High-resolution stripmap sar imaging

MULTI-MOWED BAND MAPPING SAR IMAGING

20,6

TECHNICAL FIELD OF THE INVENTION

The present invention relates to remote sensing using Synthetic Aperture Radar (SAR) in general and, in particular, to an innovative method of multi-swath strip mapping SAR imaging.

STATE OF THE ART

A typical reference geometry for generating SAR images of the Earth's surface is shown in Figure 1. In this regard, it is emphasized that in Figure 1 (and also in the following figures which will be presented and described here -after), the surface of the Earth is (and will be) shown flat for convenience and simplicity of illustration and description only, without any loss of generality.

In particular, FIG. 1 schematically represents a synthetic aperture radar (hereinafter referred to as the SAR sensor, for the simplification of the description) which moves along a flight direction d at an altitude h (with respect to the surface of the Earth) assumed to be substantially constant. As is known, the altitude h of the SAR sensor 10 is measured along a direction of the nadir Z which passes through said SAR sensor 10 (in particular, it

0 passes through the phase center of the SAR sensor antenna 10) and is orthogonal to the earth's surface and to the direction of flight d. Ideally, the SAR sensor 10 is carried in vo O bite by an air / space platform (not shown in figure 1 for simplicity of illustration), such as, for example, an aircraft or an unmanned drone (UAV). , or a satellite. The tace to the ground of the direction of flight d identifies a direction of azimuth X which is

5 parallel to said direction of flight d and orthogonal to the direction of nadir Z, while a direction transverse to track y, which is perpendicular to both the direction of nadir Z and to the direction of azimuth X, with the x azimuth direction, identifies an xy plane tangential to the earth's surface. In use, by means of a suitable antenna (not shown in Figure 1 for ease of illustration), the SAR sensor 10 transmits

0 radar pulses and receives the associated backscattered signals in an acquisition direction sr which identifies the oblique distance and which forms an altitude angle 0 with the direction of nadir

Z and an observation angle φ with the direction of flight d (or, equivalently, with the

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-2 azimuth direction x) which, in the SAR acquisition geometry shown in FIG. 1, is equal to 90 °.

In particular, the SAR acquisition geometry shown in Figure 1 relates to the mode known as band mapping, in which the SAR sensor 10 illuminates a band of the Earth's surface, known as a swath, with pulses radar and then receives the backscattered signals associated therewith, said swath extends mainly in parallel with the direction of azimuth X and having a given width w along the transverse direction y. For clarity, Figure 2 shows the geometry of SAR acquisition in stripe mapping mode in the xy plane, where it is possible to observe how the viewing angles φ are all the same (in particular, in the example shown in Figure 2, the observation angles φ 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 features and potential of it; in this regard, we can refer to:

the article by Josef Mittermayer et al. titled Bidirectional SAR Imaging Mode, IEEE Transactions on Geoscience and Remote Sensing, vol. 51, no. January 1, 2013, pages 601614, which will be indicated below, for simplicity of description, as Refl and which describes in

0 mode to create bidirectional SAR hosts;

German patent application DE 103 19 063 A1, which will hereinafter be indicated, for simplicity of description, as Ref2 and which relates to a method and a SAR antenna system having a plurality of antenna elements to generate multiple SAR beams;

٠ the article by A. Currie et al. titled Wideswath SAR, IEE Proceedings of Radar and

Signal Processing, vol. 139, no. 2, April 1, 1992, pages 122-135, which will hereafter be indicated, for simplicity of description, as Ref3 and which describes the various methods for the enlargement of the swath observable via in SAR;

١ 'غ \ لست ١ .لة أح تجهئ١.ذاً جة حة ١ ااًة' \ ٠ Advanced Concepts for High-Resolution Wide-

0 Swath SAR Imaging, 8th European Conference on 'Synthetic Aperture Radar', June 7

2010, pages 524 to 527, which will be indicated hereafter, for the simplicity of the description, as Ref4 and which presents the different concepts concerning the multi-channel SAR systems for

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create high resolution wide swath SAR images;

the book by JC Curlander and RN McDonough entitled Synthetic Aperture Radar: Systems and Signal Processing ”, Wiley Series in Remote Sensing, Wiley-Interscience, 1991, which will be indicated hereafter, for the simplicity of description, as Ref5 and which is a SAR systems manual; and the book by G. Franceschetti and R. lanarios titled Synthetic Aperture RADAR Processing ”, CRC Press, March 1999, which will hereafter be indicated, for simplicity of description, as Ref6 and which is another handbook on SAR systems.

As is known, the azimuth resolution for a SAR acquisition in band mapping mode is a function of the angular aperture (or angular deviation - delta angle) with which a target is observed by the SAR sensor; or, equivalently, the azimuth resolution can also be considered as a function of the time difference (delta time), related to the speed of the SAR sensor, with which the target is observed. In particular, the azimuth resolution can be expressed by the following equation (for more details, refer to Refi, Ref5 and Ref6):

0.886Λ res = -------------- n * delta-Cingle where res indicates the azimuth resolution, د indicates the wavelength used by the SAR sensor and deltajngle indicates the angular aperture (or angular deviation - delta angle) with

0 which the target is observed by the SAR sensor.

Assuming the angle as a 3 dB (one pass) antenna aperture (= 0.88βλ / L, where L indicates the physical or equivalent length along the azimuth direction of the SAR sensor antenna), the constraint traditionally associated with the azimuth resolution for the

5 band mapping mode can be obtained, and that is equal to L / 2 (for more details, refer to Ref3 Ref5 and Ref6).

Currently, very large antenna beams are used to improve azimuth resolution, these being achieved through the use of small size antennas or a

0 lighting or amplitude and / or phase modulation subsystem so as to reduce the equivalent size, or by using what is called the soptlight mode, the logic of the acquisition of this mode is schematically illustrated in figure 3.

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In particular, as represented in FIG. 3, the SAR acquisition logic in spotlight mode envisages using continuous or quasi-continuous piloting of the antenna beam during the flight movement of the SAR sensor 10 (by dynamically adjusting the value of the observation angle φ) so as to illuminate a same area of ​​interest with the radar pulses, and then to receive the rebroadcast signals associated with them, in this way it is possible to increase the persistence time of the SAR sensor 10 on said area of ​​interest and thus improve the azimuth resolution.

The two methods cited above used for improving the azimuth resolution have certain drawbacks. In particular, the use of very wide antenna beams requires the use of very high transmission powers, while the spotlight mode introduces lhnitations along the length in azimuth of the swaths.

As indicated in the literature, there are mathematical relationships that relate the parameters of the modes of operation. In particular, azimuth sampling requires that the emission-reception pulse repetition frequency (PRF) is related to the size of the beam and the speed of the SAR sensor (for more details, refer to Refi Ref5 and Ref6 ):

, كي &lt;ممم

L where a is a parameter dependent on the level of ambiguity desired, v indicates the speed of the SAR sensor, and L indicates the physical or equivalent length along the azimuth direction of the SAR sensor antenna.

The value of PRE limits the extension of the measured zone (mowed) in the range (for more details, refer to RefB Ref5 and Ref6):

'٤ (2٢ معم]&gt;? AT where AR indicates the extension of the measured area (mowed) in the range, τ indicates the time interval (or duration) of the emitted pulse and c indicates the speed of the light.

In order to make the most of the capabilities of a SAR system, in addition to band mapping and Spotlight modes, various other techniques have been proposed over the years.

38366Α1 years for using SAR sensors. These techniques can be grouped into three main modes:

burst mode (segment);

space division mode; and angle division mode.

The main burst mode is the ScanSAR mode, which divides the time into synchronous (ie with a regular rate) segments (bursts). A part of the scene at the nominal PRL of the antenna is acquired in each segment, so that the azimuth spectrum is correctly sampled (for more details, refer to Refi Ref5 and Ref6) ١ but for a time shorter (and therefore a smaller delta angle). This division of time allows the beam to pass, in subsequent segments, in other directions in order to increase the swath in the range. In practice, the azimuth time is divided into Nb segments and a different band is acquired at each segment. The biggest conreindication of this mode is that, by reducing the acquired delta angle, the azimuth resolution deteriorates. In particular, the constraint of continuous coverage which guarantees the absence of all in the bands implies that the best azimuth resolution cannot be less than (better) than a certain quantity (for more details, refer to Refi Ref4 , Ref5 and Ref6); in particular, we see that:

where rescanSar indicates the ScanSAR resolution and resstrip indicates the reference band mapping resolution (ie L / 2).

مل TOPS mode (for more details, refer to Ref4) is also counted among the burst modes; this mode exploits the azimuth direction capability and requires a scanning direction opposite to that of the spotlight mode, that is to say comparable to a scan rather than a recentering. That is, in each segment, the initial azimuth direction ensures that the SAR sensor looks backward and then points forward at the end of the segment. This ensures that, unlike conventional ScanSAR mode, each target is illuminated by the entire antenna pattern, allowing radiometric response equalization and azimuth ambiguities. As happens with the prescribed ScanSAR mode, in the

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TOPS (which is also a burst mode), thus, the targets are always seen by the sensor with a smaller delta angle (or delta time) compared to that typical of the strip mapping mode, n results, as well as the TOPS mode, just like the ScanSAR, (even if with different equations / constraints compared to the traditional ScanSAR), allows to extend the swath in the range at the expense of the degradation of the azimuth resolution compared to the band mapping mode . These concepts are summarized in Ref4, where in the introduction it says: examples are the ScanSAR (or TOPS) mode which allows a wide swath at the expense of impaired azimuth resolution and the Spotlight mode which allows improved azimuth resolution at the expense of non-contiguous imagery along the satellite path

As seen above, the requirements for wide swaths and high resolutions are in mutual conflict. On the one hand, a low PRP is preferable to have more time to acquire a wide scene in the transverse altitude plane, while on the other hand, a wide antenna beam would be preferable to improve the azimuth resolution. However, this latter characteristic requires a high PRP, thus being in contrast to the former condition. In order to overcome these problems, techniques have been proposed in the past which use space division modes, such as, for example, the Displaced Phase Centers (DPC) technique (for more details, refer to Ref3 and Ref4), which requires the use of several receiving antennas. This can be achieved by using multiple SAR sensors, or by segmenting a single antenna and using multiple receiving systems: a wide beam is transmitted (small antenna of size L), then received at the same time with M antennas ( small in size like that of the emission) arranged along the azimuth direction. The use of multiple receiving elements allows for a larger number of azimuth samples and therefore the use of a lower physical P RF (for more details refer to ReS and Ref4) . The biggest contraindication to this technique is the complexity; in fact, this technique requires the simultaneous use of M receivers and M small antennas (or a large antenna partitioned into M sub-blocks) and therefore requires a high transmission power to achieve adequate sensitivity of the product. In addition, the literature highlights some critical areas at the algorithm level concerning the error sensitivity of the knowledge of phase M centers, as well as the effects on the level of ambiguity. There are a few variations in the literature which

38366Α1 attempt to reduce these critical areas, such as the High Resolution Wide Swath Technique (HRWS), which also provides for altitude partitioning in order to follow the beam aloft, thereby increasing the directivity and therefore the sensitivity of the product. The purpose of techniques which use angle division modes is similar to that of techniques which use space division modes, but the additional samples are acquired by sampling in different directions. In particular, there are two main logics: angular division in altitude and angular division in azimuth.

Angular division in altitude (in connection with this, one can refer, for example, to the Multiple Altitude Beam (SEM) technique described in Ref4) contemplates simultaneous acquisition with multiple antenna / reception systems and a single transmitter (with wide swath), or several directive transmissions (for more details, refer to Refi and Ref4). In this way, a plurality of acquisitions are obtained in band mapping mode with nominal azimuth resolutions (approximately L / 2). To reduce the problems of ambiguities in the range, the literature suggests observing individual beams at altitude.

Instead, the angular division in azimuth (in relation to this, one can refer, for example, to the Single-phase Center Multi-Beam (SPCMB) technique described in ReS) considers emission via a single wide beam and simultaneous reception via M narrow beams. In this way, the wide beam is obtained (improved resolution), but similarly to the spotlight mode, the only receiving channels correctly sample a part of different angle. These channels will then be recombined during the processing in order to obtain a synthesized delta angle M times greater, consequently improving the resolution (for more details, refer to Refi and Ref4). In general, angular division in azimuth techniques has many areas critical to the level of ambiguity; in fact, the interaction of the side lobes of the transmitting antenna and of the simple receiving antennas increases the level of ambiguities.

Again, the greatest contraindication to angular division techniques is complexity; in fact, these techniques envisage the simultaneous use of M receivers and M small antennas (or a large antenna partitioned into M sub-blocks) and require

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therefore the high power of emission to obtain an adequate sensitivity of the product.

The concepts of space and angle division are well summarized in Ref4, of which section 2 states: Several proposals solve the vast dilemma of azimuth resolution versus wide swath by combining a multi-channel radar receiver with a small aperture transmitter illuminating a large area on the floor. Examples are the observable multiple beam SAR ..., the displaced phase center antenna (DPCA) technique ..., the quadrangle array SAR system ..., and the wide swath SAR system at high resolution (HRWS).

Although the objective is different, the two-way SAR imaging mode (BiDi - for more details refer to Refl) should also be counted among the angle division techniques. By exploiting the azimuth grid lobes of a phased array antenna or working at different times, this mode simultaneously acquires two different images: one relating to the front scene and the other relating to the backlog of the scene. The angular difference in azimuth is the equivalent of a time difference, namely the same scene seen at different times, and it is therefore possible to identify moving objects in the scene by comparing the images. It is important to emphasize that this technique expects the two acquisitions to be angularly separated in azimuth, that is to say, it is impossible to

0 combine the data of the two channels to reconstruct an image with a higher resolution.

In the conclusion of Refl it is stated: The short-term BiDi series can be used for speed change and detection .... The possibilities of the BiDi imaging mode in terms of speed measurement are the subject future research. Other possible applications of BiDi include the high precision measurement of longitudinal displacement by exploiting widely separated Doppler spectra.

Finally, although SAR technology can be developed with different types of antenna,

0 the one that offers the greatest flexibility of use is the so-called phase network type, which allows rapid switching of the acquisition direction. Other types of antenna can be used, such as those described in Ref2, for example.

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OBJECT AND SUMMARY OF THE INVENTION

The aim of the present invention is to provide a method of SAR imaging by band mapping which makes it possible to observe several swaths.

The above-mentioned object is achieved by the present invention insofar as it relates to an SAR imaging method and SAR system, as defined in the appended claims.

In particular, the SAR imaging method according to the present invention comprises performing N SAR acquisitions in band mapping mode of the areas of the Earth's surface by means of a synthetic aperture radar which is transported by an aerial or satellite platform and which comprises a single, non-partitioned antenna and a single receiver coupled to said single non-partitioned antenna, where N is an integer greater than one.

Each SAR acquisition in band mapping mode is performed using a respective observation angle with respect to the direction of flight of the synthetic aperture radar, said respective observation angle is either equal to, or different from the angles of 'observation used to perform the other Ν-1 SAR acquisitions in band mapping mode.

Each SAR acquisition in the band mapping mode is performed using a respective altitude angle relative to the nadir of the synthetic aperture radar, said respective altitude angle being different from the altitude angles used to achieve the others. Ν-1 SAR acquisitions in band mapping mode, thus resulting in that each SAR acquisition in the band mapping mode is linked to the respective swath of the Earth's surface which is different from the swaths observed via the other Ν-1 acquisitions made in the band mapping mode.

In addition, each SAR acquisition performed in band mapping mode includes the respective radar transmit and receive operations and which:

are the times interspersed, individually or in groups, with a single or groups of operations, radar transmission and reception of the other Ν-1 SAR acquisitions made in band mapping mode; and includes the transmission and reception of the respective radar beams in the directions

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- 10 respective acquisitions which are defined by the respective observation angle and by the respective altitude angle used for said SAR acquisition in band mapping mode, which results in said respective acquisition directions being parallel to each other and are not parallel to the acquisition directions of the others Ν-1 SAR acquisitions performed in band mapping mode.

The method further comprises generating, on the basis of each SAR acquisition performed in the band mapping mode, SAR images of areas of the respective swath, observed via said SAR acquisitions in the band mapping mode.

In particular, all of the generated SAR images have the one and the same azimuth resolution, which is half the physical length or equivalent along the azimuth direction of the single, unpartitioned synthetic aperture radar antenna. .

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, certain preferred embodiments, given by way of non-limiting example, will now be described with reference to the accompanying drawings (which are not to scale), where:

Figures let 2 schematically show a typical geometry for acquiring SAR images in band mapping mode;

0 ٠ Figures 3 show schematically a typical geometry of image acquisition

SAR in spotlight mode;

* Figure 4 schematically shows an example of the logic for SAR acquisition in band mapping mode according to a first aspect of the present invention;

Figures 5 and 6 schematically show the effects of applying a first

5 acquisition strategy in performing a SAR acquisition technique in band mapping mode according to a second aspect of the present invention; and

Figures 7 and 8 schematically represent the effects of applying a second acquisition strategy in performing the SAR acquisition technique in band mapping mode according to the second aspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

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The following description is provided to enable an expert in the field to make and use the invention. Various modifications to the embodiments shown will be immediately obvious to those skilled in the art and the generic principles described herein can be applied to other embodiments and applications without departing from the protective scope of the present invention.

Thus, the present invention is not intended to be limited only to the embodiments described and shown herein, but should be given the broadest scope consistent with the principles and features described herein and defined in the appended claims.

The present invention stems from the applicant's insight to exploit the steering capability of the antennas used in SAR sensors in an unconventional manner. The applicant then designed a multi-beam and multi-temporal SAR acquisition technique which exploits the transmission and reception characteristics of a time-shared SAR sensor.

In particular, the basic idea of ​​the present invention is the division of an SAR acquisition in band mapping mode into N elementary acquisitions in band mapping mode (with Ν&gt; 1) and combine the latter in order to obtain N sets of SAR images, in

0 which each set is related to the respective swath (i.e., N distinct swaths can be observed with the same azimuth resolution).

In detail, a first aspect of the present invention relates to making N different SAR acquisitions using N different altitude angles so as to observe N different swaths.

Particularly, the idea on which said first aspect of the present invention is based consists in making several SAR acquisitions interspersed at a pulse repetition interval (PRI) level, which represents the time between two consecutive transmitted pulses, in particularly SAR acquisitions in which the acquisition department

0 antenna altitude changes with the PRI level. In order to achieve this, a pulse repetition frequency (PRE where PRE = 1 / PRI) is used which is N times greater than the nominal PRE associated with the antenna of the SAR receiver being used. Returning

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- 12 briefly in Figure 1, for the clarity of the description, it should be remembered that the altitude angle is an angle Θ between the pointing direction sr of the antenna of the SAR receiver and the nadir direction Z of the receiver SAR.

By increasing the PRE by a factor N, the N different bands having N times less available time and therefore, in general, they will have a swath reduced by a factor N; however, by adding the N different swaths observed together, the typical swath size of the strip map mode is always obtained. As the individual bands are reduced by a factor of N, it is possible to use an antenna N times wider in altitude, and consequently, the increase in the directivity of the product, i.e. the sensitivity. In general, the antenna can be operated in several ways.

Furthermore, by increasing the PRE by a factor of N, N stripe mapping acquisitions can be obtained, individually presenting PREs compatible with the antenna size (in this mode, the azimuth ambiguity values ​​are not not altered).

Although the band mapping mode SAR acquisition technique according to the above-mentioned first aspect of the present invention can be used with a generic integer N greater than one, hereinafter, to simplify the description and without loss. of generality, examples will be displayed for N = 2, it being understood that the concepts explained below regarding the case N = 2 are also applicable mutatis mutandis in the

0 case of a generic integer N greater than one.

For a better understanding of the second aspect of the present invention, Fig. 4 schematically illustrates an example of the logic for SAR acquisition in band mapping mode according to said first aspect of the present invention in the case of N = 25 2, and in the case of the satellite application (it is understood that this SAR acquisition logic can also be used advantageously in the case of an aerial platform such as an aircraft, a drone or a helicopter).

In particular, Figure 4 (in which the Cartesian reference system used corresponds

0 substantially to that previously introduced for the figures 1 3) shows, a satellite 30 which moves along a direction of flight d and is equipped by a sensor

SAR (not shown in figure 4 for simplicity of illustration) is equipped with a

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- 13 single unpartitioned antenna (not shown in Figure 4 to simplify the illustration) which is coupled to a single receiver (not shown in Figure 4 to simplify the illustration), and is associated with a given nominal pulse repetition frequency PRFnom.

In the example shown in Figure 4, the SAR sensor is used with an operational pulse repetition frequency PRFop twice that of the nominal pulse repetition frequency of the antenna PRFnom (i.e., PRFop = 2PRFnom), so as to transmit successive pulses at a time distance PRIop = 1 / (2PRFn٠m).

In particular, as shown in FIG. 4, the SAR sensor on the satellite 30 performs a series of first SAR acquisitions using a first altitude angle and a series of second SAR acquisitions using a second altitude angle, in which said first and second SAR acquisitions are interspersed at a PRI level (i.e., the first SAR acquisition is always altered with a second SAR acquisition) and said first and second altitude angles are different from each other. In other words, the first SAR acquisition is performed using the first altitude angle, so at a time distance PRL, l./PRF „1, F2PRF ™; y١. a second SAR acquisition is performed using the second altitude angle, still at a time distance PRI1 / PRF1 / f2PRF ^ ™ ١. a first SAR acquisition is performed again using the first angle

0 altitude and so on, always alternating the execution of the first SAR acquisition with the execution of the second SAR acquisition while separating the different acquisitions ا by a period of time PRIopzl / PRFopzl ^ PRFnom). In this sense, the SAR sensor on the edge of the satellite 30 is capable of observing two distinct swaths (as shown in FIG. 4).

It is important to note that the acquisitions which concern the same swath are carried out with the nominal pulse repetition frequency PRFnom of the antenna, that is to say the first SAR acquisitions are carried out with the repetition frequency of nominal pulse PRFnom of the antenna and the second SAR acquisitions are also performed by the nominal pulse repetition frequency PRFnom of the antenna. In this sense, the values

0 azimuth ambiguity are altered.

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- 14 Again in general, since the operational pulse repetition frequency PRFop used is N times (twice for example in figure 4,) greater than the required / nominal PRFnom, the individual acquisitions will have nominal PRFs, and therefore therefore, not all product quality parameters will be altered.

For all these intentions and objectives, the technique of SAR acquisition in band mapping mode according to the first aspect of the present invention, allows the separation of the swath in the range into N swaths of reduced size (approximately 1 / Ν) without the other parameters being altered, such as for example the azimuth resolution.

All N SAR acquisitions can be performed using the same viewing angle, or each SAR acquisition can be performed using a respective viewing angle different from that used to perform the other Ν-1 SAR acquisitions, so as to obtain, for each acquisition, an integration time equal to the standard time of the strip mapping.

Consequently, the SAR acquisition technique in band mapping mode according to the first aspect of the present invention makes it possible to serve two (or N in the generic case) users who are interested in the areas of average extension and separated from each other. others in the elevation plane. With the traditional band mapping technique, these requests would conflict and therefore, would not be possible to serve them simultaneously.

In order not to alter the image quality parameters, the PRE used with the technique according to the first aspect of the present invention is greater than the natural frequency of the antenna. By increasing the PRE, the swaths in the range where they can be acquired are smaller. Thus, a second aspect of the present invention relates to a SAR acquisition technique in band mapping mode which does not use an increased PRE, or in any case is not increased by a factor of N, so as to control effects on the product and manages the degradation induced.

In particular, said second aspect of the present invention relates to what is called a burst mode band mapping technique which is not interleaved at the PRI level, i.e. where the N band mapping acquisitions are not. performed by doing

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- 15 varying the direction of acquisition in altitude of the antenna at the PRI level, but varying the direction of acquisition in altitude of the antenna in PRI blocks.

More precisely, the second aspect of the present invention relates to a burst mode band mapping technique in which the N band mapping acquisitions are performed without increasing the PRF and by varying the direction of acquisition in altitude of the antenna. , that is, the altitude angle used, in PRI blocks.

The burst mode band mapping technique with unincreased PRF and the variation of the altitude angle according to the second aspect of the present invention makes it possible to extend the swath in the range, and even up to doubling it.

In order to divide the acquisition into two (N in the generic case) and assuming the use of the natural nominal PRF of the antenna used, holes are introduced into the acquisition system. If these holes do not have the periodic characteristics, the effect will be a distributed increase of all side lobes, i.e. the ISLR (Integrated Lobe Lobe Ratio) parameter deteriorates, but the PSLR (Peak Lateral Lobe Ratio) ). Vice versa, using periodic execution patterns for two (N in the generic case) acquisition types, even echoes in a known position are created. Depending on the needs, different solutions can be chosen and then a given pattern will be applied in the acquisition logic. As a smaller number of samples will be integrated, the product will have a reduced NESZ (Noise Equivalence Sigma Zero) parameter.

By way of example, FIGS. 5 and 6 show the effects of applying a periodic execution pattern of the N types of acquisition with the band mapping technique in burst mode with a non-increased PRF according to the second aspect of the present invention, while FIGS. 7 and 8 show the effects of applying a random execution pattern of the N types of acquisition with the burst mode band mapping technique with an unincreased PRF according to the second aspect of the present invention. invention.

Compared 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

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- 16 of the antenna beam takes place at a much lower frequency.

Briefly summarizing, the present invention relates to:

the use of a PRF increased by a factor of N and the intercalated use of N different altitude angles at the PRI level to observe N separate swaths with the same azhnut resolution (in particular, each of the N swaths is observed with the nominal azimuth resolution of the traditional mode of band mapping, (i.e. L / 2,)); and the use of an unincreased PRF and the use of segmentation of N different elevation angles to extend the swath in the range by a factor of N (i.e. allow observation of N swaths, where each has a size comparable to those of swaths observed through acquisitions in conventional strip mapping mode) with azimuth resolution (in particular, each swath is observed with the nominal azimuth resolution of the traditional band mapping mode (ie L / 2)).

It is important to mention that the fact that the present invention makes it possible to carry out N continuous acquisitions in strip mapping (that is to say with integration times equal to those of the traditional mode of strip mapping), and therefore obtaining, for each swath, the maximum nominal azimuth resolution of the traditional banded mapping mode (ie L / 2)); in particular, according to the first aspect of the present invention, each acquisition is performed with the nominal PRF of the antenna of the SAR sensor used.

In conclusion, the present invention exploits the multi-beam acquisition logics which make it possible to simultaneously acquire zones which are not contiguous in the direction orthogonal to the direction of flight of the SAR sensor, unlike traditional techniques of detection. spotlight and band mapping which, on the contrary, do not allow several swaths to be observed simultaneously.

The present invention therefore relates not only to increasing the range of products

0 for systems already produced, but, above all, the introduction of a new methodology for the design of new SAR systems.

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Finally, after having compared the present invention with the traditional Spotlight and band mapping modes, the main differences from the known techniques of high resolution wide swath SAR image generation described above will now be described in detail.

In particular, unlike the present invention, the burst techniques (for example ScanSAR and TOPS) provide for a deterioration of the azimuth resolution in order to increase the swath in the range.

Unlike the present invention, which operates with a single receive channel (i.e. with a single receiver), space division techniques (e.g. DPC and HRWS) and angle division techniques (e.g. MEB and SPCMB) contemplate the use of M tracking systems for simultaneous reception and also contemplate the use of a small antenna (typically, an antenna is divided into M smaller antennas).

The BiDi mode described in Refl has a different purpose, which is Moving Target Identification (MTI) and therefore does not have the purpose of observing multiple swaths in range. In addition, the acquisition geometry is different from that of the present invention; the beam openings currently take place in the azimuth plane and not in the altitude plane.

Further, it should also be noted that Ref2 has an implementation logic at the antenna level and not as an acquisition logic of the present invention. Furthermore, as can be deduced from FIG. 3 of Ref2, the acquisitions are separated by a significant space with regard to the swath of the antenna and they have repeatability characteristics. The segments do not guarantee the continuous sampling of the azimuth spectrum with the natural frequency (PRE) of the antenna, i.e. there is no temporal continuity in the segments regarding the same swathed and therefore, contrary to what is provided by the present invention, the best achievable azimuth resolution is worse than the nominal value of the band mapping mode (ie L / 2). In particular, the geometry shown in Figure 3 of Re! 2 is typical of the ScanSAR mode, which, such as

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- 18 mentioned just before, allows to observe several swaths, but with poor azimuth resolutions compared to the nominal resolution of the band mapping mode (i.e. L / 2) and therefore poor by compared to those obtained by the mowed variables of the present invention.

Finally, it should also be noted that section 5 of Ref3 also presents the ScanSAR mode, i.e. a mode which acquires multiple sub-mows in a range with segments which are not contiguous in time. The segments are sequential and shorter in duration compared to those obtainable in 10 band mapping mode, and therefore, contrary to what is intended by the present invention, there is a degradation of the azimuth resolution by compared to the nominal resolution of the band mapping mode (i.e. L / 2). In addition, Figure 10 of Ref5 does not show the acquisition logic, but only describes the values ​​of the PRE which can be chosen depending on the distance from the scene. In particular, as shown in Figure 10 of the Refi, synchronous and spaced segments are provided on the individual areas in the range, thus forcing degradation of the azimuth resolution. In oure, figure 8 of the Refi illustrates the traditional ScanSAR mode which, as indicated previously, which makes it possible to observe several swaths, but with a mediocre azimuth resolution compared to the nominal resolution of the mode in band mapping (c ' ie L / 2) and, therefore, poor compared to those obtained for the multiple mows of the present invention.

In conclusion, it is clear that various modifications can be made to the present invention without departing from the scope of the invention, as defined in the appended claims.