Multiple-swath stripmap SAR imaging

Abstract

A SAR imaging method is provided that performs N SAR acquisitions in stripmap mode of areas of the earth's surface by means of a synthetic aperture radar transported by an aerial or satellite platform and which includes a single, non-partitioned antenna and a single receiver coupled to the single, non-partitioned antenna, N being an integer greater than one. Each SAR acquisition in stripmap mode is performed using a respective squint angle with respect to the flight direction of the synthetic aperture radar and a respective elevation angle with respect to the nadir of the synthetic aperture radar. The method may further generate SAR images of areas of the respective swath observed via the SAR acquisition in stripmap mode. All SAR images have the same azimuth resolution that is equal to half the physical or equivalent length along the azimuth direction of the single, non-partitioned antenna of the synthetic aperture radar.

Claims

1. SAR imaging method comprising performing N SAR acquisitions in stripmap mode of areas of the earth's surface by means of a synthetic aperture radar that is transported by an aerial or satellite platform (30) and which comprises a single, non-partitioned antenna and a single receiver coupled to said single, non-partitioned antenna, N being an integer greater than one; wherein the single, non-partitioned antenna of the synthetic aperture radar is associated with a nominal pulse repetition frequency; wherein each SAR acquisition in stripmap mode is performed using a respective squint angle with respect to the flight direction of the synthetic aperture radar, said respective squint angle being equal to, or different from, the squint angles used for performing the other N1 SAR acquisitions in stripmap mode; wherein each SAR acquisition in stripmap mode is performed using a respective elevation angle with respect to the nadir of the synthetic aperture radar, said respective elevation angle being different from the elevation angles used for performing the other N1 SAR acquisitions in stripmap mode, thereby resulting in that each SAR acquisition in stripmap mode is related to a respective swath of the earth's surface which is different from the swaths observed via the other N1 SAR acquisitions in stripmap mode; wherein each performed SAR acquisition in stripmap mode comprises respective radar transmission and reception operations that: are time interleaved, individually or in groups, with single, or groups of, radar transmission and reception operations of the other N1 SAR acquisitions in stripmap mode performed; and comprise the transmission and reception of respective radar beams in respective acquisition directions that are defined by the respective squint angle and by the respective elevation angle used for said SAR acquisition in stripmap mode, thereby resulting in that said respective acquisition directions are parallel to each other and not parallel to the acquisition directions of the other N-1 SAR acquisitions in stripmap mode performed; wherein the radar transmission and reception operations are performed by: using an operational repetition frequency that is increased by a factor of N with respect to the nominal pulse repetition frequency; or a burst use of the N different elevation angles to extend the swath in range by a factor of N; the method further comprising generating, on the basis of each SAR acquisition in stripmap mode performed, SAR images of areas of the respective swath observed via said SAR acquisition in stripmap mode; wherein all the generated SAR images have one and the same azimuth resolution that is equal to half the physical or equivalent length along the azimuth direction of the single, non-partitioned antenna of the synthetic aperture radar.

2. The method of claim 1, wherein the respective radar transmission and reception operations of each performed SAR acquisition in stripmap mode are individually time interleaved with single radar transmission and reception operations of the other N1 SAR acquisitions in stripmap mode performed; and wherein the radar transmission and reception operations are performed with an operational repetition frequency equal to N times the nominal pulse repetition frequency so that each of the N SAR acquisitions in stripmap mode is performed with the nominal pulse repetition frequency.

3. The method of claim 1, wherein the respective radar transmission and reception operations of each performed SAR acquisition in stripmap mode are time interleaved, in groups, with groups of radar transmission and reception operations of the other N1 SAR acquisitions in stripmap mode performed; and wherein the radar transmission and reception operations are performed with an operational repetition frequency comparable to the nominal pulse repetition frequency.

4. The method of claim 3, wherein the groups of radar transmission and reception operations of the N different SAR acquisitions in stripmap mode are time interleaved according to a periodic or random interleaving pattern.

5. Synthetic aperture radar system that comprises a single, non-partitioned antenna and a single receiver coupled to said single, non-partitioned antenna; and that is configured to perform the SAR imaging method claimed in claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) For a better understanding of the present invention, some preferred embodiments, provided by way of non-limitative example, will now be described with reference to the attached drawings (not to scale), where:

(2) FIGS. 1 and 2 schematically show a typical acquisition geometry for SAR images in stripmap mode;

(3) FIG. 3 schematically shows a typical acquisition geometry for SAR images in spotlight mode;

(4) FIG. 4 schematically shows an example of logic for SAR acquisition in stripmap mode according to a first aspect of the present invention;

(5) FIGS. 5 and 6 schematically show effects of applying a first acquisition strategy in performing a technique of SAR acquisition in stripmap mode according to a second aspect of the present invention; and

(6) FIGS. 7 and 8 schematically show effects of applying a second acquisition strategy in performing the technique of SAR acquisition in stripmap mode according to the second aspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(7) The following description is provided to enable an expert in the field to embody and use the invention. Various modifications to the embodiments shown will be immediately obvious to experts and the generic principles described herein could be applied to other embodiments and applications without departing from the scope of protection of the present invention.

(8) Thus, the present invention is not intended to be limited to just the embodiments described and shown herein, but is to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims.

(9) The present invention derives from the applicant's insight to exploit the steering capability of the antennas used in the SAR sensors in a non-conventional manner. The applicant then conceived a multi-beam and multi-temporal SAR acquisition technique that exploits the transmission and reception characteristics of a SAR sensor in time-sharing.

(10) In particular, the idea on which the present invention is based is that of dividing a SAR acquisition in stripmap mode into N elemental acquisitions in stripmap mode (with N>1) and to combine them so as to obtain N sets of SAR images in which each set is related to a respective swath (namely so as to observe N distinct swaths with the same azimuth resolution).

(11) In detail, a first aspect of the present invention concerns performing N different SAR acquisitions using N different elevation angles so as to observe N different swaths.

(12) Specifically, the idea on which said first aspect of the present invention is based is that of performing several SAR acquisitions interleaved at pulse repetition interval (PRI) level, which represents the time between two consecutive transmitted pulses, in particular SAR acquisitions in which the acquisition elevation direction of the antenna changes at PRI level. In order to do this, a pulse repetition frequency (PRF, where PRF=1/PRI) is used that is N times higher with respect to the nominal PRF associated with the antenna of the SAR sensor used. Returning briefly to FIG. 1, for clarity of description, it should be remembered that the elevation angle is the angle between the pointing direction sr of the antenna of the SAR sensor and the nadir direction z of the SAR sensor.

(13) By increasing the PRF by a factor of N, the N different strips will have N times less time available and therefore, in general, they will have a swath reduced by a factor of N; however, by adding the N different observed swaths together, the typical swath size of the classical stripmap mode is still obtained. As the individual strips are smaller by a factor of N, it is possible to use an antenna N times larger in elevation, thereby increasing product directivity, i.e. sensibility. In general, the antenna can be exploited in a more effective manner.

(14) Furthermore, by increasing the PRF by a factor of N, N stripmap acquisitions can be obtained, individually having PRFs compatible with the antenna's size (in this way, azimuth ambiguity values are not altered).

(15) Although the technique of SAR acquisition in stripmap mode according to the above-stated first aspect of the present invention can be used with a generic integer N greater than one, hereinafter, for simplicity of description and without loss of generality, examples will be shown for N=2, it being understood that the concepts explained hereinafter in relation to the case N=2 are also applicable mutatis mutandis in the case of a generic integer N greater than one.

(16) For a better understanding of the second aspect of the present invention, FIG. 4 schematically illustrates an example of logic for SAR acquisition in stripmap mode according to said first aspect of the present invention in the case of N=2 and in the case of a satellite application (it being understood that this SAR acquisition logic can also be advantageously used in the case of aerial platforms such as, for example, an aircraft, a UAV, or a helicopter).

(17) In particular, FIG. 4 (in which the Cartesian reference system used substantially corresponds to that previously introduced for FIGS. 1-3) shows a satellite 30 that moves along a flight direction d and is equipped with a SAR sensor (not shown in FIG. 4 for simplicity of illustration), which, in turn, is equipped with a single, non-partitioned antenna (not shown in FIG. 4 for simplicity of illustration) that is coupled to a single receiver (not shown in FIG. 4 for simplicity of illustration) and is associated with a given nominal pulse repetition frequency PRF.sub.nom.

(18) In the example shown in FIG. 4, the SAR sensor is used with an operational pulse repetition frequency PRF.sub.op twice that of the antenna's nominal pulse repetition frequency PRF.sub.nom (i.e. PRF.sub.op=2PRF.sub.nom) so as to transmit successive pulses at a temporal distance PRI.sub.op=1/(2PRF.sub.nom). In particular, as shown in FIG. 4, the SAR sensor on board the satellite 30 performs a series of first SAR acquisitions using a first elevation angle and a series of second SAR acquisitions using a second elevation angle, in which said first and second SAR acquisitions are interleaved at PRI level (namely, a first SAR acquisition always alternates with a second SAR acquisition) and said first and second elevation angles are different from each other. In other words, a first SAR acquisition is performed using the first elevation angle, then, at a temporal distance PRI.sub.op=1/PRF.sub.op=1/(2PRF.sub.nom), a second SAR acquisition is performed using the second elevation angle, then, always at a temporal distance PRI.sub.op=1/PRF.sub.op=1/(2PRF.sub.nom), a first SAR acquisition is performed again using the first elevation angle and so on, always alternating the execution of a first SAR acquisition with the execution of a second SAR acquisition and spacing the different acquisitions by a time period PRI.sub.op=1/PRF.sub.op=1/(2PRF.sub.nom). In this way, the SAR sensor on board the satellite 30 is able to observe two distinct swaths (as shown in FIG. 4).

(19) It is important to note that the acquisitions regarding a same swath are performed with the nominal pulse repetition frequency PRF.sub.nom of the antenna, i.e. the first SAR acquisitions are performed with the nominal pulse repetition frequency PRF.sub.nom of the antenna and the second SAR acquisitions are also performed with the nominal pulse repetition frequency PRF.sub.nom of the antenna. In this way, azimuth ambiguity values are not altered.

(20) More in general, since the operational pulse repetition frequency PRF.sub.op used is N times (two times in the example in FIG. 4) greater than that required/nominal PRF.sub.nom, the individual acquisitions will have nominal PRFs, and therefore all product quality parameters will remain unaltered. To all intents and purposes, the technique of SAR acquisition in stripmap mode according to the first aspect of the present invention enables separating the swath in range into N swaths of reduced size (approximately 1/N) without altering the other parameters, such as the azimuth resolution for example.

(21) All N SAR acquisitions can be performed using a same squint angle, or each SAR acquisition can be performed using a respective squint angle different from that used for performing the other N1 SAR acquisitions so as to obtain, for each acquisition, an integration time equal to the standard stripmap one.

(22) Therefore, the technique of SAR acquisition in stripmap mode according to the first aspect of the present invention enables serving two (or N in the generic case) users interested in areas of medium extension and separated from one another in the elevation plane. With the traditional stripmap technique, these requests would be in conflict and therefore it would not be possible to serve them simultaneously.

(23) 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 SAR 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.

(24) In particular, said second aspect of the present invention relates to a so-called burst-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 elevation at PRI level, but by varying the antenna's acquisition direction in elevation in PRI blocks.

(25) Specifically, the second aspect of the present invention concerns a burst-mode stripmap technique in which the N stripmap acquisitions are performed without increasing the PRF and by varying the acquisition elevation direction of the antenna, i.e. the elevation angle used, in PRI blocks.

(26) The burst-mode stripmap technique with unincreased PRF and variation of the elevation angle according to the second aspect of the present invention enables extending the swath in range, even to the point of doubling it.

(27) In order to divide the acquisition in two (N 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 ISLR (Integrated Side Lobe Ratio) parameter deteriorates, but not the PSLR (Peak Side Lobe Ratio). Vice versa, by using periodic execution patterns for the two (N 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 NESZ (Noise Equivalent Sigma Zero) parameter.

(28) 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 burst-mode stripmap technique with unincreased 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 stripmap technique with unincreased PRF according to the second aspect of the present invention.

(29) 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.

(30) Briefly summarizing, the present invention concerns: the use of a PRF increased by a factor of N and the interleaved use of N different elevation angles at PRI level to observe N separate swaths with the same azimuth resolution (in particular, each of the N swaths is observed with the nominal azimuth resolution of the traditional stripmap mode (i.e. L/2)); and the use of an unincreased PRF and the burst use of N different elevation angles to extend the swath in range by a factor of N (i.e. enabling the observation of N swaths, each of which has a size comparable to those of swaths observed via conventional stripmap mode acquisitions) with the same azimuth resolution (in particular, each swath is observed with the nominal azimuth resolution of the traditional stripmap mode (i.e. L/2)).

(31) It is important to underline the fact that the present invention enables performing N continuous stripmap acquisitions (i.e. with integration times equal to those of the traditional stripmap mode), thereby obtaining, for each swath, the nominal maximum azimuth resolution of the traditional stripmap mode (i.e. 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.

(32) In conclusion, the present invention exploits multi-beam acquisition logics that enable simultaneously acquiring areas that are non-contiguous in the direction orthogonal to the flight direction of the SAR sensor, unlike the traditional spotlight and stripmap techniques that, on the contrary, do not allow simultaneously observing several swaths.

(33) 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.

(34) Finally, after having compared the present invention with the traditional spotlight and stripmap modes, the main differences from the known techniques of high-resolution wide-swath SAR image generation previously described will now be described as well.

(35) In 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.

(36) Unlike the present invention, which functions with a single receiving channel (i.e. with a single receiver), the space-division techniques (e.g. DPC and HRWS) and the angle-division ones (e.g. MEB and SPCMB) envisage the use of M systems for simultaneous reception and also envisage the use of a small antenna (typically, an antenna is partitioned into M smaller antennas).

(37) The BiDi mode described in Ref1 has a different purpose, that of Moving Target Identification (MTI) and therefore does not have the object of observing several swaths in range. Furthermore, the acquisition geometry is different from that of the present invention; beam switching actually takes place on the azimuth plane and not on the elevation plane.

(38) Furthermore, it should also be noted that Ref2 has antenna-level implementation logic and not acquisition logic as the present invention. In addition, as can be inferred from FIG. 3 of Ref2, the acquisitions are separated by a significant space with respect to the antenna's swath and have repeatability characteristics. The bursts do not guarantee continuous sampling of the azimuth spectrum with the natural frequency (PRF) of the antenna, i.e. there is no temporal continuity in the bursts regarding a same swath and therefore, unlike that envisaged by the present invention, the best achievable azimuth resolution is worse than the nominal one of the stripmap mode (i.e. L/2). In particular, the geometry shown in FIG. 3 of Ref2 is that typical of the ScanSAR mode, which, as just said, enables observing multiple swaths, but with azimuth resolutions worse than the nominal resolution of the stripmap mode (i.e. L/2) and, consequently, worse than those obtained for the various swaths by the present invention.

(39) Finally, it should also be noted that section 5 of Ref3 also presents the ScanSAR mode, i.e. a mode that acquires multiple sub-swaths in range with bursts that are not contiguous in time. The bursts are sequential and of shorter duration with respect to those obtainable in stripmap mode, and therefore, contrary to that envisaged by the present invention, there is deterioration in azimuth resolution with reference to the nominal resolution of the stripmap mode (i.e. L/2). Furthermore, FIG. 10 of Ref3 does not show acquisition logic, but only describes the PRF values that can be chosen depending on the distance of the scene. In particular, according to that shown in FIG. 10 of Ref3, synchronous and spaced out bursts are envisaged on the individual areas in range, thereby forcing degradation of azimuth resolution. In addition, FIG. 8 of Ref3 illustrates the traditional ScanSAR mode that, as previously indicated, enables observing multiple swaths, but with azimuth resolutions worse than the nominal resolution of the stripmap mode (i.e. L/2) and, consequently, worse than those obtained for the various swaths by the present invention.

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