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Field of the invention

The invention relates to a synthetic-aperture interferometric radar with an antenna that is able to slide along a rotating arm, oriented with the direction of sight orthogonal to the plane of rotation.

Prior art

Interferometric radars referred to as GB-SARs (Ground-Based Synthetic-Aperture Radars) devised in particular for monitoring structures of large dimensions such as hillsides, open-cast mines, large architectural structures, such as towers and bridges have been known for some time.

GB-SAR technology is of particular interest because it constitutes a radar technique capable of providing high- precision displacement maps over an entire range of observation. In their basic conformation, conventional GB- SARs are obtained by moving an antenna along an axis, as described in the paper by M. Pieraccini , D. Tarchi, H. Rudolf, D. Leva, G. Luzi, C. Atzeni , Interferometric radar for remote monitoring of building deformations , Electronics Letters, Vol. 36, No. 6, pp. 569-570 (2000).

The patent No. US5379041 describes a SAR that makes use of an antenna fixed on a rotating arm and oriented in a radial direction, which enables creation of 360-degree images.

The system described ^' is, however, of a non-interferometric type, and the image forms in a plane parallel to the plane of rotation instead of in a plane orthogonal thereto.. The patent No. EP1145038 describes ¹ a rotating-antenna interferometric synthetic-aperture radar, designed for measuring angles, which requires at least two coherent receiving antennas and in which the image forms in a plane parallel to the plane of rotation.

The patent No. EP2194400 describes a non-interferometric radar constituted by an antenna mounted on -the sail of a windmill, which hence turns in the vertical plane and supplies an image in the space in front of the plane of rotation.

In this system, SAR synthesis occurs using the entire circumference, providing azimuth and elevation information, but with marked secondary lobes due to sampling on a closed line, and can hence be used for surveillance purposes, i.e., for detection of pointlike objects (aeroplanes or ships), but is not suitable for measuring small displacements for monitoring hillsides or buildings.

The known solutions present some significant drawbacks in the context of the systems for measuring displacements, in particular because they are able to provide only a two- dimensional image of the field of view and only one radial component .

Purpose of the invention

The purpose of the present invention is then to propose an interferometric radar equipped with an antenna that rotates with the direction of sight orthogonal to the plane of rotation that will be free from the aforementioned drawbacks of the systems of a known type.

Summary of the invention The above and further purposes are achieved with an interferometric radar according to one or more of the annexed claims .

A first advantage of the invention lies in the fact that the interferometric radar proposed is able to obtain a three- dimensional image of the field of view.

A second advantage of the invention lies in the fact that the interferometric radar proposed is able to measure the displacement vector and not only one component.

List of the drawings

The above and further advantages will be better understood by any person skilled in the branch from the ensuing description and from the annexed drawings, which are provided by way of non-limiting example and in which:

- Figure 1 is a schematic view of a rotating radar antenna according to the invention;-

- Figure 2 shows a possible path of the antenna system in the plane xz;

- Figure 3 shows, another possible path of the antenna system in the plane xz; and

- Figure 4 shows a possible division into sections of the plane scanned by the antennas so as to obtain three components of the displacement of the targets in the field of view.

Detailed description

With reference to the attached drawings, a radar R according to the invention is described, which comprises an acquisition and processing unit 10, which receives the data detected by at least one antenna 1, which rotates in the plane zx orthogonal to the direction of sight y of the antenna and is fixed to an arm 2 that can be set in rotation by a motor- drive support 3.

The antenna 1 can slide along the arm 2 by means of a motor- drive system (not shown) , which enables the two-dimensional movement of the antenna along the arm.

In various embodiments, the motor drive .11 of the antennas along the arm 2 may be independent or not of the motor drive 3 that enables rotary motion of the arm 2, and there may moreover be provided means for synchronisation of the rotary movement and of the linear movement of the antennas.

The radar R moreover comprises a data-acquisition and processing unit 10, which is operatively connected to said system of antennas 1 and is configured for acquiring a succession of images detected by the antenna during its revolution about the axis y and making differential interferometric calculations on at least two successive images of possible targets T located in the field of view of the system of antennas 1 in order to measure at least one component of the displacement thereof.

Figure 1 is a schematically representation of the case of a single antenna 1,. but the antenna 1 may be equivalently constituted by two or more antennas (one for transmitting and one for receiving) .

In preferred examples of operation, the movement of the antenna may be obtained in different ways, amongst which:

1) "stepper mode": the arm turns in steps (which are sufficiently short to prevent any angular ambiguity in reconstruction of the image) ; when the arm has described a complete revolution or circular path C, the antenna shifts along the arm (by a step that is sufficiently short to prevent any angular ambiguity in reconstruction of the image) and then performs another revolution; in this way, in discrete steps the entire surface P swept by the arm, or scanning plane, is covered; the circular steps may be of a constant angle or else, in order to reduce the acquisition time, of a constant arc;

2) "spiral mode" (Figure 2): the arm turns at a constant rate, and, the antenna moves radially at a constant rate; for each revolution, the radial movement must be sufficiently small as not to produce any angular ambiguity in the image (the pitch depends upon the lobe of the antenna; in the worst case that corresponds to the omnidirectional ^" antenna, it must be less than a quarter of the wavelength) ; the movement as a whole appears as a spiral from the periphery to the centre, or vice versa;

3) "complex-spiral mode" (Figure 3) : the arm turns at a constant rate and the antenna moves radially at a non- constant ^' rate; in this case, the antenna describes a shape L in the plane that may resemble a flower with a number of petals or other shapes depending upon how the radial velocity of the antenna varies in time.

The data of an entire acquisition may be appropriately windowed, with a radial window, in order to reduce the side lobes. In the case where sampling is obtained with the stepper mode with constant spatial spacing, the window may be for example a classic window that weights the centre more than the periphery. In the case of stepper acquisition at constant angle (or equivalently at constant time of sampling of the spiral movement) the window will have to be of an inverse type, i.e., one that weights the periphery more than the centre.

The data of an entire acquisition, processed by means of synthetic-aperture techniques, supply a three-dimensional image of the field of view that contains also the phase information. By exploiting two images taken at different time intervals (for example, in succession) it is possible to measure the possible radial displacements of the targets in the field of view by calculating the phase difference in the corresponding image point, applying the known interferometric techniques .

A possible variant of the technique makes use only of one part of the acquired data. For example, it is possible to process separately the samples of the top semicircle and the samples of the bottom semicircle. In this way, two images are obtained with an angular resolution that is lower than that of the image obtained with the entire circle, but with the advantage that two components of the possible displacement of a target in the field of view are obtained: the component from the image point of the target ^' to the phase centre of the top semicircle, and the component from the image point of the target to the phase centre of the bottom semicircle. The entire circle may also be processed in three sections (Figure 4), which may also partially overlap, so as to obtain the three components of the displacement vector.

The present invention has been described according to preferred , embodiments, but equivalent · variants may be conceived without thereby departing from the sphere of protection of the invention.