[1] [The present invention relates to a method and system for neutralizing the effect of vibrations in a rotary-wing aircraft for airborne Doppler radar. [2] The technical field of the invention is that of Doppler processing and the field of application is mainly that of helicopter radars. [3] The effect of the movements of the platform aircraft on the operation of an airborne radar is a well-known problem. In particular, these movements have a considerable effect in the field of coherent radar processing, for example the extraction of the Doppler velocity. [4] In the case of a rotary-wing aircraft, the movements of the rotors create significant vibrations in the overall structure of the platform. The vibration modes of the platform have an effect on the coherent processing that is carried out. The typical frequency range of the vibrations is from several hertz to several tens of hertz. The Doppler processing requires an integration based on the phase coherence of the signal, which increases in length as the desired velocity resolution becomes finer. The movements of the radar antenna caused by the vibrations of the platform create parasitic effects in this processing, which increase with the length of the integration time. [5] The precise determination of the ground velocity of the platform, and the detection of slow targets which are difficult to separate from ground or sea return signals, are applications that are particularly affected. [6] Compensation for the movements of the platform is particularly important in the technique of synthetic aperture radar (more widely known by the abbreviation SAR, for "Synthetic Aperture Radar" in English). In order to reconstruct correctly the images from different viewing angles, the movements of the platform must be precisely known. These measurements are usually obtained from an inertial navigation unit on the platform, and can be used to model the movement of the platform for the purpose of coherent processing. Imperfections in the correction of the platform movement can be corrected subsequently by algorithmic processing of the images obtained. The best-known method is a PGA ("Phase Gradient Autofocus" in English) algorithm, such as that described in US patent application 4924229 A, entitled "Phase correction system for automatic focusing of synthetic aperture radar", which can be used to refocus the image on the assumption of a static scene. [7] For Doppler processing, the platform movement compensation is also usually carried out by means of an inertial navigation unit. This requires a unit operating at a sufficient frequency to sample the vibratory movements correctly, while having adequate sensitivity. [8] The measurements of the platform's unit must also enable the movements of the phase centre of the radar antennas to be deduced. This depends on the areas where the radar antennas are installed and on the mechanical coupling between the inertial navigation unit and the radar antennas, and this is a drawback of this method. [9] A first technical problem resolved by the invention is that of proposing a method for neutralizing the effect of the vibrations of a rotary-wing aircraft for an airborne Doppler radar which avoids the need to use an inertial navigation unit for its implementation and avoids constraining the choice of the areas of installation of the radar antenna(s). [10] A second technical problem resolved by the invention is that of proposing a method for neutralizing the effect of the vibrations of a rotary-wing aircraft for an airborne Doppler radar in which the control of the coupling between the vibration sensor(s) and the radar antenna(s) is improved. [11 ] For this purpose, the invention proposes a method of active neutralization of the effect of the vibrations of a rotary-wing aircraft for a monostatic Doppler radar or a bistatic radar, the monostatic Doppler radar comprising: .- a transmitting-receiving radar antenna sharing the same phase centre 0, and .- a radar transmission chain, connected to a transmission input of the transmitting-receiving antenna of the monostatic Doppler radar; and .- a radar reception chain, connected to a receiving output of the transmitting-receiving antenna of the monostatic Doppler radar; or, the bistatic Doppler radar comprising: .- a first transmitting radar antenna having a transmission phase centre 01 and a second receiving radar antenna, remote from the first transmitting antenna, having a reception phase centre 02; and .- a radar transmission chain, connected to a transmission input of the first transmitting radar antenna of the bistatic Doppler radar; and .- a radar reception chain, connected to a reception output of the second receiving radar antenna of the bistatic radar. [12] The active neutralization method is implemented by an active neutralization system comprising: .- a three-dimensional vibration sensor for each radar antenna, fixed to said antenna, and near its phase centre 0, 01, 02, or a single three-dimensional vibration sensor, shared by the first transmitting radar antenna and the second receiving radar antenna and near a phase centre 01, 02, while the transmitting and receiving antennas are close to one another and strongly coupled mechanically; and .- a device for estimating the movement of each vibration sensor and for neutralizing their movement, [13] The active neutralization method is characterized in that it comprises: .- a first step of measuring and temporally extrapolating the vibration modes at the transmitting-receiving antenna or at the first transmitting antenna and the second receiving antenna near their associated phase centre(s) 0, 01 and 02; then .- a second step of estimating the expected movements of the transmitting-receiving antenna or of the first transmitting antenna and the second receiving antenna; then .- a third step of compensating the expected movements of the radar antenna(s) in the transmission chain or in the receiving chain. [14] According to particular embodiments, the method of active neutralization of the effect of the vibrations of a rotary-wing aircraft for an airborne Doppler radar comprises one or more of the following characteristics, considered separately or in combination: [15] .- the third step of compensating for the expected movements of the phase centre(s) 0, 01, 02 comprises a fourth step of calculating at least one projection of the movements in an aiming direction of the transmitting-receiving antenna or of the second receiving antenna, and a fifth step of determining at least one compensation phase shift corresponding to an aiming direction; [16] .- for each measurement sensor, the first step of measuring and temporally extrapolating the vibration modes of the vibration sensor comprises, executed successively, a sub-step of bandpass filtering of measured triaxial acceleration signals, a sub-step of complex frequency analysis of the vibration modes, and a sub-step of interpolation of the acceleration profiles by image band filtering, and temporal extrapolation to immediately subsequent instants; and the second step of estimating the expected movements comprises, executed successively, a sub-step of integration of accelerations to velocities, and a sub-step of integration of velocities to expected movements; [17] .- the active neutralization method defined above further comprises a step of learning for each boresight axis, executed on the ground or in flight on known fixed beacons, and configured to provide a model for refining the corrections to be applied in the radar transmission chain or in the radar reception chain, which allows for the variations of the vibrations correlated with the engine speed and the velocity of the rotary-wing aircraft; [18] .- the Doppler radar is a monostatic radar comprising a transmitting-receiving radar antenna with the same phase centre 0; and the active neutralization system comprises a three-dimensional vibration sensor, fixed to the transmitting-receiving radar antenna and near its phase centre; and the vibration modes of the radar antenna are measured by the vibration sensor in the first step of measurement and temporal extrapolation; and the expected movements of the phase centre 0 of the transmitting-receiving radar antenna are estimated in the second step; and the compensation for the expected movements of the phase centre 0 of the transmitting-receiving antenna is carried out during the generation of the waveform in the radar transmission chain, by calculating the projection of the expected movements of the phase centre 0 on the aiming direction of the transmitting-receiving radar antenna, then determining a compensation phase shift A