Patent ID: 12235352

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

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.

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

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 generating SAR images with enhanced azimuth resolution, while avoiding swath size reduction.

In detail, the present invention concerns a method that comprises performing, in a time division fashion, SAR acquisitions of areas of a swath of earth's surface by means of a SAR system carried by an air or space platform (e.g., an aircraft/drone/helicopter or a satellite/spacecraft).

More specifically, performing SAR acquisitions in a time division fashion includes contemporaneously acquiring, in each pulse repetition interval (PRI), a plurality of areas of the swath that are separated in azimuth (i.e., along an azimuth direction that is defined by a ground track of a flight direction of the SAR system and that is parallel to said fight direction).

Moreover, the areas acquired in T successive pulse repetition intervals (PRIs) form an azimuth-continuous portion of said swath (i.e., a continuous portion without “holes” along the azimuth direction), wherein T is an integer greater than one (i.e., T>1).

Preferably, contemporaneously acquiring, in each PRI, a plurality of areas of the swath that are separated in azimuth includes:transmitting a plurality of radar signals by contemporaneously using different transmission radar beams, and receiving a plurality of backscattered radar signals by contemporaneously using different reception radar beams, whereinthe transmission radar beams are angularly separated in azimuth (i.e., along the azimuth direction) so as to be pointed, each, at a respective one of the areas of the swath to be contemporaneously acquired, andthe reception radar beams are angularly separated in azimuth (i.e., along the azimuth direction) so as to be pointed, each, at a respective one of the areas of the swath to be contemporaneously acquired; ortransmitting one or more radar signals by using a single transmission radar beam, and receiving a plurality of backscattered radar signals by contemporaneously using different reception radar beams, whereinthe single transmission radar beam is such that to illuminate, with the transmitted radar signal(s), all the areas of the swath to be contemporaneously acquired (i.e., said single transmission radar beam has a size such that to, and is pointed so as to, “cover” all the areas of the swath to be contemporaneously acquired), andthe reception radar beams are narrower than said single transmission radar beam and are angularly separated in azimuth (i.e., along the azimuth direction) so as to be pointed, each, at a respective one of the areas of the swath to be contemporaneously acquired;

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

Preferably, contemporaneously acquiring, in each PRI, a plurality of areas of the swath that are separated in azimuth includes contemporaneously acquiring, in each PRI, P areas of the swath that are separated in azimuth, P being an integer greater than one (i.e., P>1).

Conveniently, contemporaneously acquiring, in each PRI, P areas of the swath that are separated in azimuth includes:transmitting a plurality of radar signals by contemporaneously using P transmission radar beams, and receiving a plurality of backscattered radar signals by contemporaneously using P reception radar beams, whereinthe P transmission radar beams are angularly separated in azimuth so as to be pointed, each, at a respective one of the P areas of the swath to be contemporaneously acquired, andthe P reception radar beams are angularly separated in azimuth so as to be pointed, each, at a respective one of the P areas of the swath to be contemporaneously acquired; ortransmitting one or more radar signals by using a single transmission radar beam, and receiving a plurality of backscattered radar signals by contemporaneously using P reception radar beams, whereinthe single transmission radar beam is such that to illuminate, with the transmitted signal(s), all the P areas of the swath to be contemporaneously acquired, andthe P reception radar beams are angularly separated in azimuth, are narrower than said single transmission radar beam, and are pointed, each, at a respective one of the P areas of the swath to be contemporaneously acquired;

wherein the transmission and reception radar beams used in T successive PRIs form an azimuth-continuous angular span. Conveniently, contemporaneously acquiring, in each PRI, P areas of the swath that are separated in azimuth includes using, in transmission and/or reception, an antenna of the SAR system partitioned into P different zones.

More conveniently, contemporaneously acquiring, in each PRI, P areas of the swath that are separated in azimuth includes using, in transmission and/or reception, an antenna of the SAR system partitioned in elevation into P different zones (i.e., along a nadir direction that passes through the phase center of the antenna of the SAR system and that is orthogonal to the earth's surface and to the flight direction and, hence, also to the azimuth direction).

Conveniently, the SAR acquisitions are performed by using one and the same elevation pointing (i.e., with respect to the nadir direction) corresponding to the swath to be observed.

Conveniently, the P×T areas 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.

In view of the foregoing, the present invention uses a SAR with the capability to acquire P channels at the same time (e.g., by means of a phased array with elevation partition capabilities or a multi-feed reflector antenna) along with an increased PRF (in particular, increased by T times with respect to the nominal PRF associated with the used SAR) to acquire strips of the earth's surface with performance improved by P×T times.

For a better understanding of the present invention,FIGS.5A and5Bschematically illustrate a non-limiting example of implementation of a method according to a preferred embodiment of the present invention, wherein T=2 and P=3.

In particular,FIG.5Ashows a SAR system50that is associated with a given nominal pulse repetition frequency PRFnomand is carried in flight/orbit along a flight direction d by an air/space platform (not shown inFIGS.5A and5B), such as an aircraft, a drone, a helicopter, a satellite or a spacecraft.

More specifically,FIG.5Ashows acquisition geometry in a plane xz, where x denotes an azimuth direction parallel to the flight direction d and z denotes a nadir direction perpendicular to the azimuth direction x, the flight direction d and the earth's surface. Instead,FIG.5Bshows the acquisition geometry in a plane xy, where y denotes an across-track direction that lies on the earth's surface and is perpendicular to both the azimuth direction x and the nadir direction z.

The SAR system50is used with an operational pulse repetition frequency PRFop=2PRFnom, whereby PRIop=1/PRFop=1/(2PRFnom)=PRInom/2, where PRInomdenotes the nominal pulse repetition interval and PRIopdenotes the operational pulse repetition interval.

As shown inFIGS.5A and5B:at a first time instant, the SAR system50contemporaneously acquires three first areas51separated in azimuth (i.e., along the azimuth direction x) by using radar beams that have different squint angles with respect to the flight direction d, are angularly separated in azimuth (i.e., along the azimuth direction x) and are pointed, each, at a respective one of the three first areas51, andat a second time instant, the SAR system50contemporaneously acquires three second areas52(that are different from the first areas51and are separated in azimuth (i.e., along the azimuth direction x)), by using radar beams that have different squint angles with respect to the flight direction d, are angularly separated in azimuth (i.e., along the azimuth direction x) and are pointed, each, at a respective one of the three second areas52;

wherein:the acquired first and second areas51and52form an azimuth-continuous strip (i.e., a continuous strip without “holes” along the azimuth direction x), andthe radar beams form an azimuth-continuous angular span (i.e., a continuous angular span without angular interruptions along the azimuth direction x).

Therefore, after 2PRIop(more in general, after T PRIop), a continuous overall angular span is achieved that is six (i.e., P×T) times greater than the nominal ones, thus improving azimuth resolution by a factor equal to P×T.

By increasing the PRF of F times with respect to the classical Stripmap, each of the P×T areas is acquired with the nominal PRF. Conveniently, in the same PRI, the acquisitions are separated at least of an antenna aperture (in case T=2). These features allow obtaining a correct azimuth sampling and, consequently, a good performance as for azimuth ambiguity.

FIG.6shows an example of transmission pattern illuminating two different non-contiguous zones, wherein T=P=2. Instead,FIG.7shows the two-ways azimuth pattern of each of the four channels. The two-ways azimuth pattern is minimally altered with respect to the nominal case, as shown inFIG.8.

It is important to highlight the fact that the present invention does not require antenna partition in azimuth, whereby the impact on NESZ (i.e., Noise Equivalent Sigma Zero) is manageable (e.g., if an antenna partition in elevation is used, it can be compensated by using a higher antenna—the same approach cannot be used in the space sharing techniques that need a reduced azimuth antenna in each channel).

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).

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.

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).

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

1) a partition in elevation of the antenna—namely, as shown inFIG.9, the used antenna (inFIG.9denoted 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 antenna61so 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”);

2) an exploitation of the whole antenna (as shown inFIG.10, where the antenna is denoted as a whole by62) 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.

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.

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

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 inFIG.11(where the antenna is denoted as a whole by71), 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 antenna71without introducing other necessities;

2) as shown inFIG.12, the antenna (denoted as a whole by72) 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 antenna72is divided;

3) as shown inFIG.13, the antenna (denoted as a whole by73) 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.

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

TABLE IIDIFFERENCES WITH RESPECT TOTECHNIQUETHE PRESENT INVENTIONSpace SharingTo increase of a k factor theazimuth resolution, thespace sharing techniquerequires k receivers.Instead, the presentinvention requires a lowernumber (e.g., P receiverswith a T-time increased PRFwith P × T = k).The space sharing techniquerequires that the antenna bedivided in azimuth into ksub-antennas.Instead, the presentinvention do not requireantenna partition inazimuth.The space sharing techniqueforesees the simultaneous(i.e. at the same time,inside the same PRI)acquisition (transmissionand reception) of asingle/contiguous zone fromdifferent azimuth positions.Instead, the presentinvention foresees thesimultaneous (i.e. at thesame time, inside the samePRI) acquisition(transmission and reception)of separated zones.Angular Sharing (SPCMB)To increase of a k factor theazimuth resolution, theangular sharing techniquerequires k receivers.Instead, the presentinvention requires a lower-number (e.g., P receiverswith a T-time increased PRFwith P × T = k).The angular sharingtechnique requires thetransmission of a large beamin azimuth and thesimultaneous reception ofdifferent azimuth-continuouszones.Instead, the presentinvention involvessimultaneously acquiringareas separated in azimuth.Time SharingTo increase of a k factor theazimuth resolution, the timesharing technique requiresan increase in the PRF of ktimes.Instead, the presentinvention requires a lowernumber (e.g., P receiverswith a T-time increased PRFwith P × T = k).

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.

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.