GB2285718A

GB2285718A - Detecting ships using anti-chaff radar

Info

Publication number: GB2285718A
Application number: GB8317061A
Authority: GB
Inventors: Gaston Brunet; Loic Gauthier
Original assignee: Dassault Electronique SA
Current assignee: Thales SA
Priority date: 1982-06-25
Filing date: 1983-06-23
Publication date: 1995-07-19
Anticipated expiration: 2003-06-23
Also published as: DE3322814C1; GB8317061D0; FR2709551A1; BE897026A; GB2285718B

Abstract

In a system for detecting ships at sea using a radar emitting waves polarised in a given direction (V), the energy received is detected in the same polarisation plane and in an orthogonal polarisation plane (H). Only those echoes whose degree of depolarisation, relative to the emitted waves, is within a predetermined range (N1, N2) are singled out as characteristic of the presence of ships. Preferably, the procedure is carried out with a radar possessing a frequency-agile emitter (24). The echoes are then distinguished (61) on the basis of their degree of depolarisation, pulse by pulse, in each distance gate for a defined pulse train. A statistical analysis of the number of echoes corresponding to the chosen selection criteria, for each distance gate and in the pulse train, makes it possible to distinguish the targets. <IMAGE>

Description

Procedure and device for detecting ships at sea using a radar The present invention relates to radars and in particular to airborne radars intended for detecting and pursuing ships at sea. It can be used aboard aircraft or missiles.

It is known that sea-to-sea (and/or air-to-sea) missiles intended for attacking ships on the surface are equipped with radar homing devices enabling them to be guided automatically to the target. A conventional countermeasure method for combating these homing devices consists in emitting deception devices such as chaff, namely small reflecting filaments having a length equal to half the wavelength of the radar. This chaff forms a number of small dipoles which, when struck by the radar waves, back-scatter energy which the radar is capable of picking up. In practice, a large amount of this chaff is thrown out into the atmosphere surrounding a ship. To an enemy radar, it then forms a number of apparent targets amongst which it is difficult, if not impossible, to distinguish the target ship.

The invention relates to a countercountermeasure procedure and device intended for enabling a ship forming the real target of a radar to be distinguished from reelectors or echo generators, such as chaff, whose purpose is to deceive the radar.

With this end in view, the invention relates particularly to a procedure for detecting ships at sea using a radar emitting waves polarised according to a defined pattern, characterised in that the changes in the polarisation pattern of the echoes received by the radar are analysed in comparison with the emitted waves in order to distinguish the echoes returned by a ship from those which are scattered by deception devices such as chaff.

The invention is based on the unexpected observation, as a result of measurements carried out by the Applicant, that the waves reflected by ships on the surface of the sea in response to polarised radar signals were very substantially less depolarised than the echoes returned by chaff in response to the same signals. It was found that, when chaff is irradiated with, for example, a wave polarised rectilinearly in a given direction, the echoes back-scattered by this chaff contain a high proportion of energy polarised in the orthogonal direction. Thus, if the radar is equipped with a bipolarisation antenna, echoes can be obtained from this chaff which are sufficient to enable the radar to detect them in a perfectly normal way by utilising only the waves picked up in the direction of polarisation orthogonal to the direction of emission.On the other hand, the same experiments showed that the energy reflected by reflectors such as ships comprising relatively plane and smooth plates, with a relatively limited number of ridges or angular points, possessed a polarisation component in the direction orthogonal to the direction of the emission which was substantially less than the polarisation component measured on the same reflectors in the direction of polarisation of the emission.

These findings can be applied by using a radar to emit waves polarised rectilinearly in one direction, and by equipping the radar with a bipolarisation antenna. The energy of the signats picked up in both the directions of polarisation is compared, for example by producing a signal which is a function of the ratio of these energies. It is in fact shown that this ratio, which represents the degree of depolarisation of the echoes picked up relative to the emitted waves, provides a very effective criterion for distinguishing between the echoes returned by surface ships and the echoes from deception devices. It is possible, in particular, to determine a threshold for this ratio, beyond which the depolarisation is such that the corresponding echoes could not originate from a surface ship.In addition, it is preferred to single out, as capable of corresponding to ships, only those echoes for which the ratio of the energies picked up in both the directions of polarisation is within a predetermined range.

This procedure is made easier to implement if the targets are classified on the basis not of a single measurement of the depolarisation but of several successive measurements. In this respect, if a pulse radar is used, it is desirable to carry out a statistical analysis of the depolarisation of the echoes received in response to successive pulses emitted in one direction or in a defined angular sector.

It has also been found that it is possible considerably to improve the reliability and speed of the selection carried out by the procedure which has just been indicated if a frequency-agile radar is used. It is known that, in a radar of this type, trains of successive pulses are emitted, the carrier frequency of which varies from one pulse to the next. It is remarkable that it is possible in this case to distinguish, in a statistically very certain manner, between the echoes from a ship and those which result from chaff on a train formed of a relatively limited number of pulses in a given direction of investigation.This result has proved particularly valuable, in a radar device with an antenna sweeping in a given angular field, for drawing up a radar map of the targets located in this field during each sweep of the antenna, and characterising them according to whether they correspond to a ship or to a deception device. Thus, in the context where a radar of this type is used on a sea-to-sea missile, for example, the number of pulses emitted during the time taken by the antenna to sweep an angular sector equal to its aperture half-angle is sufficient to permit, by means of an appropriate statistical analysis of the degree of depolarisation of the echoes received, the classification of these echoes in real time during the sweep time of this angular sector.

The invention also relates to a radar equipped with counter-countermeasure means for detecting ships at sea, applying the process which has just been referred to.

The explanations and the description of a nonlimiting illustrative embodiment are given below with reference to the attached drawings, in which: Figure 1 illustrates the context in which the invention is used; Figure 2 is a block circuit diagram of a radar using the invention; and Figure 3 is a diagram of an antenna suitable for use within the scope of the invention.

A missile 10 equipped with a radar antenna 12, diagrammatically shown in its nose 14, is fired over the surface of the sea 15 in the direction of a flotilla comprising two surface vessels 16 and 17. The radar antenna 12 has an angular aperture diagrammatically shown by i. It circularly sweeps an angular sector or field of investigation P during the detection phase and, if appropriate, periodically throughout the travel of the missile 10.

The radar provided with this antenna forms part of a homing system by virtue of which the trajectory of the missile 10 is a function of a target such as the ship 16. For this purpose, the radar is equipped not only with conventional detection circuits but also with distance/speed pursuit loops which can function according to known principles.

An effective countermeasure method for ships such as 16 and 17, for defending themselves against missiles with radar homing devices, such as 10, consists in emitting chaff. This is generally thrown out into the atmosphere up to an altitude from which it falls down slowly, like rain, in a relatively wide zone around the ship which has emitted it. Each of these pieces of chaff, such as shown diagrammatically by 20 in Figure 1 around the ships 16 and 17, behaves, in respect of the waves emitted by the antenna 12 of the radar, like a small dipole, the length of which is chosen to be equal to half the estimated wavelength of the electromagnetic wave used by the enemy radar. The waves returned by each of these pieces of chaff are picked up by the antenna 12.They constitute a number of echoes which are difficult for the radar to distinguish from the echoes produced by real targets such as 16 and 17.

It has been discovered, however, that if the wave emitted by the antenna 12 is linearly polarised, the targets consisting of vessels such as 16 and 17 on the surface of the sea tend to produce echoes whose polarisation is relatively unchanged compared with that of the emitted waves.

In contrast, each piece of chaff tends to produce echoes which are very strongly depolarised compared with the polarisation of the incident waves originating from the antenna 12.

The relatively weak depolarisation of the echoes by targets such as ships is a statistical observation.

In particular, the degree of depolarisation of the echoes returned by a defined target can fluctuate in the course of time with periods which can be of the order of a second.

This observation applies both to targets producing strongly depolarised echoes, such as chaff, and to targets producing relatively weakly depolarised echoes. Now, it has been found that it is possible to reduce the effect of these fluctuations if the carrier frequency of the incident waves produced by the radar varies. In particular, if the radar used operates by means of pulses, it is altogether advantageous to provide it with frequency agility, as a function of which the carrier frequency of the emitted waves varies from one pulse to the next.It has in fact been observed that, even if, because of the fluctuations referred to above, the degree of depolarisation of the echo returned by a defined target in response to a pulse of given frequency f1 does not correspond to the result normally anticipated for this target, the following echo or echoes picked up in response to pulses emitted at emission frequencies f2, f3, f4, different from the frequency fl, statistically have the normal depolarisation characteristics of this target.Thus, if the successive pulses emitted at different frequencies by a frequency-agile generator succeed one another in the form of trains, the degree of depolarisation revealed at the end of each train provides a faithful representation of the nature of the target in a reduced interval of time corresponding to the emission of this train.

Consequently, -when using a frequency-agile generator, it is not necessary, in order to analyse with sufficient certainty the nature of the targets encountered by the beam 13 from the antenna 12, to wait for the antenna to sweep its field of investigation s several times during a period corresponding to the normal fluctuation time of the echoes. On the contrary, by sending, in each angular sector 2 of the field of investigation p 2 a train of a sufficient number of pulses, with the required frequency agility, and by processing the echoes received in response to these pulses, it is possible to detect the chaff concurrently.Juxtaposing the results of the investigation of each angular sector of aperture i2 during 2 the total sweep of the antenna makes it possible to draw up a corresponding radar map.

An example of an antenna 12 is diagrammatically shown in Figure 3. It comprises two juxtaposed waveguides 150 and 151 of square cross-section, which are placed in the focal zone of a parabolic reflector 152, their front faces, or emitting-receiving faces, being turned towards this reflector. These guides form a primary source. Their rear faces 154 and 155 are short-circuited.

Each of the waveguides 150 and 151 forming a horn is doubly excited by immersed waveguides, respectively V1 and V2 for the vertical polarisation of the waves transmitted by the horns 150 and 151 and H1 and H2 for their horizontal polarisation. At one of their ends, these immersed waveguides enter perpendicularly to the lateral surface of the horns 150 and 151 of square crosssection, in vertical and horizontal directions respectively, as a function of the chosen direction of polarisation. The opposite ends of the immersed waveguides V1 and V2 are connected, in a known manner, to a coupler 158 via waveguides 156 and 157. A sum channel V(C) and a difference channel V(!) (vertical polarisation) are available at the output of the coupler 158. The channel Veto) is connected to a waveguide 32.The channel V() is connected in reception to a waveguide 30 via a duplexer 26. It can receive in emission, via the duplexer 16, the signals transmitted to the latter by the emitter 24 over the link 22.

The horizontal immersed waveguides H1 and H2 are connected to a coupler 163 via waveguides 161 and 162; at the output of the coupler, there is a sum channel H(S) (horizontal polarisation) connected to a waveguide 34 at the output of the actual antenna. It is thus seen that the channel V() functions in emission and in reception, whereas the other channels function only in reception.

If it is desired to have angular pursuit not only in a horizontal plane but also in a perpendicular plane, it is possible to use, in addition to the two horizontally juxtaposed horns 150 and 151, two other horns superimposed on the latter.

The signals received on channels 30, 32, 34 are respectively heterodyned by mixers 40, 42 and 44 supplied by a common local oscillator 45, the outputs 41, 43 and 47 of the mixers being connected, via preamplifiers 46, to respective logarithmic amplifiers 50, 52, 54 with respective outputs 51, 53, 55.

The outputs 51 and 55 are connected to two res pective inputs 56 and 58 of a double comparator 60.

If the energies received on the vertical sum channel 30 and on the horizontal channel 34 are called V and H respectively, the output signals of the amplifiers 50 and 54 can be expressed in the form: log V and log H.

The circuit 60 includes a differential amplifier making it possible to produce a signal of the form k log H and two threshold comparators with levels N1 and N2 respectively, so as to produce a binary signal at the output 61 of the circuit 60 when the condition: l < k loa xt 2 N H is satisfied. In this case, a signal of level 1, or "validated video" signal, is produced. In the opposite case, a signal of level 0 is present at the output 61.

This signal is applied to an input 62 of an AND gate 65.

The signal log V at the output 51 of the amplifier 50 is a so-caLled "radar video" signal sent to an input 64 of a threshold amplifier 66, which receives a threshold voltage Sk at its other input 67. It produces a signal at its output 68 when the radar video signal is greater than the threshold Sk, so as to eliminate the influence of the noise. The output 68 is connected to a second input 69 of the AND gate 65. The latter delivers a signal at its output 70 each time energy of a sufficient level is detected in the vertical polarisation plane, in order to correspond to an echo (excitation of the output 68 of the threshold detector 66), and each time the depolarisation of this echo, relative to the pulse of vertical polarisation from which it originates, remains within defined limits corresponding experimentally to a ship.

The output signals of the AND gate 65 (level 0 or 1) are addressed into the positions of a counting memory 104 by means of an addressing and incrementing device 102.

A control device 100, governing the operation of the radar and connected to the frequency-agile emitter 24 via a bus 105, controls the address input 101 of the addressing device 102. The Latter includes an adder (not shown) for incrementing the contents recorded in each memory position 106i addressed with the value, 0 or 1, present at the output 70 of the AND memory 65, and for replacing the new result in this memory position.

It is supposed that, while the antenna 12 is sweeping a sector equal to its aperture half-angle 2 a train of N=100 pulses is emitted. In the look-out mode, the time interval between two successive pulses is divided into m, for example 1,000, distance gates. The memory 104 contains m memory positions 106j each corresponding to a distance gate. They are addressed corresponding to the series of time gates folLowing the emission of each pulse under the control of the control device 100. For each new pulse j of the train of N pulses, the contents of the memory 106i for the distance gate of index i may or may not be incremented by one unit, according to whether an echo has been detected in this distance gate following the pulse j and whether the polarisation of this echo satisfies the conditions checked by the comparator 60.

At the end of the train of N pulses, the control circuit 100 orders the contents of the positions of the memory 104 to be searched. A threshold device 110 at the output of this memory produces a signal at its output 111 each time the number n of favourable cases in the memory position 106i searched (that is to say the number n of successive echoes received in the distance gate i which have characteristics corresponding to a desired target, out of the total number N of pulses in the train) is greater than a predetermined threshold, for example N > 300. Thus, at the end of each train of N pulses, a N signal at the output 111 is present for each distance gate in which an echo of the desired type, that is to say an echo corresponding to the presence of a ship, has been detected.The output 111 then enables the entry, into a plot memory 115 of the existence of a target in the distance band of index i, in response to a respective indication of distance on a link 116 from the control unit 100.

Thus, at the end of the train of N pulses, the plot memory 115 gathers the indications of the presence of p ships in the angular sector covered during these 100 pulses. The operation is repeated in order to cover the whole of the field of investigation of the antenna, in the look-out mode.

Of course, the circuit shown in Figure 2 for detecting the plots is very diagrammatic for the purpose of the explanation. In particular, there is no mention of the buffer memories and other circuits which are customary in data processing devices for the practical construction of the circuit whose principle is shown and described.

Moreover, the form in which the echoes leaving the intermediate frequency stage are processed can form the subject of variants, without thereby exceeding the scope of the invention. In particular, it is possible to detect and store the levels of the echoes with vertical polarisation and the levels of the echoes with horizontal polarisation for each of the distance gates during the train of 100 pulses. In that case, 2m signal integrators are provided for producing an average value of the vertical polarisation energy after N pulses and, likewise, for the echoes corresponding to the horizontal polarisation energies picked up. At the end of the trainof N pulses, the ratio of these average values is taken.

If this ratio lies within a defined range, a threshold detector produces a signal indicating the presence of a ship in the distance gate considered. It is also possible to form the ratio log V pulse by pulse and to average it H over the N pulses in order to distinguish the nature of the targets.

It is advantageous, in particular in the first embodiment which has just been described (Figure 2), to use logarithmic amplifiers 50, 52, 54 which inherently possess a large dynamic range in contrast to amplifier circuits with automatic gain control, in order to effect linear normalisation of the echoes received in both the vertical and horizontal polarisation planes. This normalisation is effected very simply by taking the difference between the output signals of the logarithmic amplifiers, in order to produce a validated video signal pulse by pulse. The existence of this signal, combined with the radar video signal, in fact makes it possible to use a system for statistical analysis of the echoes received in each distance gate for a train of N pulses, the construction of which is very simple.It effectively amounts essentially to a device for taking account of the favourable cases in each of the distance gates.

The outputs of the logarithmic amplifiers 51 and 53 are also applied to respective inputs 84 and 85 of a circuit for normalisation of separation measurement, 80, delivering, at its output 81, a signal which is a function of the values = . It is known that a signal of this L type is representative of the angular separation of the echo received at a given instant from the axis of the antenna in circulation co-ordinates.

According to an improved feature of the invention, in the look-out mode, the signal at the output 81 is used to eliminate any echo whose angular separation from the axis of the antenna is greater than the half-aperture of this antenna. This eliminates any echo picked up in the secondary lobes of the antenna. As a result, the analysis processing of the depolarisation for each train of N pulses only relates to echoes in the main lobe of the bipolarisation antenna 12.

Furthermore, if the radar video signals (at the output 51) and separation measurement signals (at the output 53) are used in pursuit in appropriate loops in accordance with the known systems, the validated video signal can also be used as a criterion making it possible to discard certain countermeasure signals, either because of their weak depolarisation or, on the contrary, because of their strong depolarisation. A statistical analysis analogous to that which has just been described above can be carried out in the angular sector towards which the antenna is pointed, in order to evade the effects of countermeasure echoes capable of causing the distance pursuit loops or angular pursuit loops to deviate from the desired target.

Claims

1. A method for detecting ships at sea using a radar emitting waves polarised according to a defined pattern, wherein changes in the polarisation pattern of the echoes picked up by the radar are analysed in comparison with the emitted waves in order to distinguish echoes returned by ships from echoes returned by deception devices such as chaff.

2. A method according to Claim 1, wherein the energy of the echoes picked up is measured in two directions of polarisation in order to determine, at least statistically, the ratio of the difference between the polarisation of the emitted waves and that of the echoes received, and those echoes for which the difference ratio is greater than a predetermined threshold are considered as not corresponding to a ship.

3. A method according to Claim 2, wherein those echoes for which the difference ratio of the polarisation is within a predetermined range of values are singled out as corresponding to a ship.

4. A method according to any one of Claims 2 or 3, wherein the polarisation of the emitted waves is rectilinear, and the said difference ratio is determined by measuring the ratio of the energy received in the direction of polarisation of the emitted waves to the energy received in an orthogonal direction of polarisation.

5. A method according to any one of the preceding Claims, wherein the radar is a pulse radar, the carrier frequency of which is varied from one pulse to the next, and a statistical analysis is carried out on the polarisation of the echoes picked up on a train of several successive pulses in a given angular sector in order to distinguish between the echoes corresponding to ships and the other echoes.

6. A method according to Claim 5, wherein the antenna of the radar is swept in a defined field and at least one train of pulses is emitted in each elementary sector of this field, corresponding to the angular aperture of the antenna, in order to obtain a radar map during each sweep of the antenna.

A A radar equipped with counter-countermeasure means for detecting ships at sea and including means for emitting polarised waves, said radar further comprising an antenna, means for picking up the energy of the echoes received by this antenna according to two orthogonal polarisation patterns, means for comparing the energies picked up in these two directions, in order to produce a signal indicating the degree of depolarisation of the echoes from each target, relative to the emitted waves, and classification means for discarding those echoes whose degree of depolarisation is greater than a determined threshold as not corresponding to ships.

8. A radar according to Claim 7, in which the arrangement is such that the emitted waves are rectilinearly polarised in one of the directions of a bipolarisation antenna and the said comparison means produce, for each target, a signal which is a function of the ratio of the energy picked up in the direction of polarisation of the emission to the energy picked up in the orthogonal direction of polarisation.

9. A radar according to Claim 7 or 8, in which the emitting means includes a pulse generator and the said comparison and/or classification means includes a device for statistical analysis of the signals picked up in response to a train of successive pulses in a given angular sector.

10. A radar according to Claim 9, in which the said pulse generator is frequency-agile.

11. A radar according to Claim 9 or 10, in which the said comparison means includes means for logarithmic amplification of the signals received in both the orthogonal polarisation planes.

12. A radar as claimed in Claim 6, substantially as described herein with reference to the accompanying drawings.

12. A radar according to any one of Claims 9 to 11, in which the arrangement is such that the said comparison means is capable of operating on the energies picked up in both the directions of polarisation, pulse by pulse, and the classification means includes a selection threshold device for producing, pulse by pulse, an indication which is a function of the output level of the said comparison means, and means for detecting whether or not the number of favourable indications for each train of successive pulses is greater than a predetermined threshold.

13. A radar according to any one of Claims 7 to 12, in which the arrangement is such that the said classification means are capable of distinguishing those signals whose degree of depolarisation is within a predetermined range of values.

14. A method as claimed in Claim 1, substantially as described herein.

15. A radar as claimed in Claim 7, substantially as described herein with reference to the accompanying drawings.

Amendments to the claims have been filed as follows

1. A method for detecting ships at sea using a pulse radar emitting waves polarised according to a defined pattern, wherein the carrier frequency of the radar is varied from one pulse to the next and changes in the polarisation pattern of echoes picked up by the radar on a train of several successive pulses in a given angular sector are analysed statistically in comparison with the emitted waves in order to determine the presence or absence of a ship in accordance with the proportion of echoes in a correspondly received train that are detected as being depolarised and thus not corresponding to a ship.

5. A method according to any one of Claims 1-4, wherein the antenna of the radar is swept in a defined field and at least one train of pulses is emitted in each elementary sector of this field, corresponding to the angular aperture of the antenna, in order to obtain a radar map during each sweep of the antenna.

6. A radar equipped with counter-countermeasure means for detecting ships at sea and having means including a frequency agile pulse generator for emitting polarised waves, said radar further comprising an antenna, means for picking up the energy of the echoes received by this antenna according to two orthogonal polarisation patterns, means for comparing the energies picked up in these two directions, in order to produce a signal indicating the degree of depolarisation of the echoes from each target, relative to the emitted waves, classification means for discarding those echoes whose degree of depolarisation is greater than a determined threshold as not corresponding to ships, and a device for the statistical analysis of the depolarisation of said echoes picked up in response to a train of successive pulses of varying frequency in a given angular sector.

7. A radar according to Claim 6, in which the arrangement is such that the emitted waves are rectilinearly polarised in one of the directions of a bipolarisation antenna and the said comparison means produce, for each target, a signal which is a function of the ratio of the energy picked up in the direction of polarisation of the emission to the energy picked up in the orthogonal direction of polarisation.

8. A radar according to Cl-aim 6 or 7, in which the said comparison means includes means for logarithmic amplification of the signals received in both the orthogonal polarisation planes.

9. A radar according to any one of Claims 6 to 8, in which the arrangement is such that the said comparison means is capable of operating on the energies picked up in both the directions of polarisation, pulse by pulse, and the classification means includes a selection threshold device for producing, pulse by pulse, an indication which is a function of the output level of the said comparison means, and for detecting whether or not the number of favourable indications for each train of successive pulses is greater than a predetermined threshold.

10. A radar according to any one of claims 6 to 9, in which the arrangement is such that the said classification means are capable of distinguishing those signals whose degree of depolarisation is within a predetermined range of values.

11 A method as claimed in Claim 1, substantially as described herein.