GB2252207A - Integrated antenna/mixer devices and weapon guidance systems - Google Patents

Abstract

A radar is fixed on board a guided projectile so that it has a fixed field-of-view relative to the projectile, the field-of-view being moved or scanned by moving the projectile as a whole, e.g. rolling the projectile. The radar system comprises a dielectric RF receiving lens 1 arranged to focus received radiation through an insulating substrate 2 onto an array of integrated antenna/mixer circuits supported on the back face of the substrate 2 and an RF generator which applies radiation via a dielectric matching device "taper" or "matching section" to the back face of the substrate so that some of the radiation is coupled into the antenna/mixer circuits to form the local oscillator for the circuits while the remainder proceeds through the lens to form the radar outgoing signal. <IMAGE>

Description

INTEGRATED ANTENNA/MIXER DEVICES AND WEAPON GUIDANCE SYSTEMS This invention reLates to sensing systems using integrated antenna/mixer circuits. It also relates to the guidance of weapons.

According to one aspect of the invention, there is provided a sensing gem comprising an integrated antenna/mixer device in which an array of antenna/mixer circuits on an insulating substrate is positioned adjacent dielectric lens means for receiving RF radiation via the lens means and said substrate, each antenna/mixer circuit comprising a received radiation responsive element, a local oscniator signal responsive element, and diode means connected between the elements for mixing the received radiation and local oscillator signal to form an IF signal, and RF radiation applying means for applying radiation to the device via dielectric matching means at the antenna/mixer side of said substrate, ie the side opposite said lens, such that some of said applied radiation becomes coupled into said antenna/mixer circuits to form the local oscillator signal therefor and some of said radiation is emitted via said lens means to be subsequently reflected from an item to be sensed by the sensing system and returned to the system as said received radiation.

AdvantageouLy, radiation applying means comprises a waveguide horn with a waveguide to dielectric matching member at its outlet adjacent the aray and said array comprises an n x 1 or n x 2 array of crossed dipole pairs in which the local oscillator reeve dimples are extended to form shorted posts extending across the waveguide horn aperture.

According to a second aspect of the invention there is provided a guidance system for a weapon comprising, on board the weapon, transmission means for illuminating a field-of-view view forward of the weapon with RF radiation and an integrated antenna/mixer device which includes a dielectric lens and an array of antenna/mixer circuits on a dielectric substrate adjacent the lens and which is arranged such that the antenna/mixer circuits are responsive to said radiation returned from within respective ones of a linearly extending pattern of portions of said field-of-view, said pattern being fixed with respect to the weapon and the guidance system including control means for scanning said field-of field-ofuiew by controlling movement of the weapon as a whole.

For a better understanding of the above-mentioned aspects of the invention, and to illustrate respective further aspects of the invention, reference will be made by way of example to the accompanying drawings, in which:- figure 1 is a diagram il lstrating the construction of one form of integrated antenna/mixer device, figure 2 is a diagram illustrating the construction of another integrated antenna/mixer device, figure 3 shows two integrated antenna/mixer devices each with means for applying RF radiation to the rear of the device, figure 4 shows an example in more detail of part of one of the devices of figure 3, figure 5 shows two views of an integrated antenna/mixer device with a polarising aperture stop, and figure 6 is a diagram for illustrating the operation of a missile guidance system.

By way of example, the invention relates to active millimetre wave homing systems for low cost antitank terminally guided submunitions (TGSM) with small body diameters (typically 100mm). These weapons usually employ steep descent angles in order to ensure penetration of target hulls. A target is acquired after a search or acition phase in which an area of ground or footprint is scanned. The antenna is normally scanned through a fixed solid angle and thus the footprint area will depend on the target detection range. In order to provide a cost effective footprint size it is desirable to be able to detect targets at ranges of the order of lkm at which the homing head receiver will be clutter limited.

This can be achieved using clutter referenced MTI provided the targets are not stationary. In many situations however, the targets will be stationary or moving in such a manner as to yield too low a radial velocity component for MTI to be effective. Thus some proposed TGSM homing heads do not include MTI and this invention concentrates on the problem of detecting stationary targets.

Conventional T GSM mm-wave active homing heads employ mechanically scanned single beam dish antennas. The beam is usually raster scanned during the target search phase and conically scanned for homing. Monopulse is avoided because of the high cost of the comparator and phase balanced receivers. Raster scanning is achieved by movement of either a dish itslf or of a subreflector (e.g. Twist Cassegrain Antenna).

In some cases the conical scanning is achieved by perturbing the dish feed whereas in others both raster and conical scanning are obtained by the same mechanism.

There are certain advantages in employing microwave radar technology to provide a staring-eye fixed head seeker, viz: Superior ground mapping coupled with monopulse techniques could give improved target recognition in cluttered environments.

Increased integration time during the target acquisition phase.

No moving parts.

Potentially lower cost, volume and masses The use of a large n x n array of microwave radar receivers to simultaneously cover the entire footprint is precluded at present due to the following difficlllei > Large n x n arrays are difficult to manufacture.

It is not practical to parallel process all the receiver outputs derived from a large n x n array.

A compatible mulbbeam transmitter is not available at present. A floodligt transmitter of sufficient power is not obtainable using current solid state devices.

In view of the above difilculees it is proposed to employ a linear array and to achieve ground search by body rotation. This method of scanning is suited to a steeply descending TGSM whereas a pusibroom scan would be used for shallow trajectories. An n x 2 rather than an n x 1 array is proposed in order to give good circumferential as well as radial resolution with the rotational scan. The receiving array operates in conjunction with a solid state transmitter providing a floodlight beam shaped to encompas the array footprint.

Linear arrays are much easier to construct compared with square because there is ample room along the edges to provide the interconnections. Hybrid circuiit linear arrays may be constructed for operation at frequencies up to about 40GHz and offer great flexibility for low cost development.

The following descrigion addresses the problem of target detection in clutter with respect to low cost steep lookudown seekers It also describes hybrid microwave array antenna/mixer circuits ailtable for 1 x n and 2 x n linear arrays operating at frequencies up to about 40GHz.

Methods of providing a floodlight transmitter and local oscillator injection are also described.

It is also shown, by way of an example, how the circuits described may be used in a rolling body submunition seeker against a stationary tank in heavy clutter.

It is also shown how a GaAs monolithic microwave array radar might permit operation at frequencies up to approximately 100GHZ.

To enable a 35GHz, 100mm TGSM to detect tank sized targets at clutter limited ranges of 300mm-lkm, up to 15dB of clutter rejection, over that obtainable using range gating and integration, is required. This is taking a back scatter coefflcient of clutter corresponding to a steep look down angle of around 60 . This can be achieved using clutter referenced dopey detection in the case of moving targets, however, for stationary targets some other form of processing must be applied.One approach is to use target recoghision algorithms which characterise the target returns in terms of phase and poisation, however this requires phase coherence for the seeker and a dual pcilarisation receiver which is not compatable with the low cost approach required for TGSM's. An alternative approach which could be achieved using simple amplitude comparison monopulse is to characterise the clutter more accurately in the form of clutter mapping and adaptive threshnlding, which allows a more accurate local CFAR detection threshold to be set, by effectively reducing the variance of the cutter.This can best be done using the full or partial stating array described later, since it allows a large area of clutter to be viewed at any one time giving the advantage of averaging clutter over a large effective footprint whilst maintaining a high antenna gain. A third approach that could be adopted again using amplitude comparison monopulse is to use a ailtable combination of sum and difference channels (e.g. or to provide some beam sharpening or improvement in angular resolution against point sources (targets) enabling them to be distinguished from distributed dutter-ike sources. The theoretical limit to this improvement is the ratio of backscatter coefficient for targets and clutter which is typify > 13dB at 35GHz.To increase azimuthal resolution to a point where it is equal to the range cell depth of 18 range gate at 600 depression angle, then a 5 to 1 improvement is required over that obtained for a 100 mum aperture at 35GHz, giving 7dB of clutter rejection. Improvements of 4 to 1 using this type of processing have been reported.

In conclllsion two types of low cost processing have been suggested which are particularly applicable to fun or partial staring arrays and could possibly provide methods of clutter rejection not possible using single beam con scan type seekers. The levels of clutter and thus the amount of rejection required apply only to the worst case of steep descent angle trapstories, for shallower descent angles the clutter problem is much less severe, received clutter power being roughly proportional to depression angle.Further clutter rejection can also be obtained, of course, from frequency and time decorrelation due to the natural spectral and temporal variance of the clutter as well as the spatial variance mentioned in the previously described processing. This can be achieved by frequency ramping the transmit waveform and integrating the received signaL Hybrid microwave radar mixer circuits have been investigated at X and J bands and have been shown to have reasonable noise figures of around 10dB with IF amplifiers mounted remote from the dipales. With this configuration, the upper frequency limit for the circuit is governed mainly by the size of the mixer diodes, and will be around 40GHz using 5:1 aspect ratio dipoles of about L5mm length.These dimensions are well within the scope of conventional thin film technology, and a circuit of this type is currently being designed at 35GHz which is expected to have a sis ar NF to the lower frequency circuits. The circuit in its simple form will self bias and needs only one IF connection and a ground connection as shown in Fig. 1. This connection method could be tl;ffi^ult for large n x n arrays but is ideal for 1 x n or 2 x n arrays with access to both sides of the linear array as in Fig. 2. In this arrangement the vertical dipoles are used for the LO and therefore distortion of the dipole pattern due to the connections is not criticaL The horizontal signal dipoles should be undisturbed.

As shown in figure 3, the radar system comprises a lens 1 made of dielictric material and to the rear of which is fixed a dielectric substrate 2. On the rear face of substrate 2, there is supported a 2 x n array of antenna/mixer circuits as shown in figures 1 and 2, ie. two rows each of n (where n is a selected number, say six) of antenna/mixer circuits. Each antenna/mixer circuit comprises two crossed dipole 5 and 6. The two halves 7 and 8 of the vertical dipole 5 and the two halves 9 and 10 of the horizontal dipole 6 are interconnected by a ring of four diodes 11.The lower half 8 of dipole 5 is connected by conductive track 12 to a ground track metaTh pattern 13 which is common to all the antenna/mixer circuits, The upper half 7 of dipole 5 is connected via track 14 to an intermediate frequency t) signal output connection pad 15 from which the IF signal is taken off to an IF amplifier (not shown). The local osailiator signal LO, linearly pa7arised in the appropriate direction, is picked up by the vertical dipole 5 while the orthogonally polarised radar return signal is picked up by the horizontal dipole 6.The two signals are mixed by the diodes 11 to form the IF signaL The outer ends of the two halves of the horizontal dipole 6 are interconnected by a bond wire 16 of which the function is to provide a return path for the unidirectional diode bias voltage developed by the circuit.

The antenna/mixer circuits could each be modified as shown in figure 7. Eere each half of the vertical dipole 70 is split, vertically, into two

and 71 and 72 connected via respective capacitors 73 and 74 to the IF output connection pad 75 while the two lower portions 76 and 77 are grounded via respective capacitors 78 and 79. The negative side of an external diode bias supply (not shown) is then connected via respective resistors 80 and 81 to the vertical dipcsse portions 71 and 76 which the postive side of the supply is supplied via respeve resistors 82 and 83 to the portions 72 and 77.Meanwhile, the bias return bond wire 16 of fIgure' 1 is replaced in figure 7 by a resistor 84 connected between the inner ends of the two halves of the horizontal dipole 85. The externally biased circuit of figure 7 is somewhat more complex than the figure 1 self biased circuit but it does rewire less local oscillator power to be coupled into the circuit and, with it, it is eater to precisely control the bias current passing through the diodes to give optimum operation.

The overall array has to be adapted to suit the modified circuit, for example because the capacitors 73, 74, 78 and 79 may well be too large to fit in place between the ground track pattern and the dipoles. One example of a suitable layout would be to delete the central portion of the ground track pattern 13 in figure 2 and to bring the IF connections from each circuit out sideways and to connect them to IF amplifiers mounted on the main part of the ground track pattern, the capacitors73, 74, 78 and 79 being provided as parts of the respective IF amplifiers.

Current methods of LO injection are all through the front of the lens and as such require a pdladsed reflection and an inconvenient sideways mounted LO source. A more convenient method of LO injection would be through the back of the lens ie. through the circuit. If this were possible it would offer a number of advantages, firstly it would provide a simple compact LO injection, but secondly and more important it would offer the possibility of increasing the power of the LO and using it to provide a floodlight transmitter through the lens for a homodyne or FMCW type of radar.The problem of injecting power into the back of the lens is firstly that the power coupled into the LO dis must be sufficient to bias the mixers and secondly the match into the high dielectric constant lens must be good enough to enable efficient transmission through the lens. Both of these must be achieved without a significant increase in received signal path loss.The ratio of power coupled forward through the high dielectric constant lens and backwards through air for a dipole at an airielectric interface, is approximately related to the ratio of dialectic constants as fallows PF/PB = EF 3/2/E approx. w Therefore for a lens of dielec'tic constant EF = 10 and air behind EB = 1 the front to back power ratio is approximately 32:1 Le. 97% of power coupled into the dipole is through the lens.This ratio and in fact the total (3600) polar pattem of the dipe assumes infinite dielectric media either side of the djpce, in practice, however, it is controlled by the nearest 'few' wavelengths of dielectric. The effective wavelength #d and hence the dipole length at the dielectric interface is a function of the mean dielectric constant and is approximately related to the wavelength in air # by: #d = '-0.5 11 (EF + EB)1/2 approx. (2) This effect, however, is dominated by the dielectic constant from 0-1/4 wavelengths array from the dipole.

Two methods of injecting power through the back of the lens using a waveguide horn have been considered and are shown in Fig. 3. First (a) is to use a tapered transition from air to dielectric filed guide of the same diectic constant as the lens, in contact with the back of the circuit.

This will give a good broadband match into the back of the lens for the transmitter and good coupling into the LO dipoles but will also reduce the coupling into the signal diodes to 50% and distort the polar pattern resulting in more than 3dB signal path loss. The dielectdc could be cut away behind the horizontal signal dipoles but this is unlikely to influence the polar pattern significantly although it should restore the effective wavelength. Further disadvantages are signiScant less in the taper and ifF'rii1ty of manufacture. The second approach (b) is to use a 1/4 wavelength matching layer of dielectric constant approximately (EF on the back of the lens similar to that required on the front refracting surface.Measurements have shown that a low less polymer such as Rexolite of the correct thickness, although it has a slightly low dielectric constant of 2.55, will give a very good narrow band match into the back of a lens from waveguide or a horn. Again the Rexotite can be cut away as a Slot behind the Signal dipole to minimise the effect on the dipole length, and since the increase in mean dielectric constant over several wavelengths behind the dipole is minimal, the effect on the polar pattern and hence front to back power ratio, should be small.However, even assuming there was a large thickness of Rexdlite behind the dipole, from equation 1 the power ratio would be 8:1 implying only signal path less of less than 0.5dB but an increase in LO gain of 6dB. Similady the maximum effect on the dipole length from equation 2 would be to reduce the centre frequency of a 35GHz dipole to 33GHz.

Fig. 4 shows how a 2 x 2 array might be realised in practice using this method of TX/LO injecdon. In this case the LO dipoles have been lengthened and become shorted posts across the waveguide enabling the LO coupling to be controlled by the width of the posts. The signal dipoles being horizontal will not couple to the LO E-field which should provide 20-30dB of LO Noise rejection. The horn must have an aperture which gives a beam width ailtable for feeding the lens and at 35GHz will be about 6mm x 8mm. The array can be lengthened in the horizontal plane for 2 x n arrays provided the feed horn is simiBy widened.This will have the effect of increasing the feed horn gain in the horizontal plane which when considering the transmitted signal will under illuminate the lens aperture and thus widen the horizontal beam width in the far field. This, however, is the assymetric beam shape required to provide a floodight transmitter for a linear array. To provide further control of the transmitter beam shape a polarised aperture stop can be used. Fig. 5 shows an example of how a 2 x 6 array might be configured with a widened feed horn and a vertically polarized aperture stop to give a 5 to 1 aspect ratio transmitter beam shape. The aperture stop is shown incorporated into a circular polariser in front of the lens aperture.

Circular polarisation is likely to be preferred for two reasons.

Firstly due to rotation of the array required for acquisition and tracking and secondly to enhance the target return, since only even numbered reflections will be rejected (e.g. dihedrals) rather than all copar reflections.

The proposed system would use a 2 x 6 array of receiver elements mounted across the diameter of the lens Fig. 6, giving an array of beams in the far field covering the field of view in one plane. Sightline information would be provided by the seeker in the roll and pitch planes relative to the body aixis, with body mounted gyro's providing the body reference. Roll information would be provided by using the six pairs of beams to generate six difference patterns which summed together would provide one difference pattern across the length of the array, with a sign change at the axis. Using this information the body can be rolled rapidly using fins in the plane of the array with the aerodynamic control surfaces driven differentially.Since rapid roil control can be achieved, the seeker can be used in the conventional null seeking mode in the roil plane. Pitch information along the line of the array is provided by first generating six sum patterns and then taking pairs of these to generate five difference patterns along the length of the array. Tracking algorithms must then be used to predict a target position along the array and hence an angle off boresight. The guidance computer must then use this information to provide pitch control using fins mounted orthogonally to the plane of the array, with aerodynamic control surfaces driven together. This control will be slow and unable to take out the body motion thus requinng a tracking law which is linear over the full field of view.

Altemative wbroom' types of scan could be adopted when a shallower depression angle is used, with the array set horiyontaZly.

Additional offset pairs of elements would probably then be used for radimetric horizon sensing to keep the array horizontaL The transmitter for the system could comprise a Gunn oscillator for example.

The feasibility of using a 35 GHz hybrid microwave array radar in a seeker to fillfill the role outlined earlier has been demonstrated. The propped partial array provides a good compromise between a large n x n nrig array and a mechanically steered system, with the only movement of the array provided by aerodynamic body rotation.

A method of injecting the combined transmitter and local oscillator for a FMCW seeker, through the back of the microwave radar array has been suggested. This offers the possibility of a compact system, and the multiple receive beams could provide advantages in terms of dutter rejection. Range predictions for the system suggest a useful performance and the possibility of a further increase if more clutter rejection could be achieved or a shallow depression angle trajectory chosen.

Although a frequency limitation of 35 GHz has been suggested for hybrid technology, the same circuit approach could be applied to a GaAs monolh:hic circuit giving the posibility of 95 GBz operation. The advantage of this over the current Si monolithic circuits is that better performance mixer diodes can be produced at these frequencies in GaAs, and with IF amplifiers mounted externally from dipole the larger FET structures regpired in GaAs are not a limitation.

Claims (3)

1. A sensing system comprising an integrated antenna/mixer device in which an array of antenna/mixer circuits on an insulating substrate is positioned adjacent dielectric lens means for receiving RF radiation via the lens means and said substrate, each antenna/mixer circuit comprising a received radiation responsive element, a local oscillator signal responsive element, and diode means connected between the elements for mixing the received radiation and local oscillator signal to form an IF signal, and RF radiation applying means for applying radiation to the device via dielectric matching means at the antenna/mixer side of said substrate, i.e. the side opposite said lens, such that some of said applied radiation becomes coupled into said antenna/mixer circuits to form the local oscillator signal therefore and some of said radiation is emitted via said lens means to be subsequently reflected from an item to be sensed by the sensing system and returned to the system as said received radiation.

2. A system according to claim 1, wherein said radiation applying means comprises a waveguide adjacent the array and said array comprises an n x 1 or n x 2 array of crossed dipole pairs in which the local oscillator responsive dipoles are extended to form shorted posts extending across the waveguide horn aperture.

3. A guidance system for a weapon comprising, on board the weapon, transmission means for illuminating a field-of-view forward of the weapons with RF radiation and an integrated antenna/mixer device which includes a dielectric lens and an array of antenna/mixer circuits on a dielectric substrate adjacent the lens and which is arranged such that the antenna/mixer circuits are responsive too said radiation returned from within respective ones of a linearly extending pattern of portions of said field-of-view, said pattern being fixed with respect to the weapon and the guidance system including control means for scanning said field-of-view by controlling movement of the weapon as a whole.