BACKGROUND OF THE INVENTION 0001 The invention pertains methods of fast exploration, by a surveillance radar antenna, on 360 in azimuth. The invention relates furthermore to surveillance radars implementing a method of fast exploration on 360 in azimuth. 0002 The invention can be applied especially to sea patrol radars or helicopter detection radars. 0003 Certain missions for 360 surveillance require fast exploration in azimuth in order to obtain several measurements on the target during the detection time. This requirement can be explained especially for the following reasons: 0004 firstly, the target may have motions of rotation on itself. In particular, if the target consists of the blades of a helicopter, a submarine periscope or a ship's mast. During rotation, the target presents a variable surface to a radar. This surface is commonly designated by the acronym SER (or service equivalent radar). Now to obtain radar coverage on 360, the approach used consists in rotating the beam of the radar antenna. Given the type of target, the faster the rotation of the radar antenna beam, the greater is the probability that the beam of the radar antenna will be directed towards the target when it has a big SER; 0005 secondly, the target may be buried in clutter with a correlation time smaller than that of the target, for example when the clutter is caused by the sea. The target can be separated from the clutter by carrying out several measurements during the detection time. 0006 The approach used to carry out fast exploration with a radar antenna on 360 in azimuth consists in making the radar antenna beam rotate at high speed. To make the radar antenna beam rotate at high speed, the prior art techniques have undergone development from techniques based on automatic control systems to electronic scanning techniques. 0007 Automatic control techniques achieve the mechanical rotation of the radar antenna by means of automatic control systems. Developments in the field of automatic control systems are used to obtain high rotation speeds. Sea surveillance radar, which is the subject of an article by J. Croney and A. Woroncow, Radar Polarization Comparisons In Sea-Clutter Suppression By Decorrelation And Constant False Alarm Rate Receivers in The Radio and Electronic Engineer, Vol. 38, No. 4, October 1969, provides a rotational speed performance of this kind. The radar in question has two back-to-back mounted antennas with the same angular aperture. The antennas have cross polarizations that are vertical in one case and horizontal in the other. The radar power is switched over at each half-revolution, from one antenna to the other, in such a way that the p.p.i. image is divided into two 180 parts. The image of the first part is repeated on the second part with an orthogonal polarization in order to choose the polarization, and therefore the antenna, that is most suitable as a function of the sea clutter. The antennas are protected by an aerodynamically-shaped radome. The rotation speed of the antenna may reach 600 rpm. 0008 Speed performance characteristics of this kind call for powerful and complex automatic control systems. The drawbacks of these systems are that they reduce reliability, are bulky and heavy and generally costly. 0009 The development of radar techniques is leading to techniques that implement electronic scanning. The electronic scanning is used for example in certain advanced technology radars for combat aircraft. However, at present, electronic scanning antennas cannot cover 360. 0010 One of the goals of the invention is to provide for fast exploration on 360 in azimuth with an antenna radar that does not have the drawbacks of the above-described techniques based on automatic control systems.

SUMMARY OF THE INVENTION 0011 To this end, an object of the invention is a method of fast exploration on 360 in azimuth by a radar antenna comprising two elementary electronic scanning antennas, installed back to back, capable of being electronically shifted by a maximum angle ad, the method consisting in: 0012 driving the antenna rotationally at a speed w1, 0013 simultaneously controlling the electronic aiming of each elementary electronic scanning antenna so that the scanning of the radar beam of the antenna is done at a speed w2 on one rotation, with w2 verifying the following relationship: 1

w_{2} = w_{1} \frac{2}{2 (\frac{}{2} -_{d})}

0014 switching over the microwave signal sent, alternatively between the two elementary electronic scanning antennas, at a determined rate, 0015 before each rotation i of the radar beam, initializing the electronic aiming of each elementary electronic scanning antenna at a determined angle i. 0016 The selection-switching rate is a function of the frequency of recurrence of the radar transmissions, the mechanical rotation speed, the aperture of the pencil beam of each antenna and the maximum aim shifting angle d. To obtain an even illumination in azimuth for each elementary antenna, the selection-switching period is smaller than the time of illumination of a target. 0017 A first preferred embodiment is characterized by a selection-switching rate identical to the frequency of the radar transmissions. 0018 A second embodiment is characterized by a selection-switching rate that is variable as a function of the aiming direction. 0019 A third embodiment is characterized by the association of the selection-switching control with polarization control. To each radar transmission, there corresponds a specified polarization. 0020 The method can advantageously be used to make the radar beam rotate at a speed greater than the speed of mechanical rotation of the antenna. 0021 Depending on its embodiments, the method is advantageously used to match the selection-switching rate to the direction of aim and choose the type of polarization at each radar transmission. 0022 In one particular embodiment, the method can be used to obtain a rotation speed of the radar beam that is variable while having a constant mechanical rotation speed. 0023 An object of the invention furthermore is a radar implementing the method. The radar comprises an antenna, an automatic control system, an aim computer and a selection-switching means. 0024 The automatic control system drives the antenna in slow rotation at a speed w1. 0025 The antenna has two back-to-back mounted elementary electronic scanning antennas that can be shifted by a maximum angle d. 0026 The aim computer simultaneously controls the electronic aiming of each elementary electronic scanning antenna so that the scanning of the radar beam of the antenna is done at a speed w2 on a rotation, w2 verifying the relationship: 2

w_{2} = w_{1} \frac{2}{2 (\frac{}{2} -_{d})}

0027 and so that, before each rotation i of the antenna radar beam, the electronic scanning of each elementary electronic scanning antenna is initialized at a determined angle i. 0028 The selection-switching means switch the microwave signal over alternatively between the two elementary electronic scanning antennas. The selection-switching means typically comprise a microwave selection switch driven by a selection switch control. The selection switch control is a function of the frequency of recurrence of the radar transmissions, the mechanical rotation speed and the aperture of the pencil beam of each antenna. A relay switches over the microwave transmission signal to one of elementary electronic scanning antennas. 0029 In a first particular embodiment, the antenna has two reflector arrays. 0030 In a second particular embodiment, the antenna has two active antennas. 0031 The radar has the advantage of not requiring any complex and costly automatic control system.

BRIEF DESCRIPTION OF THE DRAWINGS 0032 Other features and advantages shall appear from the following description. The description is made with reference to the appended figures, of which: 0033 FIG. 1 shows a method according to the invention in the form of a flow chart, 0034 FIG. 2 shows the electronic aiming at an instant to, 0035 FIG. 3 shows the electronic aiming at an instant ttoTf/2, 0036 FIG. 4 shows the electronic aiming at an instant ttoTf, 0037 FIG. 5 shows the electronic aiming after initialization, 0038 FIG. 6 shows the offset at the start of successive rotations, 0039 FIG. 7 shows an embodiment of a radar according to the invention.

MORE DETAILED DESCRIPTION 0040 The method according to the invention is shown in FIG. 1 in the form of a flow chart. 0041 The method provides for fast exploration on 360 in azimuth by a radar antenna. The radar antenna has two elementary electronic scanning antennas mounted back to back. Each elementary electronic scanning antenna may be electronically offset by a maximum relative angle d. 0042 The method consists in driving 1 the antenna in slow rotation at a speed w1. The speed w1 is typically constant. In a particular implementation of the method, the speed w1 is equal to 30 rpm. 0043 While controlling the mechanical rotation, the method controls 2 the electronic aiming of each elementary electronic scanning antenna so that the radar beam has an angular speed w2 related to the speed w1 by the following relationship: 3

\begin{matrix} \frac{w_{2}}{w_{1}} = \frac{2}{2 (\frac{}{2} -_{d})} & (1) \end{matrix}

0044 The electronic aiming control of each elementary electronic scanning antenna aims the beam of an elementary electronic scanning antenna at a relative angle that changes between the maximum angles of offset. In other words, the relative angle changes between the angle d and the angle d for the first elementary electronic scanning antenna and between the angle d and the angle d for the second elementary electronic scanning antenna, the angular reference corresponding to the normal to the first elementary electronic scanning antenna. 0045 While controlling the electronic scanning of each elementary electronic scanning antenna, the method switches over 3 the microwave frequency alternatively between the two elementary electronic scanning antennas. The selection-switching rate is a function of the frequency of recurrence of the radar transmissions, the mechanical rotation speed and the aperture of the pencil-beam of each antenna. To obtain even illumination in azimuth for each elementary antenna, the selection switching period is smaller than the illumination time for a target. 0046 A first preferred embodiment is characterized by a selection-switching rate identical to the frequency of the radar transmission. 0047 A second embodiment is characterized by a selection-switching rate that is a multiple of the frequency of recurrence of the radar transmission. 0048 A variant to the method consists of the association, with each selection switching operation, of a choice of antenna polarization. To each radar transmission, there corresponds a specified vertical or horizontal polarization. 0049 The relationship (1) expresses the fact that, when the antenna performs a rotation of 4

2 (\frac{}{2} -_{d}),

0050 the radar beam scans 2 or 360 in azimuth. 0051 After the beam has scanned 360 in azimuth, the method initializes 4 the electronic aiming of each elementary electronic scanning antenna at a determined absolute angle before starting a new exploration. 0052 FIG. 2 illustrates the electronic aiming at an instant to0 taken as the starting point of the times. The antenna 5 is seen from the top. It is aligned with a mechanical reference axis 6 taken as the absolute angular reference axis. The antenna 5 can rotate about a rotation axis 7. The relative angular reference 8 is the normal to one of the elementary electronic scanning antennas of the antenna 5, the first elementary electronic scanning antenna for example. To each elementary electronic scanning antenna, there corresponds a beam. At the instant to0, the first elementary electronic scanning antenna has a beam 9 initially offset by the relative angle d, the second elementary electronic scanning antenna has a beam 10 initially offset by the relative angle d. 0053 FIG. 3 illustrates the electronic aiming at an instant ttoTf/2. Tf is the time needed for the beam of the antenna to scan , namely 180 in azimuth. Between to and toTf/2, the beam 9, 10 of each elementary electronic scanning antenna performs a scan corresponding to half of the scanning done by the beam of the antenna 5, namely /2. Simultaneously with the electronic aiming, the antenna 5 rotates by an angle equal to /2d. The ratio between the scanning angle of the beam and the rotation angle of the antenna is equal to: 5

\frac{}{(\frac{}{2} -_{d})},

0054 corresponding to the ratio between the speeds w2 and w1 given by the relationship (1). 0055 FIG. 4 illustrates the electronic aiming at an instant ttoTf. 0056 Between the instants toTf/2 and toTf, the beam 9, 10 of each elementary electronic scanning antenna scans a zone equivalent to the one scanned between the instants to and toTf/2, giving a zone with an angle /2. 0057 Between the instants to and toTf, the beam 9 of the first elementary electronic scanning antenna scans a zone ranging between the absolute angles /2d and 3/2d and the beam 10 of the second elementary electronic scanning antenna scans a zone ranging between the absolute angles 3/2d and 5/2d. Simultaneously with the electronic aiming, the antenna 5 rotates about its rotational axis 7 by an angle equal to 2(/2d). During the period Tf, the radar beam of the antenna scans through 2. Indeed, the beam of each elementary electronic scanning antenna scans . At the instant toTf, the radar antenna explores 360 in azimuth. 0058 At the instant toTf, the radar antenna is offset by an absolute angle 2(/2d). 0059 Before the next scan rotation is done, the method consists in initializing the electronic aiming of each elementary electronic scanning antenna by a determined absolute angle . The value of the angle is determined from the number of rotations already performed, the relationship (1) and the fact that, before making a rotation, the beam of each elementary electronic scanning antenna is offset by the maximum offset value. 0060 FIG. 5 illustrates the electronic aiming of each elementary electronic scanning antenna after the initialization performed at the end of the first rotation. 0061 The beam 9 of the first elementary electronic scanning antenna is offset by a relative angle equal to d. The beam 10 of the second elementary electronic scanning antenna is offset by a relative angle equal to d. 0062 After the initialization phase, the fast exploration on 360 in azimuth is done similarly to the exploration described with reference to FIGS. 3 and 4. 0063 The antenna 5 rotates about its rotational axis 7 at a speed w1. Simultaneously with the rotation of the antenna 5, the electronic aiming control aims the beam of each elementary electronic scanning antenna at an angle that is a function of time. The relative aiming of each beam develops between the maximum offset angles of an elementary electronic scanning antenna. 0064 Between the instants to and toTf, with toTf, the beam 9 of the first elementary electronic scanning antenna aims between the relative angles d and d, namely between the absolute angles 3(/2d) and 5/23d because the antenna 5 rotates by an angle /2d between the instants to and toTf. 0065 Between the instants to and toTf with toTf, the beam 10 of the second elementary electronic scanning antenna aims between the relative angles d and d, namely between the absolute angles 5/23d and 7/23d because the antenna 5 rotates by an angle /2d between the instants to and toTf. Each beam of an elementary electronic scanning antenna scans a zone with an angle between the instants to and toTf. The radar beam of the antenna scans 2. 0066 At the instant toTf, with toTf, the radar antenna is offset by an angle 2(/2d) with reference to its position at the instant to. 0067 The absolute offset of the radar antenna at the instant toTf with toTf is equal to 4(/2d). 0068 Before undertaking the next scan rotation, the method consists in initializing the electronic aiming of each elementary electronic scanning antenna at a determined angle as described here above. 0069 FIG. 6 shows the starting offset of successive rotations. The rotation i is done with a starting point of rotation equal to i. 0070 When i1, the starting offset of the rotation i is equal to 1/2d along the mechanical reference axis 6 taken as the absolute angular reference axis. 0071 The starting offset of the rotation i1 is equal to 23(/2d). 0072 The starting offset of the rotation i2 is equal to 35(/2d). 0073 The starting offset of the rotation j is equal to (j1)2(/2d)(/2d). 0074 The starting offset of the successive rotations can easily be compensated for by means of a buffer memory to store all or part of the data corresponding to a rotation and readjust the radar images. 0075 A first embodiment of a radar for the implementation of the method according to the invention is shown schematically in FIG. 7. 0076 The radar has a plate 11, an antenna 5, a system for the automatic control of the plate 12, a rotating joint 13, a selector switch 14 and an aiming computer 15. 0077 The plate 11 can rotate 360 in azimuth. It is driven rotationally at a speed w1 by means of the automatic control system 12. 0078 The antenna 5 has a first elementary electronic scanning antenna 16 and a second elementary electronic scanning antenna 17. 0079 The first embodiment of the radar is characterized by the particular structure of the first and second elementary electronic scanning antennas 16 and 17. Each elementary antenna corresponds to a reflector array with which an emission source 18, 19 is associated. 0080 A second embodiment of the radar is characterized by an antenna formed by two active antennas. 0081 The antenna 5 furthermore comprises a control circuit 20. 0082 The two reflector arrays are installed back to back. Each reflector array may be offset electronically by a maximum angle d in the clockwise direction and the anticlockwise direction with respect to the normal of each reflector array. Each emission source 18, 19 is installed so as to face a reflector array 16, 17. The selector switch 14 switches the microwave signal over to the first emission source 18 or to the second emission source 19. The control circuit 20 is connected firstly to the first reflector array 16 and secondly to the second reflector array 17. It sequentially controls the diodes of the phase shift cells of the first reflector array 16 and the diodes of the phase shift cells of the second reflector array 17. 0083 The plate 11 bears the antenna 5 which rotates with the plate 11. 0084 The rotating joint 13 transmits different microwave signals (transmission and reception) between the plate 11 and the duplexer (not shown). It furthermore sets up the digital control link between the plate 11 and the aiming computer 15. 0085 The aiming computer 15 controls the electronic aiming of each reflector array 16, 17. The aiming control is such that the scanning of the radar beam is done at a speed w2 on a rotation, with w2 verifying the relationship (1). Before each rotation i of the radar beam, the aiming computer 15 initializes the electronic aiming of each reflector array 16, 17 at a determined angle i as described with reference to FIG. 6. 0086 According to a particular embodiment of the first mode, each reflector array may have a horizontal or vertical polarization, the associated source having the same polarization. 0087 When the two reflector arrays are crossed polarization arrays, the antenna provides for polarizing agility. The first reflector array has a horizontal polarization as does the associated emission source and the second reflector array has a vertical polarization as does the associated emission source. 0088 One variant to the embodiments of the radar comprises a means associated with a selector switch. This means is used to determine the polarization of each elementary electronic scanning antenna. The association with the selector switch determines the polarization of the elementary electronic scanning antenna at each selection switching operation.