GB2121639A - Improvements in or relating to homing systems - Google Patents

Abstract

A homing system for a missile has a detector consisting of a linear array of infra-red sensitive detector elements, and an optical system that images a field of view on the detector and includes a scanner causing the image to sweep over the detector. The output signal of each element is fed to a respective signal channel including a filter and amplifier followed by threshold gates and a peak detector to provide signals denoting the leading and trailing edges of each element output signal wave. Logic circuitry responds to the outputs of the threshold gates to provide a sampling command signal during every wave output of the detector array as a whole consequent upon the detector seeing a target, and also to provide signals indicating which elements are seeing the target and when. The signals indicating which elements are seeing the target are used separately or in combination with the signal values held by the peak detectors to form an error waveform representing the error between the target direction and the optical axis of the homing head of the missile.

Description

SPECIFICATION Improvements in or relating to homing systems This invention relates to homing systems for missiles and more particularly to the processing of signals derived by an infra-red sensing detector array and its associated optics and electronics.

In our co-pending patent application No.

11 649/78 there is described a system for distinguishing between the infra-red emission of a target and that of the background, and in our co-pending patent application No.

11 650/78 there is described an optical system which is particularly, though not exclusively, applicable for use in such a system.

An object of the present invention is to achieve means of processing the signals generated from such a multi-element detector system so that it may be determined whether or not a target is present in the field of view, and to provide error signals in the pitch and yaw axes indicating the error between the target bore-sight and the optical axis of the homing head of the missile.

According to the invention, there is provided a homing system for a missile, comprising a detector consisting of a multi-element array of radiation sensitive elements, an optical system for imaging a field of view including a target upon said detector and including means for repeatedly scanning the image across the detector, a respective threshold gate and peak detector circuit for each element of the detector to detect the peak of each signal wave output of the respective element due to target signal emission falling thereon and to determine the leading and trailing edges of said wave, processing logic to deliver target seen signals indicating which detector elements are seeing the target and when, and also to derive a sampling command signal representing the computed centre of the whole output waves of the detector due to target emission, and an error-signal-generating circuit responsive to the target seen signals, separately or in combination with the outputs of the peak detectors and the sampling signal to generate error signals for application to the missile guidance system.

The means for deriving the sampling command signal preferably comprises a counter which has a normal counting rate and a half normal counting rate, the count commencing at half rate when the leading edge of a detector output wave appears and stepping up to full rate when the trailing edge appears.

The signal processing requirements can be separated into two areas. These are (a) acquisition and tracking of long range low intensity targets and (b) tracking of short range nonuniform targets subtending large angles. In the case of a single element detector these differing requirements can be met by a single signal processing system, but for a multielement detector different logic is required to cater for the differing requirements.

The proposed processing system is based on the use of electronic filters optimised such that (a) small target sources will give rise to similar amplitude positive and negative pulses with a time separation approximately equal to the time taken for a point source to cross each element and (b) large area background sources will give rise to a reduced positive pulse and a very much reduced negative pulse with an increased time separation between positive and negative peaks. Thus by using positive and negative threshold gates and time gating of the resulting signals, the system will reject background signals whose amplitudes are up to forty times the minimum detectable irradiance (MDI) of the system.

The pitch and yaw error signals are generated, for low signal levels, by taking the target position along the strip array of detector elements and combining this with reference signal amplitudes at the time the target image crosses the strip of elements. These error signals give rise to a staircase error curve where the number of steps is equal to the number of elements in the array.

As the target range reduces, the signal level increases and the target image area increases, these changes result in higher detector signals and outputs from more than one detector element. Use is made of this extra information by taking an average position of all elements producing outputs, and as the signal outputs from the detector elements increase further the element position signals are weighted with their relative signal amplitudes. The quality of the error curve is thus progressively improved to produce an almost continuous error curve with a final aiming point biased towards the area of the target generating the highest intensity signal.

Signal processing arrangements according to the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a block diagram showing initial signal conditioning to obtain stored peak values and determine the leading and trailing edges of target signals, Figure 2 is a block diagram of processing logic to determine which detector elements have seen the target and calculate the centre of the target, Figure 3 is a block diagram of an error signal generator for deriving yaw and pitch axis error signals, and Figure 4 is a circuit diagram of an element position coder for use in the apparatus of Fig.

3.

For the purpose of description, the proposed electronics is split into three sections each described separately. The three sections are (a) signal conditioning (b) processing logic (c) error signal generation.

Referring to Fig. 1, the output signal from each detector element is applied to an individual respective signal channel including a filter 11 followed by an amplifier 12, there being a different filter for each channel. The output of the amplifier 1 2 is applied via an amplifier 1 3 to threshold gates 14 and 15, the signal to threshold gate 1 5 being inverted by an inverter 1 6. The output of amplifier 12 is also applied to threshold gates 1 7 and 18, the signal to gate 1 8 being inverted by an inverter 1 9. The output of amplifier 12 is further applied to a peak detector 20 via a gate 21 that is enabled by a signal on line 22 and reset by a signal on line 23.The threshold gates 14, 15, 17, 18 provide signal outputs 1, 2, 3 and 4, respectively.

For higher intensity targets, outputs 1 and 2 are still produced but not used. Outputs 1 and 3 are produced by the leading edge, and 2 and 4 by the trailing edge of the target signal. The time difference between leading and trailing edges is proportional to target width. Output 3 is used in the processing logic to generate a demand to open the gate 21 in the peak detector channel to allow the peak signal level to be stored.

Referring to Fig. 2, the processing logic is an all digital block. This block serves to determine which element or elements have seen a target, to define when the target is seen and to calculate the centre of the target for large targets in order to provide the error signal generator with a stepped input in polar coordinates when combined with the reference signals.

Four OR gates 24, 25, 26, 27 in Fig. 2 receive signals from the filter and threshold gate channels of all the detector elements.

The input to OR gate 24 from each channel is provided by a bistable 28 that is triggered by the output of an OR gate 29. The OR gate 29 receives as inputs the output of an AND gate 30 and output 3 of the circuitry of Fig. 1. The inputs to the AND gate 30 consist of output 2 of the circuitry of Fig. 1 applied directly and output 1 applied via a delay 31. The OR gates 25, 26 and 27 receive, respectively, as inputs the output of the AND gate 30 and the outputs 3 and 4 of the circuitry of Fig. 1. The reset signal on line 23 is provided by a timer 32 the output of which is also applied to an AND gate 33. A second input to the AND gate 33 is provided by a bistable 34 which is triggered by output 3 of the circuitry of Fig.

1, and the output of the AND gate 33 provides the gate enable signal on line 22.

The output of the OR gate 26 is applied to a bistable 35 which provides the input to the timer 32 and also provides via an inverter 36 one input of an AND gate 37 the other input to which is the output of the OR gate 25. An OR gate 38 receives inputs from bistables 39 and 40 which are triggered, respectively, by the outputs of the AND gate 37 and the OR gate 27. An oscillator 41 provides count pulses to an AND gate 42 which also receives the output of the OR gate 38 and delivers its own output to an OR gate 43. A second input to the OR gate 43 is provided by an AND gate 44 which receives both the output of the bistable 35 and also the count pulses from the oscillator 41 after division by two in a divider 45. A counter 46 receives count pulses from the OR gate 43 and a start signal from the OR gate 24.

The different modes of operation exist depending on whether outputs 3 and 4 are active or not. Some functions are common to both modes, in particular the counter 46 is used to delay the output sample command until after the trailing edge of the signal from a large target; the reference signals (Fig. 3) are phase-shifted to compensate for this delay.

For long range acquisition and tracking, inputs will only occur on 1 and 2. If these signals occur within a time t of each other a target detector input signal is produced at AND gate 30, the bistable 28 is set indicating on line 47 which element has seen the target, and the counter 46 will start counting pulses at full rate from the oscillator 41 via AND gate 42. When the counter stops, after a preset count to give the correct delay, a sampling command to the error signal generator (Fig. 3) is produced on line 48. The time t is approximately equal to the time taken for a point source target to cross one detector element, actual times being obtained from the results of filter studies.

When the input signal level is large enough to provide inputs on 3 and 4 then input 3 takes over from the combination of 1 and 2 to start the counter 46 via OR gate 29. The counter now counts pulses at half rate via AND gate 44 until input 4 occurs when the pulse rate reverts to full rate via AND gate 42 resulting in a delay to the sampling command 48 of half the interval between the leading and trailing edges on inputs 3 and 4.

Each input 3 is also used to set the bistable 34 and any input 3 starts a timer 32. The timer is used to reset the peak detectors 20 in all channels. The bistable 34 output and the timer 32 output are gated together to provide the gate enable signal to gate 21 of the peak detector in each channel where the signal level exceeds the higher threshold level. Thus on each active channel the peak signal level is stored until a further target signal is seen on any channel.

Referring to Fig. 3, this block takes information from the processing logic of Fig. 2 defining which element or elements have seen the target and at what position in the scan orbit the target is seen, and combines this with the relative signal amplitudes from the signal con ditioner and with the reference signals to provide guidance error signals in the appropriate gimbal axes.

While this Figure is appropriate to an analogue system, it will be readily appreciated that digital, analogue/digital or digital/analogue techniques could equally be applied.

An element position coder 49 receives one series of inputs 47 from the circuitry of Fig. 2 representing the elements which have seen the target, and another series of inputs 50 representing detector element signal levels from the peak detectors 20 of Figure 1, and delivers on line 51 a more or less continuous wave as the target is seen by the detector during each scan. Below a certain signal level all elements indicating a signal are assumed to produce equal signals and the output of the position coder 49 is at a computed level but when a certain threshold is exceeded gates in the position coder are switched and the output of the position coder is modified by the actual element signal levels. The signal levels 50 can also be averaged in unit 52 for application to a sample and hold unit 53 to derive a signal for automatic gain control purposes.

To derive error signals in the pitch and yaw axes, the signal output of the position coder on line 51 is separately multiplied in multipliers 54, 55 by first and second reference signals that are phase-shifted by respective phase-shifters 56, 57. The outputs of the multipliers 54, 55 are applied to respective sample and hold units 58, 59. All three sample and hold units 53, 58, 59 receive the sampling command signal 48 from the counter 46 of Fig. 2 that represents the calculated centre of the target. The sampled signal values are stored until updated by the next sampling command.

Fig. 4 shows the element position coder 49 in more detail. For this circuit use is made of the fact that if several input voltages are connected via equal resistors to a common point, and the common point is not loaded, the voltage at that common point is the average of the input voltages. Two averaging networks 60, 61 with equal resistors 63, 64 are included in the element position coder circuit shown, the first 60 being used to obtain the average of all the peak detector outputs required as a drive signal for the A.G.C. and therefore performing the function of the summing unit 52 of Fig. 3. For the second averaging network 61, potential dividers 62 with output levels proportional to the detector element number provide the input voltages.The inner detector element is defined as one and the outer element twenty; thus the average obtained is proportional to the position from the centre of the field of view of the centre of the group of elements receiving the target signal. Switches 65 and 66 are incorporated to connect to the averaging networks 60, 61 only those channels with signal levels exceeding the predetermined threshold level. Switches 67 and 68 are provided for switching between a fixed reference voltage on line 69 and the relative signal levels when the average signal level exceeds the A.G.C. threshold. This latter facility is used to weight the error signal towards the point of highest signal intensity within the target.

Whereas in the foregoing signal processing for a rotational scan and a radial linear detector element array is considered, it will be understood that a linear scan could be employed at right angles to the linear detector array in which case rectilinear co-ordinate yaw and pitch error signals can be obtained directly without the need to process an error signal in polar co-ordinates.

Claims (11)

1. A homing system for a missile, comprising a detector consisting of a multi-element array of radiation sensitive elements, an optical system for imaging a field of view including a target upon said detector and including means for repeatedly scanning the image across the detector, a respective threshold gate and peak detector circuit for each element of the detector to detect the peak of each signal wave output of the respective element due to target signal emission falling thereon and to determine the leading and trailing edges of said wave, processing logic to deliver target seen signals indicating which detector elements are seeing the target and when, and also to derive a sampling command signal representing the computed centre of the whole output waves of the detector due to target emission, and an error-signal-generating circuit responsive to the target seen signals, separately or in combination with the ouputs of the peak detectors and the sampling signal to generate error signals for application to the missile guidance system.

2. A system according to claim 1, wherein the means for deriving the sampling command signal comprises a counter which has a normal counting rate and a half normal counting rate, the count commencing at half rate when the leading edge of a detector output wave appears and stepping up to full rate when the trailing edge appears.

3. A system according to claim 2, wherein the counter has long range and short range modes of operation for deriving sampling command signals, such that in the long range mode the counter counts at full rate only for a predetermined time after both leading and trailing edges of a detector output wave are detected, and in the short range mode the count is at half rate when the leading edge appears and switches to full rate when the trailing edge appears as aforesaid to compute the centre of the output wave.

4. A system according to claim 3, wherein four threshold gates are provided in the threshold gate and peak detector circuit of each detector element, one pair detecting the leading and trailing edges of low intensity target signal waves and the other pair detecting the leading and trailing edges of high intensity target signal waves.

5. A system according to any one of the preceding claims, wherein the error-signalgenerating circuitry comprises an element position coder that uses the target seen signals, separately or in combination with the outputs of the peak detectors to generate an error waveform representing the error between the target bore sight and the optical axis of the homing system, and the error waveform is sampled under the control of the sampling signal to derive said error signals for application to the guidance system.

6. A system according to claim 5, comprising means for combining each error waveform signal sample and two phase-shifted reference signals to derive two error co-ordinate signals in the pitch and yaw axes.

7. A system according to claim 5 or claim 6, including means for combining all the peak detector outputs to derive an automatic gain control (A.G.C.) signal.

8. A system according to claim 5 or claim 6 or claim 7, wherein, for generating the error waveform, the element position coder comprises an averaging network receiving as inputs all the peak detector outputs, said peak detector outputs being individually scaled or weighted in accordance with the number or position of the respective detector element in the detector array.

9. A system according to claims 7 and 8, wherein the element position coder comprises a second averaging network averaging the peak detector outputs to derive the A.G.C.

signal.

10. A system according to claim 8 or claim 9, wherein switches or gates are provided in the element position coder to connect to the averaging network or networks only those peak detector outputs with signal levels exceeding a predetermined threshold level.

11. A system according to claims 7 and 8, or claim 9, or claims 7 and 10, wherein the element position coder includes switches on the inputs of the averaging network providing the error waveform, for replacing a reference input with the peak detector outputs when the average signal level of the peak detector outputs exceeds the A.G.C. threshold level.

1 2. A homing system for a missile, substantially as described with reference to the accompanying drawings.