Method for Performing SAR Acquisitions with Increased Swath Size

Abstract

The present invention concerns a method for performing SAR acquisitions, which comprises performing SAR acquisitions in Spotlight/Stripmap mode of areas/swaths of earth's surface by means of a SAR system carried by an air or space platform along a flight direction, whereby: an azimuth direction is defined by a ground track of the flight direction on the earth's surface, a nadir direction is defined that is orthogonal to the earth's surface, to the flight direction and to the azimuth direction, an across-track direction is defined that lies on the earth's surface and is orthogonal to the azimuth direction and to the nadir direction, and, for each acquired area/swath of the earth's surface, a respective range direction is defined that extends from the synthetic aperture radar system to said acquired area/swath. Performing SAR acquisitions in Spotlight/Stripmap mode of areas/swaths of earth's surface includes contemporaneously acquiring P areas or portions of P swaths in a pulse repetition interval having a predefined time length, P being an integer greater than one. Said P areas/swaths are separated along the across-track direction and are spaced apart from each other along the across-track direction and from the SAR system along the respective range direction by predefined distances. Said predefined time length and said predefined distances are such that to enable contemporaneous acquisition of said P areas or of portions of said P swaths in said pulse repetition interval.

Claims

1. Method for performing SAR acquisitions, comprising performing SAR acquisitions in Spotlight/Stripmap mode of areas/swaths of earth's surface by means of a synthetic aperture radar system carried by an air or space platform along a flight direction, whereby: an azimuth direction is defined by a ground track of the flight direction on the earth's surface, a nadir direction is defined that is orthogonal to the earth's surface, to the flight direction and to the azimuth direction, an across-track direction is defined that lies on the earth's surface and is orthogonal to the azimuth direction and to the nadir direction, and, for each acquired area/swath of the earth's surface, a respective range direction is defined that extends from the synthetic aperture radar system to said acquired area/swath; wherein performing SAR acquisitions in Spotlight/Stripmap mode of areas/swaths of earth's surface includes contemporaneously acquiring, in a pulse repetition interval having a predefined time length, P areas or portions of P swaths by using: P transmission radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P areas/swaths; and P reception radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P areas/swaths; wherein: P is an integer greater than one; the P areas/swaths are separated along the across-track direction and are spaced apart from each other along the across-track direction and from the synthetic aperture radar system along the respective range direction by predefined distances; and said predefined time length and said predefined distances are such that to enable contemporaneous acquisition of said P areas or of the portions of said P swaths in said pulse repetition interval.

2. The method of claim 1, wherein the predefined time length and the predefined distances are such that to enable contemporaneous acquisition of said P areas or of portions of said P swaths in each pulse repetition interval.

3. The method of claim 1, wherein the SAR acquisitions in Spotlight/Stripmap mode are performed in a time division fashion, and wherein, in each pulse repetition interval, P respective areas or portions of P respective swaths are contemporaneously acquired by using: P respective transmission radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P respective areas/swaths; and P respective reception radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P respective areas/swaths; and wherein: for each pulse repetition interval, the respective P areas/swaths are separated along the across-track direction and are spaced apart from each other along the across-track direction and from the synthetic aperture radar system along the respective range direction by respective predefined distances; the predefined time length and the respective predefined distances associated with the P respective areas/swaths contemporaneously acquired in each PRI are such that the areas or the swaths' portions acquired in T successive pulse repetition intervals form an overall region that is continuous along the across-track direction, T being an integer greater than one; and the transmission and reception radar beams used in T successive pulse repetition intervals form an elevation-continuous angular span.

4. The method according to claim 1, wherein the SAR acquisitions in Spotlight/Stripmap mode are performed by using, in transmission and/or reception, an antenna of the synthetic aperture radar system partitioned into P different zones.

5. The method of claim 4, wherein the SAR acquisitions in Spotlight/Stripmap mode are performed by using, in transmission and/or reception, the antenna of the synthetic aperture radar system partitioned into P different zones in elevation.

6. The method according to claim 1, wherein the SAR acquisitions in Spotlight/Stripmap mode are performed by using different squint angles with respect to the azimuth direction and/or orthogonal waveforms such that to increase range ambiguity performance.

7. Synthetic aperture radar system installed on board an air or space platform and configured to carry out the method for performing SAR acquisitions as claimed in claim 1.

8. Space platform equipped with a synthetic aperture radar system configured to carry out the method for performing SAR acquisitions as claimed in claim 1.

9. The space platform of claim 8, wherein said space platform is a spacecraft or a satellite.

10. Air platform equipped with a synthetic aperture radar system configured to carry out the method for performing SAR acquisitions as claimed in claim 1.

11. The air platform of claim 10, wherein said air platform is an aircraft, a drone or a helicopter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] For a better understanding of the present invention, preferred embodiments, which are intended purely by way of non-limiting examples, will now be described with reference to the attached drawings (all not to scale), wherein:

[0060] FIGS. 1A and 1B schematically illustrate an example of transmission and reception operations according to the space-sharing SAR technique named Displaced Phase Centers (DPC);

[0061] FIGS. 2A, 2B and 2C schematically illustrate an example of transmission and reception operations according to the angular-sharing SAR technique named Multiple Elevation Beam (MEB);

[0062] FIGS. 3A and 3B schematically illustrate an example of transmission and reception operations according to the angular-sharing SAR technique named Single Phase Centre MultiBeam (SPCMB);

[0063] FIGS. 4A and 4B schematically illustrate an example of transmission and reception operations according to the time-sharing SAR technique named DIscrete Stepped Strip (DI2S);

[0064] FIGS. 5A, 5B and 5C schematically illustrate a non-limiting example of implementation of a method for performing SAR acquisitions according to a preferred embodiment of the present invention;

[0065] FIGS. 6-8 show examples of features/performance of the present invention;

[0066] FIGS. 9 and 10 show possible solutions for antennas used in reception according to preferred, non-limiting embodiments of the present invention; and

[0067] FIGS. 11-13 show possible solutions for antennas used in transmission according to preferred, non-limiting embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0068] The following description is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the present invention as claimed. Thence, the present invention is not intended to be limited to the embodiments shown and described, but is to be accorded the widest scope of protection consistent with the principles and features disclosed herein and defined in the appended claims.

[0069] The present invention stems from Applicant's idea of merging peculiarities of the time-sharing DI2S technique with those of the angular-sharing MEB technique so as to reduce their respective drawbacks and to synergistically combine their respective positive aspects.

[0070] In particular, the present invention concerns a method for performing SAR acquisitions that has been named by the Applicant “DIstributed Sparse Sampling for SAR Systems” (DI4S) and that allows acquiring: [0071] SAR images in Stripmap mode of [0072] multiple swaths with nominal Stripmap azimuth resolution and nominal Stripmap swath size (more specifically, nominal Stripmap swath width), or [0073] a single swath with nominal Stripmap azimuth resolution and increased swath size (namely, swath width increased with respect to nominal Stripmap swath width); or [0074] SAR images in Spotlight mode of [0075] multiple areas with nominal Spotlight azimuth resolution and nominal Spotlight area size (more specifically, nominal Spotlight area width), or [0076] a single area with nominal Spotlight azimuth resolution and increased area size (namely, area width increased with respect to nominal Spotlight area width).

[0077] In detail, the present invention concerns a method that comprises performing SAR acquisitions in Spotlight/Stripmap mode of areas/swaths of earth's surface by means of a synthetic aperture radar (SAR) system carried by an air or space platform (e.g., an aircraft/drone/helicopter or a satellite/spacecraft) along a flight direction, whereby: [0078] an azimuth direction is defined by a ground track of the flight direction on the earth's surface, [0079] a nadir direction is defined that is orthogonal to the earth's surface, to the flight direction and to azimuth direction, [0080] an across-track direction is defined that lies on the earth's surface and is orthogonal to the azimuth direction and to the nadir direction, and, [0081] for each acquired area/swath of the earth's surface, a respective range direction is defined that extends from the SAR system to said acquired area/swath.

[0082] More specifically, performing SAR acquisitions in Spotlight/Stripmap mode of areas/swaths of earth's surface includes contemporaneously acquiring P areas or portions of P swaths in a pulse repetition interval (PRI) having a predefined time length, wherein P is an integer greater than one (i.e., P>1).

[0083] Said P areas/swaths are separated along the across-track direction and are spaced apart from each other along the across-track direction and from the SAR system along the respective range direction by predefined distances.

[0084] Said predefined time length and said predefined distances are such that to enable contemporaneous acquisition of said P areas or of portions of said P swaths in said PRI.

[0085] Conveniently, contemporaneously acquiring P areas or portions of P swaths in a PRI includes using: [0086] P transmission radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P areas/swaths, or a single transmission radar beam that is such that to illuminate, with one or more transmitted radar signals, said P areas or portions of said P swaths; and [0087] P reception radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P areas/swaths.

[0088] According to a first specific preferred embodiment of the present invention, the predefined time length and the predefined distances are such that to enable contemporaneous acquisition of said P areas or of portions of said P swaths in each PRI.

[0089] Instead, according to a second specific preferred embodiment of the present invention, an operational pulse repetition frequency (PRF) is conveniently used that is increased by T times with respect to the nominal PRF associated with the SAR system, wherein T is an integer greater than one (i.e., T>1) and wherein: [0090] the SAR acquisitions in Spotlight/Stripmap mode are performed in a time division fashion, whereby in each PRI P respective areas or portions of P respective swaths are contemporaneously acquired; [0091] for each PRI, the P respective areas/swaths are separated along the across-track direction and are spaced apart from each other along the across-track direction and from the SAR system along the respective range direction by respective predefined distances; and [0092] the predefined time length and the respective predefined distances associated with the P respective areas/swaths contemporaneously acquired in each PRI are such that the areas or the swaths' portions acquired in T successive PRIs form an overall region that is continuous (i.e., does not comprise “holes”) along the across-track direction.

[0093] Conveniently, according to said second specific preferred embodiment of the present invention, for each PRI, the respective P areas/swaths are contemporaneously acquired by using: [0094] P respective transmission radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P respective areas/swaths, or a single transmission radar beam that is such that to illuminate, with one or more transmitted radar signals, said P respective areas or portions of said P respective swaths; and [0095] P respective reception radar beams that are angularly separated in elevation with respect to the nadir direction so as to be pointed, each, at a respective one of said P respective areas/swaths;

[0096] wherein the transmission and reception radar beams used in T successive PRIs form an elevation-continuous angular span (i.e., a continuous angular span without angular interruptions/holes along the across-track direction).

[0097] Conveniently, the SAR acquisitions in Spotlight/Stripmap mode are performed by using, in transmission and/or reception, an antenna of the SAR system partitioned into P different zones.

[0098] More conveniently, the SAR acquisitions in Spotlight/Stripmap mode are performed by using, in transmission and/or reception, an antenna of the SAR system partitioned into P different zones in elevation (i.e., along the nadir direction).

[0099] Conveniently, the SAR acquisitions in Spotlight/Stripmap mode are performed by using different squint angles with respect to the azimuth direction and/or orthogonal waveforms such that to increase range ambiguity performance.

[0100] Conveniently, the P×T areas or swaths' portions acquired in T successive PRIs are individually processed, then correlated and, finally, information merging is carried out, so as to reduce/compensate for space errors, such as those related to channel synchronization and Doppler parameter estimation.

[0101] As previously explained, one of the constraints limiting swath size in range (or, equivalently, along the across-track direction that corresponds to the ground track of the range direction on the earth's surface) is that, with the known SAR techniques, it is not virtually possible to acquire and receive simultaneously. This constraint is synthesized by the following equation (already explained in the foregoing):

[00004]$\mathrm{\Delta R}\le \left(\frac{1}{\mathrm{PRF}}-2\tau \right)\frac{c}{2}.$

[0102] On the contrary, transmitting towards and receiving from zones that are separated in range (i.e., along the across-track direction), as taught by the present invention, allows to overcome such a constraint and, hence, to increase the size in range (i.e., along the across-track direction) of the acquired swath(s).

[0103] Moreover, by using a given PRF (e.g., the nominal one or an increased one) and, hence, a given PRI's time length, it is possible to acquire at the same time different zones separated in range which are spaced apart from each other along the across-track direction and from the used SAR along the respective range direction by predefined distances. In particular, the given PRI's time length and said predefined distances are selected (namely, are determined a priori) so as to enable cotemporaneous acquisition of said different zones. In other words, with the same PRF it is possible to acquire at the same time different zones, if these zones have different rank (transmission and reception distance in PRI).

[0104] Conveniently, in order to acquire the P different zones separated in range (i.e., along the across-track direction), P receivers may be used. Moreover, since the P zones are separated in range, there is no impact on range ambiguity level (anyway, it is possible to use different squint angles with respect to the azimuth direction and/or orthogonal waveforms in order to increase range ambiguity performance).

[0105] For a better understanding of the present invention, FIGS. 5A, 5B and 5C schematically illustrate a non-limiting example of implementation of a method according to a preferred embodiment of the present invention, wherein T=1 and P=2.

[0106] In particular, FIGS. 5A and 5B show a SAR system 50 that is installed on board, and is carried in flight/orbit along a flight direction d by, by an air/space platform (not shown in FIGS. 5A and 5B) such as an aircraft, a drone, a helicopter, a satellite or a spacecraft, whereby: [0107] an azimuth direction x is defined by a ground track of the flight direction d on the earth's surface, [0108] a nadir direction z is defined that is orthogonal to the earth's surface, to the flight direction d and to the azimuth direction x, [0109] an across-track direction y is defined that lies on the earth's surface and is orthogonal to the azimuth direction x and to the nadir direction z.

[0110] More specifically, FIG. 5A shows a three-dimensional acquisition geometry, while FIG. 5B shows the acquisition geometry in the plane zy.

[0111] As shown in FIGS. 5A and 5B, at a given PRI, the SAR system 50 contemporaneously acquires a first portion A1 of a first swath S1 and a second portion A2 of a second swath S2, wherein: [0112] said first and second swaths S1 and S2 are separated along the across-track direction y; and [0113] the SAR system 50 contemporaneously uses two different radar beams that have different elevation angles with respect to the nadir direction z, are angularly separated in elevation (i.e., with respect to the nadir direction z) and are pointed, each, at a respective one of the first and second portions/swaths A1/S1 and A2/S2.

[0114] Additionally, FIG. 5C shows the acquisition geometry in time domain. In particular, as shown in FIG. 5C, in each PRI (wherein all the PRIs have one and the same predefined time length), the SAR system contemporaneously transmit towards and, then, contemporaneously receive from the first and second swaths S1 and S2, which are spaced apart from each other along the across-track direction y and from the SAR system 50 along a respective range direction (that extends from said SAR system 50 to, respectively, the first or second swath S1/S2) by predefined distances. Said predefined time length and said predefined distances are such that: [0115] the radar echoes from the first portion A1 of the first swath S1 are received by the SAR system 50 after approximately three PRIs from the transmission, by said SAR system 50, of the corresponding radar signals that have illuminated said first portion A1 and, hence, have produced said radar echoes therefrom; while [0116] the radar echoes from the second portion A2 of the second swath S2 are received by the SAR system 50 after approximately five PRIs from the transmission, by said SAR system 50, of the corresponding radar signals that have illuminated said second portion A2 and, hence, have produced said radar echoes therefrom.

[0117] In other words, the SAR acquisitions are organized in time domain so that the first and second swaths S1 and S2 have substantially one and the same distance within the same PRI. Obviously, the closest swath S1 is spaced apart from the SAR system 50 by a smaller distance than the second swath S2, but the time length of the PRIs is chosen so that the residue of the distance after an integer number of PRIs (rank) is similar. This allows to contemporaneously acquire the two separate swaths S1 and S2. The ambiguity performance is guaranteed by the angular distance and, hence, by the different antenna gain values. As previously explained, in order to increase range ambiguity performance, it is possible to use different squint angles with respect to the azimuth direction x and/or orthogonal waveforms.

[0118] FIG. 6 shows an example of transmission pattern illuminating two different zones that are non-contiguous along the across-track direction, wherein T=1 and P=2. Instead, FIGS. 7 and 8 show the two-ways range pattern of each of the two channels. The two-ways range pattern is minimally altered with respect to the nominal case, as shown in FIGS. 7 and 8.

[0119] It is important to highlight that the present invention can be advantageously exploited with both Stripmap and Spotlight modes.

[0120] As previously explained, the present invention involves contemporaneous acquisition, within one and the same PRI, of P different and separate zones. This can be accomplished by means of different solutions based, for example, on multi-feed reflector antennas, active arrays or hybrid solutions (e.g., a reflector antenna fitted with an active array acting as feed thereof).

[0121] Hereinafter the case of an active array will be analyzed, remaining it clear that the same logic or equivalent ones may be applied, mutatis mutandis, also to other antenna typologies.

[0122] In particular, in the following, examples of different logic approaches usable with an active array will be described, wherein P is assumed, for simplicity, to be equal to two (i.e., P=2).

[0123] More specifically, when an active array is used in reception, two main logics may be conveniently exploited:

[0124] 1) a partition in elevation of the antenna—namely, as shown in FIG. 9, the used antenna (in FIG. 9 denoted as a whole by 61) may be conveniently partitioned into two halves (more in general, into P portions) in elevation (i.e., along the nadir direction) and each half may be conveniently exploited to receive backscattered signal(s) from a different area; since, differently from the known SAR techniques, it is not necessary to acquire a single wide zone, it is possible to increase height of the antenna 61 so that each of the two halves is sized coherently with the area to be acquired; in this respect, it is worth noting that the space division techniques require acquisition of a wide swath in azimuth and, hence, require that the antenna be partitioned in azimuth so that the single sub-antennas have a predefined size depending on the desired resolution (namely, reduced by a factor that is at least equal to the desired resolution enhancement factor); therefore, differently from the present invention that allows to compensate the partition in elevation by a higher antenna, the space division techniques cannot use a longer antenna to recover directivity loss; in some cases, in order for the directivity loss to be recovered, the use of higher antennas has been proposed in the past but, since it is necessary to acquire the whole area, it is required that a further complication of dynamic beam re-pointing in elevation be introduced (so-called “SCan On Receive”);

[0125] 2) an exploitation of the whole antenna (as shown in FIG. 10, where the antenna is denoted as a whole by 62) by digitally or analogically dividing the signal received by the single antenna elements into two parts (more in general, into P parts) and, then, by applying amplitude and phase modulations to each signal part to obtain the desired beams and, hence, to acquire the desired zones.

[0126] The first solution has an easier application but suffers a directivity loss of approximately a P factor (unless the height of the antenna is increased thereby completely preventing such a loss). On the contrary, the second solution does not affect the directivity.

[0127] Instead, in transmission, it is possible to use multiple solutions:

[0128] 1) similarly to the first solution in reception, the used antenna may be conveniently partitioned into two halves (more in general, into P portions) in elevation; as shown in FIG. 11 (where the antenna is denoted as a whole by 71), each of the two halves will illuminate the desired zone; also in this case, in order to recover directivity, it is possible to increase the height of the antenna 71 without introducing other necessities;

[0129] 2) as shown in FIG. 12, the antenna (denoted as a whole by 72) may be conveniently partitioned in homogeneous or chaotic blocks, whereby it is possible to modulate the single blocks in order to illuminate the desired areas; the impact on the directivity will depend on distribution of the single blocks and, hence, on the equivalent sampling of the single parts in which the antenna 72 is divided;

[0130] 3) as shown in FIG. 13, the antenna (denoted as a whole by 73) may be conveniently partitioned in homogeneous blocks, complying with sampling requirements, whereby it is possible to modulate the single blocks in order to illuminate the desired areas; in this case there is no directivity alteration.

[0131] The following Table II summarizes the main differences between the present invention and the known SAR techniques.

TABLE-US-00002 TABLE II DIFFERENCES WITH RESPECT TO TECHNIQUE THE PRESENT INVENTION Angular Sharing (MEB) The angular sharing technique involves the transmission of a large range beam and the simultaneous reception of different range- continuous zones and, in any case, the time constraint is not overcome. Instead, the present invention involves the contemporaneous acquisition (i.e., transmission and reception) of range- separated zones. Time Sharing The time sharing technique involves the acquisition of multiple non-contiguous zones, but not simultaneously. Additionally, the time sharing technique reduces the performance of the single acquisition (in term of swath size or of impulse response function quality). Instead, the present invention involves the contemporaneous acquisition of range- separated zones.

[0132] In view of the foregoing, the technical advantages and the innovative features of the present invention are immediately clear to those skilled in the art.

[0133] In conclusion, it is clear that numerous modifications and variants can be made to the present invention, all falling within the scope of the invention, as defined in the appended claims.