GB2102967A - Imaging systems - Google Patents

Abstract

An imaging system of a radiometer includes a reflector (1) mounted into the wing of an airborne vehicle. The reflector focuses radiation, received from the field of view below the vehicle onto an image plane (4). A detector (5) in the form of an organ pipe scanner scans a linear region in the image plane corresponding to a linear path in the field of view extending transversely of the flight track of the vehicle to generate a representation thereof. The imaging system may be a radar including a transmitter for illuminating the field of view of the receiver. <IMAGE>

Description

SPECIFICATION Imaging systems This invention relates to imaging systems and it relates particularly to a radiometric system or a radar used in conjunction with an airborne carrier (e.g. a manned or unmanned (UMA) aircraft).

It is known that an l.R. sensor provides a day and night time imaging capability having the desirable features of covertness, but such sensors suffer from the disadvantage that they are sensitive to cloud or fog, for example, which tend to inhibit their performance.

It is an object of this invention to provide an improved imaging system.

According to the invention there is provided an imaging system for use in association with an airborne vehicle comprising a receiver responsive to radiation derived from objects in the field of view to generate electrical signals indicative thereof, and circuit means for utilising said electrical signals to form an image representative of the field of view, the receiver including a reflector fixedly mounted to the vehicle and means for scanning the image plane of the reflector to detect radiation reflected thereby and to generate said electrical signals.

In order that the invention may be more readily understood and carried into effect specific embodiments thereof are now described by way of example only by reference to the accompanying drawings of which, Figure 1 illustrates in schematic form the relative positions of the reflector and corresponding scanning device, Figures 2 and 3 show respective side and plan views of a UMA incorporating a radiometric system of the kind illustrated in Figure 1.

Figure 4 shows a perspective view of an organ pipe scanner, Figure 5 shows an alternative detection device having a pyroelectric imaging face plate, Figure 6 shows a circuit for processing signals generated by the radiometer, Figure 7 shows a block schematic circuit diagram of a mode matching system for a radiometer employing a "sliding window", Figure 8 shows a block schematic circuit diagram useful for determining the absolute velocity of a target, Figure 9 shows a block schematic circuit diagram used in a "phase contrast" arrangement.

As will be described below, the present invention encompasses a radiometer or alternatively a radar system used in association with an airborne vehicle.

In a number of the embodiments described below, a radiometer, mounted to an airborne vehicle manned or unmanned (UMA) aircraft, for example-is used to detect natural millimetric radiation (typically of 94 GHz frqeuency i.e. 3.2 mm wavelength) reflected or emitted by objects, referred to hereinafter as targets, in the field of view, and electrical signals generated by the radiometer, indicative of the detected scene are utilised to provide a visual representation thereof.

A system of this kind is especially useful for surveying the terrain, for example and for determining the deployment of vehicles or other objects of interest.

The radiometer comprises a reflector which receives radiation from the field of view and focuses it onto an image plane. In accordance with this example of the present invention the reflector is fixedly mounted to the vehicle and the image plane is scanned repetitively using a suitable detector. As will be explained in greater detail below, an arrangement of this kind, constructed in accordance with the invention permits the use of relatively high scanning rates (typically 100 cycles/sec.) commensurate with a linear resolution on the ground desirable for airborne surveillance.

Figure 1 iliustrates, in highly schematic form, the relative dispositions of the reflector and the scanning detector. The reflector 1, typically a parabolic dish having a circular aperture is fixedly mounted to the underside of the aircraft, represented at 2 and focusses millimetric radiation, reflected by objects on the ground 3 onto an image plane 4 which in turn is scanned by the detector 5. In a preferred arrangement, the detector is arranged to scan the image plane along a line extending normally to the direction of flight of the aircraft (i.e. the track of the aircraft).

Since it is preferred that the aircraft, particularly a UMA, should remain within the lineof-sight of a controlling ground station it is likely that measurements would take place at a height H of at least 500 metres and so, as the aircraft progresses along the track, this permits surveillance of a relatively large swathe on the ground, typically 1 50 metres wide (i.e. in the direction normal to the track). The linear resolution AL on the ground should preferably be about 0.3 metres and, in consequence the angular resolution, AS of the system should be about AL/H, i.e. about 0.6 mulliradians in this example.

The minimum attainable linear resolution Al in the image plane, on the other hand, is typically of the order of 2--the wavelength of the radiation being sensed i.e. 3 mm-and so the image plane must be spaced from the reflector by a distance

in this example. Furthermore, since the information derived by scanning the image plane is best resolved using a display at least 500 pixels wide, the image plane should be scanned along a line about 500;1 (i.e. 1.5 metres) long, and this corresponds to a track on the ground about 500xåL metres long (i.e. N1 50 metres).It will also be appreciated that the desired angular resolution åS of the system can only be achieved provided D, the diameter of the receiving aperture is of the order A/åO which in this example is about 5 metres, so that the f-number of the system is around unity.

The dimensions set out above, which have been derived in accordance with criteria dictated by a desired operational performance of the system are compatible with a small UMA, and side and plan elevation views of such a craft, showing the relative positions of the reflector and the image plane are illustrated in Figures 2 and 3 respectively. The 5 metre diameter reflector, shown at 10 in Figures 2 and 3, is advantageously mounted into the wing area 11 of the craft, the axis of the reflector being substantially vertical, thereby minimising the frontal area presented by the wing-the total depth required to accommodate a reflector in this manner being only about 0.3 metres.As is known in the art the reflector may have a reflecting surface of a solid sheet material or a metal mesh and since the reflector should preferably be held in a fixed position to within an accuracy of A/16 i.e. 200 y metres this suggests that the structure should be relatively rigid and isolated from the airframe structure. In an attempt to reduce the depth of the reflector a Fresnel zone mirror could alternatively be used, but to achieve a performance equivalent to that of a dished reflector, of the abovedescribed kind the stepping of the reflector would tend to limit the bandwidth B to

As will be explained in greater detail below, the signal to nosie ratio, p, of the system is proportional to Xand a value of around 5 GHz is preferred.A stepped reflector, therefore, would lead to a relative signal loss of

and therefore a dished reflector is generally to be preferred.

In order to avoid blockage of the receiving aperture and a consequent reduction in the strength of the detected signal the detector used to scan the image plane is set back, away from the vertical axis so that the scanned swathe on the ground is tilted forwards in relation to that axis. The detector is conveniently contained within a streamlined aerofoil, shown at 12 which can be swung downwards and forwards on a boom 1 3 to assure an operational position, relative to the reflector, appropriate for scanning the image plane thereof.During take off, landing and transit, however, the boom is raised into the position indicated in Figure 2 by the chain dotted outline, so as to lie against the fixed body of the craft, whereas the detector can lie held steady in the operational position by means of stringers attached to the wing tips and tailpiece, for example, of the craft.

The aircraft outlined above would typically have a length of around 1 3 metres, a wing span of around 7 metres and a height, measured to the top of the tailpiece of about 5 metres and in order to avoid the possibility of enhanced "flashes" of radar cross-section the tail piece can also be provided with a dihedral angle (not shown in Figures 2 and 3).

Since, in operation, the aircraft will have a velocity V-typically about 30 metres/sec-the minimum scanning rate of the image plane, necessary to achieve the desired linear resolution, AL(=0.3 metres) on the ground will be V/AL 100 revs/sec, each such scan being achieved by successively detecting radiation received at each of a plurality (e.g. 500) of pixel regions 3 mm wide, disposed in sequence along a line in the image plane normal to the track of the aircraft.

Scanning could be performed simply by rotating a receiving horn in the image plane at a radius of 0.75 metres but an arrangement of this kind proves to be rather unsatisfactory since it yields an inefficient circular scan of the terrain and is difficult to implement in practice, the scanning rate of 100 revs/sec being rather high for such a large mechanical structure. Some amelioration of this latter problem can be achieved, however, using a switched pair of back-to-back horns or even a switched set of four quadrature horns, the rotation rates being reduced to 50 revs/sec and 25 revs/sec respectively.

A preferred scanning technique, however, is to use an organ pipe scanner of the kind illustrated in Figure 4. This device comprises a linear array of inlet apertures I arranged in side-by-side fashion which are mounted to lie in the image plane (when the aerofoil assumes the operational position) so as to receive radiation focussed by the reflector. The apertures I communicate via transmission lines T (waveguides or strip lines, for example) with corresponding outlet apertures 0, arranged to lie around the circumference of a circular track CT which is scanned using a rotating horn H, mounted at its centre. The circumference of the track CT is equal to the length of the linear track in the image plane (N1.5 metres long) and so the radius of the arm which mounts the horn

a value commensurate with the high scanning rates required.

In order to remove external gyroscopic effects, it is preferred to drive a flywheel in the opposite sense to the scanner and this could expeditiously be achieved by separating the apertures I into two groups which are scanned by two organ pipe scanners rotating in opposite senses at the same rate, a further advantage of this approach being that each scanner arm need only be 0.125 metres long. The outputs of the two scanners could either be diode switched alternately to a common receiver or alternatively a pair of receiving channels could be used.

In an alternative embodiment of the invention waveguides extending from the 500 apertures arranged along a line in the image plane normal to the track of the aircraft are grouped into a circular (or alternately square) bundle typically about 66 mm in diameter which is butted onto the face plate of a pyroelectric imaging tube, made of pvdf for example and this is illustrated in Figure 5.

It will be appreciated that the system described hitherto scans along a line in the image plane normal to the track by using a detector array.

Scanning in the direction along the track is by virtue of vehicle motion. By rotating the aerofoil and so the detector about the vertical axis, when the boom assumes the operational position, it is possible to provide an area imaging capability independent of vehicle motion, the linear array of input apertures, 500 in number, being swept over a circular region in the image plane. Alternatively the boom supporting the aerofoil could be rocked to and fro so as to generate a substantially square image having a side 500 pixels long.

It is conceivable that drag induced by the boom and/or the aerofoil could upset the aerodynamic stability of the aircraft and so a servo controlled propeller, mounted to the aerofoil could be provided so as to counteract the effect of such drag.

As is known in the art, the signal to noise ratio in the output of a radiometer at a height H, having an aperture of diameter D, scanning over an angular swathe and receiving a bandwidth B is given by the expression

where T is the difference in temperature between the target and the detector. Signals generated at the output of the detector rotating horn, for example-may be processed by means of a circuit, of known kind, illustrated in Figure 6.

Millimetric radiation sensed by the horn 20 is initially mixed at 21 with relatively low frequency signals (typically of N88 GHz) generated by a Gunn oscillator 22. The IF frequency signals generated at the output of the mixer 21 are then applied to a bandpass filter 23 prior to demodulation at 24 to thereby generate a d.c.

output signal indicative of the scanned scene.

Since the linear resolution AL on the ground =;1HID and the linear swathe has a width of NOH it follows that

and so for an optimally configured radiometer the bandwidth should be maximised and the velocity minimised.

In order to "detect" a target a relatively high thermal resolution AT is required, whereas "classification" of the target by determination of its shape, requires a relatively high linear resolution AL, typically of the order d/1 0 when d is the expected linear dimension of the target.

Inspection of Equation 1, however, shows that the linear resolution AL is proportional to T1/2 (the square root of the scanner dwell time), whereas the thermal resolution is proportional to T-1/2. It would appear therefore, that ALAT=const so that a radiometer having high spatial resolution required for target classification must of necessity have a correspondingly poor thermal resolution.

Since the energy contributions from the various parts of the target integrate noncoherently in the detection radiometer it has been found that it is possible to substantially overcome the above-mentioned problem by integrating signals derived by the detector from a "sliding window" spanning a preset area of the image plane having the spatial coverage of the classification radiometer. Integration could, for example, be effected over a substantially circular region of the detector10 pixels in diameter say, and whenever the integrated signal exceeds a preset threshold level, indicating that a detection has been made the data prevailing in the "sliding window" at that time can be utilised to classify the target.

A system using a circular sliding window (typically 10 pixel regions in diameter) is illustrated in block schematic form in Figure 7a.

Two positions of the window are representated at A and B in Figure 7b and in this example each window encompasses pixel regions derived from successive line scans (n . . . n+p) of the organ pipe scanner. In practice, the window traverses a scanned swathe on the ground, in a direction S normal to the track of the aircraft before advancing along the track to execute another traversal in the across track direction.

Data signals generated by the detector 30 (e.g.

signals, corresponding to distinct pixels sensed at the output location of an organ pipe scanner) are digitised by an A/D converter 31 and then fed to a store 32. Signals corresponding to each pixel region encompassed by a sliding window are accessed from the store to a shift register 33, integrated by cycling through an integrating circuit 34 and then returned to the store 32. Data signals corresponding to the pixel regions encompassed by the next position of the sliding window are then accessed to the shift register 33 for integration. If data signals are no longer required to construct a sliding window they may be ejected from the store, or alternatively retained therein for future use. Output signals generated by the integrator 34, and indicative of the temperature of the scanned scene are compared with a preset reference threshold signal REF using a comparator 35.When the received signal exceeds the threshold level, thereby indicating that a target has been detected, a trigger signal DET is generated by the comparator which actuates a latch 36 which permits data prevailing within the shift register to be non destructively sensed and utilised by a processing circuit 37, to effect classification of the target. By using this mode-matching principle therefore, a radiometer having a high spatial resolution, optimised for classification can be endowed with the same high thermal resolution for detection that would otherwise be associated with a radiometer having poor spatial resolution.

The thermal resolution of the mode-matched radiometer could be further improved by using a "sliding window" having a multiplicity of different shapes and thus, for example, if it were required to detect targets having rectangular and cruciform "plane" shapes (as seen radiometrically) the signals of high spatial resolution would be integrated over each of these shapes in each of a sufficient number of orientations. As before, each shape may have a linear dimension of 10 pixels, the rectangular shape being orientated into about 1 5 equiangular positions and the cruciform shape being orientated into even more orientations depending upon its precise form. It may, therefore, be necessary to compute 30 or more area integrals, each corresponding to a different orientation of the window.Each integrator would have an associated threshold level determined by the number of pixels embraced and wherever one or more integrations exceeds the corresponding threshold level a detection would be declared. In some circumstances it is possible that although a detection has been made, the thermal resolution in the classification mode may be inadequate, and in that case, in accordance with a variation of the above-described mode-matching technique, a better thermal resolution may be achieved by integrating the pixel contents over small clusters of appropriate size (4, 9, 1 6 etc. in number i.e.

square regions of side 2, 3, 4 etc. pixels). The classification system would then interact with the integrated signals to achieve the optimum degree of discrimination. It is possible that the same mix of spatial and thermal resolutions may not be appropriate for all parts of the target and so sub optimisation of each part of the target may then be necessary.

The radiometer system described hitherto is capable of generating an image of a patch of ground 1 50 metres in diameter, and when an organ pipe array is employed to scan the image plane a line scan, extending across the track of the aircraft, is produced. The time interval necessary for the aircraft to transit 1 50 metres, assuming a velocity of 30 metres/sec. is 5 secs, whereas the time interval needed to pass over a target having a linear dimension (in the direction of flight) of 3 metres is only 0.1 secs.

In accordance with a yet further embodiment of the invention, therefore, a sequential arrangement of radiometers is provided, comprising a "leading" radiometer having a relatively low spatial resolution and being used solely for detecting a target deplqyed in the 1 50 metre diameter field of view, and a "following" radiometer being arranged, in accordance with the above-described mode-matching system, to provide high spatial and thermal resolution required for classification over a relatively narrow swathe width (in the across track direction).The "leading" radiometer, therefore, provides a relatively coarse indication of the positions of targets contained within the field of view and signals generated thereby, indicative of a target, are utilised to steer the "following" radiometer, having a relatively high linear resolution into an offset angle across the track appropriate for homing in on the previously detected target.The "leading" radiometer, therefore, need have only a relatively low spatial resolution on the ground of about the same size as the target of interest i.e. 3 metres and so in contrast to the example described earlier (which required a resolution on the ground of 0.3 metres) 50 pixels (rather than 500) would be sufficient to adequately scan the image plane and this could be achieved using an array of horns, mounted in side-by-side relationship in the image plane and each having a width of 30 mm (rather than 3 mm in the earlier example). As before, the horns would feed respective waveguides which are arranged to form an organ pipe scanner, the rotation rate of the scanning horn at the centre of the arrangement being a mere 10 revs/sec in this example.In contrast, since the "following" radiometer, which is used for classification, is steered into an offset angle across the track onto a previously detected target to within an accuracy of 3 metres, and since it should have a linear resolution on the ground of about 0.3 metres, a linear arrangement of about 1 6 detecting horns, mounted to lie in the image plane and each 3 mm wide, should suffice to derive an acceptable image. Again an organ pipe scanner could be used but since the resolution on the ground is about 10 times greater than that required of the "leading" radiometer, the scanning rate would be about 100 revs/sec. Furthermore, since the number of waveguides needed is relatively low, then, in an alternative embodiment of the present invention, scanning can be achieved using a series of diode switches or other electronic switching means.

As explained above, the "following" radiometer is steered, in response to signals generated by the "leading" radiometer, and since such steering is conveniently achieved by mechanical means, the "trailing" distance of the "following" radiometer should allow a sufficient time interval for transient vibrations and movement to die away-and a period of 1 sec, corresponding to a distance on the ground of 30 metres (assuming an aircraft velocity of 30 metres/sec) should be sufficient for this purpose. This can be achieved by inclining the leading and following radiometers at an angle of 3.5 to one another in the along track direction assuming an aircraft height of 500 metres.The "following" radiometer may be mounted to a boom which is capable of being swung into an appropriate position in response to controlling signals generated by the "leading" radiometer, but such an arrangement tends to be inconvenient since the boom partially blocks the aperture of the reflector. A more convenient arrangement is to provide a carriage mounted within an aerodynamically shaped unit containing both radiometers along which the "following" radiometer may be translated.It is possible that in a practical situation the target could have a finite velocity of up to 1 5 metres/sec, say (in the case of a vehicle travelling along a road) and if the aircraft is flown along a path substantially parallel to the road the relative velocity of the aircraft (assumed to be travelling at a velocity of 30 metres/sec) and the target could range from 1 5 metres/sec to 45 metres/sec, resulting in a 3:1 variation in the measured scale of the target in the along track direction.

In accordance with another embodiment of the invention, however, it is possible, using a sequential arrangement of radiometers, similar to that described above, to determine the absolute velocity of the aircraft and target and to then correct the scale accordingly. In this embodiment two radiometers are provided, namely a "leading" radiometer used for detection and having a relatively low spatial resolution (3 metres) and a "following" radiometer having a higher spatial resolution than the "leading" radiometer (in 0.3 metres) and being spaced therefrom by a distance p along the body of the aircraft (typically of the order of 30 metres).The radiometers are mounted so as to lie along the line of flight of the aircraft, and in contrast to the previously described sequential arrangement the "following" radiometer is unsteered but scans a selected target, previously detected by the "leading" radiometer, after an elapsed time t whose magnitude depends on the relative velocity of the target and aircraft. Figure 8 shows a block schematic diagram for establishing the absolute velocity AV(=p/t-V) of the target, the "leading" radiometer (DET) being illustrated at 40 and the "following" radiometer (CLASS) being illustrated at 41-the spacing between the two radiometers being p.Since the radiometer 41 has a higher spatial resolution than radiometer 40 signals successively generated thereby, indicative of scanned locations on the ground (typically 0.3 metres long) are stored and integrated using a processor 42 so as to synthesise a new signal corresponding to the lower resolution signals derived at 40. When a selected target is detected by radiometer 40, a timer 43 is started and the signals generated by this radiometer are compared using a comparator 44 with the synthesised signals generated at 42. Eventually, after the elapsed time t the "following" radiometer will detect the selected target (sensed earlier by the "leading" radiometer) and so the input signals to the comparator 44 will then be substantially identical and an output signal O/P will be generated which is then used to stop the timer 43.A signal representing the elapsed time is then passed to a processor 45 which computes the value of the absolute velocity AV(=p/t-V) of the target and this value is used to suitably scale the image derived at 46 using the high spatial resolution signals detected at 41. If desired, the scaled image may then be displayed at 47. If the target has a significant velocity across the track of the aircraft the classification radiometer could be translated across the line of flight to compute the transverse velocity of the target in the abovedescribed manner. To accomplish this, however, the swathe width of the classification radiometer must be relatively wide to ensure that the target is sensed.

Having established the along and/or across track velocities of the target it is thus possible to utilise this information to rescan the target more slowly, thereby giving an enhanced thermal resolution (i.e. AT, the thermal resolution n'1/r1I2) although in view of the increased scanning time needed, the expected target density should preferably be rather low. The relatively slow scan can be achieved by translation of the classification radiometer (i.e. the "following" radiometer) in a sense appropriate to cancel the relative velocity of the target and aircraft (i.e.

along or across the line of flight of the aircraft) and the rate of translation, t would be given by the relative velocity of the aircraft and the target p/t less the required slow scan rate S, where S=p/t-T. If scanning of a target (typically 3 metres across) is to be of 1 second duration, the slow scan rate S would be 3 metres/sec. For a stationary target, therefore, the effective scan rate S of the classification radiometer is reduced from 30 metres/sec (the aircraft velocity) to 3 metres/sec, the translation rate T of the classification radiometer in this case being 27 metres/sec, towards the rear of the aircraft.The thermal resolution of a dynamic system of this kind can, therefore be improved by a factor

In order to achieve two-dimensional tracking motions, the "following" radiometer is preferably mounted to a two dimensional traverse assembly.

Alternatively, instead of using a line scanning arrangement (provided by an organ-pipe scanner, for instance) a two dimensional pyroelectric array could be used and the entire array would then be translated to achieve the relatively low effective scan rate commensurate with high thermal resolution.

The systems described hitherto all employ an antenna which is used passively to perform radiometric measurements. The antenna could also be utilised, however, in a millimetric radar and if the transmitted power is W then, as is known in the art, the single pulse signal-to-noise ratio Pr of the receiver is

for the terrain below the aircraft, where a0 is the radar back-scatter coefficient, F is the noise figure of the radar system, L is the system loss, D is the diameter of the aperture, H is the height, ss is the bandwidth, k is Boltzman's constant, To is 3000k, w is peak power.

If the aircraft is flying at an altitude of 500 metres then a suitable pulse repetition interval PR is about 5 ,us i.e. a 2 ,us pulse length followed by a 3 ps receiving period. As explained above, the time interval necessary for the aircraft to pass over a patch on the ground 150 metres in diameter is about 5 secs (assuming a velocity of about 30 metres/sec) and if the corresponding image is scanned using an organ pipe scanner, of the kind described earlier, having 500 feed horns (each 3 mm across) at a rate 1 00 revs/sec then the pixel dwell time will be 20 sss and so four 5 s radar pulses could be received by each horn giving a post detection integration improvement of about 5 dB. If therefore, B=0.5 MHz W=1 mW,L=10 dB, F=8 dB and assuming, as before, an altitude H and a reflector diameter D of 500 metres and 5 metres respectively, then Pr =51 dB.

Oro If the minimum usable S/N ratio Pr is 15 dB then the system would be capable of measuring radar returns from the ground or extended targets with back-scatter coefficients down to -36 dB m2/m2 and so a radar of this kind, used in conjunction with a radiometer of the kind described earlier, constructed in accordance with the invention, would be both practicable and sensitive.

It would be possible to use separate organ pipe scanners for the radar and radiometric systems, but alternatively the two systems could be time multiplexed into a common scanner. If the two systems share the 20 ,us dwell time equally the sensitivity of the radiometer and radar is reduced by about 1.5 dB and 3 dB respectively, and such redudtion can be tolerated. With a compound arrangement of the above-described kind radiometric temperature and radar cross-sectional measurements can be derived, together with radiometric spatial measurements which serve to classify the target.

The use of a radar does of course render the system vulnerable to interception, but this can be reduced by actuating the radar only when the radiometer has detected a potential target which requires further classification. The boom, supporting the scanner, could then be repositioned to rescan the selected area on the ground using the radar which is turned on momentarily. The boom can then be returned to its original position appropriate for use in conjunction with the radiometer.

The use of a radar also provides the possibility of extracting information relating the relative range (from the aircraft) of different portions of a detected target (i.e. its variation in depth) using a "phase contrast" technique of the kind disclosed in our copending British Patent Application No.

43783/78, and such information provides an additional classification facility. As detailed in the above-mentioned patent application, the phase contrast technique may be applied in a variety of ways and in one example a carrier signal of millimetric wavelength (typically 94 GHz) is mixed with a phase coherent reference signal having a sufficiently long wavelength to enable the required range discrimination (typically 20 metres, in this example, corresponding to a frequency of 1 5 MHz i.e.

C metres/sec 2x10 metres and the modulated signals are then transmitted.

The received signals, reflected by a target of interest on the ground are then demodulated and the phase of the low frequency component compared with that of the reference signal, a variation in phase difference derived from different parts of the target providing a measure of the variation of the depth of that target. Clearly it is only possible to discriminate between depths of up to half the wave length of the reference signal, i.e. 10 metres.

In an alternative approach phase contrast information may be derived by interrogation of the beat frequency between two RF signals having frequencies separated by 1 5 MHz, and an example of a system to achieve such interrogation is illustrated in block schematic form in Figure 9.

In this arrangement, a pair of stable Gunn oscillators 50, 51 is used to respectively provide the transmitted power at a frequency of 94 GHz, and an LO signal at 84 GHz, the difference frequency corresponding to a 10 GHz IF signal.

The output of the 94 GHz oscillator is passed through a SSB digitally-controlled modulator 52 driven at 53 to add and subtract 7.5 MHz on alternate pulses. After gating at 54, the resultant signal is transmitted via the organ-pipe scanner, OS.

On receipt, the echoes are heterodyned against the L.O. as is the transmitter oscillator, the two resultant signals being filtered and amplified before the received signals are phase-detected with reference to the transmitter oscillator. The resultant analogue components will comprise 2 ps pulses which are digitised at 55, 56 and passed to a computer 57 for temporary storage.

The first function of the computer is to extract the amplitude by deresolving the two components, the resulting output being the oro feature. The normalised components (sin 8,, cos 0,) from the first pulse will then be stored until the corresponding components (sin 02, cos 02) have been derived from the second pulse.The "phase range" of the content of the pixel, sX, can then be computed from Sin 0=sin 0, cos 02-cos O sin 62 Cos =cos O cos 02+sin O sin 62 and if the pixel dwell corresponds to multiples of two pulses then clearly the observations should be averaged at some stage.

It will be appreciated that phase contrast measurements of this kind, which provide an indication of the variation in depth of a detected target can be used in conjunction with the radiometric and radar facilities described earlier to further improve the classification of the target.

Moreover, in addition to a "phase contrast" facility the use of a fully coherent radar also provides the possibility of deriving relative velocities associated with the target using Doppler frequency detection, and such techniques are well known (See for example, Chapter 3 p.

72-86 "Introduction to Radar Systems" Skolnik, McGraw Hill). If, for example, the slow scan sequential radiometer arrangement described earlier, is used having a pixel dwell time of 1/1 62=4 ms and the PRI is again of the order of 5 ys, then the maximum unambiguous Doppler frequency is about 100 KHz, and the frequency resolution is of the order of 250 Hz.When associated with a 3 mm radar (94 GHz, say) these frequencies correspond to relative velocities of 540 kph (150 m/s) and 0.2 kph (0.375 m/s), and since a target vehicle may have a distinctive Doppler signature due to wheel rotation, cooling fan velocity etc. falling within this range, detection of these velocities further improves classification of the target If the maximum velocity of the target vehicle is 60 kph, therefore, the maximum value of the corresponding Doppler velocity will be 120 kph which can, therefore, be resolved into 600 channels-although a lower resolution than this may suffice. If desired, the Doppler analysis may be time multiplexed with the phase contrast analysis described earlier, and the Doppler velocity resolution would then be 0.4 kph.

The above-described invention shows how a radiometer used in conjunction with an airborne carrier-an aircraft such as a UMA, for example, can provide high resolution thermal and/or spatial images suitable for detecting and classifying objects in a field of view. Such a system may, if desired, be used in association with a radar which provides additional information useful for classification, namely the radar cross-section, the height profile of the objects and the relative velocity of the target or parts attached thereto. It is apparent, therefore, that it is possible to provide a comprehensive range of classification facilities within the same system.

Claims (Filed 13 July 1982) 1. An imaging system for use in association with an airborne vehicle comprising a receiver responsive to electromagnetic radiation reflected at, or emanating from a field of view to generate an electrical signal, means for utilising said electrical signal to form an image representative of the field of view, such receiver including a reflector fixedly mounted on the vehicle, means for scanning a detector relative to the image plane of the reflector, the detector being responsive to radiation reflected into the image plane to generate said electrical signal.

2. An imaging system according to Claim 1 wherein said scanning means is arranged to scan the detector repetitively relative to a linear region in the image plane, the scanned region corresponding to a linear path in the field of view, extending transversely of the vehicle track and the rate of scanning being selectable so that successively scanned paths are contiguous.

3. An imaging system according to Claim 1 or Claim 2 wherein the detector is mounted on a support member capable of deploying the detector, for operation, at a position appropriate for scanning the image plane.

4. An imaging system according to Claim 2 or Claim 3 wherein the scanning means comprise organ pipe scanner including a linear array of receiving elements located, in operation, along said linear region.

5. An imaging system according to Claim 2 or Claim 3 wherein the scanning means comprises a plurality of waveguides arranged to communicate radiation, incident at the image plane, to a pyroelectric detection device, the respective input ends of the waveguide being arranged in side-byside fashion to extend along said linear region in the image plane.

6. An imaging system according to any one of Claims 1 to 5 wherein the receiver is mounted into the wing of the airborne vehicle.

7. An imaging system according to any one of Claims 2 to 6 comprising means for strong electrical signals resulting from a succession of scans in the image plane, and corresponding to a swathe in the field of view extending transversely of the vehicle track, means for processing selected ones of the stored signals, corresponding to a region in said swathe of predetermined shape and position to generate an output signal representing the temperature in that region of the field of view, and comparison means for indicating whenever the output signal represents a temperature exceeding a reference temperature, thereby to indicate the presence of a target of interest in said region of the field of view.

8. An imaging system according to Claim 7 wherein the processing means comprises a selection circuit for selecting said signals corresponding to said region in the swathe, and means for integrating the signals, so selected, to generate said output signal representing the temperature.

9. An imaging apparatus according to Claim 8

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. feature. The normalised components (sin 8,, cos 0,) from the first pulse will then be stored until the corresponding components (sin 02, cos 02) have been derived from the second pulse. The "phase range" of the content of the pixel, sX, can then be computed from Sin 0=sin 0, cos 02-cos O sin 62 Cos =cos O cos 02+sin O sin 62 and if the pixel dwell corresponds to multiples of two pulses then clearly the observations should be averaged at some stage. It will be appreciated that phase contrast measurements of this kind, which provide an indication of the variation in depth of a detected target can be used in conjunction with the radiometric and radar facilities described earlier to further improve the classification of the target. Moreover, in addition to a "phase contrast" facility the use of a fully coherent radar also provides the possibility of deriving relative velocities associated with the target using Doppler frequency detection, and such techniques are well known (See for example, Chapter 3 p. 72-86 "Introduction to Radar Systems" Skolnik, McGraw Hill). If, for example, the slow scan sequential radiometer arrangement described earlier, is used having a pixel dwell time of 1/1 62=4 ms and the PRI is again of the order of 5 ys, then the maximum unambiguous Doppler frequency is about 100 KHz, and the frequency resolution is of the order of 250 Hz.When associated with a 3 mm radar (94 GHz, say) these frequencies correspond to relative velocities of 540 kph (150 m/s) and 0.2 kph (0.375 m/s), and since a target vehicle may have a distinctive Doppler signature due to wheel rotation, cooling fan velocity etc. falling within this range, detection of these velocities further improves classification of the target If the maximum velocity of the target vehicle is 60 kph, therefore, the maximum value of the corresponding Doppler velocity will be 120 kph which can, therefore, be resolved into 600 channels-although a lower resolution than this may suffice. If desired, the Doppler analysis may be time multiplexed with the phase contrast analysis described earlier, and the Doppler velocity resolution would then be 0.4 kph. The above-described invention shows how a radiometer used in conjunction with an airborne carrier-an aircraft such as a UMA, for example, can provide high resolution thermal and/or spatial images suitable for detecting and classifying objects in a field of view. Such a system may, if desired, be used in association with a radar which provides additional information useful for classification, namely the radar cross-section, the height profile of the objects and the relative velocity of the target or parts attached thereto. It is apparent, therefore, that it is possible to provide a comprehensive range of classification facilities within the same system. Claims (Filed 13 July 1982)

1. An imaging system for use in association with an airborne vehicle comprising a receiver responsive to electromagnetic radiation reflected at, or emanating from a field of view to generate an electrical signal, means for utilising said electrical signal to form an image representative of the field of view, such receiver including a reflector fixedly mounted on the vehicle, means for scanning a detector relative to the image plane of the reflector, the detector being responsive to radiation reflected into the image plane to generate said electrical signal.

2. An imaging system according to Claim 1 wherein said scanning means is arranged to scan the detector repetitively relative to a linear region in the image plane, the scanned region corresponding to a linear path in the field of view, extending transversely of the vehicle track and the rate of scanning being selectable so that successively scanned paths are contiguous.

3. An imaging system according to Claim 1 or Claim 2 wherein the detector is mounted on a support member capable of deploying the detector, for operation, at a position appropriate for scanning the image plane.

4. An imaging system according to Claim 2 or Claim 3 wherein the scanning means comprise organ pipe scanner including a linear array of receiving elements located, in operation, along said linear region.

5. An imaging system according to Claim 2 or Claim 3 wherein the scanning means comprises a plurality of waveguides arranged to communicate radiation, incident at the image plane, to a pyroelectric detection device, the respective input ends of the waveguide being arranged in side-byside fashion to extend along said linear region in the image plane.

6. An imaging system according to any one of Claims 1 to 5 wherein the receiver is mounted into the wing of the airborne vehicle.

7. An imaging system according to any one of Claims 2 to 6 comprising means for strong electrical signals resulting from a succession of scans in the image plane, and corresponding to a swathe in the field of view extending transversely of the vehicle track, means for processing selected ones of the stored signals, corresponding to a region in said swathe of predetermined shape and position to generate an output signal representing the temperature in that region of the field of view, and comparison means for indicating whenever the output signal represents a temperature exceeding a reference temperature, thereby to indicate the presence of a target of interest in said region of the field of view.

8. An imaging system according to Claim 7 wherein the processing means comprises a selection circuit for selecting said signals corresponding to said region in the swathe, and means for integrating the signals, so selected, to generate said output signal representing the temperature.

9. An imaging apparatus according to Claim 8

wherein the selection circuit is arranged to select electrical signals corresponding to a plurality of different regions, which have the same shape but different respective orientations relative to a reference axis in the swathe, means for integrating the selected signals to generate a plurality of output signals representing the temperatures of the respective regions and the comparison means generates an indication whenever any one of said output signals represents a temperature exceeding a reference temperature indicative of a target of interest.

10. An imaging system according to Claims 7 to 9 comprising a second receiver, responsive to electromagnetic radiation reflected at, or emanating from the field of view to generate electrical signals indicative thereof and having a spatial resolution less than the spatial resolution of the first receiver, electrical signals generated by the second receiver being applied to guidance means arranged to respond to said signals to steer the first receiver in a direction appropriate to scan a region in the field of view containing a target detected by the second receiver.

11. An imaging system according to Claim 10 wherein the reflector of the second receiver is mounted on the body of the vehicle in a forward position relative to the reflector of the first receiver.

12. An imaging system according to Claims 7 to 9 comprising a second receiver including a reflector spaced, along the body of the vehicle, from the reflector of the first receiver, the second receiver being responsive to electromagnetic radiation reflected at, or emanating from, the field of view to generate electrical signals indicative thereof and having a spatial resolution less than the spatial resolution of the first receiver, the system also including means for integrating signals, generated by the first receiver to form an integration signal indicative of the temperature in the field of view of the first receiver, means for indicating the elapsed time between generation by said second receiver of a signal indicative of a target and generation by said first receiver of an integration signal indicative of the same target and means for varying, in dependence on said elapsed time, a representation of the target, of relatively high spatial resolution, derived from electrical signals generated by the first receiver.

13. An imaging system according to Claim 12 including means, responsive to said indication of elapsed time, for translating the reflector of the first receiver in a sense, and at a speed, appropriate to offset a relative velocity of said reflector of the first receiver and a target detected by the first receiver.

14. An imaging system according to any one of Claims 1 to 6 in the form of a radar system including transmission means for illuminating with r.f. radiation the field of view of the receiver, the receiver being capable of receiving a corresponding return to generate an electrical signal indicative of a target.

1 5. An imaging system according to Claim 14 including means for inhibiting transmission of r.f.

radiation until the said receiver has generated an electrical signal indicative of a target in the field of view.

1 6. An imaging system according to Claim 14 including means for modulating the transmitted r.f. signal, means for comparing the phase of the modulation of the transmitted signal with the phase of the modulation of the corresponding return to generate a comparison signal indicative of the range of a target.

17. An imaging system according to Claim 14 comprising means for generating electromagnetic radiation at a first radio frequency, means for transmitting pulses of said radiation modulated alternately at different respective modulation frequencies, means for generating electromagnetic radiation at a second radio frequency, means for heterodyning radiation at said second radio frequency with radiation at said first radio frequency and with returns corresponding to the transmitted pulses to generate respective difference signals, and means for comparing the phases of the difference signals to generate a comparison signal indicative of the range of a target.

1 8. An imaging system substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.