GB2211371A

GB2211371A - Position warning system

Info

Publication number: GB2211371A
Application number: GB8724386A
Authority: GB
Inventors: Bryan Morgan
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1987-10-17
Filing date: 1987-10-17
Publication date: 1989-06-28
Also published as: GB8724386D0

Abstract

In a warning system for a travelling body 63 carrying a receiver RX and crossing one or more boundaries B1 defined between a transmitter TX and transponders TP1, transmitter TX repetitively emits pairs of signals separated by a predetermined interval and each transponder TP1 responds to the first by emitting a signal after a delay. The locus of points at which the transponder signal and second transmitter signals arrive simultaneously forms a boundary B1 which, by choice of transmitter signal interval and transponder delay, can be located mid-way between them and extends orthogonally to a line joining them. A safety region around and above a firing range 60 is formed by boundaries B1 etc. and a missile forms the travelling body 63; if the missile strays to the wrong side of any boundary it self destructs. <IMAGE>

Description

POSITION WARNING SYSTEM This invention relates to position warning systems and in particular to a system for a travelling body to warn of its crossing a predefined boundary.

Such a boundary may define an open or closed two dimensional region, such as an area of land or sea, or a three dimensional region such as air space or, more particularly, an area of land or sea and air space above it.

Closed regions may be used to contain the body and warn of its exit or to exclude the body and warn of its intrusion whereas an open region may define a dimensionally limited pathway for, or a region associated with, a body constrained more or less to a particular direction of travel.

An example of a three dimensional closed region, and one within which the invention is particularly, but not exclusively, concerned is a missile firing range wherein missiles are launched from one end of the range to fly to a target at the other, an important requirement of such a range being the ability to neutralise, usually destroy, any missile which goes off course and crosses the boundaries of the range, possibly including a 'ceiling over the range.

Warning systems are known for such missile ranges within so-called flight termination systems employed to track missiles in flight down the range by steerable electro-optic or radar devices and respond to detection of a missile position outsi4e of a notionally defined range boundary to cause self destruction of the missile by ceasing the transmission of a destruct-inhibiting signal thereto. For reasons which will become apparent it may be remarked that such warning systems have disadvantages, both economically because of their complexity and operationally because of their inability to deal satisfactorally with a salvo of missiles, both in tracking them all and destroying only those individuals which stray across the range boundary.

When dealing with travelling bodies which behave individually there are clearly advantages for any individual body to be made aware of its transgression of a boundary either as a general warning or an individual warning to enable the body to take corrective action. Such corrective action may, on the part of an 'intelligent' body such as an animal or self guided vehicle, be to move back within the bounded region or may, on the part of a dumb -or inanimate body be to stop travel or, in the case of a missile within a flight termination system, destroy itself.

It is an object of the present invention to provide a warning system of simple construction for providing at a travelling body a warning of reaching a position that results from crossing a boundary of a region defined thereby.

It is a further object of the present invention to provide a flight termination system for missiles simpler and more useful than known systems and which utilises such a position warning system for effecting controlled activities, including self neutralisation, of missiles travelling across a boundary of a region define by the warning system.

According to a first aspect of the present invention a position warning system for a travelling body to indicate its crossing of a defined boundary comprises a transmitter operable to transmit repetitively a pair of event signals separated by a predetermined time interval, at least one transponder responsive to receipt of at least the first transmitted event signal to emit a transponder signal and disposed with respect to the transmitter to define therebetween a boundary extending across a line joining the transmitter and transponder at a position denoting equality between the time taken, from transmission of the first event signal of the pair, for the second event signal of the pair to reach the boundary and the time taken for the transponder emission, in response to reception thereby of the first event signal of the pair, to reach the boundary, and receiver means of transmitter and transponder emissions, adapted to be carried by the body, arranged to be inhibited from responding to the second event signal of a transmitted pair by reception of the transponder emission signal and responsive to a change of signal reception from that when the body is to a desired side of a boundary, between the transmitter and transducer to provide a warning signal that the body has crossed the boundary.

According to a second aspect of the present invention a flight termination system for a missile firing range includes a position warning system as defined in the preceding paragraph in which the body carrying the receiver means comprises a missile, to be launched down the range to a target point, containing neutralisation means, responsive to a warning signal from the receiver means to neutralise the missile, and in which each boundary defined between a transmitter and transponder comprises a boundary to the range the crossing af which effects neutralisation of the missile.

Embodiments of the present invention, both as a position warning system in general and a flight termination system in particular, will now be described by way of example with reference to the accompanying drawings, in which: Figure l(a) is a schematic illustration of the definition of a boundary between a transmitter and transponder, of a position warning system according to the present invention, Figure l(b) shows time relationships between signals transmitted and received by components of the position warning system, Figures 2(a) to 2(c) show block circuit diagrams of the transmitter, transponder and receiver means respectively of a position warning system according to the invention, Figure 3(a) shows in plan view the disposition of transmitter and transponders, located at the earth's surface defining parallel planar boundaries and a corridor between them, Figures 3(b) to 3(d) show respectively corresponding plan views of further dispositions of the transmitter and transponders and open and closed regions defined by the intersection of planar boundaries between the transmitter and transponders, Figure 3(e) is a view in elevation of the disposition of Figure 3(d) illustrating the addition of an aerial transponder and the definition of a ceiling boundary to the closed region defined by the other transducers, Figures 4(a) to 4(c) are plan views of a flight termination system in accordance with the present invention for a missile firing range, illustrating different shapes of open and closed regions defined by boundaries between a transmitter and plurality of transponders, Figure 5 shows a perspective view of the boundaries and region defined in the system of Figure 4(c) and including ceiling boundaries defined by aerial transducers, Figure 6 shows a perspective view, similar to of Figure 5 of a bounded region suitable for missiles air-launched at a ground target, Figure 7 shows in elevation a further form of bounded region, in space, and surrounded by boundaries defined in space by a transmitter and transducers, and Figure 8 is a plan view, similar to Figure 4, but of an alternative form of flight termination system in which a plurality of bounded regions are defined by different transmitters at different parts of the missile firing range.

Referring to Figures l(a) and l(b) a transmitter TX of electromagnetic energy in the radio frequency band, preferably in the UHF band, is caused to emit a signal comprising or containing a pair of distinctive event signals with a predetermined interval between them.

In a simple form the event signals may comprise short pulses 10, 11 of r.f. radiation of duration te, say of the order of 1 microsecond long, with an interval t. of several microseconds between them, (waveform (i)), preferably effected by changes in a longer duration carrier signal 12 by modulation or coding of the transmission, although discrete transmission bursts could be used. Such transmission bursts may benefit from a transmitted preamble 10', 11' before each event signal pulse to enable a simple receiver to lock on to the transmission. Any such emission additional to an event signals of interest may be considered as a conditioning signal to condition the receiver to event signal reception.

The transmission of the pair of event signals is repetitive with a repetition period long in comparison with the interval between event signals, say of the order of one millisecond.

A transponder TPl is responsive to the UHF radiation and in particular to the event signals and in response to the first event signal 10 of the pair emits, after a short (and possibly variable) retransmission delay td an r.f. transponder signal 13 (waveform (ii)) which may extend for much longer than the event ' signal interval t. but less than the repetition L period, conveniently the duration of the event signal interval tri .

L Considering a body travelling between the transmitter TX and transponder TP1 and carrying means RX for receiving the r.f. emissions from both transmitter and transponder, the timing of signal reception is shown in waveform (iii). It will be seen that the first event signal of the pair arrives and is received at the body before its reception at the transponder.

Subsequently the second event signal 11 may arrive before or after transponder signal 13, depending upon the event signal interval ti, the retransmission delay td and the position of the receiver between the transmitter and transponder in relation to their separation (in determining the transit times of the signals).

The receiver RX is arranged to receive the second event signal and also for the transponder signal to inhibit recognisable reception of a second event signal arriving subsequently to it and a receiver position exists between the transmitter and transponder at which the second event signal from the transmitter is received simultaneously with the jamming signal from the transponder, such a position defining a boundary between the transponder and transmitter.

The situation therefore exists wherein if the second signal to reach the receiver is the second event signal then the receiver is between the transmitter and the boundary position whereas if the second signal to reach the receiver is the transponder signal, possibly manifested by failure to recognisably receive the the second event signal subsequently, then the receiver is between the boundary position and the transponder. It will also be seen that if one reception condition is set to be satisfied by the signal received, excluding and subsequent to the first event signal, being either the second event signal or the jamming signal and the signal actually received changes to the other one then this is indicative of the receiver having crossed the boundary position.

Any number of boundary positions may be found for a particular set of event signal intervals and retransmission delays and a locus of boundary points, conveniently called a boundary can be generated which in general assumes a three dimensional hyperbolic surface. For different values of event interval and retransmission delay a set of boundary loci may be defined at different distances between the transmitter and transponder.

Such sets of hyperbolic surfaces are in themselves familiar from navigation systems employing timed master and slave transmitters or transmitters and transponders.

Because of the complexity of shapes with such hyperbolic surfaces, the boundary mid way between the transmitter and transponder is of particular interest in comprising a plane extending orthogonally to the line joining the transmitter and transponder.

In the embodiments hereinafter described such a mid-way planar boundary is defined, unless otherwise specified, for ease of implementation, and therefore of description and illustration, although it will be appreciated that boundaries having any of the hyperbolic contours may be employed.

Thus in accordance with the present invention ' a transmitter and transponder, or more usefully a plurality of transponders, are disposed. with respect to each other to define between the transmitter and the, or each, transponder a boundary in space which can be recognised by receiver means crossing it in terms of the change in reception conditions, between reception of a second transmitted event signal and reception (or at least the effects) of transponder signals and the receiver means can issue a warning signal indicative of such a crossing.

Referring to Figure 2(a) this shows in block form the transmitter TX. This comprises in basic form a clock 21, a timer 22 responsive to the clock to define the event signal durations t , event signal interval t. and the interval tR representing the transmission repetition period. A radio frequency (r.f.? generator 23 is connected by way of a modulator circuit 24 to an antenna 25. The r.f. generator emits a continuous UHF carrier wave and the modulator 24 is controlled by the timer 22 to permit in each repetition period tr two short modulated bursts of r.f. radiation to be transmitted as a pair of event signals each of duration t and separated by e interval ti.

It will be appreciated that the continuous r.f. carrier transmission may be omitted other than when serving as a conditioning signal and a preamble to the event signals and r.f.

gating or switching means 26 may be provided, controlled also by the timer 22, to provide interruption of the r.f. carrier.

Instead of providing a simple carrier r.f. as the conditioning signal, the 'carrier' or any parts comprising specific preambles to an event signal may be modulated or coded in a manner characteristic of the transmitter, say by modulation generator 27, with the event signal defined by a change in modulation or coding, by modulator 28, effected by switch 29 under control of timer 22.

Figure 2(b) shows in similar block form the transponder TP1 which comprises a reception antenna 30, a receiver 31 of the transmitted event signals, a clock 32, timer 33 responsive to the clock to determine time intervals, such as the duration of the transponder emission, and an r.f. generator 34 connected to feed a transmission antenna 35 by way of first gating means 36, responsive to timer 33, and second gating means 37, responsive to delay means 38. The delay means 38 is responsive to a signal received from receiver 31 and characteristic of a transmitter event signal and to the clock 32 to provide a delay, the retransmission delay td, before the gating circuit 37 is opened to permit commencement of the transponder signal emission. In the simple form the emission is a continuous r.f.

jamming signal and the timer 33 may determine the duration of the jamming emission by receiving also the output signal of the delay means 38. The timer may be set to provide a jamming emission duration much longer than any possible interval event signals and transit times of the signals but much less than the transmitter repetition period or, conveniently, reception of the second event signal by the transponder may be used to stop transponder emission, possibly after a fixed delay.

It will be appreciated that the transponder emission may also take different forms. Instead of being a simple jamming signal, which relies upon a higher energy content than the transmitter, at least up to a desired boundary point between them, to effect jamming of the transmitter signal, it may be an emission of shorter duration modulated or coded to enhance its reception and identify its origin, relying upon the receiver means to perform the identification and utilise it to inhibit reception of, or at least response to reception of, the second transmitter event signal. However such forms of transponder signal may require a more complex receiver means to receive and act upon, including means of accommodating simultaneous reception of transponder signals from more than one transponder, than does a simple jamming signal.

The regular reception of first event signals or transmitted conditioning signals may serve to provide synchronisation between the clock 32 of the transponder and the transmitter.

Figure 2(c) shows in block form basic receiver means RX comprising antenna 41, r.f. receiver and signal detector 42 and decision means 43. The receiver is of simple construction consistent with its use in large numbers and expendibility and benefits from being able to lock onto the transmitter by means of the conditioning signal(s) additional to the event signals.

It will be appreciated that although the first event signal reaches the receiver means its main use is at the transponder and the decision of the receiver means as to its position with respect to a boundary is made on the basis of which of the second event signal or transponder signal arrives first.

To enable reception of the second event signal the transmitter transmits a conditioning signal in the form of a preamble, conveniently comprising r.f. carrier extending from before the first event signal and for the interval. between them. The receiver may respond to reception of a conditioning signal in any form but conveniently to the first event signal if this is included in the conditioning signal. The decision means 43 as shown comprises a clock 44, a bistable flip-flop 45 connected to the output of receiver 42 to be triggered thereby, a counter 46 of clock pulses and an output latch 47. The output stage 48 of the counter is connected to a reset input 49 thereof as is one output 50 of flip-flop 45, the other flip-flop output 51 being connected to a counter start input 52.The flip-flop 45 is also reset by the output of final counter stage 48 and responds to reception and detection of a conditioning signal from the transmitter, conveniently the first event signal, to produce an output at 51 to start the counting of clock pulses. The number of counter stages and/or clock frequency is chosen such that the counter is filled and provides an output at 48 only after a counting time greater than a maximum time interval t. between transmitted event signals.

If the receiver detects a second event signal the flip flop changes state removing the output at 51 and providing an output at 50 to reset the counter. In the next transmitter period the process is repeated and the second signal normally received, that is, after the conditioning signal, is the second event signal consistent with the receiver means being between a transmitter and boundary. If, however, the second signal received in the period is the jamming signal from the transponder the receiver 42 is jammed before receipt of the second event signal and so does not provide a detection signal. The counter 46 continues the count until full when an output signal sets output latch 47 and resets the counter and flip flop for the next transmitter period.

The signal available from output latch 47 may thus comprise the warning signal and in this particular configuration indicate that the receiver means has been carried across a boundary between transmitter and a transponder and is between the boundary and transponder. Such a configuration may therefore be employed where it is desired to maintain a body carrying the receiver means between the transmitter and the boundary.

The receiver means may of course be used in respect of maintaining the body between the boundary and transducer, wherein an output from counter 46 is expected for each transmitter period but where the absence of such a counter output, reset by reception of a second event signal, is indicative of it crossing the boundary and being closer to the transmitter.

The decision means 43 may also be re-configured to provide an output signal only in the boundary crossing conditions lastly described rather than rely upon the absence of a counter output to indicate such conditions.

If desired the decision means may be arranged not to respond to a conditioning signal of the transmitter, which is employed to ready the receiver, but may be operated on a timed basis to derive a suitable time window for the reception of second event signals and respond to reception or failed reception of a second event signal in the time window.

Additionally, the receive means RX may include a further counter 53 (shown by broken lines) operable to count signal outputs from the decision means 43 and provide the warning signal only when the body is detected as being across the boundary for a predetermined number of transmitter periods. The further counter 53 may be arranged to be reset by the output 50 of flip flop 45, that is reset in any transmitter period during the count if a second transmitter event signal is received, thereby providing a warning signal output only when the boundary transgressions detected are maintained for the predetermined number of consecutive transmitter periods.

Referring now to Figure 3(a) this shows in plan view the relative dispositions of transmitter TX and a plurality of transponders TP1 and TP2 at the earth surface, called for convenience 'ground level' although it includes the surface of a body of water.

The transponders are disposed at opposite sides of the transmitter from each other and the separations and time intervals chosen as described above to give planar boundaries B1 and B2 respectively mid-way between transmitter and respective transponders.

A body 55 carries receiver means RX as shown in Figure 2(c) which provides a warning signal when between a boundary and a transponder so that the boundaries B1 and B2 define between them a corridor along which and across which the body can travel without the receiver means producing a warning signal unless it crosses one of the boundaries.

The corridor may be of any length depending only upon the reception of adequate transmitter and transponder signals by the receiver means to function correctly. The body may also travel at or to any height within the corridor subject of course to the correct reception of signals.

Figure 3(b) shows a similar. plan view but with a further transponder TP3 defining a third boundary B3 which intersects boundaries B1 and B2 and defines a corridor closed at one end. A further transponder TP4 and boundary ss4 may with the other intersecting boundaries B1 to B3 define a closed region within which the body 55 is constrained to travel without issuing a warning signal.

Tt will be readily understood, and Figure 3(d) illustrates, that such a closed region need not be regular and the. juxtaposition of boundaries depends upon, and is as variable as, the number of, and positioning of, the transponders with respect to the transmitter.

The disposition of transmitter and transponders at ground level defines the open or closed region for a body travelling at or above ground level and the mid way boundaries extend vertically upwardly from the ground as shown in the sectional elevator view of Figure 3(d) in Figure 3(e).

Figure 3(e) also shows the provision of an aerial transponder TPA above the bounded region, in this case above the transmitter, the separation between them and transponder delay permitting a further mid-way planar boundary BA to be defined which forms a ceiling to the bounded region. Such an aerial transponder may be fixed with respect to the ground, by mounting on a post or the like, or, suited to higher altitudes, carried by a small tethered bolloon. Alternatively, the transponder could be carried by a small remotely piloted vehicle caused to take up, and hover at, any position determined by a remote controller.

The use of a non-planar hyperbolic boundary may be particularly useful in defining such a ceiling boundary, examples of sections through possible such boundaries being shown in Figure 3(e) at BA' and BA". Furthermore, lateral displacement of the aerial transponder from directly above the transmitter as at TPA' permits an assymetrically shaped ceiling over the region as illustrated by chain dot boundary lines By''' and BA"".

It will be seen that any closed or open region can be defined particularly simply when using planar mid-point boundaries and of virtually any dimensions depending only upon the setting of time intervals and delays in the transmitter and transponder the use of adequate transmission powers and receiver sensitivity to ensure reception at the body.

The simplicity and convenience of setting out the shape of any closed region is further enhanced by defining the planar mid-point boundaries at ground level and considering the transmitter and transponders to be in the same plane orthogonal to the planes of the boundaries. It will be appreciated that the transponders and transmitter need not be at ground level, enabling a floor to be provided corresponding to the ceiling of Figure 3(e), nor need they share a common plane. For instance, the transponders may be in one plane spaced above the transmitter when the boundary walls hitherto extending vertically upwards would be inclined towards each other in the manner of a pyramid. Alternatively, a transmitter disposed above transponders at ground level would define a region with an inverse pyramid, or funnel-shaped, boundaries.The transponders may of course be at different heights giving more complex three dimensional bounded regions.

Furthermore, a single boundary such as BA in Figure 3(e) defined by the transmitter and a single transponder may define by its intersection with the ground plane a closed region at and above ground level.

Any region provided by boundaries defined between a transmitter and transponders is therefore not limited in the number of boundaries enclosing the region, their separation nor the angles made between them, this being for any transmitter whose event signal interval is set in accordance with defining other boundaries simply a function of the separation of transmitter and transponder, the direction of a line joining them (across which the boundary extends) and the retransmission delay of the transponder.

It will also be appreciated that in some circumstances boundaries defined for initial operation of the system may require to be changed during operation.

With the transmitter and transponders fixed in their positions the retransmission delays of the individual transponders and/or the event signal interval of the transmitter may be changed to define a different bounded region including, by resorting to any hyperbolic, rather than planar, boundaries, a region of different shape. Additionally, or alternatively, the relative dispositions of the transponder and transmitter may be changed.

Such changes, both to timing parameters and transponder positions, may be effected automatically as a result of measurements and calculations of operation or as part of a timed sequence, conveniently at a remote location and revised operating parameters transmitted by cable or wirelessly to the transmitter andtor transponders of the system. Naturally, a change of transponder position assumes the provision of a mechanism for effecting position change in response to such remote control signals. Many positioning mechanisms are known and mechanisms or descriptions thereof available elsewnere. As they do not form an integral part of the present invention no further description is considered appropriate.

As outlined hereinbefore the transmitter, transponder and receiver means may be made simple or more complex, using well known techniques for radio transmission and reception to improve its operating performance.

It will be appreciated that the definition of regions illustrated in Figure 3 for confining a body may similarly apply to excluding a body. The crossing of boundary into this region, that is, between boundary and transmitter, may also be used to produce the warning signal that a boundary has been crossed.

The warning signal is produced at the receiver means carried by the body and is individual to the body and it is convenient for the body also to utilise its individual signal for issuing a warning at the body, such as an audible or visual warning or to effect a control function on the body, such as stopping any motion thereof. The body may of course have other means for transmitting the warning signal to a remote location.

Also, the transmitted energy may be other than radio frequency electromagnetic radiation. Electromagnetic radiation may be of a different part of the spectrum, such as optical radiation, although greater problems may be expected with transmissions from the transmitter in plural directions and with occlusions of any transmission paths. The system may employ different forms of energy, such as acoustic energy, although the velocity of its propagation through the medium between transmitter, transponders and receiver means which is a factor in defining boundary positions may present problems of accuracy of determination or require continuous monitoring.

It will be appreciated that such a position warning system may be employed for any defined region and any body capable of carrying a receiver means, which travels within (or without) sucn a region.

Such a position warning system may be used to particular effect in a flight termination system for a missile firing range.

Flight termination systems are employed to prevent any missile on an incorrect trajectory from leaving the range by detecting that it has crossed a notional boundary enclosing the range and causing the missile to self destruct.

In employing a position warning system as described hereinbefore it is contemplated that a missile is neutralised, not necessarily by destroying it, although such destruction may be the most common form of neutralisation. The missile represents a travelling body and carries therein receiver means as outlined above and neutralisation means, such as an explosive self destruction charge, responsive to the warning signal given by the receiver means.

It is normal in flight termination systems to operate a fail-safe scheme wherein the neutralisation means is arranged to try normally to effect self destruction out is inhibited by continuous or continual reception of a control signal transmitted to it from remote observation means. Intentional cessation of the control signal or loss of reception for any other reason results in the fail-safe destruction of the missile.

As outlined above, the receiver means may be made to deliver a warning signal to neutralise the missile only when the receiver means is outside of a boundary of a defined region, or may be made to deliver signals continually when the reception means is inside the boundary D ehe continually produced signals serving to inhibit neutrarisalon of the missile until they cease if the missile crosses a boundary, the cessation conprising a warning signal in the traditional manner.

Although neutralisation in the former case relies upon initial production of a warning signal, as opposed to an absence, the warning signal effectively results from the absence of hitherto continually received second event signals from the transmitter and has a directly corresponding fail-safe aspect to its production.

In either case the warning signal is produced at the missile and does not rely upon a destruction control signal transmitted thereto from a remote location. Also, if the transmitter and the transponders are disposed at the part of the firing range such as the target. region where such crossing of the boundaries is most likely then the IR proximity to the receiver means improves the ability of the receiver means to receive their emissions, which determine the fate of the missile, with more certainty then when a remote transmitter is used to control self destruction.

The ability of the position warning system to define open or closed regions of any shape also lends itself to use in a flight termination system.

Referring now to Figure 4(a) this shows in plan view a missile firing range 60.

The range comprises a missile launcher 61 at a launch point and a target point (or area) 62 down range of the launch point. The launcher 61 is arranged to launch a missile 63 towards the target point and, may comprise a gun for launching unpowered projectiles, such as artillery shells, or a vehicularor man-carried launcher of guided or unguided self-powered missiles.

A transmitter TX is disposed down range of the firing point and with surrounding identical transponders TP1 to TP3 defines boundaries B1 and B2 forming a corridor extending from the firing point to beyond the target point, and a boundary B3 terminating the corridor. Receiver means RX is carried by the missile 63 which, of course, in operation comprises the travelling body.

The transmitter TX and transponders TP1 to TP3 are conveniently disposed at ground level but the firing range may be provided with a ceiling boundary by means of an aerial transponder TPA or TPA', as shown in the elevation of Fig.

3(e). Where the firing range is used to fire missiles along a ballistic trajectory a hyperbolic ceiling, as shown at BA' in Figure 3(e) is particularly convenient.

When a missile comprises a gun-launched unpowered projectile then there is no danger of it going other than generally down range and the region bounded, as in Figure 4(a) may be a corridor open in the vicinity of the gun. The corridor need not extend all the way to the launch point and if the transmitter/transponder radiated power is to be limited it may be preferred to mount the transmitter in the vicinity of the target point, to increase the certainty of reception towards the end of the projectile trajectory whilst reducing the certainty towards the firing point.

If the missile is a self-powered missile, particularly a guided missile able to change direction, then it may be preferred to enclose both the firing and target point within a closed region as illustrated in the plan view of Figure 4(b) when a fourth ground based transponder TP4 is employed.

In keeping with the ability to define regions by appropriately intersecting boundaries the plan view of Figure 4(c) shows a closed region containing a firing point 61 and target point 62 defined by transmitter TX and three transponders TP1 to TP3 with transponders TP1 and TP2 disposed to provide non-parallel corridor boundaries B1' and B2", saving not only an additional transponder but also defining a region more suited to the natural dispersion pattern of missiles.

Figure 5 shows a perspective view'of the volume of the region enclosed in Figure 4(c) when including a hyperbolic ceiling boundary by means of an aerial transponder TPA.

The bounded region defined in Figures 4(c) and 5 is typical for a well defined ground based firing point in tapering to minimum dimensions at the firing point.

Figure 6 shows a perspective view of a bounded value suited to use with air-launched missiles, self powered or projectile, and represents a funnel-like region leading from a more extensive possible launch region to a less extensive region about a ground target point. The funnel may be 'open ended', that is, not have a boundary wall For may be provided with such a wall by which receiver means provides a signal to the aircraft that it has entered the bounded region and can safely launch the missile.

Alternatively, the receiver means may be arranged such that it responds to initial crossing of a boundary such as B4', or even boundaries B1 and B2, not by producing a warning signal but arming any detonation circuits of a missile warhead, thereby causing the missile to remain unarmed if it should fail to enter the region, and setting the receiver means such that if it does cross a further boundary to leave the region the warning signal is produced and neutralisation of the armed missile is effected.

It will be appreciated that such arming control operations may be employed with any missile, including ground launched ones, if it is desired to arm them only upon entering into a boundary defined region in the vicinity of a target point but neutralise them if they subsequently leave the bounded region.

As indicated hereinbefore neutralisation of the missile may be effected, by disarming the warhead and/or disabling any propulsion means of such a missile leaving the bounded region rather than destroying it. Whilst forms of neutralisation other than self destruction are not within the strict and traditional interpretation of the name 'flight termination system' they are believed to be so allied to the function of and philosophy of such systems in preventing damage by errant missiles as to be considered within the ambit of flight termination systems.

In all the flight termination systems hitherto described the transmitter and most of the transducers are mounted at ground level and define a region bounded on one side by the ground. Any aerial transponder is for preferenace carried by a free flying remotely piloted vehicle to eliminate any possible sources of trajectory interference by a support or tether structure. It will be understood that by mounting the transmitter and at least some of the transponders above the ground a bounded region may be defined in space with boundaries on all sides, as shown in Figure 7, including a ceiling at BA and a corresponding floor BF at a predetermined height above ground level defined by ground transponder TPG. Such a region may find uses in respect of aerial target points.

In all of the above described embodiments of flight termination systems a single transmitter and plurality of transponders provides boundaries for a complete firing range between the firing point and target point. Where the firing range is particularly long or transmitter/transponder power has to be kept low, or for other reasons relating to the shape of the boundaries, it may be desired to employ a plurality of transmitters each with a plurality of transponders to define several adjacent bounded regions between firing and target points.

Referring to Figure 8 this shows a plan view of range 70 having firing point 71 and target point 72.

The firing point is contained within a bounded region 73, open down range, formed by transmitter TX1 and transponders TPll to TP13 to accommodate a missile going astray at firing.

A further bounded region 74 is formed at the vicinity of the target point by transmitter TX2 and transponders TP21 to TP3 and forming a closed-end corridor open up-range. The corridor walls may be parallel as described above or planar and divergent to form a 'funnel' to the target point. The walls may of course be hyperbolic and effect a funnel or other shape thereby.

The target point bounded region 74 could of course be closed by a boundary 75, such that only by initial crossing thereof has the missile become armed, and/or, any further corridors, such as 76 be defined by a third transmitter TX3 and transponders TP31 and TP32 intermediate 73 and 74.

Other techniques available for defining boundaries, and thus regions, may be . adopted as outlined above, such as different boundary orientations in space by separating transmitter and transponder planes or changing transmitter event interval and/or transponder retransmission delay or the relative positions of transmitters and transponders remotely whilst the system is in operation.

A flight termination system according to the present invention and employing the position warning system also benefits in providing the boundary-crossing warning signal for each missile within the receiver means carried by the individual missile. The missile may thus be one of many fired in a salvo and operate independently of the others in respect of flight termination. That is, unlike tradiational systems errant, and only errant, projectiles of a salvo are individually neutralised if they cross the boundaries set up around the range while leaving unaffected any correctly flying missiles.

It will also be appreciated that although the simple receiver means carried by each missile is a consumable item destroyed with each missile it need not be significantly more expensive than the self-destruction control receiver required to be carried by each missile in a conventional flight termination system, and indeed may be less so if simplified and more local transmissions are involved, and the cost of the fixed transmitter(s) and transponders compared with complex electro-optic tracking devices and boundary computers results in less complex and therefore cheaper and more reliable system with the added ability of dealing with salvo firings.

Claims

Claims:

1. A position warning system for a travelling body to indicate its crossing of a defined boundary comprising a transmitter operable to transmit repetitively a pair of event signals separated by a predetermined time interval, at least one transponder responsive to receipt of at least the first transmitted event signal to emit a transponder signal and disposed with respect to the transmitter to define therebetween a boundary extending across a line joining the transmitter and transponder at a position denoting equality between the time taken, from transmission of the first event signal of the pair, for the second event signal of the pair to reach the boundary and the time taken for the transponder emission, in response to reception thereby of the first event signal of the pair, to reach the boundary, and receiver means of transmitter and transponder emissions, adapted to be carried by the body, arranged to be inhibited from responding to the second event signal of a transmitted pair by reception of the transponder emission signal and responsive to a change of signal reception from that when the body is to a desired side of a boundary between the transmitter and transducer to provide a warning signal that the body has crossed the boundary.

2. A position warning system as claimed in claim 1 in which the transponder signal is caused to jam the receiver to inhibit the reception of further transmitter event signals at least for the duration of the transmitter event signal pair interval.

3. A position warning system as claimed in claim 1 in which the transponder signal is coded with information indicative of the transmission by the transponder and the receiver means is responsive to the information to inhibit the reception of further event signals at least for the duration of the transmitter event signal pair interval.

4. A position warning system as claimed in any one of the preceding claims in which the transmitter is operable to modulate each event signal transmitted in a manner to characterise the transmission.

5. A position warning system as claimed in any one of the preceding claims in which the transmitter is operable to transmit a conditioning signal prior to the second event signal and the receiver means is responsive to reception of the conditioning signal to condition the receiver means to subsequent reception of either the second event signal of the pair or the transponder signal.

6. A position. warning system as claimed in claim 5 in which the conditioning signal includes the first event signal.

7. A position warning system as claimed in claims 4 and 5 in which the conditioning signal is a carried signal transmitted at least between the first and second event signals.

8. A position warning system as claimed in any one of the preceding claims in which the event signals comprise pulse signals.

9. A position warning system as claimed in any one of the preceding claims in which the transmitter and transponder are arranged to emit, and the transponder and receiver receive, electromagnetic energy in the radio frequency part of the spectrum.

10. A position warning system as claimed in any one of the preceding claims in which the receiver means is arranged to produce a warning signal only after an indication of change of signal reception between second event signal and transponder signal for a plurality of transmission repetition periods.

11. A position warning system as claimed in claim 10 in which the receiver means is arranged to produce a warning signal only after a change is indicated and maintained for a plurality of consecutive transmission repetition periods.

12. A position warning system as claimed in any one of the preceding claims in which the receiver means is operable to inhibit production of the warning signal until two successive changes of signal reception are indicated, characteristic of the body crossing and then re-crossing the boundary.

13. A position warning system as claimed in any one of the preceding claims arranged to produce a warning signal upon crossing a boundary between the transmitter and a transponder from between the transmitter and boundary to between the boundary and transponder, ' the, receiver means being arranged normally to receive in each transmitter repetition period the second event signal of each transmitter pair and responsive prior reception of the transponder signal to indicate crossing of the boundary.

14. A position warning system as claimed in any one of the preceding claims in which the time interval between transmission of the pair of transmitter event signals and the separation of transmitter and each transponder are chosen having regard to a transponder delay between event signal reception and emission by the transponder to define the position of the boundary associated therewith.

15. A position warning system as claimed in any one of the preceding claims in which the transponder includes variable delay means operable to be set to define the delay between event signal reception and transponder emission.

16. A position warning system as claimed in claim 14 or claim 15 in which a boundary- is arranged to be mid way between the transmitter and a transponder responsible therefor and .extending as a plane orthogonal to the line joining the transmitter and transponder.

17. A position warning system as claimed in any one of the preceding claims in which there are at least two transponders disposed at opposite sides of the transmitter from each other to define by the boundaries between them and the transmitter a corridor in which the transmitter is located.

18. A position warning system as claimed in claim 17 in which there is at least one further transponder disposed to define by intersection of the boundary defined thereby with the corridor boundaries a corridor closed at at least one end.

19. A position warning system as claimed in claim 18 in which the transponders are disposed such that the boundaries defined between each and the transmitter intersect each other to form a closed region.

20. A position warning system as claimed in any one of claims 17 to 19 in which the transmitter and at least some of the transponders are disposed substantially at the earths surface and define by the boundaries therebetween a region of space around the transmitter extending upwardly from the surface.

21. A position warning system as claimed in any one of the preceding claims in which the transmitter is disposed substantially at the earth surface and including at least one aerial transponder disposed above the earth surface to define a boundary in space above the transmitter.

22. A position warning system as claimed in claim 21 in which the aerial transponder is disposed with respect to the transmitter to define a hyperbolic boundary extending at its periphery to the ground and defining a ceiling for a region defined around the transmitter.

23. A position warning system substantially as herein described with reference to Figures l(a) to 3(e) of the accompanying drawings.

24. A flight termination system for a missile firing range including a position warning system as claimed in any one of the preceding claims in which the body carrying the receiver means comprises a missile to be launched down the range to a target point and containing neutralisation means, responsive to a warning signal from the receiver means to neutralise the missile, and in which each boundary defined between a transmitter and transponder comprises a boundary to the range the crossing of which effects neutralisation of the missile.

25. A flight termination system as claimed in claim 24 in which there are at least two transponders disposed at opposite sides of the transmitter from each other and which define between them and the transmitter boundaries forming a corridor extending down range with respect to a missile launching point and including the target point.

26. A flight termination system as claimed in claim 22 in which the corridor-forming boundaries contain the missile launching point.

27. A flight termination system as claimed in claim 25 or claim 26 in which the transmitter is disposed in the vicinity of the target point.

28. A flight termination system as claimed in any one of claims 25 to 27 in which the transmitter and transponders are disposed such that the boundaries defined between them intersect to form a closed bounded region containing both launch and target points.

29. A flight termination system as claimed in any one of claims 25 to 27 including a plurality of position warning systems, each having a transmitter and associated transponders, disposed with respect to the missile firing range so as to provide a succession of corridors extending down the range and receiver means, carried by the missile, responsive to transmissions from each system.

30. A flight termination system as claimed in claim 29 including two position warning systems disposed one each to define a bounded region at a missile launch point and a bounded region at a target point, at least one of the bounded regions being unbounded in a direction along the range towards the other region.

31. A flight termination system as claimed in any one of claims 25 to 30 in which the receiver means is arranged to respond to crossing a boundary to between the boundary and transmitter to arm the missile and set the receiver means to deliver a warning signal upon subsequent crossing of a boundary to between boundary and transmitter.

32. A flight termination system as claimed in any one of claims 24 to 31 in which the neutralisation means is adapated to effect neutralisation of the missiles unless inhibited by a continually received inhibiting signal and the receiver means is adapted to be set to produce a continual inhibiting signal when between a boundary and transmitter by continual reception of the second event signal of each pair and to produce by the cessation thereof upon crossing the boundary a warning signal permitting neutralisation of the missile.

33. A flight termination system as claimed in claim 32 when dependent on claim 31 in which the neutralisation means is responsive to crossing a boundary to between the boundary and transmitter to be rendered operative, and subject to inhibition.

34. A flight termination system for a missile firing range substantially as herein described with reference to, and as shown in, the accompanying drawings.