HIGH-RESOLUTION STRIPMAP SAR IMAGING TECHNICAL FIELD OF INVENTION The present invention relates to remote sensing by means of Synthetic Aperture Radar (SAR) in general and, in particular, to an innovative method for high-resolution stripmap SAR ima n . STATE THE ART A typical reference geometry for generating SAR images of the earth's surface is shown in Figure . With regard to this, it is wished to underline the fact that in Figure 1 (and also in the following figures that will be presented and described herei a er), the earth's surface is (and will be) shown "flat" only for convenience and simplicity of illustration and description, without any loss of generality. In particular, Figure 1 schematically shows a synthetic aperture radar (hereinafter called a SAR sensor, for simplicity of description) 10 that moves along a flight direction d at an altitude (with respect to the earth's surface) assumed to be substantially constant. A s is known, the altitude h of the SAR sensor 10 s measured along a nadir direction that passes through said SAR sensor 10 (in particular it passes through the phase centre of the antenna of the SAR sensor 10) and is orthogonal to the earth's surface and the flight direction d . Conveniently, the SAR sensor 10 is transported in flight/orbit by an aerial/space platform (not shown in Figure 1 for simplicity of illustration) , such as, for example, an aircraft, or an unmanned aerial vehicle (UAV) , or a satellite. The ground trace of the flight direction d identifies an azimuth direction x that is parallel to said flight direction d and orthogonal to the nadir direction z , while a cross -track direction y , which is orthogonal to both the nadir direction z and the azimuth direction x , together with the azimuth direction x , identifies an x-y plane tangential to the earth's surface. n use, by means of an opportune antenna (not shown in Figure 1 for simplicity of illustration) , the SAR sensor 10 transmits radar pulses and receives the associated back:- scattered signals in an acquisition direction sr that identifies the slant range and which forms an elevation angle with the nadir direction z and a squint angle p with the flight direction d (or, equivalently, with the azimuth direction x ) that, in the SAR acquisition geometry shown in Figure 1 , is equal to 90°. In particular, the SAR acquisition geometry shown in Figure 1 regards the so-called stripmap mode, in which the SAR sensor 10 illuminates a strip of the earth's surface, known as swath, with radar pulses and then receives the associated backscattered signals therefrom, said swath principally extending parallel to the azimuth direction x and having a given width W along the cross-track direction y . For greater clarity, Figure 2 shows the geometry of SAR acquisition in stripmap mode in the x-y plane, where it is possible to observe how the squint angles p are all the same (in particular, in the example shown in Figure 2 , the a squint angles are all right angles). SAR technology can be considered a mature technology,- in fact, nowadays there are countless articles, manuals, patents and patent applications that describe the characteristics and potential thereof; in this regard, reference can be made to: the article by Josef Mittermayer et . a . entitled "Bidirectional SAR Imaging Mode", IEEE Transactions on Geoscience and Remote Sensing, vol. 51, no. 1 , 1 January 2013, pages 601-614, which hereinafter will be indicated, for simplicity of description, as R e and which describes a mode for creating bidirectional SAR images; German patent application DE 103 1 063 Al, which hereinafter will be indicated, for simplicity of description, as Re and which relates to a SAR antenna method and system having a plurality of antenna elements for generating multiple SAR beams,- the article by A . Currie et al, entitled "Wide-swath SAR", EE Proceedings of Radar and Signal Processing, vol. 139, no. 2 , 1 April 1992, pages 122-135, which hereinafter will be indicated, for simplicity of description, as Ref3 and which describes various methods for widening the swath observable via a SAR; * the article by G . rieger et al. entitled "Advanced Concepts for High-Resolution. SAR Imaging", 8th European Conference on synthetic Aperture Radar, 7 June 2010, pages 524-527, which hereinafter will be indicated, for simplicity of description, as Ref and which presents various concepts regarding multi-channel SAR systems for creating high-resolution wide-swath SAR images; the book by J . c . Cur lander and R . . McDonough entitled "S et c Aperture Radar: Systems and Signal Processing", Wiley Series in Remote Sensing, Wileyinterscience , 1991, which hereinafter will be indicated, for simplicity of description, as Ref5 and which is a manual on SAR systems; and the book by G Franceschetti and R . Lanari entitled "Synthetic Aperture RADAR Processing" , CRC Press, March 1999, which hereinafter will be indicated, for simplicity of description, as Ref6 and which is another manual on SAR sy em . As is known, the azimuth resolution for a SAR acquisition in stripmap mode is a function of the angular aperture (or angular difference - delta angle) with which a target is observed by the SAR sensor; or, equivalently, the azimuth resolution can also be seen as a function of the time difference (delta time) , related to the velocity of the SAR sensor, with which the target is observed. In particular, the azimuth resolution can be expressed by the following equation (for further details, please refer to Ref3, Re and Re ) : 0.886 res =—— — , 2+delta _ angle where res indicates the azimuth resolution, indicates the wavelength used by the SAR sensor and delta angle indicates the angular aperture (or angular difference - delta angle) with which the target is observed by the SAR sensor. Assuming the angle as a 3 dB aperture (one-way) of the antenna (= 0.886 / , where indicates the physical or equivalent length along the azimuth direction of the antenna of the SAR sensor) , the constraint traditionally associated with the azimuth resolution for the stripmap mode can be obtained, which is equal to / 2 (for further details, please refer to Re , RefS a d Re£6) . Currently, very wide antenna beams are used to improve the azimuth resolution, these being achieved through the use of antennas of small size or under- illuminated or with amplitude and/ r phase modulation such as to reduce the equivalent size, or by using the so-called spotlight mode, the acquisition logic o f which is schematically illustrated in Figure 3 . n particular, as shown in Figure 3 , the SAR acquisition logic in spotlight mode envisages using a continuous, or quasicontinuous, steering of the antenna beam during the flight movement of the SAR sensor 10 (by dynamically adjusting the value of the squint angle 90 ) ; and • at the second time instant t , a second SAR acquisition forwards (namely with p <90°) . As shown in Figure , the two radar beams transmitted and received in the two acquisition directions r and sr are contiguous along the azimuth, in this way allowing the integration times to be increased (in particular, doubled) with respect to those of traditional stripmap techniques for a given antenna. The two SAR acquisitions shown in Figure 4 represent the elemental acquisitions of a total SAR acquisition in stripmap mode that is shown in Figure 5 , where it can be seen how a series of backward SAR acquisitions (hereinafter also referred to as odd acquisitions for simplicity of description) and a series of forward SAR acquisitions (hereinafter also referred to as even acquisitions for simplicity of description) are performed with interleaving at PR level, that is to say by always alternating a backward SAR acquisition with a forward SAR acquisition, i.e. by alternating use of the first squint angle with use of the second squint angle A s previously mentioned, the radar beams of odd and even acquisitions performed at immediately successive time instants are contiguous along the azimuth so as increase (in particular, to double) the integration times with respect to those obtainable via traditional stripmap techniques for a given antenna. n other words, the azimuth width of the radar beams and the variation of the squint angle are such that to guarantee azimuth contiguity of the radar beams and, consequently, an increase (in particular, a doubling) of the integration times. A flowchart is shown Figure s that represents a method of processing the data acquired using the technique of SAR acquisition in stripmap mode shown in Figure . In particular, as shown in Figure 6 , said processing method comprises: applying a Fast Fourier Transform (FFT) to both the raw data obtained from the odd acquisitions (block 61) and the raw data obtained from the even acquisitions (block 62) , so as to obtain, respectively, a first raw spectrum and a second raw spectrum; • estimating the values of the Doppler centroid on the basis of the raw data obtained from the odd and even acquisitions and, conveniently, also on the basis of the nominal pointing values of the antenna (block 63) •performing filtering and frequency alignment of the first raw spectrum (block S4) and of the second raw spectrum (block 65) on the basis of the estimated values of the Doppler centroid and the nominal pointing values of the antenna so as to obtain, respectively, a first spectrum correctly positioned in frequency and a second spectrum correctly positioned in frequency; applying an equalization of the amplitude modulation to both the first spectrum correctly positioned in frequency (block 66) and the second spectrum correctly positioned in frequency (block 67) so as to obtain, respectively, a first equalized spectrum and a second equalized spectrum,- frequency combining the first equalized spectrum with the second equalized spectrum (block 68) so as to obtain final spectrum having twice the size of that of the first and second raw spectra; and forming a SAR image on the basis of the final spectrum (block 6$) , said SAR image having an azimuth resolution that is half i .e . two times better) that of the SAR image formed on the basis of just the raw data obtained from the odd acquisitions o just the raw data obtained from the even acquisitions. It is important to underline the fact that with the proposed technique it is possible to correlate the values of the Doppler centroids of the odd and even acquisitions via the nominal values linked to the variation in pointing of the antenna and, in consequence, improve the estimate of the Doppler centroid. n order to better understand the characteristics and potentiality of the technique of SAR acquisition in stripmap mode according to the first aspect of the present invention, Figures 7 and 8 show the results of simulations performed by the applicant, assuming operation in the X-band with a planar antenna carried by a satellite and having a length of 5.6 metres (and always in the case where N=2) . Xn particular, Figure 7 shows the intensity of the two-way pattern regarding response of a single target . As can be inferred from the graph in Figure 7 , the technique of SAR acquisition in stripmap mode according to the first aspect of the present invention enables doubling (multiplying by a factor of in the generic case) the persistence time of the SAR sensor on the target (i.e. doubling the integration time) and therefore halving (or dividing by a factor of N in the generic case) the resolution (i.e. improving it by a factor of . Figure 8 shows the azimuth response obtained by simulating the presence of two targets 2.8 metres apart, i.e. half the size of the antenna. In the graph in Figure the two targets are quite distinct as the achieved resolution is significantly less (approximately half, or approximately a quarter of the antenna's size) than the value contemplated by traditional techniques (approximately half the physical or equivalent length of the antenna) . A s is known, the parameter that describes the sensitivity characteristics of a SAR image, a parameter from which the characteristics of the antenna, as well as the principle radar parameters such as transmission power are derived, is NESZ (Noise Equivalent Sigma Zero) , for which the following law of proportionality holds : where GT indicates the antenna gain in transmission, • G indicates the antenna gain in reception, L p indicates the loss due to integration of the antenna's non-ideal pattern, and • P indicates the transmitted power. As previously described, the method of SAR acquisition according to the first aspect of the present invention allows achieving a better (up to N times) azimuth resolution for acquisitions in stripmap mode with respect to that obtainable with conventional techniques (approximately half the physical or equivalent length of the antenna) . The difference will now be analysed, in terms of sensitivity, between two systems that have the same resolution, but with one using a traditional technique based on using an antenna having half the physical or equivalent length and the other using the innovative technique according to the first aspect of the present invention and based on the use of an antenna that is longer by a factor of N that is to say having a length NxL. An antenna N times longer results in an V- ld increase in gain, both in transmission and reception. As previously described, the PRF used in processing is N times smaller, i.e. the single target is focused using a number of samples N times smaller . Due to the presence of the cusp (with regard to this, reference can be made, for example, to Figure 7 ) the integration m igh be slightly higher with respect to the traditional case. However, this value, which depends on the shape of the antenna beam and the value of N, is not igni can Summarizing, it follows that: NESZinv _ 1 NESZtrad * N ' where NESZinv indicates the NESZ associated with the technique according to the first aspect of the present invention and NESZtrad indicates the NESZ associated with the traditional stripmap technique. Therefore, with the technique according to the first aspect of the present invention there is a considerable increase in product sensitivity, namely it is possible detect a signal with an intensity N times smaller. In consequence, said technique according to the first aspect of the present invention can also be used to reduce the transmission power, and therefore to reduce the technological complexities. The following Table 1 summarizes the advantages and drawbacks, for the same performance in azimuth resolution, of the technique according to the first aspect of the present invention with respect to the traditional one. TABLE 1 __ __ Advantages I Drawbacks 1 P T/IB2014/058872 - 21 - n order to better highlight the advantages obtained by using the technique according to the first aspect of the present invention with respect to the traditional one, comparisons for the same product performance and the same size in elevation are listed in Tables 2 and 3 below. In particular, the data shown below in Table 2 has been obtained by the applicant assuming a satellite application with a satellite height of approximately 6X9 Km, an antenna size in the range of approximately 1.5 m , a resolution of 1 m x 1 m , a swath width greater than 10 Km (between 13 and 15 Km depending on the elevation angle) , a NES2 of approximately -24 dBm /m and a PR in the range between 9300 Hz and 10500 H TABLE 2 Traditional Stripmap S ripmap strip a technique technique technique according to according to with a the first the first antenna aspect of aspect of having an the present the present azimuth size invention invention of 1.9 with an with an antenna antenna having an having an azimuth size azimuth size of 3.8 m and of m and with =2 with N=3 Transmitted «24 14 « power ] Transmitted power density « 9 «2.7 «1.1 [ W/ 2] Compatibility with azimuth resolution of YES YES YES 1 and swath width of 13 Km Compatibility with azimuth resolution of NO NO YES 3 and swath width of 40 Km Minimum FRF for Spot/Sean. «9000 «4500 «3000 [H ] Furthermore, the data shown below in Table 3 has been obtained by the applicant assuming a satellite application with a satellite height of approximately 61 Km, an antenna size in the range of approximately 1.5 , a resolution f 1.5 x 1.5 m , a swath width of approximately 20 Km, a NESZ of approximately -24 dBm / 2 and a PRF in the range between 6200 Hz and 7000 Hz. TABLE 3 As can be inferred from the data shown in the foregoing tables, the technique according to the first aspect of the present invention enables producing, or in any case significantly reducing the critical areas of, SAR systems with stripmap products having metric resolution and increases the type of products that can be obtained with already designed/operational SAR systems. As has hitherto been described, the technique according to the first aspect of the present invention enables simultaneously acquiring N stripmap images. In particular, these images, according to said first aspect of the present invention, are obtained with different squint angles to increase the azimuth resolution. In order not to alter the image quality parameters, the PRF used with the technique according to the first aspect of the present invention is greater than the natural one of the antenna. By increasing the PRF, the swaths in range that can be acquired are smaller. Thus, a second aspect of the present invention regards a technique of S R acquisition in stripmap mode that does not use an increased PRF, or in any case not increased by a factor of N , so as to control the effects on the product and manage the induced degradation. n particular, said second aspect of the present invention relates to a so-called burs -mode stripmap technique that is not interleaved at PRI level, i.e. where the N stripmap acquisitions are not performed by varying the antenna's acquisition direction in azimuth at PRI level, but by varying the antenna's acquisition direction in azimuth in PRI blocks. Specifically, the second aspect of the present invention concerns a burst -mode stripmap technique in which the stripmap acquisitions are performed without increasing the PRF and by varying the azimuth acquisition direction of the antenna, namely the squint angle used, in PRI blocks. The burst-mode stripmap technique with unincreased PRP and variation of the squint angle according to the second aspect of the present invention enables improving the azimuth resolution N times without deteriorating the swath in range, that is to say without altering the size of the swath in range. In particular, this technique enables achieving azimuth resolutions N times smaller than half the physical or equivalent length of the antenna used (i.e. azimuth resolutions N times better with respect to those of traditional stripmap techniques) . n order to divide the acquisition in two (if in the generic case) and assuming to use the natural nominal PRF of the antenna used, ^holes'' are introduced in the acquisition scheme . If these holes do not have periodic characteristics , the effect will be a distributed raising of all the lateral lobes, i.e. the SLR (Integrated Side Lobe Ratio) parameter deteriorates, but not the PSLR (Peak Side Lobe Ratio) . ice versa, by using periodic execution patterns for the two in the generic case) types of acquisition, paired echoes in a known position are created. Depending on requirements, various solutions can be chosen and then a given pattern applied in the acquisition logic. Since a lower number of samples will be integrated, the product will have an impaired NES2 parameter. By way of example, Figures 9 and 10 show the effects of applying a periodic execution pattern of the N types of acquisition with the burst-mode stripmap technique with unincreased PRF according to the second aspect of the present invention, while Figures 11 and 12 show the effects of applying a random execution pattern of the N types of acquisition with the burst-mode stripmap technique with unincreased PRF according to the second aspect of the present invention. With respect to the technique according to the first aspect of the present invention, the technique according to the second aspect introduces less technological constraints because the switching of the antenna beam takes place at a considerably lower frequency. Briefly summarizing, the present invention concerns • the use of a PRF increased by a factor of N and the interleaved use of N different squint angles at PR level to improve the azimuth resolution N times, i.e. to obtain an azimuth resolution with a numerical value smaller by a factor f N with respect to the numerical value of the nominal azimuth resolution of the stripmap mode {said numerical value of the nominal a i resolution of the stripmap mode, as previously explained, being equal to L/2, where indicates the physical or equivalent length along the azimuth direction of the SAR antenna used) ; and the use of an unincreased PRF and the burst use of N different squint angles to improve the azimuth resolution N times, i.e. to obtain an azimuth resolution with a numerical value smaller by a factor of iV with respect to the numerical value of the nominal azimuth resolution of the stripmap mode (i.e. L/2) . n conclusion, the present invention exploits multi-beam acquisition logic that enables achieving stripmap acquisitions with extreme resolutions comparable to those of spotlight mode, thereby overcoming the constraint on azimuth resolution linked to the width of the antenna beam. The present invention also enables managing the energy radiated in a more efficient manner, reducing the power required to guarantee the sensitivity value established for the product (reducing the power in transmission and the power density in transmission) . The present invention therefore not only increases the range of products for systems already produced, but, above all, introduces a new methodology for designing new SAR systems. Finally, after having compared the present invention with the traditional spotlight and stripmap mod , the main differences from the known techniques of high- resolution wide- swath SAR image generation previously described will now be described as well . n particular, unlike the present invention, the burst techniques (e.g. ScanSAR and TOPS) envisage deterioration of azimuth resolution in order to increase the swath in range. Unlike the present invention, which functions with a single receiving channel (i.e. with a single receiver), the spacedivision techniques (e.g. PC and S) and the angle-division ones (e.g. EB and SPC B ) envisage the use of M systems for simultaneous reception and also envisage the use of a small antenna (typically, an antenna is partitioned into smaller antennas) . The BiDi mode described in e has a different purpose, that of Moving Target Identi cat on