FR2751420A1 - RADAR FIXED ANTENNA PULSE MONITORING SYSTEM - Google Patents

Abstract

A fixed antenna low frequency pulse surveillance radar system is disclosed. This system comprises a fixed antenna with 2P plane networks (1 to 4) covering adjacent sectors of the space to be monitored, and a single transmitter 20, supplying each sequence to two opposite networks, in an interlaced manner via a switch (50), and controlled by a management unit (30) to electronically scan each of the sectors associated with the supplied network. This system further comprises two receivers (7.12, 7.34) connected respectively to the networks being transmitted by two switches (6.12, 6.34) and a device for forming beams by calculation (8) connected to the two receivers. The invention applies in particular to very low altitude 360 deg aerial surveillance radars. in deposit.

Description

RADAR MONITORING SYSTEM

FIXED ANTENNA PULSES

  The present invention relates to a radar radar surveillance system with low frequency transmission and fixed antenna. In the context of battlefield surveillance and the detection of threats, including airborne, it is necessary to provide radar surveillance systems located near the line of contact with the enemy to detect all threats at very low o10 altitude, such as airplanes, helicopters, missiles, drones etc ... The detection of threats constituted by aircraft flying at very low altitude implies being able to exercise surveillance 360 in the field. In addition, it is necessary to be able to detect the threat constituted by helicopters

  masked behind vegetation.

  A known solution for detecting this second type of threat is to use a radar with a relatively

  low, for example in V.H.F., covering a limited area of space.

  One of the problems then is the size of the antennas to use.

  The use of network antennas is a possible solution but it is limited to a network consisting of a small number of radiating sources if we want to remain within reasonable limits of cost and space. This results in broad beam antennas which, given their low directivity, do not in themselves give a sufficient angular resolution. A first object of the invention is therefore a radar system capable of detecting all threats at very low altitude, even hidden by vegetation, with a satisfactory resolution and a limited space requirement.

allowing his mobility.

  Another object of the invention is a low frequency radar transmission system and fixed array antenna using an electronic scanning on transmission and beamforming by calculation on reception. According to a first aspect of the invention, there is therefore provided a radar surveillance system with a low frequency transmission pulse and a fixed antenna, characterized in that the fixed antenna comprises 2P flat gratings respectively covering adjacent sectors of the antenna. space to be monitored, in that said radar system comprises transmitting means for supplying said planar arrays sequentially, management means for controlling said transmitter means so as to perform an electronic scanning of each sector by said associated plane network and receiving means for performing the reception of the radar signals from the successive directions illuminated by said planar networks and said transmitter means, said management means controlling said

  receiving means for effecting beamforming by calculation.

  Moreover, given the long operating wavelengths and the reflections on the ground which, for very low altitude targets, create a phenomenon of light and dark fringes due to the interference between the direct and reflected waves, the The fringes obtained have very large differences in distance, which leaves non-illuminated valleys where target detection is almost impossible. To remedy this drawback, the invention provides for transmitting and receiving in each direction in bearing successively at several different transmission frequencies. According to another aspect of the invention, it is therefore provided, in a radar system as defined above, that said transmitting means comprise generating means controlled by said management means for emitting in each beam direction several bursts

  pulse of different transmission frequency.

  The invention will be better understood and other characteristics and

  The advantages will be apparent from the following description and drawings

  O-joints: - Figure 1 is an example of a network antenna used in a system according to the invention; FIG. 2 is an embodiment of a fixed antenna for the radar system according to the invention; FIG. 3 illustrates a transmitter diagram that can be used in the radar system according to the invention; FIG. 4 partially shows a fixed antenna radiation pattern of the system according to the invention; FIG. 5 illustrates the operating sequences of the system for 360 space coverage; FIG. 6 is an explanatory diagram of the operation at emission and

  FIG. 7 is a diagram of the radar system according to the invention.

  As already mentioned, we will consider the case of a radar surveillance system covering 360 in the field although the invention can also be applied to the coverage of a sector of space

more reduced if necessary.

  On the other hand, we consider a radar system operating in V.H.F as an example to allow the detection of a masked target by

o vegetation.

  Figure 1 illustrates an exemplary V.H.F.

  to cover a limited area of space. This network is composed of a flat panel 10 formed here, by way of example, of four identical radiating sources 11 to 14. Each source is formed of two elementary dipoles 15, 16 constituted by doublets folded into a plate fed by a line 18, itself fed centrally at 19 by a coaxial cable. The doublets and the line 18 are made on a double-sided printed circuit arranged in front of a reflective plane (not shown). The energy coupling with the line 18 takes place at the slots 17 on the

figure 1.

  Such an antenna, arranged vertically as in the figure, provides a wide beam because of the small number of sources but retains reasonable overall dimensions of the order of a few wavelengths of operation. The beam has a width of the order of 30 in

deposit and 40 in site.

  FIG. 2 represents, in perspective, a possible embodiment of a fixed antenna for a radar system according to the invention, making it possible to cover 360 in the field. Here, we chose to use four panels 1, 2, 3, 4 each identical to the panel of Figure 1. These four panels are

  arranged in square, each panel to cover 90 in the field.

  More generally, one could choose any number of panels arranged in a regular polygon. Preferably according to the invention, an even number 2P of panels or planar gratings is chosen as will be seen later. Each network must then cover a sector in 2P deposit Of course, if necessary, one can 2P just to cover a angle in field of deposit inferior to 360 with the 2P networks. In order to illuminate a sector of the 90 "space in a field using a panel such as that of FIG. 1, an electronic scanning is used with at least three different scores since the antenna beam has a

width of 30.

  This is obtained by supplying the sources of a network by a conventional transmitter such as that of FIG. 3. The sources 11 to 14 are connected through circulators 201 to 204 to the transmitter of FIG. on the other hand to a receiver (not shown). The transmitter comprises a low-level wave generator 240 determining the transmission frequency, followed by a low power stage 230 dividing in four the power supplied to four phase-shifters 221 to 224 controlled by a management unit 5. L Phase-shifted radiofrequency energy is then amplified in power stages 201 to 204 connected to the circulators. This arrangement allows the phase-shifters to work at a low level, which simplifies their

  achievement due to lower power performance requirements.

  The direction of the beam emitted by the antenna panel 10 is determined by the choice of phase shifts controlled by the unit of

management 5.

  FIG. 4 shows a partial diagram of the radiation of the antenna of FIG. 2, essentially limiting itself to the coverage of the sector of 90 × 0 × 'by the network 1. We have chosen the case where the scanning electronics makes it possible to point the beam in three different directions. The three beams formed correspond to curves D1-1, D2-1 and D3-1 respectively. Beyond Ox ', the first

  D1-2 beam provided by the network 2.

  It should first be noted that it is hardly possible to extend

  beyond 90 the coverage of a network such as that of Figure 1. Indeed, for emission angles, compared to the normal to the network, greater than 45, there is a rapid drop in antenna gain related on the surface

  effective decreasing of this antenna.

  On the other hand, one can choose a number N of emitted beams larger than three. For example, the use of four beams per plane grating 1, 2, 3, 4 makes it possible to very slightly reduce the variations of the antenna gain as a function of the angle of exploration. But this has the disadvantage of leading to a loss of the processing gain because a given target is observed for a shorter time and this loss is greater than the increase in antenna gain.

  in the directions where it is weakest.

  We can also determine the score of the different beams to balance the best antenna gains in the disadvantaged directions. At the reception, the antenna beam is pointed in the direction of the emitted beam by means of a processing technique of

ol0 beam by calculation.

  According to one characteristic of the invention, a single radar transmitter is used to supply successively all the planar networks. For this, the radar transmitter will be used to feed each sequence two

opposed planar networks.

  Thus, FIG. 5 represents the succession of the six sequences T1 to T6 making it possible to supply each of the four panels 1 to 4 according to the three

  successive beam directions.

  Thus during the sequence T1 the transmitter provides signals with phase shifts such that each of the two panels 1 and 3 provides a

  beam (F1.2, solid line, F3.4 dashed) in the first direction.

  In sequence T2, we move on to the next direction. From the sequence T4, we switch to the power supply of the panels 2 and 4. At the end of T6 we have illuminated by electronic scanning and switching all the

360 directions.

  It is clear that, since at each sequence, the transmitter feeds two networks, it must be connected to these two networks via a power divider with two outputs (or rather four dividers, one

  for each pair of opposing sources).

  However, an advantageous solution is to interleave the emissions of two opposite networks. This makes it possible to use the transmitter to supply a single network at any moment by switching the transmitter on the opposite network between two pulses applied to a network, according to the diagram of FIG. 6. In this diagram, the hatched pulses correspond to the pulses. supplying the network 1, the radar repetition period being TR. Between two successive transmissions towards the network 1, the transmitter is switched on the network 3 to emit an impulse

  (with dotted lines in the figure). This embodiment has two advantages.

  On the one hand, this makes it possible to dispense with power dividers (on the other hand, switches are used). On the other hand, it allows not to double the peak power of the transmitter, since at every moment it feeds only one network and not two. On the other hand, we double the average power

must provide the transmitter.

  With regard to reception, however, two receivers must be used since the reception periods for two opposite networks are

I overlap in time.

  As already mentioned in the introduction, monitoring at very low altitude leads to the existence of light and dark fringes strongly spaced in distance thus leaving non-existent hollows

  illuminated. The position of the fringes depends on the wavelength used.

  According to another aspect of the invention, it is approximately

  non-illuminated hollows at a first frequency by emitting at several other frequencies such that the corresponding fringes are properly intertwined so as not to leave

shade "important.

  For this, several bursts of pulses are emitted at

  different frequencies, for example F1, F2, F3.

  One can think to emit three successive bursts to

  F1, F2, F3 during each T1 to T6 sequence.

  Thus, we arrive at the radar system diagram according to the invention of Figure 7. The four planar networks 1 to 4 have not been shown nor the associated circulators. This system includes, on transmission, a transmitter 20 such as that of FIG. 3 with the elements 211 to 214, 221 to 224 and 230. The wave generator is replaced by a synthesizer / driven chain assembly which provides at the desired moment the low level signal at the desired transmit frequency. The four outputs of the transmitter are connected to a switch 50 which switches these four outputs to one of the planar networks

1 to 4.

  On reception, two switches 6.12 and 6.34 respectively connect the signals of two opposite networks (1 and 3 or 2 and 4) to two receivers 7.12 and 7.34. The received signals obtained corresponding to the four sources for each network and associated receiver are sent to a beam forming device by the calculation 8 which can be double or even be unique if it is used in time sharing for the two receivers. The target signals corresponding to the various directions of the space are then sent to a user and display unit 9

of any known type.

  A management unit 30 manages the different blocks of the radar system. The management unit 30 first controls the driven chain 40 to determine the pulse transmission frequency and the overall timing. Unit 30 controls transmitter 20 and in particular phase shifters

  to determine at each instant the beam direction to be emitted.

  The management unit also controls the switch 50 to alternately select two opposed planar gratings according to the

sequence running.

  On the other hand, the management unit synchronously controls the switches on reception 6.12 and 6.34 to connect the networks being transmitted to the receivers 7.12 and 7.34. Finally the unit 30 also controls the choice of coefficients for the device 8 which correspond to the direction

swept at every moment.

  Thanks to these features of embodiment according to the invention, in particular the use of a single transmitter and only two receivers with a fixed antenna, a coverage of 360 is obtained with a radar system that is sufficiently compact and compact to be used as

  radar from the front, especially on the vehicle.

  Of course, the embodiments described are not

  in no way limitative of the invention.

Claims (8)

  A radar surveillance system with low frequency transmit pulses and fixed antenna, characterized in that the fixed antenna comprises 2P plane gratings (1, 2, 3, 4) respectively covering adjacent sectors of the space at monitoring, in that said radar system comprises transmitter means (201, 212-214; 221 ... 224, 230, 240; 20,, 50) for supplying said planar networks sequentially, management means (5; 30); for controlling said transmitting means so as to perform an electronic scanning of each sector by said associated plane network and receiving means (6.12, 6.34, 7.12, 7.4, 8, 9) for receiving the radar signals from the directions successive illuminated by said planar networks and said transmitting means, said management means (30) controlling said means   receivers for performing beamforming by calculation.   2. Radar system according to claim 1, characterized in that said management means (30) are provided for controlling said transmitting means (20, 40, 50) so that each plane network emits N successive adjacent beams covering the associated sector and for controlling said receiving means (6.12, 6.34, 7.12, 7.34, 8, 9) of   to form N corresponding beams for each of said sectors.   3. Radar system according to claim 2, characterized in that said transmitting means comprise generating means (40) controlled by said management means (30) for transmitting in each beam direction several transmission frequency pulse bursts (F1, F2, F3) different.   Radar system according to one of claims 1 to   3, characterized in that said transmitter means comprise a single transmitter (20) and switching means (50) so that, at each sequence (Tn) corresponding to the emission of a beam, said transmitter supplies the p and P networks + p, with p varying from 1 to P. 5. Radar system according to claim 4, characterized in that said management means (30) are provided for controlling said switching means (50) and said transmitter (20) so as to feed said network p and said network P in an interlaced manner. + p during the associated sequences (Tn). 6. Radar system according to claim 5, characterized in that said receiving means comprise two receivers (7.12, 7.34), switching means (6.12, 6.34) for respectively connecting said receivers to the p and P + p networks which are in progress. transmission and computing means (8) for carrying out for each network in question the formation of a   beam in the direction in which it emits.   Radar system according to one of Claims 1 to 6,   characterized in that it comprises a fixed antenna covering 360 "in the field and formed of four planar arrays (1, 2, 3, 4) each comprising a panel of four radiating sources (11 to 14), said panels being arranged according to the vertical faces of a cube to cover each 90 "in the field and in that said emitting means are controlled so as to scan electronically by each network three directions   Separately distributed in the associated 90 sector.   8. Radar system according to claim 7, characterized in that each radiating source (11 to 14) comprises two doublets folded in   plate (15, 16) forming a vertical sub-array and supplied in parallel.