GB2042694A - Fuzes for Guided Missiles - Google Patents

Abstract

A pulse-radar proximity fuze for detonating a warhead of a flying body such as a guided missile when the flying body is at a required distance from the target comprises a pulsed radio frequency transmitter 13 which transmits signals towards a target. Reflected signals are received and passed to a range gate 18 which opens between successive transmitted pulses for a time interval which corresponds to the time interval during which a received signal generated by reflection from the target of the last transmitted pulse would be received were the target at a range at which detonation of the warhead is required. Signals passing the range gate are used to detonate the warhead. <IMAGE>

Description

SPECIFICATION Fuzes for Guided Missiles A previously proposed pulse radar fuze for detonating a warhead of a flying body when the flying body is at a required range from a target comprises the transmission of short pulses of radio frequency energy towards the target and the reception of reflected signals. A proportion of the pulsed transmission signal is taken via a delay line, which delays the signal for a time equal to that taken for a transmitted signal to reach a target at the required range and return to the flying body. The delayed signal acts as a local oscillator for a balanced mixer. If the target is at approximately the correct range, a Doppler beat frequency is obtained from the mixer output which can be used to cause detonation of the warhead.

This previously proposed fuze has two disadvantages. First, there is uncertainty as to the precise range represented by a particular Doppler frequency due to the range of velocities and relative dispositions between a flying body and a target encountered in use. Secondly, this previous proposed fuze requires the use of a homodyne receiver which can be confused by carrier wave interference.

It is an object of the present invention to mitigate these disadvantages.

According to a first aspect of the invention, there is provided a method of warhead detonation in a flying body using a fuze and comprising transmitting a train of time spaced pulses of radio frequency energy from the fuze of the flying body, receiving a reflected signals from a target, passing the received signals to a range gate of the fuze, opening the range gate between the transmission of successive pulses of radio frequency energy for a time interval which corresponds to the time interval during which a received signal generated by reflection from the target of the last transmitted pulse would be received were the target at a range at which detonation of the warhead is required, a signal being emitted from the range gate when the received signal for a transmitted pulse is received while the range gate is open for that signal, and then passing an emitted signal from the fuze to detonate the warhead.

According to a second aspect of the invention, there is provided a fuze for detonating a warhead of a flying body and comprising a transmitter for transmitting from the flying body a train of time spaced pulses of radio frequency energy, a receiver for receiving reflected signals from a target, a range gate for receiving reflected signals and a range gate opening circuit for opening the range gate between the transmission of successive pulses of radio frequency energy for a time interval which corresponds to the time interval during which a received signal generated by reflection from the target of the last transmitted pulse would be received were the target at a range at which detonation of the warhead is required, a signal being emitted from the range gate in use, when a received signal is received at the range gate while the range gate is open for that signal, the emitted signal, in use, being for detonation of the warhead.

The method and apparatus of the invention have the advantages that by measuring range directly they can be altered readily to accommodate changes in proximity range requirements. In addition, the use of time spaced pulses of radio frequency energy and the range gating technique offer the potential of obtaining good electromagnetic compatability performance and a high resistance to electronic countermeasures.

The following is a more detailed description of one embodiment of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a block diagram of a pulse radar proximity fuze for detonating a warhead of a guided missile and diagrammatic wave forms of signals generated in the fuze, Figure 2 is a longitudinai cross-section of a nose cone of a guided missile incorporating the pulse radar proximity fuze of Figure 1, Figure 3 shows the nose cone of the missile of Figure 2 and four polar diagrams of an aerial mounted on the nose cone, and Figure 4 shows, in the upper half, a wave form of a pulse of radio frequency energy emitted from the aerial of Figures 2 and 3 and, in the lower half, a wave form of a signal received by said aerial.

Referring first to Figure 1, the pulse radar proximity fuze comprises a pulse generator 10 having two outputs 11, 12. A first of the outputs, 11, is connected to a pulsed oscillator 1 3 whose output is fed to a matching network 14 and then to a transmitting and receiving aerial 1 5. The matching network 14 has an output connected to a detector 1 6 which in turn is connected to an amplifier 1 7. The output to the amplifier 1 7 is fed to a range gate 1 8.

The second of the outputs, 12, of the pulse generator 10 is fed to a monostable delay 19 whose output controls the range gate 1 8. The output to the range gate 18 is fed to a target detection circuit 20 which is in turn connected to a proximity delay 21. A preselect switch 22 is provided for the proximity delay 21. The output to the proximity delay 21 leads to an output circuit 23 which also receives signals from a piezoelectric impact switch 24 and a touch sensor 25. The output 26 to the output circuit 23 leads to a detonation device, not shown.

Referring next to Figure 2, the fuze is mounted in a piastics nose cone 28 of a guided missile 27.

The aerial 1 5 is formed on the exterior of the apex of the nose cone 28 by a printing or plating method. The remainder of the exterior surface of the nose cone 28 is covered by a metallic earth sheet 29 printed or plated thereon.

A thin metal centre web 30 extends from the body of the missile 27. Two printed circuit boards 31, 32 rest one on either side of the web 30 and are secured thereto by an outer wrapping of resilient foam (not shown). One printed circuit board 31 carries the components forming the pulse generator 10, the pulsed oscillator 1 3 and the matching network 14. The other printed circuit board 32 carries the components forming the detector 16, the amplifier 17, the range gate 18, the target detection circuit 20, the proximity delay 21 and the control circuit 23. The nose cone 28 pushes over the foam wrapping to locate the circuit boards 31,32 in position.

In use the guided missile 27 is fired at a flying target which may be stationary, such as a helicopter, or moving such as an aeroplane. The missile is guided towards the target and the fuze is required to detonate a warhead when the missile is at a range from the-target which is optimum for destroying the target. This distance is generally accepted to be 3 metres or less.

In air, a radio wave travels at about 30 centimetres per nanosecond. Thus, at the optimum distance of 3 metres the time taken for a radio wave to travel from the missile to the target and return will be 20 nanoseconds. If the radio wave is of 10 nanoseconds duration, the return signal at an optimum target distance should be received 20 to 30 nanoseconds later.

Circuit design considerations limit the duration of the radio wave to 10 nanoseconds which allows a minimum approach of 1.5 metres.

However, if a closer approach is required a delay line may be inserted in the aerial 1 5 to increase the apparent range of the target.

To produce a radio wave pulse of 10 nanoseconds duration, the pulse generator 10 comprises a multivibrator pulse repetition frequency pulse generator which produces a 10 nanosecond duration sinusoid pulse and thus has a band width of 100 MHz and will occupy a transmission spectrum of 200 MHz.

The pulse repetition frequency should be sufficiently high to ensure an adequate number of echo returns during the brief time period when the missile is at or near the optimum distance from the target. If this distance is considered to be 0.5 metres either side of the optimum distance and the relative closing speed between the missile and the target is assumed to be 760 metres per second, the missile will cover this one metre around the optimum distance in 1.3 milliseconds. If it is assumed arbitrarily that 100 return echos are required from the target during this period for positive identification of the target and that the minimum time for passing through this one metre gate is one millisecond, then a pulse repetition frequency of about 100 KHz is required.

The 10 nanosecond pulses at 100 KHz pulse repetition frequency produced by the multivibrator are fed to three monostable circuits (not shown). The output to a first of the monostable circuits is the output 11 to the pulsed oscillator 1 3 and is shown as wave form A in Figure 1.

The pulsed oscillator 1 3 employs a single RF transistor to form a low Q oscillator having a peak power of 2 watts. The oscillator tank circuit is mainly provided by the transistor internal reactances. This results in a circuit having fast start-up and decay times of less than 5 nanoseconds. Pulse modulation is achieved by using a high speed transistor to switch the ground return supply line to the oscillator transistor emitter. The high speed transistor is controlled by the 10 nanosecond pulses received from the pulse generator 10. Due to the very small mark/space ratio of the pulses (less than 1 :1000) the oscillator 1 3 consumes negligible current.

The choice of the frequency of the RF transistor is influenced by the ratio of carrier frequency to modulation band width, aerial dimensions and component size and cost. The ratio of carrier frequency to modulation band width should not be less than 5:1. This gives a minimum carrier frequency of about 1 GHz (30 centimetres wave length). A quarter or even half wave aerial 1 5 at this frequency can be readily accommodated in the nose cone 28. Up to 1.5 GHz most RF circuits can be realised in discrete component form, even though some of the components may be represented by internal reactances and lead inductances of transistors. Above this frequency, it is necessary to use transmission line techniques which can require considerable space. In addition, above this frequency the cost of semiconductor components rises rapidly.Accordingly a frequency of 1.25 GHz is provided by the RF transistor. An advantage of this frequency is that it is not subject to significant attenuation or reflection effects in the presence of rain, hail or snow. The wave form of the output from the pulsed oscillator 13 is shown at B in Figure 1.

The matching network 14 intercouples the pulsed oscillator output, the aerial -15 and the input to the detector 1 6. The network 14 provides a maximum RF attenuation outside the required band, thus aiding rejection of unwanted RF signals for example signals produced by electronic counter-measures from the target. The matching network 14 passes the output from the pulsed oscillator 1 3 to the aerial 1 5.

The aerial 1 5 is a quarter wave (6 centimetres) unipole marked A/4 in Figure 3. Referring to the polar diagrams of Figure 3, the aerial null (Z axis), is along the longitudinal axis of the missile and its maximum sensitivity normal to that direction (i.e.

in the X-Y plane). The aerial 1 5 has full 3600 coverage.

The aerial 1 5 transmits a train of time-spaced pulses of radio frequency energy produced by the pulsed oscillator 1 3. One pulse is shown at B in Figure 1 and in the upper half of Figure 4. The aerial 1 5 receives reflected pulses which are passed to the detector 1 6. The received wave form is shown at C in Figure 1 and in the lower half of Figure 4 and is preceded by a transmitter break through signal also shown in these figures.

The detector 1 6 comprises a single hot carrier diode which forms a simple half wave detector circuit having a threshold sensitivity approaching 40 dBm. Hot carrier diodes have a high immunity to radar burn out. The detector 1 6 rectifies and filters the received signal from the aerial 1 5 thus recovering the original pulse envelope. The detector will reject any signal below its minimum sensitivity which is such that the detector rejects signals which are below the level of a received signal from a transmitted pulse at detonation range. The output signal from the detector 1 6 includes both the transmitter break through signal and the received signal and has a wave form as shown at D in Figure 1.

This output signal is amplified by the amplifier 1 7 which is a 200 MHz linear integrated circuit amplifier. An input and output coupling network of this amplifier constrain the total circuit band with to 100+50 MHz. This band width is adequate to cope with a 10 nanosecond pulse while ensuring rejection of lower frequency modulated interfering signals and carrier wave interference. The amplifier 1 7 has a gain of about 50.

The amplifier output is fed to the range gate 1 8 which employs 2 high-speed diodes acting as an analogue switch for the range gate. The range gate 1 8 is also connected to the third monostable circuit of the pulse generator 10 which acts as the delay 19. The third monostable circuit is connected to the first and second monostable circuits and an output signal from the first monostable circuit triggers the second monostable circuit which in turn triggers the third monostable circuit to produce a pulse of 10 nanoseconds duration shown at E in Figure 1.The triggering delay between the monostable circuits determines the start time of the pulse from the third monostable circuit and this delay is chosen to be 20 nanoseconds so that the pulse from the third monostable circuit opens the range gate 1 9 for a 10 nanosecond period 20 nanoseconds after the production of a pulse by the first monostable circuit. This delay is sufficient to allow the pulse from the first monostable circuit to be transmitted to a target at the optimum range and to return to the range gate 1 9.

If a received signal arrives at the range gate 1 9 when it is open, i.e. conductive, the signal, shown at F in Figure 1, will pass through the range gate 1 9 to the target detection circuit 20. The transmitter break through signal will not pass the range gate 1 9 because as it reaches the range gate 19, the range gate will be closed. The target detection circuit 20 provides further discrimination against interference effects. It is possible that an interfering pulse will arrive at the range gate 1 9 when the range gate 19 is open.

However, it will be unlikely that a long succession of interference pulses will be in synchronism with the repeated opening of the range gate 1 9. Thus interference will at the output of the range gate take the form of intermittent pulses whereas a proper target return will take the form of a succession of pulses at the pulse repetition frequency.

Accordingly, the target detection circuit 21 comprises an 8 microsecond monostable circuit followed by a dual slope integrator. An output from the range gate 1 9 above a predetermined threshold level will trigger the monostable circuit to produce an 8 microsecond pulse shown at G in Figure 1 which is fed to the integrator. The monostable circuit output switches the integrator slope from "fast negative" in the "off" condition to "slow positive". Thus with normal received signals from the target of 10 nanoseconds at 100 KHz pulse repetition frequency the integrator runs "slow positive" for 8 microseconds and "fast negative" for 2 microseconds as shown at H in Figure 1.If approximately 100 mostly consecutive pulses at the pulse repetition frequency, as shown at G in Figure 1, are received by the target detection circuit the integrator output will rise as shown at H in Figure 1 to a sufficient level to cross a trigger threshold of the proximity delay circuit 20. If there is a prolonged gap in the pulses the integrator output will decay rapidly.

An output produced by the target detection circuit 20 is fed to the proximity delay 21. The purpose of this circuit is to delay detonation of the warhead of the missile until the missile has had an opportunity to either optimise the position of the warhead for maximum damage or to allow the missile to impact with the target. This will involve delaying detonation for a time which depends on the closing rate of the missile and the target. The two extreme rates are the closing rate against a stationary target of less than 30 centimetre per millisecond and the closing rate against a fast diving aircraft of about 76 centimetres per millisecond. Accordingly, the proximity delay 21 provides two selectable delays one of which is chosen before the missile is fired by operation of the preselect switch 22.For this purpose the proximity delay 20 may include two capacitors connected in series and returning to ground. One capacitor is shorted by a fusible link with the link with intact, the maximum delay would be 10 milliseconds for use against stationary targets.

With the link blown by operation of the preselect switch 22, the maximum delay would be 5 milliseconds for use against moving targets.

The output to the proximity delay 21 is fed to the output circuit 23, described in more detail hereinafter.

The impact switch 24 comprises a high output ceramic piezoelectric material connected to a seismic mass 34 (Figure 2). When subjected to the fast high level deceleration shocks likely to be experienced in an impact between the missile and a target, whether a direct impact or a significant graze between the missile and the target, the material will generate a sufficient voltage to actuate the output circuit in a manner described below.

The touch sensor 25 comprises a piece of a conductive elastomer on the nose of the missile.

This material changes from a high resistance state (greater than 5 Megohm) to low resistance state (fractions of an ohm) when subjected to a pressure greater than 1 or 2 pounds. The elastomer employs the aerial plating as a ground connection with an inner face of the elastomer being connected to a bias network so as to operate the output circuit when crushed, in a manner described below. The touch sensor 25 may be provided in addition to the impact switch 24 or as an alternative to the impact switch 24.

The output circuit 23 comprises a silicon controlled rectifier in a circuit configuration which accepts inputs from the proximity delay 21 or the impact switch 24 or the touch sensor 25. When such a signal is received, the output circuit produces an output signal which detonates the warhead.

Claims (27)

Claims

1. A method of warhead detonation in a flying body by use of a fuze and comprising transmitting a train of time spaced pulses of radio frequency energy from the fuze of the flying body, receiving a reflected signals from a target, passing the received signals to a range gate of the fuze, opening the range gate between the transmission of successive pulses of radio frequency energy for a time interval which corresponds to the time interval during which a received signal generated by reflection the target of the last transmitted pulse would be received were the target at a range at which detonation of the warhead is required, a signal being emitted from the range gate when the received signal for a transmitted pulse is received while the range gate is open for that signal, and then passing an emitted signal from the fuze to detonate the warhead.

2. A method according to Claim 1 and comprising feeding received signals to a detector of the fuze having a threshold level which rejects received signals which are below the level of a received signal from a transmitted pulse at detonation range, rectifying and filtering with the detector a received signal above the detector threshold level before passing the signal to the range gate.

3. A method according to Claim 2 and comprising amplifying the signal between the detector and the range gate.

4. A method according to any one of Claims 1 to 3 and comprising feeding the signal emitted from the range gate to a target detection circuit of the fuze, counting in the target detection circuit the receipt of emitted signals and producing an output signal for detonation of the warhead only when a predetermined number of emitted signals have been counted in a predetermined time interval.

5. A method according to any one of Claims 1 to 4 and comprising feeding the output signal or the emitted signal to a proximity delay circuit of the fuze, delaying the signal for a time which allows the flying body to optimise its position relative to the target to maximise the damage caused by detonation of the warhead and then, after said delay, passing the signal to cause detonation of the warhead.

6. A method according to any one of Claims 1 to 5 and comprising producing the train of time spaced pulses of radio frequency energy by producing a train of trigger pulses which are spaced by a time which is the required time spacing of the radio frequency pulses and which are of a duration which is the required duration of the radio frequency pulses and then triggering a radio frequency oscillator of the fuze with the trigger pulses to produce said train of time spaced pulses of radio frequency energy.

7. A method according to Claim 6 and further comprising feeding the train of trigger pulses to a delay circuit of the fuze, delaying each trigger pulse for a time which is the time taken for a received signal generated by reflection from the target of the radio frequency pulse generated by that trigger pulse to reach the range gate were the target at a range at which detonation of the warhead is required and then, after said time interval, applying the trigger pulse to the range gate to open the range gate for the duration of the trigger pulse and thus for the duration of a received signal.

8. A method of warhead detonation in a flying body by use of a fuze and substantially as hereinbefore described with reference to the accompanying drawings.

9. A fuze for detonating a warhead of a flying body and comprising a transmitter for transmitting from the flying body a train of time spaced pulses of radio frequency energy, a receiver for receiving reflected signals from a target, a range gate for receiving reflected signals and a range gate opening circuit for opening the range gate between the transmission of successive pulses of radio frequency energy for a time interval which corresponds to the time interval during which a received signal generated by reflection from the target of the last transmitted pulse would be received were the target at a range at which detonation of the warhead is required, a signal being emitted from the range gate, in use, when a received signal is received at the range gate while the range gate is open for that signal, the 'emitted signal, in use, being for detonation of the warhead.

10. A fuze according to Claim 9 and further comprising a detector connected between the output of the receiver and the input of the range gate and having a threshold level which rejects received signals which are below the level of a received signal from a transmitted pulse at detonation range, the detector, in use, also rectifying and filtering a received signal above the threshold level before passing the received signal to the range gate.

11. A fuze according to Claim 10 wherein the detector comprises a hot carrier diode.

12. A fuze according to Claim 10 or Claim 11 and further comprising an amplifier between the detector and the range gate for amplifying the received signal output of the detector before passing the received signal to the range gate.

13. A fuze according to any one of Claims 9 to 12 wherein a target detection circuit is provided for receiving signals emitted from the range gate and-then counting the receipt of emitted signals before producing an output signal for detonation of the warhead only when a predetermined number of emitted signals have been counted in a predetermined time interval.

14. A fuze according to Claim 1 3 wherein the target detection circuit comprises a monostable circuit which in use is triggered by each emitted signal received thereby to produce a pulse and an integrator circuit which in use receives and integrates the pulses, the integrator circuit having a rapid decay time so that unless the predetermined number of emitted signals are received in a predetermined time, the integrator circuit output will not exceed a predetermined level to produce the output signal.

1 5. A fuze according to any one of Claims 9 to 14 and further including a proximity delay circuit for receiving the emitted signal from the range, or the output signal from the target detection circuit where such is provided, and for delaying the signal for a time which allows the flying body to optimise its position relatively to the target to maximize the damage caused by detonation of the warhead and for passing the signal to cause detonation of the warhead after said delay.

1 6. A fuze according to Claim 1 5 wherein the proximity delay circuit is operable to effect a chooseable one of two different delays on the signal, one delay being suitable for use when the flying body is aimed at a stationary target or a substantially stationary target and the other delay being suitable for use when the flying body is aimed at a moving target.

17. A fuze according to any one of Claims 9 to 16 and further comprising an output circuit for receiving an emitted signal from the range gate or, where provided an output signal from the proximity circuit or the target detection circuit and, on receipt of such a signal, for producing a detonation signal for detonation of the warhead.

18. A fuze according to Claim 1 7 and further comprising an impact sensor connected to the output circuit and for producing a signal which operates the output circuit when subjected to the shock of high level deceleration caused by impact of the flying body with the target.

19. A fuze according to Claim 18 wherein the impact sensor is a high output piezoelectric device.

20. A fuze according to any one of Claims 1 7 to 19 and further comprising a nose tip touch sensor connected to the output circuit and for producing a signal which operates the output circuit when the sensor contacts a target.

21. A fuze according to Claim 20 wherein the nose tip sensor comprises a generally nonconductive elastomer which becomes electrically conductive on the application thereto of pressure.

22. A fuze according to any one of Claims 9 to 21 wherein the transmitter comprises a trigger circuit for producing a train of trigger pulses which are spaced by a time which is the required time spacing of the transmitted radio frequency pulses and which are of a duration which is the required duration of the radio frequency pulses, the trigger circuit triggering a radio frequency oscillator of the transmitter to produce said train of time spaced pulses of radio frequency energy.

23. A fuze according to Claim 22 wherein the range gate opening circuit receives the output of the trigger circuit and, in use, delays each trigger pulse for a time which is the time taken for a received signal generated by reflection from the target of the radio frequency pulse generated by that trigger pulse to reach the range gate were the target at a range at which detonation of the warhead is required, the range gate opening circuit then, in use, after said time interval, applying the trigger pulse to the range gate to open the range gate for the duration of the trigger pulse and thus, for the duration of a received signal.

24. A fuze according to any one of Claims 9 to 23 wherein the transmitter and the receiver are connected to a single aerial for transmitting the train of pulses of radio frequency energy and for receiving the reflected signals.

25. A fuze according to Claim 24 wherein the fuze is mounted in a nose cone for a flying body and wherein the aerial is arranged on an outer surface of the nose cone.

26. A fuze for detonating a warhead of a flying body substantially as hereinbefore described with reference to the accompanying drawings.

27. A guided missile having a warhead and a fuze for detonating the warhead according to any one of Claims 9 to 25.