Radar retroreflector devices such as radar corner reflectors are generally known, for example for calibrating a radar measurement setup or for determining a specific distance to a probed structure using phase information.

It is an object of the present invention to provide a radar retroreflector device for further application. Thereto, according to the invention, the device comprises a base unit having a first and a second side edge, opposite to each other, and two radar retroreflectors mounted on said base unit, a first radar retroreflector being located at the first side edge of the base unit, and a second radar retroreflector being located at the second side edge of the base unit, wherein the first radar retroreflector has a first focus orientation and the second radar retroreflector has a second focus orientation that is different from the first focus orientation.

By providing a single device provided with two radar

retroreflectors, the device can advantageously be applied for performing radar measurements on the Earth's surface using spaceborn technology.

The invention is at least partially based on the insight that satelhtes following a low Earth orbit, LEO, travel along two different orbit paths. Then, by providing a device with radar retroreflectors that can be directed to mutual different orientations, radar equipped geodetic exploitation satelhtes are enabled to perform optimal radar measurements, such as interferometric synthetic aperture radar, InSAR, measurements.

By using a radar corner reflector on surface placing the radar retroreflector device described above in an area to be measured, relative spatial height information of said measured area can be interrelated to an absolute position that is e.g. measured by a GNSS receiver on the device. Then, advantageously, absolute height information of the complete measured area can be obtained.

It is noted that the article entitled "Submillimeter Accuracy of InSAR Time Series: Experimental Validation" by Alessandro Ferretti et al. discloses a mobile assembly of dihedral reflectors used for interferometric synthetic aperture radar measurements. The dihedral reflectors are mounted upon two sliding platforms by means of a robust clamping system so that rotations of the reflectors around both the vertical and horizontal axes can be performed.

It is further noted that patent publication CN 201 654 225 discloses a pair of dihedral corner reflectors that are pivotable around a vertical rotating shaft on a supporting structure.

The invention also relates to a method of preparing a radar retroreflector device.

By way of example only, embodiments of the present invention will now be described with reference to the accompanying figures in which

Fig. 1 shows a schematic perspective view of a radar retroreflector device according to the invention;

Fig. 2 shows a schematic top view of the radar retroreflector device of Fig. 1;

Fig. 3 shows a schematic perspective view of another radar retroreflector device according to the invention, and

Fig. 4 shows a flow chart of an embodiment of a method according to the invention.

It is noted that the figures merely show preferred embodiments according to the invention. In the figures, the same reference numbers refer to equal or corresponding parts.

Figure 1 shows a schematic perspective view of a radar

retroreflector device 1 according to the invention. The radar retroreflector device 1 is provided with a base unit 2 having two side edges 3a,b, opposite to each other, and two radar retroreflectors 4, 5 mounted on the base plate 2. Here, a first radar retroreflector 4 is located at a first side edge 3a of the base plate 2, while a second radar retroreflector 5 is located at a second side edge 3b of the base plate 2. The first radar retroreflector 4 has a first focus orientation Fl, while the second radar retroreflector 5 has a second focus orientation F2 that is different from the first focus orientation.

It is noted that within the context of the present application, a focus orientation F of a radar retroreflector is a three-dimensional direction wherein the retroreflector provides a maximal reflection energy. Generally, a radar retroreflector acts as a reflector reflecting an incident radar beam I as a reflected radar beam R having the opposite direction as the incident radar beam, thus reflecting the incident radar beam back to the radar source over a range of incident beam angles. However, due to a limited extension of the retroreflector, the amount of reflected radar energy varies is a function of the incident beam angle relative to the orientation of the retroreflector. If the incident beam angle coincides with the focus

orientation F of the radar retroreflector, a maximum amount of the incident radar energy is reflected.

In the shown embodiment, the radar retroreflectors 4, 5 are radar corner reflectors each having three mutually perpendicular oriented, intersecting triangular flat reflecting surfaces 4a,b,c; 5a,b,c joined via connecting edges 4d,e,f; 5d,e,f of a tetrahedron, the reflectors being formed as trihedral retroreflectors. The three connecting edges 4d,e,f; 5d,e,f extend between a top vertex 4g; 5g, also called apex, joining the three reflecting surfaces 4a,b,c; 5a,b,c, and three basis vertices 4h,i,j; 5h,i,j, respectively, at the open side of the retroreflectors 4, 5. The three basis vertices 4h,i,j; 5h,i,j are mutually connected via three basis edges 4k,l,m; 5k,l,m defining an opening section 4n; 5n of the respective retroreflectors 4, 5. Then, the focus orientation Fl, F2 of the respective radar retroreflector is directed from the respective apex 4g, 5g through the respective opening section 4n, 5n outwardly, generally transverse or approximately transverse to said opening section 4n, 5n.

It is noted that, in principle, other radar corner reflectors are applicable such as corner cubes having three intersecting surfaces formed as square shapes. Further, generally, other retroreflectors could be applied such as a dihedral reflector, a so-called cat's eye including refracting optical elements, or a phase-conjugate mirror.

In the shown embodiment, the radar corner reflectors 4, 5 are located at the side edges 3 of the base plate 2 such that an edge of a reflecting surface 4c of the first radar corner reflector 4, viz. a basis edge 41 that is remote from the apex 4g of the radar corner reflector 4, adjoins the first edge 3a, and that an edge of a reflecting surface 5c of the second radar corner reflector 4, viz. a basis edge 51 that is remote from the apex 5g of the radar corner reflector 5, adjoins the second edge 3b.

The device 1 further includes a first hinge 6a and a second hinge 6b at the first and second edge 3a,b, respectively, of the base plate 2 hingeably mounting the corner reflectors 4, 5 at the side edges 3a,b of the base plate 2 so that the orientation of the corner reflectors can be set relative to the base plate 2. Preferably, the reflecting surface edge 41, 51 adjoining the first and second edge 3a, b, respectively, is substantially parallel to said first and second edge 3a,b such that the radar corner reflectors 4, 5 can swivel or pivot around a swiveling axis Si, S2 parallel to the first and second edge 3a,b, respectively, of the base plate 2, thereby providing a back -flipped position wherein a basis vertex 4h, 5i is located lower than the apex 4g, 5g so that any collection of moisture and dirt in the corner reflectors 4, 5 is counteracted. The hinges 6a,b are preferably one -dimensional and may extend along said first and second edge 3a,b, respectively, of the base plate 2. Then, the reflecting surface edge 41, 51 adjoining the first and second edge 3a,b may be parallel to the respective swivehng axis Si, S2 and the respective edge 3a,b of the base plate 2. The device 1 shown in Fig. 1 also includes fixations means for fixing the orientation of the corner reflectors 4, 5 relative to the base plate 2 by fixing the angular position of the corner reflectors 4, 5 around the swiveling axis Si, S2 relative to the base plate 2. As an example, the fixation means include a setting bolt or screw in the first and second hinge 6a,b. By securing the corner reflector orientation, the focus orientation F is made stationary for enabling an optimal reflection performance relative to a selected radar source. The fixation can be permanent or temporarily. In the latter case, the orientation of the corner reflectors 4, 5 can be released, e.g. for re-use of the device 1. In another embodiment, the orientation of the corner reflectors 4, 5 relative to the base plate is pre-fixed, for making an installation procedure easier, e.g. in case the device 1 is pre-selected for reflecting a pre-determined radar source.

The device 1 also includes a support structure for positioning the base plate 2 on a surface to be radar probed. In the shown embodiment, the support structure includes a pillar 7 provided at a lower end 7a with a first connecting flange 8 for connection with a surface structure such as a basement. The pillar 7 is further provided at an upper end 7b with a second connecting flange 9 connected to the base plate 2. Preferably, the device 1 includes adjusting means for adjusting the orientation of the base plate 2, more preferably for levelling the base plate 2. Such adjusting means may e.g. include a setting element such as setting screw at the first or second connecting flange 8 for adjusting the orientation of the connecting flange 8 relative to the surface structure or the base plate, respectively.

Generally, the support structure can be modular, e.g. for providing a support with different heights.

Figure 2 shows a schematic top view of the radar retroreflector device 1. The two opposite side edges 3a,b of the base plate 2 define a tapered or non-parallel, diverging side contour of the base plate 2. Then, also the pivoting axes Si and S2, parallel to said side edges 3a,b, are tapered, slightly converging. As shown in Fig. 2 the side edges 3a,b are not parallel but are slightly tapered including a tapering angle a that may e.g. range between circa 30° and circa 90°. Optionally, the tapering angle a is adjustable so that the tapering angle a can be set when preparing the device 1 for operational use. However, the base plate 2 may also have a permanent side contour, e.g. by implementing the base plate 2 as an integral unit.

Then, also the mutual orientation of the side edges 3a,b is permanent.

Alternatively and/or supplementary, the orientation of the pivoting axes S i, S2 of the hinges 6a,b can be adjusted to a desired tapering angle a. The base plate 2 may have a tetragon contour such as a trapezoid shown in Fig. 2 having two tapered opposite side edges 3a,b and two further side edges 3c, d that are parallel to each other.

Generally, the corner reflectors 4, 5 can be mounted to the base plate 2 in another way, using other pivoting mechanisms such as a two- dimensional pivoting structure, or pivoting around another axis or at another orientation or location.

It is noted that the base unit 2 can be implemented as a mainly flat base plate 2 having side edges as shown in Fig. 1 and Fig. 2 or as another structure, e.g. as a frame or other two-dimensional or three-dimensional object having, in a top view, a first side edge and a second side edge opposite to the first side edge. The first radar reflector 4 and the second radar reflector 5 are hingeably mounted to the first side edge and the second side edge, respectively, of the base unit, using a first one -dimensional hinge 6a having a first swiveling axis S i and second one -dimensional hinge 6b having a second swiveling axis S2, respectively, the axes Si, S2 extending parallel to the first side edge and the second side edge, respectively, of the base unit. Preferably, during operational use of the device, the axes S i, S2 also extend in a mainly horizontal plane, for optimal setting the orientation of the radar reflectors. In the shown embodiment, the device 1 further comprises a number of other geodetic attributes including a GPS receiver 11 for absolute retrieving absolute position information of the device 1, a marker 12 for airborne photogrammetry, a levehng benchmark 13 for determining a height of the device 1 relative to a standard height map in the area where the device is located, and a triangulation benchmark 14. In the shown embodiment, the attributes 11, 12 are located on the base plate 2 of the device 1. However, at least one or more attributes can be mounted at another location on the device 1, e.g. on the pillar 7. As an example, the leveling benchmark 13 is formed by a bulge on the pillar 7. It is noted that additionally or alternatively, instead of the GPS receiver 11 another global navigation satellite system, GNSS, receiver can be applied such as a

GLONASS, Galileo or Beidou receiver. The marker 12 is formed by a coloured top segment of the GPS receiver 11, e.g. as a white hat. The triangulation benchmark 14 can be implemented as an optical retroreflector such as a prism for tachymetry. It is noted that the device 1 may include more geodetic attributes e.g. for performing redundant measurement to increase measurement results. As an example, aerborn laser scanning ALS technology can be applied for obtaining location information of the device 1. Further a leveling element can be mounted. On the other hand, the device 1 may include less geodetic attributes than the embodiment shown in Fig. 1.

Figure 3 shows a schematic perspective view of another radar retroreflector device 1 according to the invention. Here, the corner reflectors are brought in a back -flipped position wherein a basis vertex 4h, 5i is located lower than an apex 4g, 5g of the respective tetrahedron, so that the lowest connecting edge 4d of the tetrahedron runs from the apex

downwardly thus allowing any moisture and/or dirt in the corner reflectors 4, 5 to move or flow outwardly.

Further, the device 1 shown in Fig. 3 includes an upper pillar portion 27 extending from base plate 2 upwardly. The upper pillar portion 27 can be integrally formed with the pillar 7 supporting the base plate 2, or can be formed in a different manner, e.g. as a separate pillar structure mounted to the base plate 2. The upper pillar portion 27 is provided with a GPS receiver 11 and a marker 12 formed as a white helmet-shaped top segment.

During operational use of the device 1, both radar corner reflectors 4, 5 are oriented such that their focus orientation Fl, F2 is directed to positions on respective orbit paths of a satellite that is equipped with a radar system probing the device 1.

In principle, during a radar measurement process, the radar corner reflectors 4, 5 and the base plate 2 are rigidly connected forming a single structure such that the reflectors 4, 5 and the base plate 2 can not move relative to one another. Then, the integral structure formed by the base plate and the reflectors is such that a movement performed by one of the radar corner reflectors 4, 5 or the base plate inherently induces a similar movement to the other elements of the device 1 for providing reliable, precise, reproducible and consistent measurement data, both in horizontal and vertical deformations.

Typically, a geodetic exploitation satellite is provided with a so- called interferometric synthetic aperture radar, InSAR, or synthetic aperture radar, SAE, that is arranged for performing spaceborne radar measurements on the Earth's surface. By placing the device 1 described above in an area to be measured, relative spatial height information of said measured area can be interrelated to an absolute position measured by a GNSS receiver on the device 1 resulting in a connection to a geodetic reference frame. Then, advantageously, absolute height information of the complete measured area can be obtained.

Usually, geodetic exploitation satellites follow a low Earth orbit, LEO. Then, at a particular location on Earth, radar measurements can be performed when a line of sight to the satelhte is realized. It appears that the satellite follows a first orbit path when travelling roughly from North to South, and a second orbit path when travelling roughly from South to North. The heading of the first, descending and the second, ascending orbit path may depend on the latitude where the device is located on Earth.

For optimal radar performance, the focus orientation Fl of the first radar corner reflector 4 can be aligned to a position in the first orbit path, while the focus orientation F2 of the second radar corner reflector 5 can be aligned to a position in the second orbit path. Then, in principle, radar measurements can repeatedly be performed with the satellite.

During setting up the radar retroreflector device 1, preparing the device for operation use, the base plate 2 is positioned on the surface to be radar probed such that the focus orientation Fl of the first radar corner reflector 4 and the focus orientation F2 of the second radar corner reflector 5 is directed to the respective orbit path positions.

The step of positioning the base plate 2 may include setting the first radar corner reflector 4 with a first orientation relative to the base plate 2 and setting the second radar corner reflector 5 with a second orientation relative to the base plate, e.g. by adjusting the angular positions of the respective corner reflectors relative to the base plate 2, preferably into a back -flipped position wherein a basis vertex is located lower than an apex of the respective tetrahedron.

Said positioning step may also include setting the tapering angle a between the two pivoting axes Si, S2 of the base plate 2. Then, the orientation of the corner reflectors 4, 5 in two degrees of freedom can be realized.

Figure 4 shows a flow chart of an embodiment of the method according to the invention. The method 100 is applied for preparing a radar retroreflector device for operational use, the device comprising a base plate having two side edges, opposite to each other, and two radar corner reflectors mounted on said base plate, a first radar retroreflector being located at a first side edge of the base plate, and a second radar

retroreflector being located at a second side edge of the base plate. The method 100 comprises the step of positioning 110 the base plate on a surface to be radar probed such that a focus orientation of the first radar

retroreflector is directed with an orbit position of a radar equipped satellite traveling roughly northwards and a focus orientation of the second radar retroreflector is directed to an orbit position of the radar equipped satelhte traveling roughly southwards.

The invention is not restricted to the embodiments described above. It will be understood that many variants are possible, wherein a radar retroreflector device is provided including a base unit and two radar retroreflectors, the retroreflectors preferably being pivotally mounted around pivot axes relative to the base unit such that the pivot axes deviate from each other. Advantageously, the pivot axes extend in a levelled plane and/or a tapering or deviating angle between the pivot axes is adjustable.

It is noted that the base plate can be replaced by another base unit, e.g. by a box-shaped structure provided with side surfaces on which the corner reflectors are hingeably mounted. Further, the base unit can be implemented as a two-dimensional or three-dimensional open or closed base frame or as an composition of base elements forming the base unit.

These and other embodiments will be apparent for the person skilled in the art and are considered to fall within the scope of the invention as defined in the following claims. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments. However, it will be appreciated that the scope of the

invention may include embodiments having combinations of all or some of the features described.