The present invention relates to a method for calibrating a sectorized active antenna. It also relates to a measurement device for the implementation of such a method. The invention applies in particular to the field of electronic-scanning active antennas used in particular to equip radars or seekers of missiles and 10 architectured in sectors to perform computational beamforming (CBF). More generally, it applies to any sectorized electronic-scanning antenna, whatever its application. An electronic-scanning antenna is an antenna with an array of radiating 15 elements which comprises electronic devices allowing changes of states of the shape of the transmitted beam. Depending on their nature, these electronic devices (phase-shifters, attenuators), which are connected to the N radiating elements so as to form N active channels, act on the shape, the direction, the frequency or the polarization of the wave formed. 20 An active electronic-scanning antenna corresponds to an antenna whose devices for amplifying the transmitted or received signals are included in the antenna and distributed as close as possible to the radiating elements, forming an array of active transmit and receive channels. Conventionally, in reception the signals arising from the N radiating elements are grouped 25 together after amplification, weighting and phase-shifting by physical devices called distributers or summators so as to form four sectors of identical general shape commonly called "antenna quarters". These 4 quarters are thereafter combined together by a physical device incorporated into the antenna and called a "channel formatter" which forms the conventional 30 deviometry channels used in radars or seekers in particular, these channels being the sum channel I, the bearing difference channel LlB and the elevation difference channel LlE. A sectorized active electronic-scanning antenna is an antenna which does not group the N radiating elements together directly to form the four sectors 35 and then the conventional deviometry channels but performs M partial 2 groupings thereof, each containing a certain number of channels. These partial groupings go by the name of sector channels and are available at antenna output. A sectorized antenna can continue to give access to the conventional deviomelry channels as a supplement to the outputs of the 5 sector channels. The deviometry signals are processed conventionally while the signals arising from the sectors are used by the system whatever it be (radar, seeker, ... ) to form, by computation a certain number of channels: the conventional deviometry channels for example but also any other type of combination of the signals arising from the M sectors which are intended to 10 improve the information processing. Thus, in addition to the fomnulation of the conventional deviometry channels, a sectorized antenna formulates output signals of sector channels corresponding to the partial and disjoint groupings of a certain number of active channels of the antenna. The fact of having access to these various sector signals makes it possible to recompose by 15 computation a CBF signal according to all combinations of the type: Signal JBF = :E~1 BieN'. Channel_sector(i) (1) Where M is the number of sectors, Channel_sector(i) the signal arising from 20 the sector of order i, and BieN' a complex weight assigned to sector i. In practice M is a multiple of 4, M is for example equal to 16 or 32. 25 30 35 Hereinafter, equation (1) will be called the CBF equation. Hereinafter also, the following terminology will be used: Active channel: one of the N unitary channels of which the antenna is composed; Conventional deviometry channel: one of the three deviometry channels conventionally formulated in a traditional electronic-scanning antenna; Sector channel: one of the M sector channels formulated by a sectorized antenna; Antenna Channel: generic term designating a sector channel or a conventional channel. 3 The adjustment of active antennas involves a so-called calibration phase which consists in measuring on the assembled antenna the dispersions of amplitude and of phase in transmission or reception on the N various active channels of the antenna, these dispersions being due in particular to the 5, dispersions of the components included in the active channels as well as to the assemblage dispersions. These dispersions are thereafter compensated by appropriate corrections applied to the amplitude control and phase control of each active channel by virtue of the pointing computer. In the case of a conventional antenna, this calibration is traditionally done at the antenna 10 output on the sum channel E. Within the framework of a sectorized active antenna, the conventional deviometry channels and the sector channels must be optimized simultaneously, a problem which does not actually arise for a conventional antenna. 15 Once calibrated with the required precision, the active antennas, be they sectorized or not, have to be tested according to a certain number of parameters (patterns, EIRP, G/T, ... ). In the case of a sectorized antenna, the number of measurements to be made is much more significant than for a conventional antenna and increases nearly in proportion to the number of 20 sector channels. 25 30 Thus for a sectorized antenna, the following problems arise in an acute manner: calibration of the antenna, since several Antenna channels have to be optimized simultaneously (the conventional channels and the various sector channels) in contradistinction to a conventional antenna where the calibration is generally performed on the sum channel L:; testing, since a lot of data must be measured: with a precision at least equal,. or indeed increased, with respect to a conventional antenna; in a measurement time which is of the same class as for a conventional antenna so that the schedules of the test phases do not increase in duration. 4 Several documents deal with the calibration of conventional scanning antennas, this being the case in particular for the documents FR0904045, FR 1103143 and FR 1302928 as well as the documents by C Renard: · "Calibration et mesure faible coOt d'une antenne a balayage electronique 5 active" [Low-cost calibration and measurement of an active electronicscanning antenna] CNES Workshop, December 2008, by E.GRORUD et al "Design and Measurement of an active array antenna for an airborne X-band SAR/MTI Radar", EUCAP Nice 2006 and by C. Renard: "Etalonnage des antennes aeroportees" [Calibration of airborne antennas], CCT CNES 10 January 2009. In the case of sectorized antennas, few or no satisfactory known solutions exist. 15 As regards the test measurements, the number of channels to be measured simultaneously in reception in a conventional electronic-scanning antenna is limited to the three conventional deviometry channels and the problem of the simultaneity of the measurements is conventionally solved: 20 25 either by using a multiport array analyzer capable of measuring these three conventional channels at the same time, the number of measurement ports of the analyzer not being able to be increased indefinitely and being limited in a standard manner to 8 currently; or by multiplexing the three measurements on a conventional array analyzer comprising a reception port, the multiplexer being linked to the three outputs of the antenna to be measured and comprising switches making it possible to orient the desired channel The application of these schemes to an antenna that comprised for example 16 or 32 sectors to be tested in reception is not possible, either because the 30 number of Antenna channels to be tested exceeds the simultaneous capacity of current array analyzers, or because of the trail effect in the case of multiplexed measurements. Indeed the antenna being in motion during the measurement, the Antenna channels are not all actually measured at the same location of the antenna pattern, this not being serious when the number 5 of channels is low but which becomes crippling when the number of channels increases greatly, then causing the so-called "trail" effect Stopping the antenna, which is necessary for the time the measurement is being performed, leads to crippling measurement times and cannot be an 5 acceptable solution either. An aim of the invention is in particular to enable the CBF signals to be of the same quality as the conventional deviometry signals formulated by the antenna, and to do so in a sectorized-antenna calibration and measurement time comparable to that for a conventional electronic-scanning active 10 antenna. To this end, the subject of the invention is a method for calibrating an electronic-scanning antenna composed of radiating elements each forming an active channel in which the signals arising from said channels are grouped together on the one hand to form deviometry signals, each 15 deviometry signal forming a deviometry channel, and are grouped together on the other hand according to sectors (S1, S2, SM) to form a sectorbased signal forming a sector channel, a computation-based beam being fonrned on the basis of the signals arising from each of said sectors, said method comprising at least the following steps: 20 25 30 measuring the response to a calibration signal of each active channel at one and the same time on the sum deviometry channel and on the sector channel to which it belongs, the responses on the sum channel making it possible to obtain the calibration of the sum channel; computing, for each active channel, the disparity between the mean of the responses measured on the sum channel and the mean of the responses measured on the sector to which it belongs; in the operational phase, forming said beam by computation on the basis of the measurements of the signals arising from the sector channels calibrated with the calibration obtained for the sum channel, and correcting the computation of said beam as a function of said disparity. The measurement of the response to a calibration signal of each active channel at one and the same time on the sum deviometry channel and on the 6 sector channel to which it belongs is for example performed in one and the same instantaneous measurement sequence. The signal n1easurements arising from the sector channels are for example performed in one and the same instantaneous measurement sequence. 5 In a particular mode of implementation, the signal received by the beam formed by computation, denoted Signal_ CBF, is obtained according to the following relation 10 15 20 25 M Signal_CBF = L B; eN'. Channel_sector(i). Cor _sector(i) i.=l where: M is the number of sectors; Channel_sector(i) is the measurement of the signal arising from the channel of the i1 h sector calibrated with the calibration obtained for the sum channel, i varying from 1 to M; B;eN' is a complex weight assigned to the ith sector; Cor _sector(i) represents the disparity between the mean of the responses measured on the sum channel and the mean of the responses measured on the i1 h sector The invention also relates to a measurement device for the implementation of said method, which comprises at least: a multipart array analyzer comprising a total number P of receivers, said analyzer being able to measure the signals arising from said active channels in response to said calibration signal; a set of K switches each having Q input channels and an output channel to which one of said input channels is wired, said switches being driven by said analyzer, said input channels being able to each receive a signal arising from an active channel of said antenna in response to said calibration signal, said output channels of said each being linked to one of the P receivers; said analyzer triggering successively Q measurement according to the 30 following sequence: 5 7 on reception of a measurement order signal, said analyzer driving said switches so that they each switch their first input channel onto their output channel and then triggering the measurement on its P receivers of the signals arising from P first active channels; and then said analyzer driving said switches so that they each select their second input channel on their output channel, and then triggering the measurement on its P receivers of the signals arising from P following active channels; and so on and so forth until said analyzer driving said switches makes them 1 o each wire up their Q1 h input channel to their output channel. An amplifier is for example disposed between each switch output and the input of the receiver to which it is linked. A port of said analyzer comprises for example at least one transmit module for generating the transmit signal intended to be transmitted toward said channels. 15 Said device comprises for example a coupler at the output of said transmit module, said port comprising at least one receiver for receiving a reference signal arising from said transmit signal via said coupler. Said port comprises for example a second receiver receiving a signal arising from an array formed of a few radiating elements of said antenna. 20 Other characteristics and advantages of the invention will become apparent with the aid of the description which follows, given in relation to appended drawings which represent: Figure 1, an illustration of the principle of realization of a channel 25 formatter used in a sectorized electronic-scanning antenna; 30 Figure 2, various possible steps of the method according to the invention; Figure 3, an exemplary embodiment for the implementation of the method according to the invention. Figure 1 illustrates the principle of realization of a channel formatter 1 used for a sectorized active electronic-scanning antenna. 8 In a conventional active antenna, the conventional deviometry channels 2:, llE, llB, are formed physically by the antenna through a channel formatter. The calibration measurement in respect of each active channel is made between the input of the radiating element and the output of the sum channel 5 2: of the channel formatter in the calibration operation. The quality of the calibration on the 2: channel associated with the precision of operation conventionally encountered in channel formatters suffices to guarantee correct operation of the set of three deviometry channels. In the case of a sectorized antenna, the simultaneous formation of the 1 o conventional deviometry channels and of the sector channels is ensured by a specific-channels formatter presented schematically by Figure 1. It comprises for example a for:matter 2 of conventional channels which is intended for the formulation of the conventional deviometry channels 2:, llE, llB and a part 3 which manages the sectors S1, S2, ... SM. 15 Each active channel of the antenna arising from a radiating element emerges simultaneously on the conventional deviometry channels and on the output of the sector to which it belongs but the path that it follows is not the same. More precisely, the path difference between the sum channel 2: and the sector channel is the same for all the active channels belonging to one and 20 the same sector. The paths are common as far as the channel formatter 1 and differ thereafter inside this device 1. If one chooses to continue to calibrate the antenna on the sum channel 2: as on a conventional antenna, the corrections made during the pointing of the antenna lead to well-adjusted deviometry channels but the signals arising 25 from the various sectors are not mutually coherent since the path disparity between the sun'1 channel 2: and the corresponding sector output is not the same from sector to sector. If one chooses to do the converse, that is to say to calibrate on · the sector channels, the deviometry channels 'formed physically by the' antenna are not good (regrowth of the sidelobes ahd axis 30 deviation in particular) and therefore cannot be used . . This therefore constitutes a first problem encounter. The various antenna channels must indeed be optimized simultaneously. Moreover, application of the CBF computation formula brings to light a need for increased precision in the measurement of the sectorized antenna with 35 respect to a conventional antenna. Indeed, the summation of errors per 9 antenna sector produces significant effects on the regrowth of the first sidelobes and on the axis deviation. This problem has no equivalent in conventional antennas which concentrate the efforts to be made on the physical devices for grouping the channels and not on the measurement 5 Finally, the quantity of measurements to be performed in a sectorized antenna is nearly proportional to the number of sectors of the antenna while the development and test schedules for this type of antenna quite naturally imposes test timescales which are of substantially the same class as for conventional antennas. 1 o As indicated previously, a problem posed is therefore to develop a calibration scheme making it possible to simultaneously optimize the conventional deviometry channels and the sector channels, with a corresponding test means whose main characteristics are very good measurement precision and a capacity to make a multiplicity of measurements in a time comparable 15 with that required for a conventional antenna, 5 to 10 times as many measurements for example. 20 25 Figure 2 illustrates the possible steps of the method according to the invention. These steps in fact carry out the following operations: Undertake the calibration on the sum channel 2: in a conventional manner to optimize the deviometry channels by virtue of the channel formatter inserted into the antenna; Measure the defect in adjustment between the sum channel L and the sector channels; Correct the mutual composition of the sectors by a :factor which compensates this differential adjustment defect Thus, in a first step 21, the calibration measurements are performed simultaneously on the sum channel L and the sector channels. More 30 precisely, to simultaneously optimize the antenna on the conventional deviometry channels and the sector channels serving for the computational beamforming CBF, the response of each active channel to a test signal is measured simultaneously, or almost simultaneously, at one and the same time on the sum channel Land on the sector channel to which it belongs. 35 10 In a following step 22, the calibration on the sum channel 2: is stored. More particularly this calibration takes the form of a table, consisting of the measurements of the responses of each active channel to the test signal, is for example stored in the pointer of the antenna just as for conventional 5 antennas. This calibration is applied to the antenna. Under these conditions the functional operation of the antenna is optimized on its deviometry channels but not on its sector channels. In this step also, the difference between the calibration table of the sum channel L: and the calibration table on the sector channels is stored. The calibration table of a sector consists, for 10 each· sector, of the responses of each active channel belonging to this sector. The calibration table on the sector channels groups together the set of calibration tables of the sectors. In the current art, these calibrations on the sector channels do not allow optimized operation on these sector channels. According to the invention, the CBF equation defined by claim 1 is modified 15 as a function of this difference between the tables. In a following step 23, a correction is therefore determined to be performed on the CBF signals in the operational phase, that is to say when the antenna is in operation. More precisely, one determines the correction to be applied to 20 the signals obtained according to the CBF equation defined by relation (1 ). With this correction, a CBF signal is obtained according to the following relation (2): M Signal_CBF = L B; eit/Ji, Channel_sector(i). Cor _sector(!) i::::l 25 Just as for relation (1 ), B;eN' is a complex weight assigned to sector i. Channel_sector(i) is the measurement of the signal at the output of sector i in response to the test signal on the basis of which the ca'libration of the sum channel 2: is performed. It is therefore a non-optimize;d measurement in respect of the calibration of the CBF signal. 30 The s'ignal Cor _sector(i), of each sector i, makes it possible to optimize the CBF signal, that is to say the calibration. It corresponds to the disparity between the calibration of an active channel on· fhe sum channel 2:, Cal_ channeli:(i), and the calibration of this active channel on the 11 sectori to which it belongs, Cal_sector(i). More precisely, the , quantities Cal_ channeli(i) and Cal_ sector(i) are averaged over the set of . active channels belonging to one and the same sector i: 5 Cor _sector(i) = Cal_channeli:(t)- Cal_ channell:(t) (3) In the corrected CBF equation of relation (2), it is .necessary to be able to measure the elements Cor _sector(i), and Channel_sector(i) with very good precision so that the contribution of the measurement errors to the weighted summation of the sectors is negligible. io , A device according to the invention advantageously enables these accuracy levels desired in the CBF equation to be attained, both for the correction term and for the actual measurement of the sector channels. ' Figure 3 presents a possible exemplary embodiment of a device according to 15 the invention. Its aim is in particular to allow simultaneous or almost 20 25 simultaneous measurements of the key parameters of the CBF equation. To , this end: the response of each active channel is measured on the sum channel L: and on its sector output in one and the same instantaneous measurement sequence, thereby minimizing the error incurred in the response difference and therefore in the term Cor _sector(i); the responses of the M sectors on each point of the near-field mapping, plotted according to cartesian coordinates X, Y, or of the farfield pattern, plotted in polar coordinates (8,