high-power pulse laser is emitted over the whole of the scanned area.Thus, by measuring the time elapsing between reception of the reference radiation pulse and the pulse of radiation which is received as the C.W. laser beam tracks over the missile position, receiving and timing apparatus on-board the missile can determine its position within the scanned area and guide the missile towards the target.

Described herein with reference to the drawings are principles whereby, in an optical beam rider missile guidance system, the missile position information is obtainable from the incidences of the C.W. laser beam on the missile, i.e. without reference to any reference radiation pulse. However, the said principles are not only applicable to missile guidance, Instead, by way of example, they could be used in connection with surveillance systems and for guidance of objects other than missiles, the aforementioned aircraft, spacecraft or satellites say.

According to one aspect of the invention, there is provided position determining apparatus comprising controllable projecting means for projecting a beam of radiation in a controllable direction within a field-of-view of the projecting means, and control means connected to control the projecting means and operable for so varying the beam direction that the beam traces out a path during which the beam twice becomes incident on a point within said field-of-view and the time between the two incidences is dependent upon the position of the point within the field-of-view.

Advantageously, the control means is operable for causing the beam to trace out a first scanning path over the whole of an area within said fieldof-view and subsequently to trace out a second scanning path over the whole of said area such that the time between incidences of the beam on said point during the execution of the respective patterns is dependent upon the position of the point within said area.

The first and second scanning paths may comprise respective substantially identical patterns of parallel lines but with the beam moving in one direction to execute the lines of one pattern and in the opposite direction to execute the lines of the other. In such a case, it is preferred that the control means should be operable to cause the beam to trace out third and fourth scanning paths over said area, these paths again comprising substantially identical patterns of parallel lines and the beam again moving in opposite directions to execute the lines of the respective patterns, but the lines of the third and fourth paths being transverse to the lines of said first and second paths. It is also preferred that the control means should be operable to cause the beam to trace out the scanning paths in repeated cyclic sequence.

Advantageously, the execution of successive scanning paths, and the execution of successive sequences of scanning paths where these sequences are repeated cyclically, are separated by respective delay times whereby the successive paths may be identified from the time elapsing between successive incidences of said beam on said point.

Said controllable projecting means may comprise a laser beam generator and acoustooptical beam deflector means for varying the direction of the beam.

According to a second aspect of the invention there is provided a scanning waveform generator for use in the control means specified for the first aspect of the invention, the generator being operable for producing in cyclic sequence respective scanning waveforms of which one is the reverse of the other such that a radiation beam of which the direction of projection is controlled in accordance with said waveform traces out a path twice crossing a point within a field-of-view into which the beam is projected and the time between the two crossings is dependent upon the position of the point within the field-ofview.

The scanning waveform generator may have two outputs and, during a first time interval, it may supply at one of said outputs an ascending staircase waveform for indexing the lines of a linewise scanning pattern while at the other output it supplies a succession of ramp waveforms for executing the respective lines and then, during a second time interval, it supplies a descending staircase waveform at said one output and a succession of reversed ramp waveforms at the other. Advantageously, during respective third and fourth time intervals, the generator supplies ascending and descending staircase waveforms at said other output and successions of ramp waveforms of opposite gradient at said one output.

According to a third aspect of the invention there is provided a beam rider missile guidance system comprising position determining apparatus according to the first aspect of the invention and at least one missile incorporating apparatus for sensing the incidence of said beam upon the missile, for determining the time elapsing between successive incidences and for guiding the missile accordingly.

For a better understanding of the invention reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram illustrating the operation and, in simplified form, the construction of an optical beam rider missile guidance system, Figure 2 is a diagram showing a sequence of scan patterns executed by a laser beam projected by the Figure 1 system, Figure 3 is a timing diagram showing pulses produced on board a missile guided by the Figure 1 system, Figure 4 is a simplified diagram of a drive unit control circuit used in the system of Figure 1, Figure 5 is a diagram showing the waveforms of several signals produced within the circuit of Figure 2, and Figure 6 is a diagram showing a modified scan pattern sequence.

Referring to Figure 1, the illustrated guidance system comprises a ground station incorporating a continuous wave laser 1 with an associated power supply 2, two acousto-optic deflector cells 3 and 4, a halfwave plate 5 and a switchable mirror 6. As is known, an acousto-optic deflector cell is operable to receive a beam of light, such as a beam 7 from laser 1, and in response to a high frequency drive signal, in the MegaHertz or GegaHertz range say, to deflect some of the light energy in a single plane to form a so-called "first order" beam, the defection angle being substantially proportional to the frequency of the drive signal. The cell 3 in the Figure 1 is arranged for receiving beam 7 and for the first-order beam 8 produced by the cell to pass via the half-wave plate 5 to the cell 4.The function of plate 5 is to rotate the polarisation plane of beam 8 and hence render it correct for the proper operation of cell 4 as will be understood by those skilled in the art.

The first-order beam 9 produced by cell 4 passes to switchable mirror 6. The zero-order beam (not shown) from each cell, i.e. the undeflected portion of the beam received by each cell, is passed to a respective energy absorbing medium (not shown).

The switchable mirror 6 is controllable to pass the first-order beam 9 from cell 4 to a first output optical system 10 or, via a further mirror 1 to a second output optical system 12. One of the optical systems 10 and 12, called the "gather optics", has a comparatively side field-of-view and is used to pick-up a just launched missile and guide it into the smaller field-of-view of the other system, the "tracker optics", which is then used to guide the missile through the remainder of its flight.

The cell 3 is arranged so that variation of the angle through which this cell deflects beam 8 varies the elevation direction of the output beam 13 which is actually emitted from whichever of the two optical systems 10 or 12 is in use.

Meanwhile the cell 4 control the azimuth direction of the output beam 1 3. The drive signals for the two cells are provided by respective drive units 14 and 1 5 each comprising a gate circuit, a voltage controlled oscillator and possibly also an amplifier output stage (the elements of each drive unit are not separately shown). In each unit, the gate circuit is operable in response to a common enable signal E from a drive unit control circuit 1 6 to pass the output of the voltage controlled oscillator to the associated deflector cell, the frequency of that output being substantially proportional to the magnitude of a respective one of two control voltage signals Vx and Vy produced by circuit 1 6.

When the drive signals to cells 3 and 4 are gated off, substantially all of the energy received by each cell emerges with the respective zero order beam, i.e. undeflected, and passes to the energy absorbing medium. During such times therefor, the output beam 1 3 is shut off. When the drive signals are gated on, the beam 13 is emitted with its elevation and azimuth controlled by the respective magnitudes of the signals Vx and Vy.

In use, the signals Vx and Vy are varied to cause beam 13 to scan repeatedly a field 101, of rectangular cross-section, within the field-of-view of the operative output optical system. The successive scans are executed according to a cyclic sequence of four linewise scan patterns A, B, C and D as shown in Figure 2. The first pattern A commences at the top left-hand corner of the scanned field, this position of beam 1 3 being produced by high values of signals Vx and Vy, and comprises horizontal lines in the execution of which the beam 13 moves from left to right and between which, as shown by the dashed lines in the Figure, the beam notionally flies back from right to left and is at the same time decremented downwardly by one interline space.The fly back paths are only notional because in fact, during the fly back period, the drive signals to cells 3 and 4 are gated off and hence beam 13 is not produced at this time. Scan pattern A is completed with the beam 1 3 directed towards the bottom right-hand corner of the scanned field 101 whereupon, after a short delay t1 during which the cell drive signals are again gated off, the beam traces out scan pattern B which is the exact reverse of pattern A, i.e. with lines proceeding from right to left and fly back from left to right and upwards by one step. After time t2 following completion of patterri B, the beam traces out the vertical line pattern C and then, after a delay equal to the delay t1 between the execution of patterns A and B, pattern D is traced out, this pattern D being the reverse of pattern C. After a further delay t3 the sequence restarts with the beam tracing out pattern A again, then B and so on.

During the execution of each scan pattern, a missile (not shown) fitted with a suitable detector and within the scanned area will catch sight of the laser radiation for an instant as the beam 13 scans through the missile position. Thus, as shown in Figures, at some time following the instant Toe when scan pattern A started, the radiation detector on board the missile will form a signal pulse a as the laser beam scans through the missile position. The end of scan pattern A occurs at time Tea, there then follows a short delay t1 whereupon, a time Tob, scan pattern B starts. During execution of pattern B, the raser again becomes incident on the missile and the onboard radiation detector forms pulse b. Scan pattern B is completed at time Teb, delay time t2 follows, and scan pattern C starts at time Toc.

During execution of this pattern, the missile radiation detector forms pulse c and the pattern is completed at time Tec. Delay time t, follows, then pattern D starts at Tod and, during execution of pattern D, pulse d is formed. The sequence then repeats itself as shown with delay time t3 occurring between the end (Ted) of scan pattern D and the start of the second execution of pattern A.

Suitable timing circuitry on board the missile can then measure the time tAB between the respective sightings during scan patterns A and B and thereby determine the position of the missile relative to the right-hand side of the scanned field, and can measure the time tcd between the respective sightings during scan patterns C and D and thereby determine the missile position relative to the bottom of the scanned field.

The function of the delays t1 between the execution of scan patterns A and B and between C and D is to ensure that the signal processing circuitry on board a missile which is very close to the bottom righthand corner of the scanned field, i.e. the point where pattern A ends and the beam "reverses" to start pattern B and where pattern C ends and D starts, can properly resolve the resultant sightings. If the delays were not provided, the sighting during patterns A and B and those during patterns C and D might overlap to form an apparent single sighting or might be so close together that the missile does not resolve them into two. By way of example, the delay t, might be around 25 Secs. say.The function of the delay times t2 and between the execution of scan patterns B and C and between two pattern sequences respectively is to resolve any ambiguities for a missile which has just entered the scanned field, i.e. to enable such a missile to determine which scan pattern is which. Thus t2 can be made somewhat larger than t, while t3 can be even larger than t2; preferably t3 is made a little longer than the time required to execute any one scan pattern.

It will be realised that due to the sequence of scanning patterns, with pattern A being followed by reverse pattern B and pattern C being followed by reverse pattern D, and missile is able to determine its altitude and azimuth relative to the scanned field without requiring to receive any time reference pulse of the kind that would be needed if a single scanning pattern is executed repeatedly over the scanned field. Accordingly, the illustrated system does not comprise any additional reference laser device, i.e. the highpower pulse laser which has been proposed for providing reference pulses in known systems.

Referring now to Figures 4 and 5 together, the operation of the drive unit control circuit to produce one complete scanning sequence of patterns A, B, C and D is initiated by the arrival of a separately generated start pulse S.T.P. which is applied to the set input of a set/reset bistable 20 and the ieading edge of which sets the bistable output signal SC to logic zero. The bistable is reset at the end of the sequence by a pulse from monostable 21. Pulse S.T.P. could possibly be generated by a manually controlled device but would best be formed by a suitable generator (not shown) which forms a train of such pulses at intervals of say 1 3T/4 where T is the time elapsing between the start of scan A and the end of scan B (or between the start of scan C and the end of scan D).This pulse interval thus covers the complete sequence with delays t, and t2 and gives a suitable value of t3. Signal SC is applied to a control input of a logic circuit 22 which includes a frequency-dividing counter and which becomes operable when signal SC is at logic zero to derive from the output of an oscillator 23 a timing signal ST. Signal ST, parts of which are shown magnified in time in Figure 5, consists of a series of logic one pulses each of a duration equal to the time required to execute one line of any of the scan patterns and separated by intervals corresponding to the line fly back periods. The timing signal ST is fed to a counter 24 having a capacity such that it can count to a number exceeding the total number of horizontal lines to be scanned when executing patterns A and B (i.e.

twice the number of lines in A or B) plus the total number of vertical lines to be scanned when executing patterns C and D (i.e. twice the number of lines in C or D) plus a number equivalent to the number of lines that could be executed during the delay time t2 between patterns B and C. The parallel content outputs of the counter 24 are connected to respective address inputs of a read only memory 25 wherein data has been prestored so that, as a counting sequence proceeds, there appear at the five used data outputs of the memory respective signals SM, SN, SO, SP and SQ.

The signal SP which takes the logic value one during each of the delay times t, and t2 and after the completion of the scanning sequence (i.e.

during time t3) is fed with the timing signal ST from circuit 22 to two gates 26 and 27 which each form the logical association ST. SP. The output of gate 26 is taken off to the drive units 14 and 1 5 as the common enable signal E. When this signal has the logic value one, the drive signals formed in units 14 and 15 are gated through to the cells 3 and 4.

A staircase waveform SW for incrementing the elevation of the laser output beam during execution of scan patterns A and B and the beam azimuth during patterns C and D is formed by a staircase generator comprising an up/down counter 28 of which the parallel content outputs are fed to a digital to analogue converter 29. The clock input of counter 28 is fed with the output of gate 27, i.e. with a signal identical to the enable signal E, and its content is incremented by each logic zero going edge of this output. The up/down control input of the counter is fed with signal SN from memory 25 this signal having the logic value one, and causing the counter to count up the zero going edges of the output of gate 27, while scan patterns B and D are being executed and having the value zero, and causing the counter to count down, at all other times.The logic zero going edge of signal SC from bistable 20 at the start of the scanning sequence sets the content of counter 28 to a number equal to the number of lines in pattern A while the zero going edge of signal SQ, which consists simply of a short positive going pulse with its rear flank coincident with the start of the pattern C scan, sets the counter content to the number of lines in pattern C.

A ramp waveform R for causing the scan lines of each scan pattern to be traced is produced by an integrator 30 which is fed with the sum, formed by adder 31, of a unidirectional offset voltage from a generator 32 and the output of an exclusive-or gate 33. The two inputs of gate 33 are fed with the output of gate 27, i.e. a signal identical to enable signal E, and the signal SN.

Thus, this gate simply passes the output of gate 27 on to the adder 31 while signal SN is at logic zero, i.e. while scan patterns A and C are being executed, and forms the inverse of gate 27 output while scan patterns B and D are being executed.

As a result, integrator 30 forms the required lowgoing ramp with high-going flybacks for executing patterns A and C, and the high-going ramp with low-going flybacks for patterns B and D. The reason for adding the offset voltage from generator 32 to the integrator input is to maintain the average d.c. level of the integrator output at zero. The generator 32 produces appropriate different levels of offset voltage V, an V2 for the two kinds of ramp waveform produced by the integrator and a zero voltage, i.e. no offset, during the interscan delay times t1, t2 and t3 under the control of signal SN, signal SM which has a logic one value only while scan patterns A and C are being executed, and signal SO which takes the logic value one when pattern C is started and goes to zero when pattern D is completed.The zero going edge of signal SO initiates the generation of the pulse from monostable 21 which resets bistable 20 and signifies the completion of the scanning sequence.

The staircase waveform from the digital to analogue converter 29 and the ramp waveform from the integrator 30 are passed to a changeover switch 34 which, under the control of signal SO, passes the staircase waveform to the Vy output of the drive unit control circuit to control drive unit 1 5 and hence the y-plane deflection cell 4 and passes the ramp waveform to the Vx output of the control circuit to control drive unit 1 3 and x-plane defiection cell 3, hence producing scan patterns A and B, and which then changes over so that the staircase waveform is directed to the Vx output and drive unit 14 while the ramp waveform is directed to the Vy output and drive unit 15, hence producing scan patterns C and D.

As will be appreciated by those skilled in the art, the construction and operation of drive unit control circuit 1 6 is described above by way of example only and this circuit could be implemented in various other ways. One possibility would be to simply cdntrol the drive units by means of a suitable computer which has been programmed to obtain the desired scan patterns, possibly a series of different patterns which can be selected as desired.

It will be further realised that the exact form of the scan patterns shown, or the described sequence thereof, are not essential. Rather, the scanning can follow any suitable pattern provided that, at some point, not necessarily immediately afterwards, the beam moves so as to become incident on the missile again and the missile can determine its position by reference to the time between the two incidences. For example, one suitable modified scanning pattern sequence is shown in Figure 6. The sequence comprises a first scan E which commences at the top left-hand corner 100 of the scanned field 101 and the azimuth direction of the beam is then varied so that it scans across towards the right of the field.

After d short delay time TL, it then executes a reverse scanning movement i.e. not a flyback movement as before with the same elevation so that it comes back to its starting point whereupon the beam elevation is stepped downwards and, following a further short delay TI a further sequence of a right-going forward scan, short delay TL, and a left-going reverse scan is executed. The beam elevation is then stepped down again, a further forward and reverse scan executed, and so on. The beam ends up at the bottom left-hand corner 102 of the field 100 and, after a suitable delay TO, starts to execute scan F each comprising a series of up and down scan movements, the azimuth direction of the beam being stepped from left to right between each pair of up and down scans. As before, each upwards scan and the following downwards scan are separated by delay TL while each downward scan and the following upward scan are separated by delay TI (during which the azimuth direction is stepped). The beam then ends up at the bottom right hand corner 103 of the field. From this position it returns to the original starting point 101 in Figure 6a and, after a further predetermined delay TF, repeats the whole sequence. A missile within the field 100 thus receives two closely spaced laser sightings while the pattern A is executed and, from the time between these two sightings minus the known delay time TL, can determine its position relative to the right-hand edge of field 100. While pattern B is being executed, the missile again gets two closely spaced glimpses of the laser beam and therefrom can determine its position relative to the top edge of field 100.As before, ambiguities and such are resolved by the guidance apparatus within the missile on the basis of the known delays TO, TF, TI and TL.

The time between respective sightings, during patterns A and B gives not only azimuth information but, to an accuracy of a line spacing or so, also elevation information. Correspondingly, the time between sightings during patterns C and D gives at least rough azimuth information as well as accurate elevation information. Thus, possibly, only the scan patterns A and B or the patterns C and D need be executed. This is not often likely to be true for a missile guidance system where accuracy to within a line spacing will rarely be good enough either for the azimuth or the elevation measurements. However, this invention is not of course limited to missile guidance but is instead applicable to many situations where some object is to be guided or to guide itself relative to a defined position.By way of example, a scanning system according to the invention might be used say for guiding a spacecraft from a ground position or from a position on board another spacecraft, or it might be used for guiding say a helicopter trying to land on an offshore oil platform. In the latter case, probably the position information would be simply presented to the helicopter pilot rather than being used for automatic control as would be the case with a missile and probably also a spacecraft.

Finally, it will be realised that it may be possible to use, perhaps with some adaptation, a mechanical type of scanning mechanism, e.g. one incorporating moving mirrors, to provide a sequence of such scan patterns that the time between incidences of radiation on a point within the scanned field is dependent upon the position of the point. However, the use of a nonmechanical deflection system, particularly the acousto-optical deflector system described herein and shown in the drawings is much preferred since thereby it may be that synchronisation of the various movements making up the chosen scan patterns is made simpler and the achievability of scan pattern changes, the speed of scanning and the scan repetition rate, achievability of control and programmability of the scanned field-of-view and other parameters, and the accuracy of the positional information are all improved.

Claims (10)

Claims

1. Position determining apparatus comprising controllable projecting means for projecting a beam of radiation in a controllable direction within a field-of-view of the projecting means and control means connected to control the projecting means and operable for so varying the beam direction that the beam traces out a path during which the beam twice becomes incident on a point within said field-of-view and the time between the two incidences is dependent upon the position of the point within the field-of-view.

2. Apparatus according to claim 1, wherein said control means is operable for varying the beam direction to trace out a first scan pattern over the whole of an area within said field-of-view and subsequently to trace out a second scan pattern over said area such that the time between incidences of said beam on said point during the execution of the respective patterns is dependent upon the position of the point within said area.

3. Apparatus according to claim 2, wherein said first and second scan patterns comprise respective substantially identical patterns of parallel lines but with the beam moving in one direction to trace out the lines of one pattern and in the opposite direction to trace out the lines of the other pattern.

4. Apparatus according to claim 3, wherein said control means is further operable to cause said beam to trace out respective third and fourth scan patterns over said area, these paths again comprising substantially identical patterns of parallel lines and the beam again moving in opposite directions to execute the lines of the respective patterns, but the lines of the third and fourth patterns being transverse to those of the first and second patterns.

5. Apparatus according to claim 2, wherein said control means is operable to cause said scan patterns to be traced out in repetitive cyclic sequence.

6. Apparatus according to claim 1, wherein said control means is operable for causing the beam to trace out scanning patterns repetitively, and for introducing predetermined identifying time delays within the scanning.

7. Apparatus according to claim 1, wherein said projecting means comprises a laser beam generator and acousto-optical beam deflecting means for varying the beam direction.

8. Apparatus according to claim 1, wherein said control means is operable for varying the beam direction to trace out a scan pattern comprising a series of pairs of successive scanning paths, the successive paths of each pair being the same but traversed in opposite directions.

9. A beam rider missile guidance system comprising position determining apparatus according to claim 1 and at least one missile incorporating apparatus for sensing incidences of said beam at the missile, for determining the time elapsing between successive incidences and for guiding the missile accordingly.

10. Position determining apparatus substantially as hereinbefore described with reference to the accompanying drawings.