GB2458723A

GB2458723A - Radiation field sensor

Info

Publication number: GB2458723A
Application number: GB8918885A
Authority: GB
Inventors: Huw David Rees; Geoffrey Lewis Moule
Original assignee: Qinetiq Ltd; UK Secretary of State for Defence
Current assignee: Qinetiq Ltd
Priority date: 1988-09-20
Filing date: 1989-08-18
Publication date: 2009-10-07
Anticipated expiration: 2009-08-18
Also published as: GB8918885D0; GB2458723B; IT8948363A0; DE3931400A1; GB8822011D0

Abstract

The sensor (10,30,40) incorporates a substrate-mounted dipolar antenna array 30 coupled to microwave radiation by a lens 12. Each antenna (26) has two dipole limbs (26a,26b) connected together via a respective Schottky barrier detector diode 28. Low frequency output signals are fed to respective signal channels each containing a video amplifier (42), an analogue to digital converter or ADC (44) and a multiplier (46). A read-only memory or ROM (48) stores coefficient sets obtained by calibrating the sensor 10, (30,40 figs 2,3) with radiation beams of known parameters such as direction, frequency and/or polarisation. The multipliers (46 fig 3) receive respective coefficients from successive coefficient sets. Each coefficient set provides a respective set of multiplier output products which are summed by an adder tree (56,58,60). A computer (64) establishes which coefficient set yields the maximum sum of products, ie the maximum degree of correlation with the antenna array outputs. Identification of the best correlated coefficient set indicates predetermined radiation beam parameters. The antennas (26) in the array (30) may be mutually parallel, or may be arranged in a variety of inclinations to one another to provide sensitivity to beam polarisation, (Figs 11, 16-19). Dipoles may have differing lengths (fig 12).

Description

RADIATION FIELD SENSOR

This invention relates to a radiation field sensor, and more particularly to a sensor for determining any one or more of the direction, power, frequency and polarisation parameters of an incident microwave radiation beam.

Apparatus for determining parameters such as power, direction, frequency or polarisation of a microwave signal or beam is known. One such apparatus comprises an array of microwave horn antennas each with a respective mixer.

� Mixer output signals are processed to provide one or more of the required incident beam parameters. Each horn antenna typically has a square signal receiving aperture about 5 across, where X is the operating wavelength, and tapers over lOX or more to an antenna output. The output of the antenna is normally fed to a waveguide and thence to a coaxial cable for connection to processing circuits. At 10 GHz, Ic K = 3 cm, each horn antenna in the array is at least 15 cm across, and has a taper of 30 cm long. It weighs 1-3 kg, depending on constructional metal, eg brass or aluminium. For adequate characterisation of a 10 GHz microwave beam, a square array of nine or sixteen horn antennas would be employed, together with output waveguides etc. A nine antenna array would be at least 45 cm by 45 cm in frontal area, and would extend to the rear 30 cm. The output waveguides would extend a further 3 cm.

If constructed in brass, it would weigh approximately 30 kg. Its volume would be over 6 x i� cm3, over 2 cuft, and its frontal area 2 x I0 cm2, over 2 sqft.

The prior art microwave horn array is undesirably bulky for many uses, and is unacceptably so for use in missile guidance systems for example. One possible form of missile guidance system would incorporate a radar beam sensing device to enable location of a missile in space relative to a controlling ground station.

However, missile design requirements include a very small cross-sectional area to enable the missile to reach high speeds. A horn antenna array of the kind previously indicated could not be accommodated within a typical missile because of size limitations. L -2-

It is an object of the invention to provide an alternative device for determining microwave radiation beam parameters.

The present invention provides a radiation field sensor comprising: (a) a substrate retaining an array of dipolar antennas and a respective mixing means for each antenna; (b) a dielectric lens module arranged closely adjacent to the substrate so that individual antenna centre positions in the array correspond to respective beam directions for radiation incident on the lens, and the lens and substrate dimensions and dielectric constants being in combination such as to provide for each antenna to couple predominantly to radiation passing through the lens; and (c) signal processing means responsive to degree of correlation between mixing means output signals and sets of predetermined coefficients corresponding to known incident radiation parameters.

The invention provides the advantage that a radiation beam is characterjsed by a process of coefficient matching, a straightforward signal processing procedure.

Only one receiving aperture is required, as opposed to an array of horn apertures in the prior art, the aperture being defined by the lens. The radiation pattern produced by beam transmission through the lens is received by the antenna array, and the form of the radiation beam is ascertained with the aid of predetermined coefficients arising from calibration of the sensor using known beam parameters.

There is no requirement for prior knowledge or independent measurement of one or more beam parameters such as frequency or phase in order to determine other parameters such as beam direction and/or polarisation.

The antenna array may be located within the lens depth of focus for maximum Sensitivity to beam direction. p ( -3-

The signal processing means may include a respective amplifier, analogue to digital converter (ADC) and multiplier for each array output. Each multiplier may be arranged to multiply the corresponding digitised array output by a respective coefficient in each of the series of coefficient sets to generate product values. Summing means may be provided for adding multiplier output products corresponding to individual coefficient sets. The summing means may be associated with comparing means arranged to determine the coefficient set yielding the maximum sum of multiplier products and to indicate beam parameters associated therewith. The comparing means may comprise a computing device io arranged to compare summing means outputs with one another. The signal processing means may also include a storing device arranged to store coefficient sets at respective addresses, and to furnish them to the multipliers when addressed. The storing device may be addressed by a counter activated in response to signal receipt by the antenna array.

The antenna array may incorporate dipoles arranged mutually parallel to facilitate close packing and provide for beam direction and/or frequency determination.

The array may alternatively comprise separate dipoles having a variety of inclinations to one another to provide beam polarisation sensitivity. As a further alternative, each dipole may be crossed by two additional and relatively inclined dipoles each with respective mixer diodes, each of the crossed dipoles being preferentially sensitive to radiation polarised parallel to it. For sensitivity to radiation of circular polarisation, the antenna array may instead incorporate pairs of crossed dipoles of generally S shape, one of each pair being a mirror image of and rotated relative to the other.

The lens and substrate may be of similar dielectric constants, eg an alumina lens ( = 9.8) may be employed with a silicon substrate (E = 11.7) or an alumina substrate. The antenna array dipoles may be of metal deposited on a substrate surface remote from the lens, the substrate acting as an extension of the lens through which radiation passes. Discrete detector diodes or integrated Circuits incorporating them are then bonded to respective metal antenna layers. A silicon substrate may be formed as an integrated circuit, detector diodes, low frequency amplifiers and diode bias circuitry being integrated in the substrate. The 3 substrate may alternatively retain components on both sides; ie antenna dipole layers may be deposited on one surface and detector diodes and associated circuitry arranged on the other with diode-antenna connections being made through the substrate thickness. In this embodiment the antennas are sandwiched between the lens and substrate, and the lens has higher dielectric constant than the substrate to provide for radiation coupling to the antennas to be predominantly via the lens. The lens dielectric constant is preferably at least twice that of the substrate. A.n alumina substrate may be employed with a lens of barium nonatitanate. (Ba2Ti9O20, e = 39).

The invention may form part of a guidance system for a missile. In this embodiment, the antenna array includes dipoles with a variety of orientations to provide polarisation sensitivity. The invention is located at the rear of the missile to receive radiation from a range of solid angle about the rearward direction. The array receives polarised signals from a controlling station, which transmits three overlapping beams both to detect targets and to provide array receive radiation. Each beam has a respective modulation code identifying it.

Signal processing circuitry on board the missile determines the missile range from the controlling station, and its direction and attitude relative to co-ordinates defined by the transmitted beam directions and polarisation. The beams are also coded with signals appropriate to furnish target location information to the missile; using this information, a computer in the missile controls its flight so as to intercept the target. Conventional means for achieving this incorporates a gyro, which is both more expensive and more bulky, and also it severely limits missile agility because of robustness limitations.

In order that the invention might be more fully understood, embodiments thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a microwave receiver unit for a sensor of the invention; Figure 2 illustrates antenna dispositions in the Figure 1 unit; Figure 3 is a circuit diagram of a signal processing device for the Figure 1 unit; 1' -5-.

Figures 4 to 9 illustrate radiation patterns in the focal plane of the Figure 1 unit, and indicate the effect of varying direction and frequency of an incident radiation beam; Figures 10 to 12 illustrate in more detail antenna positions and circuitry for use with the Figure 1 unit; Figures 13 to 15 are schematic sectional views of alternative microwave receiver units for a sensor of the invention; Figure 16 shows a microwave receiver unit arranged for detection of incident beam polarisation; and io Figures 17 to 19 illustrate crossed dipole antenna elements for use in a microwave receiver unit.

Figures 1 to 3 collectively provide a schematic illustration of a microwave radiation field sensor of the invention. Parts of these drawings which are common to two or more are like-referenced. Figure 1 shows a microwave receiver unit 10 in section, Figure 2 is a plan view of an antenna array 30 incorporated in the unit 10, and Figure 3 is a block diagram of a circuit 40 for processing output signals from the array 30. The receiver unit 10 is of modular construction, and incorporates an alumina lens module 12 having a spherical surface 14, a frusto-conical surface 16 and a plane surface 18. As illustrated, the frusto-conical surface 16 has a half-angle of about 30. Preferably, however, the surface 16 should have a half-angle of 50'. An alumina sheet substrate module 20 is mounted on the plane lens surface 18, the substrate 20 having a plane surface 22 contiguous with that of the lens. The substrate 20 has a reverse plane surface 24 bearing planar metal dipole antennas 26, of which two are illustrated. Each dipole antenna 26 has two limbs 26a and 26b connected together via a respective microwave detector diode 28. The detector diodes 28 are discrete Schottky barrier devices which are bonded to respective antennas 26.

The antennas 26 are located in the focal plane of the lens 12 (or at least within its depth of focus, ie where antenna coupling to incident radiation is within 3 dB of the optimum).

Parallel radiation from differing directions indicated by chain lines 32 and dotted lines 34 is incident on the lens spherical surface 14, and is focussed by the lens 12 at respective antennas 26 through the thickness of the substrate 20. Each antenna centre position in the lens focal plane therefore corresponds to a respective direction for radiation incident on the lens 12, and the substrate 14 acts as an extension of the lens.

Referring now also to Figure 2, the antenna array 30 contains fourteen dipole antennas 26 (not all referenced) aligned mutually parallel. Each antenna 26 has a respective detector diode 28 connected between its limbs as shown in Figure 1, but the diodes are not shown in Figure 2 to reduce illustrational complexity.

The antennas 26 are shown disposed over a grid 36 graduated from +40' to 40' in both elevation (vertical) and azimuth (horizontal), each square such as 38 of the grid corresponding to 10. The grid 36 represents the angular field of view of the lens 12. Antenna centre positions on the grid 36 correspond to radiation incident from directions indicated by respective grid positions. For example, the lowermost antenna 26 is centred at -24' (elevation) and O (azimuth), which indicates the direction of radiation incident on the lens 12 and subsequently refracted to the centre of that antenna. It is of course understood that in practice antennas have finite beamwidths, and each responds to radiation from a range of directions.

Low frequency output signals from the detector diodes 28 are fed to the processing circuit 40 of Figure 3. The circuit 40 incorporates a respective signal channel for each of the fourteen antennas 26, reference numerals being applied only to items in the uppermost channel to reduce complexity. Each antenna 26 is connected to a respective video amplifier 42, analogue to digital converter (ADC) 44 and multiplier 46. Each diode 28 is connected between the respective antenna output and earth. Each ADC 44 is a sample-and-hold device activated by a strobe signal and providing a 12-bit signal to the respective multiplier 46.

A read-only memory (ROM) 48 is connected to each of the multipliers 46. The ROM 48 has a respective 12-bit data output for each of the multipliers 46, the data outputs being indicated collectively at 52. The ROM 48 has a 168-bit output, of which the first multiplier 46 receives bits 0 to 11, the second 12 to 23 and so on. Each 168-bit output therefore corresponds to a respective set of fourteen 12-bit coefficients. The ROM has an 8-bit address space, which provides 256 coefficient sets. The outputs 52 are combined to form a coefficient 3 bus 54 connected to each of the multipliers 46. Multiplier output signals are fed to an adder tree containing full adders arranged in four vertical ranks. Each adjacent pair of multipliers 46 provides input signals to a respective first rank adder 56, of which there are seven. The three uppermost pairs of first rank adders 56 provide inputs to three second rank adders 58. An upper third rank adder 60 sums signals from the upper pair of second rank adders 58, and a lower third rank adder 60 sums signals from the lowermost first and second rank adders 56 and 58. A single fourth rank adder 62 sums the third rank adder signals.

io Output signals from the fourth rank adder 62 are fed to a microprocessor (zP) 64 having a strobe signal output 64a, a beam characterisation output 64b, and a start/reset output 64c. The strobe output 64a is connected to each of the ADCs 44. The start/reset output 64c is connected to the like input of a counter 66, the counter being connected via an address bus 68 to address inputs 70 and 72 of the P 64 and of the ROM 48 respectively.

The radiation field sensor illustrated in Figures 1 to 3 operates as follows. Each antenna 26 receives microwave radiation via the lens 12, and has a respective radiation beam in free space which is that of a dipole modified by the lens 12, the latter being approximately 2.5 radiation wavelengths in diameter. Radiation received by an antenna 26 is rectified by its associated detector diode 28.

Microwave radiation is generated and received in pulses, so each diode 28 produces a pulse with a low frequency envelope in response to antenna signal reception. The diode outputs are amplified by amplifiers 44 responsive to the modulation frequencies, typically in the range 0.1 to 10 MHz. These are commonly termed video amplifiers.

The tP 64 initially holds the start/reset output 64c low, which resets the counter 66 to zero. The corresponding address on the address bus 68 is 0... .0. At this address, the ROM 48 stores fourteen coefficients each of which is unity, ie 0.. .01 in digital form. (Some other set of equal, non-zero values may of course be employed.) While the output 64c is low, the P 64 supplies a succession of strobe pulses to each of the ADCs 44. The strobe pulse spacing is sufficiently large to accommodate the time required for signals to be received and analysed 3 by the pP 64. Each strobe pulse activates ADC sampling and output to respective multipliers 46 for multiplication by unity. The multiplier outputs are summed in the adder tree 56 etc. The output of the fourth rank adder 62 to the zP 64 is proportional to the radiation power level received by the antenna array 30, by virtue of multiplication of individual ADC outputs by unity. This provides a power measurement.

The P 64 supplies strobe pulses to the ADCs 44 until it receives an output from adder 62 exceeding a pre-arrangeci threshold indicating a significant level of radiation reception. The strobe pulses are then terminated, leaving each ADC 44 with its existing sample output. The 11P 64 switches the start/reset input of the counter 66 to high to begin a count. The initial count value changes from 0... .0 to 0... .01, and coefficients stored at ROM address 0... .01 are furnished to respective multipliers 44. The fourth rank adder consequently produces a weighted sum of ADC outputs, weighting being performed by the coefficient set addressed in the ROM 48. The weighted sum is received by the P 64, which also receives the corresponding ROM address via the address bus 68.

The 1tP 64 compares the weighted sum with an existing stored value (initially zero). If the weighted sum is greater than the stored value, the sum and the corresponding ROM address replace the stored value and its address in the pP internal store. The counter 66 subsequently counts in unit steps through each of the ROM addresses while the signals sampled and held by the ADCs 44 remain unchanged. When the counter 66 has counted through all ROM addresses, it stops and is reset to zero by the pP 64. When the pP 64 has received the maximum count or final ROM address (256), and has implemented its comparison/replacement operation, its internal store contains the ROM address corresponding to the coefficient set yielding the maximum sum of multiplier outputs. This coefficient set is that which is most closely correlated with the ADC outputs. The coefficient set address may be output at 64 by the pP 64.

Alternatively, the pP 64 may have an internal look-up table in which incident radiation beam parameters are stored at respective addresses, and are output at when addressed. As will be described later in more detail, the identification of the best coefficient correlation may be employed to measure any one or more 3 of the polarisation, frequency and direction of a radiation beam incident on the

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array. The coefficient sets are obtained from prior calibration of the receiver unit 10, and each set is associated with a respective radiation pattern received by the antenna array 30. The highest correlation determined by the P 64 therefore indicates a closest match to a predetermined radiation pattern. After a convenient interval, the pP 64 may re-initiate ADC strobing to characterise subsequent radiation.

Figures 4, 5 and 6 illustrate variation in radiation intensity at the focal plane of the lens 12 (the antenna array position) for differing directions of radiation io incident on the lens 12. The radiation intensity and frequency is constant in all three drawings. The beam shape is the normal so-called sync2 function, ie in section through the beam maximum in each case the profile is (sinx/x)2 where x is measured from the beam maximum.

i5 In Figure 4, a beam maximum 80 is centred at the lower left of the drawing.

Figures 5 and 6 show beam maxima 82 and 84 located centrally and centre right respectively. The positions of the maxima 80, 82 and 84 correspond to different incident radiation directions.

Figures 7, 8 and 9 illustrate the effect of variation in incident radiation frequency for radiation directed centrally (along the boresight) of the lens 12 and antenna array 30. The radiation frequency increases from Figure 7 to Figure 9, which corresponds to diminishing diffraction maximum width.

Although Figures 4 to 9 relate to radiation patterns in the lens focal plane, it is not in fact essential to locate the antenna array 30 in this plane. Sensitivity to incident beam direction is improved by array location in the focal plane, but this is not necessarily the case for other parameters such as frequency. It is required that an incident radiation beam produce a pattern at the antenna array which can be associated with a predetermined pattern for which antenna Output signals have been obtained by calibration. Departures of the antenna array 30 from the focal plane, optical imperfections in the lens 12 or its surrounding environment or other departures from the optical ideal are compensated for by calibration. In the case of a comparatively short wavelength radiation beam producing a principal diffraction maximum received by one antenna when focussed, it is advantageous -10 -

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to defocus the system so that the maximum is received by a plurality of antennas.

The antenna array 30 provides the microwave equivalent of individual pixel signals in a radiation field as illustrated in Figures 4 to 9. Each antenna 26 has a respective radiation beam centred around a respective central direction of radiation incident on the lens 12. The antenna beams necessarily overlap somewhat, but each antenna 26 receives a respective radiation intensity. The antenna array 30 therefore provides fourteen samples of the radiation field at the lens focal plane. This is sufficient to determine the radiation field pattern to high accuracy for signal processing as described with reference to Figure 3. In particular, an embodiment of the invention was constructed in accordance with Figures 1 and 2, for the frequency range 7-17 0Hz. It employed a computer to carry out the Figure 3 function. This embodiment proved capable of determining incident radiation beam direction to better than 1% of the radiation beamwidth of the lens/antenna combination. This accuracy was maintained over the field of view of the antenna array 30 through the lens 12, and is comparable with that of a high quality monopulse receiver employed as a tracking radar. The embodiment also proved capable of measuring frequency to � 0.5 0Hz.

The invention may be arranged to detect both the frequency and the direction of incident radiation simultaneously. This requires the ROM 48 to be effectively two dimensional. It would store a respective set of coefficients for each beam maximum position at each frequency. The counter 66 would once more count through or address the coefficient sets sequentially. However, where the interval containing the frequencies is sufficiently narrow, the beam direction may be determined first as previously indicated at a frequency at the centre of the interval. The frequency is subsequently measured by correlation with coefficient sets corresponding to constant incident beam direction but varying frequency.

This greatly reduces the time required for the correlation operation. Beam direction may subsequently be verified by checking degree of correlation with coefficient sets corresponding to constant frequency and incident beam direction near that originally determined. This is a successive approximations approach.

It requires the P 64 to take over control of addressing the ROM 48 after an initial beam direction estimation, so an additional address bus is required.

-10 - L -Il-Figure 10 is a reproduction of an engineering drawing of the antenna array 30 shown in plan and about three times actual size for a centre frequency of 0Hz. Parts previously referred to are like-referenced. Individual antennas 26 are 1 mm wide and 6 mm long, and are made by thick film deposition of gold on an alumina substrate 20. The antennas 26 have respective chip diodes 28 indicated by squares. Each diode 28 is mounted on and connected to one dipole limb, and is connected to the other respective limb by a lead 90. One dipole limb of each antenna 26 is connected to earth by a lead such as 92, the earth connection being made in the case of inner antennas via respective outer io antennas. The other dipole limb of each antenna is connected to a respective low frequency signal output such as 94 by a lead such as 96. Leads 96 from inner antennas pass via respective outer antenna centres under diode/antenna leads 90, from which they are insulated. A polygon 98 approximating to a circle indicates the perimeter of the region of the focal plane of the overlying lens 12 which is substantially free of optical distortion.

The antenna array 30 of Figure 10 is produced by standard thick film deposition techniques on an alumina substrate. Thick film procedures are well known and will not be described in detail.

The antennas 26 in the array 30 are aligned parallel to one another, and have maximum sensitivity to radiation polarised parallel to them. They have low (theoretically zero) sensitivity to perpendicularly polarised radiation. Figure 11 is a reproduction of an engineering drawing of an antenna array 100 equivalent to that of Figure 10, except that it includes antennas such as 102 with varying orientations. The array 100 includes twelve antennas 102 arranged in inner and Outer rings and each inclined at 60' to its neighbours. There are two antennas fewer than the equivalent for the array 30 because of the difficulty of providing space for coplanar low frequency output leads 104. In other respects, the array 100 is equivalent to that shown in Figure 10.

The antenna array 100 incorporates four mutually parallel antennas 102, and eight other antennas 102 arranged in parallel pairs each inclined to the remaining ten. Irrespective of the polarisation of incoming radiation, it will couple to a 3 minimum of eight antennas 102. This provides more than adequate numbers of -11 -independent signals for direction, polarisation and power determination as described with reference to Figure 3. The array 100 is particularly appropriate for determination of direction and linear polarisation of an incident beam.

Figure 12 shows a further antenna array 110 equivalent to the array 30 shown in Figure 10, except that the array 110 incorporates parallel antennas such as 112, 114, 116 and 118 of differing lengths. The array 110 is mounted on a substrate 120, and is shown approximately three times actual size As in the array 30, the antenna width is 1 mm, but the antenna lengths are 7.6 mm, 5.8 mm, 4.5 mm and 1.9 mm for antennas 112 to 118 respectively. As will be described later in more detail, the effective antenna length for radiation reception purposes is affected by the dielectric constants of adjacent media. In the present example, antenna positioning between air ( = 1) and an alumina substrate (E = 9.8) provides for the antennas 112 to 118 to be half-wavelength resonant at 8 GHz, 10.7 0Hz, 13.3 GHz and 16 0Hz respectively.

By virtue of the length variation of the antennas 112 to 118, power coupled to any individual antenna depends on frequency in addition to position in the array and incident beam direction. At lower frequencies below 10 0Hz, radiation coupling is largely confined to the longest antennas 112. As the frequency increases, the number of antennas coupling appreciably also increases. In effect, the packing density of coupled antennas increases with frequency.

The antenna/substrate assembly described with reference to Figures 1, 2 and 10 to 12 provided for the lens 12 and substrate 20 to be of like material (alumina) and dielectric constant. Moreover, the substrate thickness lay between the lens 12 and diode/antenna circuitry. The substrate 20 consequently acted as an extension of the lens 12, and the antennas 26 were bounded on one side by alumina (c = 9.8) and on the other side by air ( 1). It is advantageous for antennas 26 to be close-packed in the array 30, in order to couple a large fraction of the incident radiation power to them irrespective of direction of incidence. It can be shown that the centre to centre spacing of adjacent (equal length) antennas should be approximately 0.5 X to provide good coupling efficiency for antennas mounted on a substrate with a dielectric constant greater 3 than 5; here >e is an effective radiation wavelength derived by taking into -12 - ç -13-account the dielectric properties of the media adjacent the antennas 26. It is given by X Xo/fle (1) where X0 is the free space wavelength and is an effective refractive index given by: -6 12] (2) and 2 being the respective dielectric constants of the two media adjacent the antenna array 30, Ic air and alumina.

The resonant frequency r of a dipole antenna is such that its length L is half the effective wavelength Xe, Ic: 1'r -c/Xn -c/2Ln, where c is the velocity of light (3) for air and alumina respectively = 1 and = 9.8, which gives n = 2.43.

For an antenna dipole 5 mm in length, r is approximately 13 GHz. This ignores the effect of finite dipoie width to length ratio. Increasing this ratio reduces the resonant frequency.

-13 -Referring now to Figures 13 to 15, there are shown alternative Constructions of lens/substrate assemblies for use in a radiation field sensor of the invention. In Figure 13, a lens 120, substrate 122 and antennas 124 are arranged as in Figure 1. However, each of the antennas 124 is surmounted by a respective integrated circuit (IC) chip 126 incorporating a detector diode, diode biasing circuit and video amplifier (not shown). The IC chip 126 is easily accessible for connection to processing circuitry of the kind shown in Figure 3. In Figure 14, a lens 130 is attached to a silicon wafer 132 in which diodes 134 are integrated and on which antennas 136 are deposited by planar metallisation. The wafer 132 j has high resistivity (p 100 ohm Cm) to reduce absorption of radiation passing through it, and has video amplifiers and diode bias circuitry (not shown) integrated within it. The active components (diodes, amplifiers, bias circuitry) are fabricated in a layer I to 3 zm thick of the wafer 132 adjacent the surface on which the antennas are deposited. This form of Construction is highly suitable for frequencies in the order of or above 30 GHz. Silicon has a dielectric constant of 11.7 which is approximately matched to that of alumina, so reflections at the lens-substrate wafer interface are not serious. The substrate wafer 132 may alternatively be of GaAs.

In Figure 15, a lens 140 and an alumina antenna support substrate 142 are disposed so that antennas 144 on the substrate surface are sandwiched between lens and substrate. Integrated circuits 146 containing respective detector diodes, biasing circuits and video amplifiers (not shown) are bonded to the reverse side 148 of the substrate 142. Plated-through connections 150 are formed between the antennas 144 and circuits 146 on the side walls of holes through the substrate 142. The substrate 142 is accordingly double sided, having antennas 144 on one surface adjacent the lens 140 and circuitry on the other. This has the advantage that Connections (not shown) to signal processing circuits (eg ADCs 44 etc in Figure 3) may be provided on the substrate surface without conflict with the antenna metal pattern. This allows closer antenna packing and avoids disturbance of the antenna radiation beam patterns by additional conducting tracks in the antenna array plane.

-14 -The invention employs a lens such as 140 to impose one sided or directional radiation coupling on a dipole antenna array, which would be substantially omnidirectional in the absence of a lens. The Figure 15 arrangement requires radiation coupling to the antenna array to be predominantly via the lens 140, as opposed to via the substate 142. This in turn requires that the lens dielectric constant be two or more times that of the substrate 142 at least in the lens region adjacent the antenna array. An alumina substrate 142 ( = 9.8) may be associated with a lens 140 of barium non-atitanate ceramic (Ba2Ti9Q20, 39).

The lens 140 may alternatively be a compound lens having a high dielectric constant portion adjacent the antenna array and a low dielectric constant portion relatively more remote from the array.

Referring now to Figure 16, there is schematically illustrated a further embodiment 160 of the invention suitable for detection of radiation polarisacion.

The device 160 incorporates a lens 162 adjacent a dielectric block 164 divided along a diagonal. The diagonal division incorporates a polarisation selective mirror 168. Two antenna array support substrates 170 and 172 are disposed to receive radiation reflected and transmitted by the mirror 168 respectively.

Antennas and Circuit components associated with the substrates 170 and 172 are not shown, but these may be disposed as in embodiments described earlier. The lens, block and substrate dielectric constants are selected to provide for both antenna arrays to couple predominantly to radiation received via the lens 162.

The mirror 168 consists of parallel metal strips (not shown) each arranged longitudinally perpendicular to the plane of the drawing. Radiation polarised perpendicular to the strips is transmitted to the substrate 172, whereas radiation polarised parallel to the strips is reflected to the substrate 170. Radiation with intermediate polarisation is partly transmitted and partly reflected, and is received by both substrates 170 and 172. Signals from the antenna arrays associated with the substrates may be processed as described with reference to Figure 3.

Referring now to Figures 17, 18 and 19 there are shown various composite antenna structures in schematic form, and providing alternative means for sensing radiation polarisation. In Figure 17, a pair of orthogonal crossed antenna dipoles are shown each with a respective diode 182. The diode outputs will be equal for two different electric vector polarisations equally inclined at 45 to the -15 - -16 -dipoles. These polarisations are indicated by chain lines E1 and E2. There will consequently be a twofold ambiguity as regards direction of polarisation This ambiguity may be resolved by the arrangement shown in Figure 18, which employs three antenna dipoles 190 with respective diodes 192. The dipoles are mutually inclined at 60, and collectively provide a unique combination of three signals for any linear polarisation of incident radiation.

Figure 19 shows two superimposed and generally S-shaped antenna dipoles 200 and 202 with respective diodes 204 and 206. The antennas 200 and 202 are io mirror images of one another with a relative rotation of 90, and both couple to circularly polarised radiation. However, antenna 200 couples to radiation polarised in the opposite rotational sense to that to which antenna 202 couples.

The antenna configurations of Figures 17 to 19 are replicated to form arrays of like composite or multipole antenna elements on substrates as previously described for individual antenna dipoles. They are employed with signal processing circuitry as set out in Figure 3, in which the ROM 48 is required to store a respective set of coefficients for each incident beam direction at each beam polarisation. This is equivalent to storage of coefficients for different frequencies for simultaneous position and frequency determination as described earlier. Each individual antenna and associated mixer diode of each composite antenna element is provided with a respective signal channel incorporating a video amplifier 42, ADC 44 and multiplier 46, together with input to the tree of adders 56 etc. An array of composite antenna elements of the kind shown in Figures 17, 18 or 19 consequently requires more signal channels than the equivalent for earlier embodiments.

The processing circuit 40 of Figure 3 executes a correlation operation to determine incident beam parameters. Individual amplified and digitised antenna signals are multiplied by respective coefficients in each of a series of stored sets of coefficients corresponding to precalibrated incident beam parameters. The multipliers 46 and adders 56 etc may be replaced by a digital correlator circuit of known kind. The correlation procedure is mathematically equivalent to a least squares fitting operation involving ADC output signals and calibration coefficient sets. The square of the error en obtained between the ADC outputs and the -16 - c -17-nth set of coefficients is given by: e2 -(v1_br1)2 (4) where: v1 = output signal from the ith ADC 44; = ith coefficient of the nth coefficient set expressed as a fractional value, b = a parameter to be determined and proportional to power received by the antenna array; and m = number of antenna signal channels.

The values of n and b which minimise 4 are required; b determines the received signal power, and n determines the coefficient set corresponding to the best fit thereby indicating radiation parameters such as direction, polarisation and/or frequency. It is convenient to scale the calibration coefficient sets so that the sum of the squares of the coefficients in each set is unity, ie: m 2 r1 -1 (5) Equation (4) may be rewritten: m m m 2 V2 V 2V2 e -/ v. -2 / bv.r. + b 1 r. (6) fl t I L. mi fli 1-1 i-I i-I If it is defined that m V2

S -1-1

-17 - (sum of squared ADC output signals) and x)v.r fl L ml i-I (sum of weighted ADC output signals produced by the fourth rank adder 62), then: e2 -S -2bx + b2 (7) For any given n, e in equation (7) is a minimum when b = x. This minimum e is given by: e2 -S-x2 (8) The value of n yielding the best least squares fit is that for which x is a maximum, that is, when the output from adder 62 is a maximum. Consequently, the signal processing procedure described with reference to Figure 3 yields the coefficient set or value of n providing the least squares fit. The best value of b is equal to the maximum x.

required, the incident power may be estimated from the value of b X. Ifl addition, the Figure 3 circuit 40 may be modified by switching the multipliers 46 so that they receive respective ADC signals on both inputs, and provide respective values of vj for summation in the adder tree to produce S. Since the zP 64 determines x, e in Equation (8) may be determined. This squared error may be compared with a pre-arranged value by the pP 64, and discarded if insufficiently low as indicating an inadequate fit. This might for example arise from a radiation beam having signal parameters outside the range for which predetermined calibration coefficient sets are available.

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The invention may be employed to form part of a guidance system for a missile.

This embodiment incorporates an antenna array 100 (see Figure 11) having dipoles 102 at a variety of orientations to provide polarization sensitivity. The array 100 is mounted at the rear of a missile to receive radiation from a range of solid angle about the rearward direction. A radar ground station transmits a polarised radiation beam for reception by the array 100 and determination of the array orientation relative to the radar beam direction of polarisation as previously decribed. Since the array is attached to the missile, this also determines the missile attitude relative to the radar beam. This avoids the need for an attitude control gyro on board the missile.

The ground station may transmit three (or more) overlapping radar beams which illuminate both the missile and a target of interest. The beam centres are spaced apart by distances in the region of half a beamwidth. Radar returns i from the target are received from the ground station, which determines the target range, bearing and elevation. The ground station subsequently transmits this target information to the missile by coding or modulating the radar beams. The beams also have differing modulation codes for identification purposes. The missile receives the three radar beams and determines their relative amplitudes.

This determines the missile position relative to the line between the ground station and target. The missile attitude is determined as has been said relative to the polarisation of the transmitted radar beam. Having obtained positional and attitude information, a computer on board the missile calculates flight path corrections to produce target interception. Such a missile does not require on board gyros.

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Claims

CLAIMS1. A radiation field sensor comprising:(a) a substrate sheet module retaining an array of dipolar antennas and a respective mixing means for each antenna; (b) a dielectric lens module arranged closely adjacent to the substrate so that individual antenna centre positions in the array correspond to respective beam directions for radiation incident on the lens, and the lens and substrate dimensions and dielectric properties being in combination such as to provide for each antenna to couple predominantly to radiation passing through the lens; and (c) signal processing means responsive to degree of correlation between mixing means output signals and sets of predetermined coefficients corresponding to known incident radiation parameters.
2. A sensor according to Claim 1 wherein the antenna array is within the lens depth of focus.
3. A sensor according to Claim 1 wherein the signal processing means includes a respective amplifier and analogue to digital converter for each mixing means, means for multiplying each analogue to digital converter output signal by a respective coefficient in each of a series of sets of coefficients to provide product values, means for summing product values, and means for determining the coefficient set yielding the maximum sum of product values and thereby indicating corresponding incident radiation parameters.
4. A sensor according to Claim 1 wherein the antennas are mutually parallel.
5. A sensor according to Claim 1 wherein each antenna is inclined to at least some of the other antennas in the array to provide sensitivity to incident radiation polarisation.

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6. A sensor according to Claim 5 wherein each antenna is associated with two other antennas superimposed on it and inclined both to it and to one another at angles of substantially 60.
7. A sensor according to Claim I wherein the lens and substrate have similar or equal dielectric constants and the lens is arranged to couple radiation to the antenna array through the substrate thickness.
8. A sensor according to Claim I wherein the mixer diodes are integrated in the substrate material.
9. A sensor according to Claims I wherein the antenna array is sandwiched between the substrate and the lens, and the mixer diodes are spaced from the lens by the substrate thickness and are connected to respective antennas by connections extending through the substrate. -21-Amendments to the claims have been filed as follows1. A radiation field sensor comprising:(a) a substrate sheet module supporting an array of dipolar antennas and a respective diode mixing means for each antenna; (b) a dielectric lens module arranged closely adjacent to the substrate so that individual antenna centre positions in the array correspond to respective beam directions for radiation incident on the lens, and the lens and substrate dimensions and dielectric properties being in combination such as to provide for each antenna to couple Predominantly to radiation passing through the lens; and (C) signal processing means arranged to characterise a radiation beam incident on the lens in terms of degree of correlation between diode mixing means output signals and sets of predetermined coefficients corresponding to known incident radiation parameters.2. A sensor according to Claim 1 wherein the antenna array is within the lens depth of focus.3. A sensor according to Claim I wherein the signal processing means includes a respective amplifier and analogue to digital converter for each diode mixing means, means for multiplying each analogue to digital converter Output signal by a respective coefficient in each of a series of sets of coefficients to provide product values, means for summing product values, and means for determining the coefficient set yielding the maximum sum of product values and thereby indicating corresponding incident radiation parameters.4. A sensor according to Claim 1 wherein the antennas are mutually parallel.5. A sensor according to Claim 1 wherein each antenna is inclined to at least some of the other antennas in the array to provide sensitvity to incident radiation polarisation.6. A sensor according to Claim S wherein each antenna is associated with two other antennas superimposed on it and inclined both to it and to one another at angles of substantially 60.7. A sensor according to Claim 1 wherein the lens and substrate have similar or equal dielectric constants and the lens is arranged to couple radiation to the antenna array through the substrate thickness.8. A sensor according to Claim 1 wherein the diode mixing means are integrated in the substrate material.9. A sensor according to Claim 1 wherein the antenna array is sandwiched between the substrate and the lens, and the diode mixing means are spaced from the lens by the substrate thickness and are connected to respective antennas by connections extending through the substrate.
10. A sensor according to Claim 1 including a second substrate sheet module supporting a second dipolar antenna array and polarization selective reflecting means arranged to reflect one polarization of radiation passing through the lens to the second antenna array.
11. A sensor according to Claim 1 including a respective second dipolar antenna crossed with each antenna of the array.
12. A sensor substantially as herein described with reference to the accompanying Figures 1 to 9.
13. A sensor substantially as herein described with reference to Figures 1, 3 to 9 and any one of 10, 11 and 12.
14. A sensor substantially as herein described with reference to Figures 3 to 9, any one of 10, 11 and 12 and any one of 13, 14 and
15.