GB2316234A - A low frequency fixed antenna pulse search radar system - Google Patents

Abstract

This invention relates to a low frequency search pulse radar system using a fixed antenna. This system features a fixed antenna with 2P planar networks (1 to 4) covering adjacent sectors of the space to be surveyed, and a single emitter, 20, feeding two opposing arrays at each sequence in an interleaved manner through a switch (50) and controlled by a management unit (30) so as to electronically scan each of the sectors associated with the array being fed. This system also features two receivers (7.12, 7.34) respectively connected to the arrays emitting by two switches (6.12, 6.34) and means for beam formation by calculation (8) connected to the two receivers. The invention applies particularly to very low attitude, 360{ aerial search radars.

Description

"A LoW FREQUENCY FIXED ANn1BNNa Mn-L SFAIMI RADAR SYSTEM" This invention relates to a low frequency search pulse radar system using a fixed antenna.

In the context of battlefield surveillance and threat detection, particularly aerial threats, search radar systems must be provided close to the line of contact with the enemy to detect all very low altitude threats such as aircraft" helicopters, missiles, drones etc.. Detection of the threat presented by aircraft flying at very low altitude requires a 360 surveillance capability. In addition, it must be possible to detect the threat presented by helicopters concealed behind vegetation.

A known solution for the detection of this second type of threat consists in using a radar with a relatively low frequency, in V.H.F., for example, covering a limited sector of space.

One of the problems with this type of system is the size of the antenna required. One possible solution is to use array antennas, but this must be limited to an array consisting of a small number of radiating elements if the system is to remain within reasonable limits in terms of cost and dimensions.

One is hence left with solutions using wide-beam antennas which, given their low directivity, are not alone sufficient to provide the angular resolution required.

A first object of this invention is therefore a radar system capable of detecting all very low altitude threats, even those concealed by vegetation, with satisfactory resolution and overall dimensions reasonable enough to make the system mobile.

Another object of the invention is a low frequency, fixed antenna radar system using an electronic scanning process during emission, and digital beam forming during reception.

According to a first aspect of the invention there is provided a low frequency, fixed antenna pulse search radar system, Wherein the fixed antenna consists of 2P plane arrays, each one covering adjacent sectors of the spice to be surveyed, said radar system including emitter means to feed said plane arrays in sequence, controlling means to control said emitter means so as to carry out electronic scanning of each sector by said associated plane array and reception means to receive the radar signals arriving from each direction successively illuminated by said plane arrays and said emitter means, said controlling means controlling said reception means to carry out digital beam forming.

In addition, given the long operating wavelengths and reflections off the ground which, for very low altitude targets, create a phenomenon of light and dark fringes due to interference between the direct and reflected waves, the.

fringes obtained are very widely spaced in tenns of distance, which leaves non-illuminated zones in which detection of a target is almost impossible. To remedv this oroblem. the invention emits and receives in each bearinq direction at several different frequencies in succession.

According to a feature of the invention, it is therefore envisaged that, in a radar system such as that defined above, said emitter means comprise generator means controlled by said controlling means for transmitting several pulse bursts at different frequencies in each beam direction.

Further characteristics and advantages of the invention vill be more clearly explained in the following description and the appended dravings, wherein: - Figure 1 is an example of an array antenna which could be used in a system which eidies the invention; - Figure 2 is an embodiment of a fixed antenna for a radar system in which the invention is enibodied; - Figure 3 illustrates an emitter which could be used in a radar system in Which the inention is elibodied; - Figure 4 avs a partial representation of the radiation diagram of a fixed antenna in which the invention is eWied; - Figure 5 illustrates the operating sequences of the invention for 3600 coverage of space; - Figure 6 is a diagram Yhich explains the operation of the invention during emission, and - Figure 7 is a diagram of a system in Fhidi the invention is embodied.

As has already been mentioned, the case considered is that of a search radar system covering a 360 field in bearing, althrough the invention necessary.

also be applied to the coverage of a more restricted sector of space if/ On the other hand, the following description shall consider a radar system operating in V.H.F., by way of example, for the detection of a target concealed by vegetation.

Figure 1 illustrates an example of a V.H.F. array antenna for the coverage of a limited sector of space. This array consists of a plane panel 10 made up in this case, by way of example, of four identical radiating elements.

11 to 14. Each element consists of two elementary dipoles 15, 16 made up of flat-folded doublets and fed by a line 18, which is itself centrally fed by a coaxial cable at 19. The doublets and line 18 are installed on a double-sided printed circuit disposed in front of a reflective plane (not shown in the diagram). Power coupling with line 18 is at the level of the notches such as 17 shown in Figure 1.

When disposed vertically as shown in the figure, such an antenna provides a wide beam because of the limited number of elements, but retains reasonable overall dimensions of the order of a few operating wavelengths.

The beam has an angular width of the order of 30 in bearing and 40 in elevation.

Figure 2 shows a perspective view of a possible embodiment of a fixed antenna for a radar system in which the invention is embodied to cover a 3600 field in bearing. In this case it has been chosen to use four panels 1, 2, 3, 4, each one identical to the panel shown in Figure 1.

These four panels are arranged in a square, each panel covering a 90 field.

More generally, the system could use any mumber of panels arranged in the form of a regular polygon. It should he preferable to use an even mrmber 2P of panels or plane arrays as will be seen bellow. Eadi panel 360 should hence cover a angle sector of 2P. It is, of course,possible to cover a field angle of less than 3600 with the 2P arrays.

To illuminate a sector of space cospondiz to a 90 angle with a panel such as that illustrated in Figure 1, electronic scanning with at least three different antenna aimings is used as the antenna beam has an angular width of 30".

This is obtained by feeding the elements of an array with a conventional emitter such as that shown in Figure 3. Elements 11 to 14 are linked by circulators 201 to 204 to the emitter of Figure 3 on the one hand, and to a receiver (not shown in the diagram) on the other. The emitter features a low level wave generator 240 which determines the emission frequency, followed by a low power stage 230 which divides the power supplied between four phase shifters 221 to 224 which are controlled by a controlling unit 5. The phase-shifted radio-frequency energy is then amplified in power stages 201 to.

204 connected to the circulators. This arrangement allows the phase shifters to operate at low level, which means that their construction is simplified because of the consequent reduction in power performance requirements.

The direction of the beam emitted by antenna panel 10 is determined by the value of the phase-shifts, which are controlled by controlling unit 5.

Figure 4 shows part of the radiation diagram of the antenna of Figure 2, essentially limited to the coverage of the 90" sector xOx' by array 1. In the example used, the electronic scanning enables the beam to be aimed in three different directions. The three beams thus formed are represented by curves D1-1, D2-1 and D3-1 respectively. Beyond Ox', the first beam D1-2 provided by array 2 has also been shown.

It should be noted first of all that it is not practically possible to extend the coverage of a single array such as that illustrated in Figure 1 beyond 90 .

For emission angles greater than 45" with respect to the perpendicular to the array, antenna gain falls rapidly due to the reduction of the effective area of this antenna.

Additionally, a number N of emitted beams greater than three can be used. For example, using four beams for each plane array 1, 2, 3, 4 enables a slight reduction of the variation of antenna gain according to exploration angle. This, however, has the disadvantage of leading to a loss of processing gain, as a given target is observed for a shorter time, and this loss is greater than antenna gain increase in those directions where antenna gain is at its weakest.

The aiming of the different beams can also be determined so as to balance out most efficiently antenna gains in the weakest directions.

During reception, the antenna beam is aimed in the direction of the emitted beam using a processing technique of the digital beam forming type.

According to one characteristic of the invention, a single radar emitter is used to feed all the plane arrays in succession. To achieve this, the radar emitter will be used to feed two opposing plane arrays at each sequence.

Thus, Figure 5 shows the succession of the six sequences T1 to T6 used to feed each of the four panels 1 to 4 in the three successive beam directions.

Hence, during sequence T1 the emitter generates signals with phase-shifts such that each of the two panels 1 and 3 generates a beam (F1.2 shown as a solid line, and F3.4 shown as a dotted line) in the first direction. During sequence T2, the same process is carried out for the second direction. With sequence T4 feeding of panels 2 and 4 begins. At the end of sequence T6, all directions over a 3600 angle have hence been illuminated by electronic scanning and switching.

Clearly, since the emitter feeds two arrays for each sequence, it must be connected to these two arrays through a power divider with two outputs (or, to be more precise, four dividers, one for each pair of opposing elements).

However, one advantageous solution consists in interleaving the emissions of the two opposing arrays. This enables the emitter to be used to feed either array of a pair at any given moment by switching the emitter to the opposing array between the application of two successive pulses to a given array1 as shown in the diagram in Figure 6. The pulses with diagonal shading in this diagram correspond to those pulses feeding array 1, the repetition rate of the radar being TR. Between two successive emissions to array 1, the emitter is switched to array 3 and emits one pulse (shown in dotted shading in the figure). This embodiment has two advantages. Firstly, the system no longer requires power dividers (switches are used instead). Secondly, this solution means that the peak power of the emitter does not have to be doubled, because it is only feeding one array at any given moment and not two. On the other hand, the mean power supplied by the emitter is doubled.

As far as reception is concerned, however, two receivers must be used since the reception periods for two opposing receivers overlap in time.

As has already been mentioned in the introduction, surveillance at very low altitude leads to the existence of very widely spaced light and dark fringes which hence leave non-illuminated zones. The position of the fringes depends on the wavelength used.

According to another aspect of the invention those zones which are left unilluminated at one frequency are approximately covered by emitting at several other frequencies in such a way that the corresponding fringes are suitably interleaved so as not to leave Ndark zones1 of any significant size.

To achieve this, several bursts of pulses are emitted at different frequencies, for example, F1, F2, F3.

We might consider the particular case of emitting three successive bursts at Fl1 F2, F3 during each sequence T1 to T6.

This produces the diagram of the radar system ody the invention shown in Figure 7. The four plane arrays 1 to 4 are not shown, and nor are the circulators. This system consists for emission of an emitter 20 such as that shown in Figure 3 with elements 211 to 214, 221 to 224 and 230.

The wave generator is replaced by a synthesizer/driven channel assembly 40 which supplies the low level signal at the required emission frequency at the desired moment. The four outputs of the emitter are connected to a switch 50 which switches these four outputs to one of the four plane arrays 1 to 4.

During reception, two switching devices 6.12 and 6.34 respectively connect the signals of two opposing arrays (1 and 3 or 2 and 4) to two receivers 7.12 and 7.34. The received signals obtained corresponding to the four elements for each array and the associated receiver are sent to a device for digital beam forming 8 which can be a dual device, or even a single device if it is shared in time between the two receivers. The target signals corresponding to the various directions in space are then sent to a utilization and display unit 9 of any known type.

A controlling unit 30 manages the various blocks of the radar system.

Controlling unit 30 first controls driven channel 40 to determine the emission frequency of the pulses and overall synchronization.

Unit 30 controls emitter 20 and in particular the phase-shifters to determine the direction of the beam to be emitted at any given moment.

The controlling unit also controls switch 50 to select altemately the pairs of opposing plane arrays according to the sequence in progress.

In addition, the controlling unit also controls the synchronization of switches 6.12 and 6.34 during reception to connect the emitting arrays to receivers 7.12 and 7.34. Finally, unit 30 also controls the choice of coefficients used by device 8, which correspond to the direction being scanned at any given moment. particularly, The characteristics of a system in w h the imYtion is eCodied,/ the use of a single emitter and just two receivers with a fixed antenna, mean that 360" coverage can be obtained with a radar system which has sufficiently compact dimensions to be used as a forward area radar, particularly mounted on a vehicle.

The example embodiments described above are, of course, in no way limiting with respect to the invention.

Claims (8)

1. A low frequency, fixed antenna pulse search radar system, wherein the fixed antenna consists of 2P plane arrays each one covering adjacent sectors of the space to be surveyed, said radar system including emitter means to feed said plane arrays in sequence, controlling means to control said emitter means so as to carry out electronic scanning of each sector by said associated plane array and reception means to receive the radar signals arriving from each direction successively illuminated by said plane arrays and said emitter means, said controlling means controlling said reception means to carry out digital beam forming.

2. A radar system as claimed in claim 1, wherein said controlling means control said emitter means in such a way that each array emits N successive adjacent beams covering the associated sector and control said reception means in such a way that they form N corresponding beams for each of said sectors.

3. A radar system as claimed in claim 2, wherein said emitter means comprise generator means controlled by said controlling means for transmitting several pulse bursts at different frequencies in each beam direction.

4. A radar system as claimed in any of claims 1 to 3, wherein said emitter means comprise a single emitter and switching means such that, for each sequence corresponding to the emission of a beam, said emitter feeds arrays p and P + p, where p varies from 1 to P.

5. A radar system as claimed in claim 4, wherein said controlling means control said switching means and said emitter in such a way as to feed said array p and said array P + p in interleaved fashion during associated sequences

6. A radar system as claimed in claim 5, wherein said reception means comprise two receivers second switching means to connect said receivers respectively to arrays p and P + p during emission and calculation means to carry out beam forming for each array in question in the direction in which this array is emitting.

7. A radar system as claimed in any of claims 1 to 6, comprising a fixed antenna covering 360" in bearing and consisting of four plane arrays each one consisting of a panel of four radiating elements said panels being disposed on the vertical faces of a cube so as to cover 90".

each, and wherein said emitter means are controlled such that three separate directions equally spaced over the associated 90 sector are electronically scanned by each array.

8. A low frequency, fixed antenna pulse search radar system substantially as described hereinbefore, with reference to accompanying drawings and as illustrated in Figure 1 or Figures 1 and 2, or Figure 3 or Figure 7 of those drawings.

8. A radar system as claimed in claim 7, wherein each radiating element consists of two flat-folded doublets forming a vertical subarray and fed in parallel.

9. A law frequency, fixed antenna pulse search radar system substantially as described hereinbefore, vith reference to the accaw anying drawings and as illustrated in Figure 1, or Figures 1 and 2, or Figure 3 or Figure 7 of those drawing.

Amendments to the claims have been filed as follows 1. A low frequency, fixed antenna pulse search radar system in which the fixed antenna consists of 2P plane arrays, each one covering adjacent sectors of the space to be surveyed, said radar system including emitter means to feedsaid plane arrays in sequence, controlling means to control said emitter means so as to carry out electronic scanning of each sector by said associated plane array and reception means to receive the radar signals arriving from each direction successively illuminated by said plane arrays and said emitter means, said controlling means controlling said emitter means in such a way that each array emits N successive adjacent beams covering the associated sector, wherein said controlling means control said reception means to carry out digital beam forming in such a way that they form N corresponding beams for each of said sectors.

2. A radar system as claimed in claim 1, wherein said emitter means comprise a single emitter and switching means such that, for each sequence corresponding to the emission of a beam, said emitter feeds arrays p and P + p, where p varies from 1 to P.

3. A radar system as claimed in claim 2, wherein said controlling means control said switching means and said emitter in such a way as to feed said array p and said array P + p in interleaved fashion during associated sequences.

4. A radar system as claimed in claim 3, wherein said reception means comprise two receivers, second switching means to connect said receivers respectively to arrays p and P + p during emission and calculation means to carry out beam forming for each array in question in the direction in which this array is emitting.

5. A radar system as claimed in any one of claims 1 to 4, wherein said emitter means comprise generator means controlled by said controlling means for transmitting several pulse bursts at different frequencies in each beam direction.

6. A radar system as claimed in any of claims 1 to 5, comprising a fixed antenna coverning 3600 in bearing and consisting of four plane arrays,each one consisting of a panel of four radiating elements,said panels being disposed on the vertical faces of a cube so as to cover 900 each, and wherein said emitter means are controlled such that three separate directions equally spaced over the associated 90 sector are electronically scanned by each array.

7. A radar system as claimed in claim 6, wherein each radiating element consists of two flat-folded doublets forming a vertical sub-array and fed in parallel.