GB2346752A

GB2346752A - Radar system

Info

Publication number: GB2346752A
Application number: GB8918485A
Authority: GB
Inventors: Kenneth Grantham-Wright; Clifford John Hope
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-08-12
Filing date: 1989-08-14
Publication date: 2000-08-16
Anticipated expiration: 2009-08-14
Also published as: GB2346752B; GB8819216D0; GB8918485D0

Abstract

A Staring-Array Multi-Functioning, Intelligent, Radar-Imager (acronym "SAMFIRI"). A radar system for target detection and location employing a staring multi-element essentially non-coherent array. The transmitting antenna has a multiplicity of transmitting elements illuminating the target sector substantially uniformly but non-coherently. The receiving antenna has a similar multiplicity of individually directional receivers (15) providing a degree of angle discrimination. Measurement cells (volumes in space) are defined by the receiver beams and range gates (25) in each receiver. The content of each measurement cell is assessed against an adaptive threshold (27) and integrated over a large number of pulse periods. The count for each cell is compared in time and space with adjacent ones and sudden perturbations detected as targets.

Description

Radar System This invention relates to a radar system for general purpose target detection and location. It will be understood that the term"target' includes any object that will provide a radar echo and which it is desired to detect.

The particular embodiment of the invention that will be described subsequently may be seen as a Staring-Array, Multi-Functioning, Intelligent, Radar-Imager, referred to by the acronym"SAMFIRI".

Radar systems generally, provide target location by phase analysis of target echo and/or relative magnitude of signals received by directional antenna elements. Amplitude and phase comparison monopulse radars are examples of such systems.

An object of the present invention is to provide a radar system in which phase information is completely disregarded, so providing considerable simplification in manufacture-phase tolerances etc.

According to the present invention, a radar system employs multiple antenna elements to provide randomly phased illumination of a target sector and target location within a three-dimensional array of measurement cells defined by the directivity of individual antenna elements and a series of range gates associated with each of the individual antenna elements, the system further comprising storage means for storing periodically a signal representative of the magnitude of reflection from each of the measurement cells and means for detecting a target content of a measurement cell by virtue of a comparison of the stored signal with other stored signals associated with measurement cells adjacent in time or space.

The system preferably comprises a transmitting antenna having a multiplicity of antenna elements and a receiving antenna having a multiplicity of antenna elements, as opposed to dual function elements The receiving antenna may be a regular polyhedron each face of which comprises a plurality of antenna elements.

Each receiving antenna element preferably comprises a monolithic solid state circuit including local oscillator, mixer, I. F. amplifier, detector, threshold circuit and storage means.

Preferably each transmitting antenna element comprises a monolithic solid state circuit including a diode oscillator and a pulse modulator.

The system preferably comprises a pulse modulator for the transmission of pulses of radar energy, integrating means operable over a plurality of periods of the pulse modulator to provide accumulated values of the reflection magnitude in respect of each measurement cell, the storage means being adapted to store the accumulated values, each accumulated value being stored as an update of the preceding one for the same measurement cell, and means responsive to discrepancy between each accumulated value and the preceding one for the same measurement cell and between each accumulated value and the accumulated values for adjacent measurement cells, to indicate the presence of a target.

There are preferably included means for determining the reflection magnitude in respect of a single pulse reflection and a single measurement cell as a two valued quantity related to a threshold value, the threshold value being controlled to divide pulse reflections between the two values in substantially constant proportion, and means for assessing the average value of the reflection magnitude in terms of change in the threshold value.

The threshold value is preferably arranged to be controlled in logarithmic steps.

The system is preferably operative at at least two different frequencies and comprises means for deriving the signal representative of reflection magnitude at each of the different frequencies independently.

Alternatively, the system may comprise means for deriving the signal representative of reflection magnitude as a combined signal in respect of all the different frequencies.

The system, in use on an unstable platform preferably comprises means for sampling the platform attitude periodically with respect to a plurality of reference axes and means for adjusting the accumulated values in dependence upon the sampled values of the platform attitude.

One embodiment of a radar system in accordance with the invention will now be described, by way of example, with reference to the acompanying drawings, of which: Figure 1 is a diagrammatic side view of combined transmitting and receiving antennas; Figure 2 is a block diagram of transmitting circuit components for a single one of a multiplicity of transmitting antenna elements; Figure 3 is a block diagram of receiving circuit components for a single one of a multiplicity of receiving antenna elements; Figure 4 is a block diagram of a target imaging and tracking system employing the systems of Figures 1,2 and 3.

A basic feature of the present radar system is the randomly phased illuminatin of a target sector, that is, a volume of space in which a target may lie. The present example is intended for use aboard ship in which circumstance a target, eg an attacking aircraft or missile, may occupy any position within a hemisphere. The target sector is thus hemispherical.

The radar antenna at the centre of this hemisphere is required to illuminate all targets in this hemisphere with randomly phased radar energy ie with a non-coherent signal.

Figure 1 illustrates one combined transmitting/receiving antenna suitable for this purpose. It comprises a receiving antenna 1 mounted above a transmitting antenna 3 both being symmetric about a common vertical axis 5.

The transmitter 3 is of'diabolo'form and its curved surface is uniformly covered with transmitting antenna elements (not shown), each element being a monolithic solid state element including a triple-crossed dipole providing distributed polarisation. The monolithic element also includes all of the basic components of a radar transmitter-exclusive to the particular element -as will be explained with reference to Figure 2.

There is a multiplicity of such elements on the antenna surface, 120,000 in the particular example although there might be as few as 10,000 and as many as several hundred thousand. As will appear, the greater the number the more uniform the illumination of the target sector and the greater the suppression of phased peaks and troughs. As mentioned above, each element has its own independent radar pulse generator so that the individual transmissions are non-coherent, even though the pulse modulation is synchronised.

The operating frequency is typically 15GHz and each elemental antenna is designed to give a 10 milliradian 3dB beam width, ie a narrow beam. Referring to the plane of Figure 1 the beams will converge to a point at the centre of the curved surface of the'diabolo'and then diverge to cover substantially all directions simultaneously, although there will be some obscuration in the axial direction. A'staring'ie non-scanned, array is thus provided.

The diabolo form of the antenna provides that axial (or nearly axial) radiation is emitted from points off axis so reducing the masking effect of the receiving antenna above it.

While the operating frequency of each transmitting element is centred on a value fo = 15GHz (in this example), there is a frequency spread about this figure of, typically, plus and minus 0.1%. A limit to this frequency spread is imposed by the I. F. bandwidth of the receiving system.

This frequency spread gives an inherent degree of polychromatic, ie multi-frequency, operation, with the result that the target sector is phase-scanned. The peaks and troughs occurring throughout the target sector resulting from phase summation and cancellation, and which would be a fixed, albeit very complex, pattern in space with a single spot value of fo, change and shift non-coherently given the range of frequencies in the transmitting band. Even in the case of a single spot frequency this value would drift very slightly between the thousands of transmitting elements, so changing the radiation pattern of peaks and troughs considerably.

The fundamental antenna gain determined by the aperture size in wavelengths is increased considerably by the most favourable arrangement of phasing that can occur in the above myriad changing patterns. These being random, some point in space will have exceptionally high levels of illumination, others the reverse; this is to say that with multiple transmitter elements randomly phased there will be points in which the illumination is additive and others in which it is substractive, as explained above. This will change with time because there is a spread of frequency, albeit small, and there is no control over relative phases from each transmitter element. Over many pulses, the chance that every'measurement cell'in space will experience a very high level of illumination is a near certainty. The term'measurement cell'will be explained subsequently.

The polychromatic effect is enhanced by the use of two or more different operating frequencies. For example, two frequencies within 5% of 15 GHz may be used, the total array of transmitting elements comprising two sets working on the respective frequencies, the two sets being interspersed throughout the'diaboloidal'array either singly or in groups. The two frequencies will each have a frequencfy spread as for the single fo case. As a further variation, sets of transmitting elements can be switched differentially between different operating frequencies on a continuous basis.

It may be seen that the vector sum of the elemental contributions at any particular point in space may be described as a randomly fluctuating composite vector with respect to time. The effective polarisation relationship between the transmitter elements may be considered to be a function of the phase relationship. The composite vector can be expected to vary in polarisation with respect to time and space and may thus be described as multipolarised over some region of time or space.

In the event that the composite vector is integrated over matched bounds, and in particular over a large number of pulses, then there will be no random fluctuation on the magnitude of the integral.

Turning to Figure 2, this shows the components that are incorporated in each of the 120000 transmitting elements and which are therefore exclusive to the particular transmitting antenna element.

The triple-crossed dipole antenna is shown at 7, fed by an R. F. oscillator diode 9 controlled by a pulse modulator 11. A pulse modulating signal is derived from a common PRF generator not shown. There is also a common power supply to the transmitting element. Local control of the operation is effected by a processor 13 in turn controlled by a system processor not shown.

Pulsing of the non-coherent transmissions is provided to permit range determination and reduce power rating. A high pulse repetition rate is necessary to give a high duty rate for integration purposes but a sufficiently low one is necessary to avoid'overlay'at significant range. A PRF of 5000 pulses per second is employed in the present embodiment. An integration period of one second will then incorporate this large number of pulses and so avoid random fluctuations.

Avoidance of ambiguity, in a ship context, can also be provided by using co-lateral information where this is available, ie confirmation of long range targets by a separate long range radar system, measurement of angle and range rates to exclude impracticable'targets'etc.

Turning now to the receiving antenna shown in Figure 1, this is of substantially spherical form although it is convenient to manufacture it in the form of a polyhedron. In the particular example considered it has 20 faces, each an equilateral triangle of side approximately one hundred wavelengths. The'sphere'is conveniently mounted on one of its 20 faces, as shown.

The number of receiver elements may conveniently lie between, say, 80 and two thousand, and in the present embodiment is 900. Each receiver element, incorporating the components of Figure 3, is supporte on a separate 6-sided substrate.

Referring to Figure 3, each receiver element includes a set of multi-wavelength, triple-crossed dipoles 15, designed to give a 200 milliradian 3dB beam width. The width of the beam can be designed to suit specific requirements but must of course be of sufficient width in view of the number of beams to cover the total target sector. The receiver element beam width is greater than that of the transmitting elements although still relative narrow.

The received signal is applied to a mixer 17 supplied by a local oscillator 19 to produce an intermediate frequency signal for an I. F. amplifier 21 of bandwidth 200 MHz thus encompassing the frequency spread of 0.2% around the operating frequency fo. The I. F. signal is then applied to a series of 250 range gates 25 each defining a range cell of 100 metres and thus covering a range of 25 kilometres. The range gates 25 are controlled by a clock generator operating at lGHz the spread being optimised for correlated integration.

The content of each range gate 25 is assessed by a threshold circuit 27 on a binary basis and if the threshold is crossed the pulse sample is stored in a range bin. Each of the 5000 samples from the particular range gate is assessed against the threshold and accumulated in the range bin if the threshold is crossed. The number of crossings in the integration period (1 second) is counted and the current compared against a predetermined optimum value of say 50% of the 5000 samples. According to whether the count exceeds the 50% figure (threshold too low) or does not reach the 504$', figure (threshold too high) a step change is made in the threshold figure to reduce the disparity. The step change is logarithmic, being large for a large disparity and decreasing as the disparity decreases.

At the end of each integration period the range bin content is zero'ed, the number of threshold crossings is stored, and the size of the threshold correction step is stored.

Significant changes in the statistical values occur when a target happens to receive a high level of illumination-as occurs with the randomly phased transmitter-and this assists considerably in the subsequent image processing.

It will be apparent that the geometric form of the receiving antenna provides an inherent target direction location capability without reference to any phasing of the signal since a localised target will only be detected by the receiver elements approximately facing the target, ie by those receiver elements in whose beam widths the target lies. Thus a measurement cell in space is defined by the intersection volume between a receiver element beam width and a range cell determined by a receiver range gate.

The array of receiver elements each with several hundred range bins form several hundred thousand such'measurement cells'. Each measurement cell contains a set of complex data about all parts of the scene and the statistics of the sensor system and its environment. The data in all measurement cells is needed to extract the essential outputs, even from those cells which have little or no incident energy from clutter, targets or other sources, since they contain samples of noise peculiar to the system. To minimise illuminator power the output of all cells is integrated over a large number of illuminator pulses, as explained above, to obtain as near stationary values as possible or random noise statistics in the sensor system and its environment. By statistically stationary is meant no variation in population means and standard deviation as a function of any significant time shift. The maximum integration time can cover several output data intervals, but in the present embodiment the basic integration time is set equal to the data output rate period of 1 sec (5000 pulses).

The presence of any reflecting source is indicated by perturbations in the statistically stationary noise background. Its angular position is found by establishing the centre of symmetry over the most relevant patch of measurement cells. The number and extent of the symmetry sets used is governed by the system tasks at the time and by the local co-lateral information available.

Rapid target detection is achieved by concentrating the image processing upon the analysis of differential discontinuities in adjacent cells on a pulse group integrating period basis. These discontinuities are obtained by comparing the current integrating period outputs with those held in store from a previous period taking out into a second store any differences in thresholds and crossings, and at the same time replacing the old with the new in the first store.

The SAMFIRI system so far described thus uses radar in a full staring mode, that is with no scanning by moving parts, which provides a full hemisphere surveillance and can simultaneously track a large number, for example 400, or disparate targets.

The outputs of the image processing are tailored to the functional requirement. Inter alia these can be a two dimensional (angular) view or a three dimensional representation of the surrounding scene, any changes in the scene of size, position, frequency, amplitude or displacement movement, weather, alien and/or known'target'detection and tracks, external co-frequency power sources, etc. The resolution of the images depends upon the range cell size selected in the receiver design and the signal to background power ratio in angle. In the present embodiment these are 100m and better than 5 milliradians in azimuth and elevation.

There is thus provided the use of a general purpose sensor system, which is intelligently adaptive in its processing, to meet the particular specified demands of its function, which in turn are selectable on demand.

This is to say that because the information extraction is achieved by a set of parallel array processors which operate on an adaptive, rather than a fixed basis, the input data can be supplied by a general (or multi) purpose sensor. The adaptive process is conditioned by the particular information requirements and the environment at the time. The environment and the requirements can change. Constraints on reactions to the environment and changes to the particular information priorities can be selectively controlled.

When mounted on a moving platform, which may for example be a ship, aircraft, land vehicle or satellite, the scene perceived by the receiver is not spatially stable. Rather than attempting to stabilise the antenna against six degrees of motion, a threshold crossing correction factor is applied to compensate for motion in the processing.

The correction factor can be derived by reference to fixed objects in the scene or from a small stabilised table recording the antenna motions on six shift registers. The sampling time of each input to the registers is determined by the maximum likely movement rate of the antenna and the required target angular discrimination. The antenna angular movement in the sampling time must be small compared to the required target angular discrimination. Threshold crossings from each measurement cell are recorde as totals in the movement sampling time and stored in shift registers. At the end of each processing integrating period, the number of theshold crossings for each measurement cell is adjusted between adjacent cells, in three dimensions, by a factor determined by the six antenna motion shift registers. The resulting adjusted counts are then passed to what has become a spatially stabilised scene store for subsequent image processing.

This stabilisation system is shown incorporated in the overall system shown in Figure 4. The transmitter 3 (see Figure 1) is controlled by local processors 13 and weapon system processor 29 in conjunction with a PRF generator 31.

The receiver 1 supplies the local processors 28 which in turn provide the threshold crossing information. This is adjusted as explained above by a stabiliser system 33 to provide space stable target information.

The stabilised table 35 provides information as to its six degrees of motion relative to the antenna to six registers 37. The register contents are processed by stabiliser 33 to provide the correction factors for the threshold crossing data.

The resulting stabilised image scene 39 is provided to the weapon system processor for target assessment, display and weapon control, assuming that the platform (ship etc) does have means of defence or attack.

It will be clear that an important factor in the cost of the described system will be the large scale application of VLSI (very large scale integration) integrated circuits in view of the very large number of replaced components, ie small but comprehensive transmitter and receiver elements.

Claims

CLAIMS 1. A radar system employing multiple antenna elements to provide randomly phased illumination of a target sector and target location within a three-dimensional array of measurement cells defined by the directivity of individual antenna elements and a series of range gates associated with each of said individual antenna elements, the system further comprising storage means for storing periodically a signal representative of the magnitude of reflection from each of said measurement cells and means for detecting a target content of a said measurement cell by virtue of a comparison of the stored signal with other stored signals associated with measurement cells adjacent in time or space.
2. A radar system according to Claim 1 comprising a transmitting antenna having a multiplicity of antenna elements and a receiving antenna having a multiplicity of antenna elements.
3. A radar system according to Claim 2, wherein said transmitting and receiving antennas are axially symmetric about a common axis to permit uniform coverage of an azimuthal target sector.
4. A radar system according to Claim 2 or Claim 3, wherein said receiving antenna is substantially spherical and is substantially uniformly covered with receiving antenna elements.
5. A radar system according to Claim 4, wherein said receiving antenna is a regular polyhedron each face of which comprises a plurality of antenna elements.
6. A radar system according to any of Claims 2 to 5 wherein each receiving antenna element comprises a monolithic solid state circuit including local oscillator, mixer, I. F. amplifier, detector, threshold circuit and storage means.
7. A radar system according to Claim 6 as appendent to Claim 5, wherein said polyhedron has equilateral triangular faces and each receiving antenna element is of hexagonal form.
8. A radar system according to Claim 2 or Claim 3, wherein said transmitting antenna is of diabolo form the curved surface of which is substantially uniformly covered with transmitting antenna elements.
9. A radar system according to Claim 8, wherein each transmitting antenna element comprises a monolithic solid state circuit including a diode oscillator and a pulse modulator.
10. A radar system according to any preceding claim, wherein each of said antenna elements comprises a triple-crossed dipole.
11. A radar system according to any preceding claim, and comprising a pulse modulator for the transmission of pulses of radar energy, integrating means operable over a plurality of periods of said pulse modulator to provide accumulated values of the reflection magnitude in respect of each of said measurement cells, said storage means being adapted to store said accumulated values, each accumulated value being stored as an update of the preceding one for the same measurement cell, and means responsive to discrepancy between each accumulated value and the preceding one for the same measurement cell and between each accumulated value and the accumulated values for adjacent measurement cells, to indicate the presence of a target.
12. A radar system according to Claim 11, including means for determining said reflection magnitude in respect of a single pulse reflection and a single measurement cell as a two valued quantity related to a threshold value, said threshold value being controlled to divide pulse reflections between said two values in substantially constant proportion, and means for assessing the average value of said reflection magnitude in terms of change in said threshold value.
13. A radar system according to Claim 12, wherein said threshold value is arranged to be controlled in logarithmic steps.
14. A radar system according to any preceding claim, operative at at least two different frequencies and comprising means for deriving said signal representative of reflection magnitude at each of said different frequencies independently.
15. A radar system according to any of Claims 1-13, operative at least two different frequencies and comprising means for deriving said signal representative of reflection magnitude as a combined signal in respect of all said different frequencies.
16. A radar system according to Claim 11, for use on an unstable platform and comprising means for sampling the platform attitude periodically with respect to a plurality of reference axes and means for adjusting said accumulated values in dependence upon the sampled values of said platform attitude.
17. A radar system substantially as hereinbefore described with reference to the accompanying drawings.

17. A radar system substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A radar system employing multiple antenna transmitter elements to provide randomly phased illumination non-coherent from element to element of a target sector and target location within a three-dimensional array of measurement cells defined by the directivity of individual antenna receiving elements and a series of range gates associated with each of said individual antenna receiving elements, the system further comprising storage means for storing periodically a signal representative of the magnitude of reflection from each of said measurement cells and means for detecting a target content of a said measurement cell by virtue of a comparison of the stored signal with previous stored signals from the same measurement cell or with stored signals from adjacent measurement cells.

2. A radar system according to Claim 1 comprising a transmitting antenna having a multiplicity of antenna elements and a receiving antenna having a multiplicity of antenna elements.

3. A radar system according to Claim 2, wherein said transmitting and receiving antennas are axially symmetric about a common axis to permit uniform coverage of an azimuthal target sector.

4. A radar system according to Claim 2 or Claim 3, wherein said receiving antenna is substantially spherical and is substantially uniformly covered xith receiving a tenna elements.

5. A radar system according to Claim 4, wherein said receiving antenna is a regular polyhedron each face of which comprises a plurality of antenna elements.

6. A radar system according to any of Claims 2 to 5 wherein each receiving antenna element comprises a monolithic solid state circuit including local oscillator, mixer, I. F. amplifier, detector, threshold circuit and storage means.

7. A radar system according to Claim 6 as appendent to Claim 5, wherein said polyhedron has equilateral triangular faces and each receiving antenna element is of hexagonal form.

8. A radar system according to Claim 2 or Claim 3, wherein said transmitting antenna is of diabolo form the curved surface of which is substantially uniformly covered with transmitting antenna elements.

9. A radar system according to Claim 8, wherein each transmitting antenna element comprises a monolithic solid state circuit including a diode oscillator and a pulse modulator.

10. A radar system according to any preceding claim, wherein each of said antenna elements comprises a triple-crossed dipole.

11. A radar system according to any preceding claim, and comprising a pulse modulator for the transmission of pulses of radar energy, integrating means operable over a plurality of periods of said pulse modulator to provide accumulated values of the reflection magnitude in respect of each of said measurement cells, said storage means being adapted to store said accumulated values, each accumulated value being stored as an update of the preceding one for the same measurement cell, and means responsive to discrepancy between each accumulated value and the preceding one for the same measurement cell and between each accumulated value and the accumulated values for adjacent measurement cells, to indicate the presence of a target.

12. A radar system according to Claim 11, including means for determining said reflection magnitude in respect of a single pulse reflection and a single measurement cell as a two valued quantity related to a threshold value, said threshold value being controlled to divide pulse reflections between said two values in substantially constant proportion, and means for assessing the average value of said reflection magnitude in terms of change in said threshold value.

13. A radar system according to Claim 12, wherein said threshold value is arranged to be controlled in logarithmic steps.

14. A radar system according to any preceding claim, operative at at least two different frequencies and comprising means for deriving said signal representative of reflection magnitude at each of said different frequencies independently.

15. A radar system according to any of Claims 1-13, operative at least two different frequencies and comprising means for deriving said signal representative of reflection magnitude as a combined signal in respect of all said different frequencies.

16. A radar system according to Claim 11, for use on an unstable platform and comprising means for sampling the platform attitude periodically with respect to a plurality of reference axes and means for adjusting said accumulated values in dependence upon the sampled values of said platform attitude.