imaging reference station

Abstract

The present invention relates to an imaging reference station (1) comprising a frame (3) supporting a reflecting structure (2) in the form of a pentagonal pyramid with equilateral faces, each of the five faces of the reflecting structure (2) comprising four radar wave reflecting corners (10a, 10b, 10c, 10d), the reflecting corners (10a, 10b, 10c, 10d) on a face being arranged with their apexes oriented towards the center of the reflecting structure (2) and opening outwards from the reflecting structure (2), three of the four reflecting corners (10a, 10b, 10c) on a face being arranged respectively at each of the vertices of the respective face and the fourth corner (10d) being arranged at the center of the respective face, each reflecting corner at vertex (10a, 10b, 10c) being secured to the reflecting corner at center (10d) by a fish bone,the edges of the reflecting corners of each face of the reflecting structure (2) being truncated such that the edges of the reflecting corners (10a, 10b, 10c, 10d) of a face of the reflecting structure (2) belong to the same plane corresponding to the plane of that face of the reflecting structure (2). Figure to be published with the abbreviation: Figure 1,

Description

Title of the invention: Imaging reference station

[0001] The present invention relates to the field of georeferencing and imaging, and more particularly to an imaging reference station.

[0002] Images, especially terrestrial ones, can be obtained nowadays by several means: radar imaging, optical imaging (images taken by airplanes or drones), and satellite imaging.

[0003] Each of these means has advantages and disadvantages, and obtaining accurate and usable images may require combining images obtained by at least two of these means.

[0004] In this case, the question then arises of the alignment of images from the different means and therefore the geo-referencing and calibration of the different images by tags.

[0005] Georeferencing tags using reflective corners are known.

[0006] Corner reflectors are indeed commonly used for calibrating radar satellite images. They generally consist of a single, large trihedral element that reflects the emitted wave in a single direction. Corner reflectors made of two perpendicular surfaces, similar to those used in US patent US1384014A, also exist. Originally used primarily for radar, they have recently become particularly important for light, such as that from lasers. These corner reflectors produce a bright spot (reflection point) back towards the source in the direction of the incident waves and a weak return in other directions, which constitutes a significant limitation.

[0007] The amount of energy returned to the source by a three-sided reflector corner depends on many factors such as the design and orientation of the reflector corner, the distance to the source and the power of the source.

[0008] Polarization also plays a role. If these factors remain constant, the reflected energy will increase proportionally to its size. The radar cross-section (RCS) of a reflecting wedge for a triangular trihedron is proportional to the size L:

[0009] [Math.l] ÇFR , =----

[0010] where L is the length of the sides of the reflecting corner and X is the incident wavelength.

[0011] The larger the reflective corners, the greater the energy reflected back to the source, and the narrower the beam.

[0012] Several patents have been filed for arranging a row of reflective corners so that, for a given angle of incidence, there is at least one reflective corner with all three of its inner faces visible. Examples include US patents US2769977A, US4673934A, and US4551726A. However, this is not sufficient to guarantee a strong return of the wave for all angles of incidence. Wave interactions require constructive interference. Structures made with multiple reflective corners can interfere destructively. These returns can then cancel each other out, rendering the total return unusable.

[0013] The present invention aims to overcome the drawbacks of the prior art, by proposing an imaging reference station allowing in particular a strong return of the incident wave, regardless of the angle of incidence.

[0014] The present invention therefore relates to an imaging reference station, characterized in that it comprises a chassis supporting a pentagonal pyramid-shaped reflection structure with equilateral faces, each of the five faces of the reflection structure being a radar wave reflection face comprising four radar wave reflector corners, the reflector corners being identical within the same face of the reflection structure and the faces of the reflection structure being identical to each other, the reflector corners on a face being arranged with their apex oriented towards the center of the reflection structure and opening outwards from the reflection structure, three of the four reflector corners on a face being arranged respectively at each of the vertices of the respective face and the fourth corner being arranged at the center of the respective face, each vertex reflector corner being joined to the center reflector corner by an edge,the edges of the reflecting corners of each face of the reflecting structure being truncated in such a way that the edges of the reflecting corners of a face of the reflecting structure belong to the same plane corresponding to the plane of that face of the reflecting structure.

[0015] Due to the pentagonal shape of the imaging reference station of the invention, there is no direction which generates destructive backscattering interference and whatever the orientation of the incident ray, there is always one or more dihedral or trihedral angles which backscatter, for optimal backscattering in all directions.

[0016] Reflection on each of the lateral sides of a reflecting corner can be approximately described by the condition that the angle of incidence is equal to the angle of reflection. The reflecting corner consists of three lateral faces perpendicular to each other. A ray incident towards the interior of a reflecting corner is reflected on all three faces of the reflecting corner and is then directed towards the incident wave. For different ray paths, the ray bounces off two or three faces of a reflecting corner and is also reflected off the faces of the other reflecting corners.

[0017] The original orientation of the radar reflector corners allows the imaging reference station to be visible in the up and down passes during image acquisition by monostatic or airborne SAR radar satellites (X, C, and L bands), as well as in bistatic configurations. Coherence is achieved by an arrangement that allows the continuity of the return of the incident wave from a dihedral-trihedral reflector corner and the returns from several other dihedral-trihedral reflector corners.

[0018] The imaging reference station according to the invention satisfies three properties to obtain backscattering from a set of reflective corners.

[0019] The first property is that several small reflective corners are lighter than one large reflective corner, because the sheet metal used to manufacture the small reflective corners is thinner. The weight is therefore reduced compared to a single reflective corner of equivalent surface area.

[0020] The second property is that there are more return angles of emitted Fonde, and therefore that the backscattering is higher, due to the fact that at least two reflecting corners are oriented differently to produce strong backscattering.

[0021] The third property is that the different reflective corners produce in-phase backscattering to ensure constructive interference.

[0022] Indeed, electromagnetic waves interfere constructively provided that their phases are substantially equal. If their phases are opposite, they interfere destructively. In the reference station according to the invention, when two faces of two reflecting corners are visible from a given direction, the two reflecting corners interfere constructively because the wavelengths of the round-trip paths for reflection from these two reflecting corners do not differ by more than a quarter of a wavelength.

[0023] Thus, the proposed imaging reference station allows for strong backscattering of the incident wave in the direction of incidence, but also in other directions. Backscattering the incident wave in the direction of incidence produces a bright spot in the synthetic aperture radar (SAR) image acquired by a monostatic satellite. This spot is used for calibrating and georeferencing the image. Backscattering in other directions is used for calibration during image acquisition in the case of bistatic satellite configurations, or ground-based radar satellites and radar aircraft.

[0024] The convex pentagonal structure of the reflecting corners ensures uniform and high-intensity backscattering.

[0025] Each satellite does not see only one face of a triangle as classically, but several, which leads to a unique spectral signature specific to the imaging reference station according to the invention.

[0026] The elevation angle of each face of the imaging reference station according to the invention is advantageously but not necessarily independently modifiable.

[0027] Several imaging reference stations according to the invention, used for example in a triangulation objective, with faces set identically between the imaging reference stations would lead to identical spectral signatures for the imaging reference stations.

[0028] According to one embodiment, each vertex reflector corner is arranged such that one of its edges is the bisector of the corresponding vertex angle.

[0029] According to one embodiment, the reflective corners are joined together by welding at their common edge.

[0030] According to one embodiment, the vertices of the reflecting corners of a face of the reflecting structure belong to the same plane parallel to the plane of that face of the reflecting structure.

[0031] According to one embodiment, the reflective corners are made of steel. However, the invention is not limited in this respect, and other materials suitable for reflecting electromagnetic waves can be considered within the scope of the present invention. Thus, although the corners are preferably made of sheet metal (steel), other materials such as faces made of composite materials (carbon fiber reinforced polymers) coated with aluminum can be considered.

[0032] According to one embodiment the chassis comprises a base supporting five posts, each post being intended to support one face of the reflective structure.

[0033] The reference station of the invention is thus easily transportable and demountable and can advantageously be installed on all types of terrain.

[0034] According to one embodiment, each face of the reflective structure is mounted on a post of the corresponding frame by means of an adjustable bracket, allowing the inclination of each face of the reflective structure to be adjusted independently of each other.

[0035] Phase coherence is thus obtained through the azimuth and elevation orientation of the imaging reference station.

[0036] According to one embodiment, the imaging reference station further comprises a mast extending vertically from the base of the chassis and passing through the top of the reflection structure.

[0037] According to one embodiment, the top of the mast carries a satellite positioning beacon.

[0038] The installation of a satellite positioning beacon of the type of geolocation module (type GPS, GLONASS or GALILEO) continuous differential at the top of the mast of the imaging reference station allows its geolocation on the ground permanently thanks to the corresponding satellite system, with an accuracy on the order of the centimeter.

[0039] According to one embodiment, the imaging reference station further comprises wired or wireless data transmission / reception equipment. This allows it to exchange data with other entities. Normally passive, if the imaging reference station includes such wireless transmission / reception equipment or a satellite positioning beacon, the imaging reference station will then include a power supply (mains or battery(ies)) and computing means (processor, microprocessor, microcontroller, digital signal processor (DSP), field-programmable gate array (FPGA), or application-specific integrated circuit (ASIC)) associated with read-only or random-access memory for the operation of the imaging reference station.

[0040] According to one embodiment, a dome made of material transparent to radar waves is formed to cover the reflection structure.

[0041] The dome may in particular be made of composite materials (carbon fiber reinforced polymers).

[0042] The hemispherical dome covers the imaging reference station and makes it insensitive to weather conditions (rain, snow, wind).

[0043] According to one embodiment, an optical marker is formed on the external surface of the dome.

[0044] Such an optical marker can, for example, consist of a black cross (or other visible marker) on a light-colored dome, preferably white, and serves as a reference for image acquisition by optical satellites.

[0045] To better illustrate the object of the present invention, an embodiment will now be described in detail, with reference to the attached drawings.

[0046] On these drawings:

[0047] [Fig. 1] represents an imaging reference station according to the present invention in exploded view.

[0048] [Fig.2] represents the imaging reference station of [Fig.1] in the mounted state for use.

[0049] [Fig.3] represents the imaging reference station of [Fig.2] without the dome.

[0050] [Fig.4] is a schematic view of the reflection structure of the imaging reference station according to the invention.

[0051] [Fig.5] is a detail view of a reflector corner used in the reflection structure of [Fig.4].

[0052] [Fig.6] is a view of the chassis supporting the reflecting structure of [Fig.4].

[0053] Referring to Figures 1 to 6, it can be seen that a station has been represented imaging marker 1 according to the present invention.

[0054] The imaging reference station 1 according to the present invention comprises a reflection structure 2, a chassis 3 supporting the reflection structure 2, a mast 4 carrying a geolocation module 5, and a protective dome 6.

[0055] As can be seen more clearly in [Fig.4], the reflecting structure 2 is in the shape of a pentagonal pyramid, each face of the pentagonal pyramid being made up of four reflecting corners generically designated by the reference number 10. The base of the reflecting structure 2, forming the base of the pentagonal pyramid, is intended to be fixed to the frame 3, as will be explained below.

[0056] As can be seen more clearly in [Fig.5], each reflecting corner 10 consists of three orthogonal faces, respectively 11, 12 and 13 forming a corner C, each of the three edges forming the corner C being constituted by the intersection of two respective faces 11, 12 and 13.

[0057] The three faces 11, 12 and 13 are of the same shape, substantially triangular, with the angles opposite corner C truncated, to form, on the edge opposite corner of each face, respectively 11, 12, 13, a longitudinal edge, respectively 11a, 12a, 13a, and two transverse edges, respectively 11b, 11c, 12b, 12c, 13b, 13c, forming a right angle return on one of the corresponding edges forming corner C.

[0058] Each reflective corner 10 is made of a material suitable for reflecting electromagnetic waves, in particular metal, especially steel or sheet metal. Other materials, such as composite materials (carbon fiber reinforced polymers), may be considered.

[0059] A face 2a of the reflection structure 2 has been isolated in dotted lines on [Fig.4], the other faces of the reflection structure 2 being identical to this face 2a.

[0060] As can be seen, face 2a consists of four reflecting corners, respectively 10a, 10b, 10c and lOd, reflecting corners 10a, 10b and 10c being arranged at the corners of face 2a, reflecting corner lOd being located at the center of face 2a. Each reflecting corner with vertex 10a, 10b and 10c is arranged such that one of its edges is the bisector of the corresponding vertex angle of the reflection structure 2.

[0061] The four reflective corners 10 on one face are identical, and the reflective corners 10 are also identical on all faces of the reflection structure 2.

[0062] Two adjacent corners on face 2a are joined along one of their longitudinal edges by a weld 14. In practice, only the central reflecting corner 10d is joined to the three other reflecting corners 10a, 10b, and 10c. Each of its longitudinal edges is therefore linked, for example by weld 14, to the adjacent longitudinal edge of the adjacent reflector corner 10a, 10b and 10c.

[0063] Since the reflecting corners 10 are identical, the vertices of the reflecting corners of a face of the reflection structure 2 belong to the same plane parallel to the plane of that face of the reflection structure 2.

[0064] The reflective structure 2 is supported by the chassis 3.

[0065] The chassis 3 consists of five feet 15 arranged in a star shape.

[0066] Each foot 15 carries an upright 16 on which are mounted two longitudinal supports 17, mounted transversely with respect to the upright 16 in such a way that the supports 17, once mounted on the upright 16, protrude on either side of the upright 16 along their longitudinal direction, orthogonal to the direction of the upright 16.

[0067] The supports 17 are mounted on the upright 16, with one support 17 mounted on the lower part of the upright 16, on the outside side of the chassis 3 and one support 17 mounted on the upper part of the upright 16 on the inside side of the chassis 3, a longitudinal through bore 17a being formed in each of the supports 17.

[0068] Two identical triangular plates 9 are mounted on either side of the uprights 16 by screws 18 and nuts 19, the two triangular plates 9 forming a bracket suitable for receiving one of the faces of the reflecting structure 2.

[0069] Notches 9c and 9d are formed on the edge of the triangular plates 9 intended to receive one face of the reflective structure 2, each notch 9c, 9d allowing the rear of a reflective corner 10 to be received.

[0070] Rounded through grooves 9a, 9b are formed through the triangular plates 9, and allow the face of the reflecting structure 2 to be tilted at will when mounted on the chassis 3.

[0071] It is understood that the chassis 3 can take other forms, without departing from the scope of the present invention.

[0072] It will be understood that, by its structure, the reference station 1 of the invention is thus easily transportable and demountable and can advantageously be installed on all types of terrain.

[0073] The faces of the reflective structure 2 can then be joined together, for example by welding the faces of the reflective corners in contact once the reflective structure 2 is mounted on the chassis 3.

[0074] In practice, each face of the reflective structure 2 is formed by welding four reflective corners 10, then the faces are mounted one by one on each of the brackets formed by the triangular plates 9 on the frame 3, the inclination of the faces of the reflective structure 2 is adjusted, and optionally the faces of the reflective structure 2 are joined together.

[0075] A mast 4 can be positioned at the center of the imaging reference station 1, fixed to the chassis 3, and carry a geolocation module 5 of the GPS beacon type. The mast 4 is preferably transparent to radar waves and may, in particular, be made of carbon fibers.

[0076] In this case, the module may be powered either by a battery integrated into the module, by photovoltaic cells, or by an external power supply not shown. The geolocation module 5 may also include computing means, memory, and / or wired or wireless communication means for exchanging data with other entities.

[0077] Finally, a dome 6 of substantially hemispherical shape may be provided to cover the reflective structure 2 mounted on the chassis 3, and thus protect it from the elements. The dome 6 is preferably transparent to radar waves and may, in particular, be made of composite materials (carbon fiber reinforced polymers).

[0078] A hole 8 is formed in the center of the dome 6 to allow the passage of the mast 4. A seal can be provided in the hole 8 for the passage of the mast, in order to prevent moisture from forming under the dome 6.

[0079] Support elements for the dome 6 (not shown to avoid complicating the drawing) mounted on the frame 3 may be provided to support the dome 6 so that it does not rest directly on the reflecting structure 2.

[0080] Finally, an optical marker 7, in the non-limiting example shown a cross, can be formed on the external surface of the dome 6, to allow optical tracking of the imaging marker station 1 by optical imaging. In the non-limiting embodiment shown, the dome 6 is light-colored and the optical marker 7 is a dark-colored cross, but those skilled in the art believe that any optical marker can be used, provided it can be optically detected.

Claims

Demands

1. - An imaging reference station (1), characterized in that it comprises a frame (3) supporting a pentagonal pyramidal reflecting structure (2) with equilateral faces, each of the five faces of the reflecting structure (2) being a radar wave reflecting face comprising four radar wave reflecting corners (10a, 10b, 10c, 10d), the reflecting corners (10a, 10b, 10c, 10d) being identical within the same face of the reflecting structure (2) and the faces of the reflecting structure (2) being identical to each other, the reflecting corners (10a, 10b, 10c, 10d) on a face being arranged with their apex oriented towards the center of the reflecting structure (2) and opening outwards from the reflecting structure (2), three of the four reflecting corners (10a, 10b, 10c) on a face being positioned respectively at each of the vertices of the respective face and the fourth corner (lOd) being positioned at the center of the respective face,Each vertex reflector corner (10a, 10b, 10c) is joined to the center reflector corner (lOd) by an edge, and the edges of the reflector corners on each face of the reflection structure (2) are truncated such that the edges of the reflector corners (10a, 10b, 10c, lOd) of a face of the reflection structure (2) lie in the same plane corresponding to the plane of that face of the reflection structure (2).

2. - Imaging reference station (1) according to claim 1, characterized in that each vertex reflector corner (10a, 10b, 10c) is arranged such that one of its edges is the bisector of the corresponding vertex angle.

3. - Imaging reference station (1) according to claim 1 or claim 2, characterized in that the reflector corners (10) are joined together by welding at their common edge.

4. - Imaging reference station (1) according to any one of claims 1 to 3, characterized in that the vertices of the reflecting corners of a face of the reflecting structure (2) belong to the same plane parallel to the plane of that face of the reflecting structure (2).

5. - Imaging reference station (1) according to any one of claims 1 to 4, characterized in that the reflector corners (10) are made of steel.

6. - Imaging reference station (1) according to any one of claims 1 to 4, characterized in that the chassis (3) comprises a base carrying five posts (16), each post (16) being intended to support a face of the reflection structure (2).

7. - Imaging reference station (1) according to claim 6, characterized in that each face of the reflection structure (2) is mounted on a post (16) of the corresponding frame (3) by means of an adjustable bracket (9), allowing the inclination of each of the faces of the reflection structure (2) to be adjusted independently of each other.

8. - Imaging reference station (1) according to any one of claims 1 to 7, characterized in that it further comprises a mast (4) extending vertically from the base of the frame (3) and passing through the top of the reflecting structure (2).

9. - Imaging reference station (1) according to claim 8, characterized in that the top of the mast (4) carries a satellite positioning beacon (5).

10. - Imaging reference station (1) according to claim 8, characterized in that it further comprises wired or wireless data transmission / reception equipment (5).

11. - Imaging reference station (1) according to any one of claims 1 to 10, characterized in that a dome (6) of material transparent to radar waves is formed to cover the reflection structure (2).

12. - Imaging reference station (1) according to claim 11, characterized in that an optical reference (7) is formed on the external surface of the dome (6).

Images