DE102016208508A1 - Angle reflector device with adjustable height ambiguity for SAR applications - Google Patents

Abstract

The angle reflector device (10) for SAR applications is provided with an angle reflector (12) having at least two mutually orthogonally aligned reflector surfaces for reflecting radar waves and a coupling element (39) for mechanically coupling the angle reflector (12) with a variable in its vertical extent and when changing its vertical extension of the angular reflector (12) entraining object (29) on the ground, on and / or on which the coupling element (39) can slide along additionally. Furthermore, the angle reflector device (10) has a fixedly arranged frame (30) with a guide rail (36) inclined at an acute or obtuse angle to the vertical extent of the object (29) and a guide element guided slidably along the guide rail (36) (38) mechanically coupled to the angle reflector (12). By means of the guide rail (36) and the guide element (38), a change of the vertical extent of the object (29) is converted into a movement of the angle reflector (12) along the guide rail (36).

Description

The invention relates to an angle reflector device for SAR applications.

One field of application of such angle reflector devices is the measurement of water surfaces with regard to their height changes. For this purpose, angle reflectors are installed in wetlands where they float, for example, on the surface of the water, moving them up and down depending on the water level. Angle reflectors of this type are realized in the simplest case by an L-shaped reflector angle, the two reflection surfaces orthogonal and vertical to the water surface. The triple reflection necessary for the reflections of radar waves is achieved by the two reflection surfaces of the reflector angle and by the water surface.

SAR devices and in particular interferometer SAR (InSAR devices) as well as differential interferometer SAR (DInSAR devices) provide high resolution in terms of detection of deformations in, for example, vegetation such as water surface height changes. The distance measurement in SAR systems is based on a phase measurement and is therefore unique only over the size of maximum of the wavelength of the SAR waves used. To measure height changes of vegetation or artificial objects (eg buildings) clearly with a SAR device, so it requires more information about the object in question. If, for example, the temperature or season-dependent change in the height of objects is to be measured without additional information, then the maximum height difference occurring must be known and, in particular, smaller than the resolution achievable with the SAR device. In a SAR device operating in the C-band and at an angle of incidence of, for example, 40 °, only height changes of less than about 3.7 cm can be clearly measured. Multiple multiples of this size are unfortunately not detectable without additional information.

If, for example, one wishes to use SAR devices for the measurement of water levels in, for example, lakes, this can basically fail due to the fact that the water level changes are greater than the area within which height ambiguity can be avoided.

The object of the invention is to provide an angle reflector device for SAR applications whose scope goes beyond previously known angle reflectors for such applications.

• – einem Winkelreflektor mit mindestens zwei orthogonal zueinander ausgerichteten Reflektorflächen zur Reflektion von Radarwellen,
• – einem Kopplungselement zur mechanischen Kopplung des Winkelreflektors mit einem hinsichtlich seiner vertikalen Erstreckung veränderlichen und bei Veränderung seiner vertikalen Erstreckung den Winkelreflektor mitnehmenden Objekt auf der Erde, an dem und/oder auf dem das Kopplungselement zusätzlich entlang gleiten kann,
• – einem ortsfest angeordneten Gestell mit einer um einen spitzen oder stumpfen Winkel zur vertikalen Erstreckung des Objekts geneigt verlaufenden Führungsschiene und
• – einem an der Führungsschiene und entlang dieser verschiebbar geführten Führungselement, das mechanisch mit dem Winkelreflektor gekoppelt ist,
• – wobei mittels der Führungsschiene und des Führungselements eine Veränderung der vertikalen Erstreckung des Objekts in eine Bewegung des Winkelreflektors längs der Führungsschiene umsetzbar ist.

To solve this problem, the invention proposes an angle reflector device for SAR applications, which is provided with

• An angle reflector with at least two orthogonally oriented reflector surfaces for the reflection of radar waves,
• A coupling element for the mechanical coupling of the angular reflector with an object that varies with respect to its vertical extent and that, when changing its vertical extent, takes the angle reflector along the ground, on which and / or on which the coupling element can additionally slide along,
• - A stationary arranged frame with an inclined or an obtuse angle to the vertical extent of the object extending guide rail and
• A guide element which is displaceably guided on the guide rail and along this guide and which is mechanically coupled to the angle reflector,
• - By means of the guide rail and the guide element, a change in the vertical extent of the object in a movement of the angle reflector along the guide rail is implemented.

The device according to the invention has an angle reflector with at least two orthogonally oriented reflection surfaces for the reflection of radar waves. The angle reflector is connected to a guide element, which is guided displaceably on a guide rail. The guide rail itself is held by a frame which is stationary and therefore arranged independently of the object to be measured. Furthermore, the inventive angle reflector device has a coupling element, with which the angle reflector is movably coupled to the object mechanically. If the height of the object changes, the angle reflector moves along the guide rail accordingly.

According to the invention, the guide rail is inclined to the vertical extent of the object. Thus, according to the invention, a deflection of a vertical displacement of the angle reflector, which experiences this as a result of a change in the height of the object, in an obliquely directed displacement of the angle reflector, d. H. in a shift directed obliquely to the vertical, so that the distance change "seen" by the SAR device is ideally smaller depending on the orientation of the guide rail than in the case where the angle reflector moves vertically only, as in the known systems. when the object undergoes a change in its vertical extent.

In principle, the angle reflector device according to the invention can be used for the measurement of deformations on those surfaces in which a coupling with the coupling element and with the coupling element sliding along on the deforming surface is possible. In particular, a water surface is suitable as a "deformed" surface, the coupling element in this example being a floating body. The frame is fixed in the water, whose water surface is to be measured, installed. The float floats on the water surface. As soon as their height changes, the float moves not only vertically but also horizontally along the surface, being guided by the guide element and the guide rail. With the float moves accordingly also the angle reflector.

In a corresponding manner, the inventive angle reflector device can also be used for level measurement of free-flowing material. In both cases, it is expedient if the angle reflector device is additionally weighted (for example by a ballast weight) to ensure that it "drops" following gravity as the surface to be measured lowers. In an advantageous embodiment of the invention can be provided that the guide element is mechanically coupled to the coupling element.

In a further advantageous embodiment of the invention can be provided that the angle reflector is arranged to change its orientation relative to the direction of incidence of radar waves in particular automatically adjustable.

In a further advantageous embodiment of the invention, a holding element is provided, with which the angle reflector, the coupling element and the guide element are connected. This construction simplifies the assembly of the angle reflector device.

In a further advantageous embodiment of the invention can be provided that the angle reflector has three pairs of mutually orthogonally arranged reflector surfaces and / or that the reflector surfaces are hinged and, where appropriate, are automatically erectable again.

The reflector surfaces of the angle reflector need not necessarily be formed as closed, continuous surfaces. It is particularly appropriate that the reflector surfaces of the angle reflector perforated and in particular are formed by grid elements. This has the advantage that, depending on the intended use of the angle reflector device whose angle reflector is less susceptible to wind, is significantly easier and that further can not accumulate in him water.

In a further advantageous embodiment can be provided that the guide rail is arranged with respect to their inclination adjustable on the frame. This opens up the possibility of optimally adapting the expected height difference measurement to the construction of the angle reflector device as well as to the sensitivity of the SAR system due to the wavelength. Thus, the angle reflector device can be used by varying the slope for SAR systems of different frequency bands.

Specifically, the invention may be used for the operational monitoring of wetland or flood-prone areas to monitor the level of flood (civil protection). In addition, further level measurements are conceivable, provided that attachment structures for the frame are present, whose height does not change. Finally, the invention can also be used to calibrate DInSAR devices to resolve ambiguities directly from a measurement (rather than phase unwrapping).

In a further advantageous embodiment of the invention can be provided that it is the frame to a first component of the device, which is anchored in the ground (which is believed to be stable), namely z. B. by steel cables or additional struts (both not shown).

An angle reflector formed of metal plates is mounted on a structure which is connected to the mounting structure by means of a vertical strut. A connection between the two structures is made by plain bearings. After this structure has been placed in the water, the buoyancy of the buoy and the weight of the angle reflector and a counterweight are in such equilibrium that the angle reflector is above the water surface, but the counterweight is below the water surface.

A change in the water level results in an upward (upward or downward) vertical force, which is deflected in the oblique direction predetermined by the oblique guide rails. The angle α between the vertical and the oblique guide rail leads to an improved unique height h ' _u , as already explained.

The angle reflector should be rotated so that the direction of its maximum backscatter substantially coincides with the line of sight of the satellite.

The implementation described above is to be considered merely as an example, and numerous variations are possible, provided that the above-described principle of operation is observed.

The invention will be explained in more detail using an exemplary embodiment and with reference to the drawing. In detail, they show:

1 and 2 a comparison of the previously used measuring geometry with the improved measuring geometry for measuring the surface height change h, wherein a height difference h results in an oblique distance difference d, wherein in the improved geometry, the height difference h is effectively reduced by the measured difference in the skew distance d to d ' , where the angle of incidence is given by θ,

3 the unique height h _u for the known geometry (displacement of the mirror point only in the vertical direction) as a function of the angle of incidence θ in the C-band (wavelength λ = 5.6 cm),

4 and 5 the improvement of the uniqueness of the height h ' _u (when using the inclined guide rail) as a function of the angle of incidence θ in the C-band (wavelength λ = 5.6 cm) for the geometry according to the invention, the original measuring geometry of a vertical alignment of the guide rail, ie α = 0 °, and

6 a possible construction for the inventive angle reflector device for use in the Wasserspielgelhöhenbestimmung of waters or flood-prone areas to monitor the extent of the flood (civil protection).

Synthetic aperture (SAR) radar systems have found many new applications in recent years. In particular, the interferometric SAR (InSAR) has led to an expansion of SAR applications, as this principle can be used to quantify terrain heights and surface deformities. A recent application based on the InSAR principle is to monitor water level changes in wetlands.

The approach of InSAR or Differential Interferometric SAR (DInSAR) provides high resolution in detecting strain changes, ie. H. a resolution on the order of a fraction of the SAR wavelength. The InSAR measurements are phase measurements, and therefore, unambiguous measurements are only possible within 2π, which may limit the suitability of the InSAR approach for some applications. For example, at the C-band (wavelength of 5.6 cm) at an incident angle of 40 °, multiples of 3.7 cm (= 5.6 cm x sin 40 °) of vertical deformation result in the same phase measurement and are without additional information not distinguishable from each other. For applications such as wetland monitoring or high water level reporting, where greater altitude changes are expected, additional unambiguous measurement may be required.

The following is an overview of the inventive concept of a three-surface angle reflector device, by means of which the degree of uniqueness at the same wavelength and angle of incidence is effectively improved by transforming a vertical movement of a reference point into an oblique motion. The apparatus is useful for detecting unique phase measurements for monitoring the water level of wetlands, or for similar applications where differences in magnitude greater than 2π are expected. Considering that the invention can be manufactured at a relatively low cost, the concept is applicable not only to a few calibration measurements, but also in an operating environment where simple angle reflectors have previously been used.

Although the invention is also applicable to measurements other than observing the level of water in wetlands, this use shall serve as an example in the following explanations.

The current approach to measuring the water level with an InSAR image pair is by utilizing the dual reflection of existing vegetation. The double reflection is caused by the water surface and a vertical (biological) shaft. The skew distance, as seen by the radar for this two-face configuration, is equal to the line-of-sight distance to the vertices of the two-face shape, ie, where the water surface and the shaft contact each other. As the water level changes, this vertex moves vertically a distance h, and the SAR instrument then measures a phase change that is proportional to h, projected to the skew distance d. The geometry is in 1 shown.

The SAR instrument detects phase changes Δφ in a range direction with a sensitivity Δφ = 4π / λd, where λ is the wavelength. As with any phase measurement, multiples of 2π of the measured phase can not be distinguished so that there is only a limited range of heights for which a unique phase measurement can be determined. The height should be designated as a unique height h _u = λ / 2cosθ, where θ is the angle of incidence. 3 shows the unique height for a C-band system as a function of the angle of incidence.

To increase the unique height h _u for water level measurements, you can use the in 2 use schematically shown embodiment according to the invention. Here, a water level change not only leads to a vertical shift of the mirror point, but also to an additional horizontal shift. This can be z. B. can be achieved by an angle reflector is installed on a guide rail, wherein the height of the angle reflector is adjusted by a floating buoy. Details of a possible realization are listed below.

With this improved geometry, the unique height, ie the uniqueness of the height, can be improved, depending on the inclination angle α of the guide rail, such as 2 shows. The resulting improved unique height h ' _u is h ' _u = λ / 2 * cos α / cos (α + θ)

A graph of the improved unique height versus angle of incidence θ and guide rail pitch angle α is shown in FIG 4 and 5 shown. Theoretically, the unique height can be increased by an arbitrarily large factor, although in practice it is expected that a factor over z. Example 10 would cause too high a sensitivity to alignment errors, so that factors in the range of a fraction of 1 to 10 are useful. It should be noted that at α + θ = π / 2, the effect of a vertical shift is not recognized by the SAR instrument, so this combination should be avoided.

A construction embodiment of the invention shows 6 ,

The reflector device 10 has an angle reflector 12 with three each in pairs orthogonal aligned angle reflector surfaces 14 . 16 . 18 on. The angle reflector 12 is on a holding element 20 in the form of a handrail 22 attached to the below the angle reflector 12 a float 24 as a buoy 26 is attached. Below the float 24 is located on the retaining element 20 another ballast weight 28 ,

In the water 29 (for example, by founding) is a frame 30 fixedly arranged, that in this embodiment consists of two posts 32 . 34 is formed. Between the two posts 32 . 34 extends an obliquely aligned in space guide (especially double) rail 36 , at the one (double) guide element 38 slidably guided is arranged. With the guide element 38 preferably connected rotatably about a horizontal axis is the holding element 20 ,

The float 24 acts as a coupling element 39 for mechanical coupling of the angle reflector 12 with the waters 29 , A change (see the double arrow 40 ) of the water surface 42 leads to a shift (see the double arrow 44 ) of the angle reflector 12 along the guide rail 36 , This will, as well as in 2 shown schematically, the change in the distance of the water surface 42 to the SAR device 46 used in this embodiment as a satellite platform 48 is shown in contrast a reduced change in the distance of the angle reflector 12 to the SAR device 46 implemented.

• – Der Winkelreflektor kann aus nur zwei vertikalen Platten ausgebildet sein, die orthogonal zueinander ausgerichtet sind. Die dritte Winkelfläche wird dann durch die Wasseroberfläche gebildet. Obwohl dieser Ansatz ein kostengünstigeres und leichtgewichtigeres Design ermöglichen kann, gilt es, folgendes zu beachten: – Der Winkelreflektor kann nicht mehr zu dem SAR-Instrument hin gekippt werden, was je nach dem Einfallswinkel zu einer geringeren Rückstreuung bei identischen Plattenbemessungen führt. – Die Wasseroberfläche ist möglicherweise während einer sie überstreichenden Bewegung, z. B. aufgrund von Wind, nicht glatt. In diesem Fall fehlt der durch die Boje bewirkte ”Mittelungs”-Effekt, und der Streumechanismus ist nicht mehr ideal (die Wasseroberfläche verläuft nicht notwendigerweise orthogonal zu den beiden vertikalen Platten).
• – Abgesehen von der Verwendung zur Beobachtung von Feuchtgebieten kann die Winkelreflektorvorrichtung auch zur Beobachtung von Höhenveränderungen in anderen Umgebungen verwendet werden. Das Erfordernis besteht darin, dass die Vorrichtung auf einer konstanten Höhe angeordnet werden kann und dass die Boje (o. dgl.) gleitbar an der Oberfläche befestigt ist, die gemessen werden soll. Mögliche alternative Anwendungen sind: – Messen des Füllpegels von Gastanks, deren Oberseite sich zusammen mit der enthaltenen Gasmenge bewegt. – Messen des Füllpegels von Kornsilos in abgelegenen ländlichen Gebieten, wo der Einsatz anderer Mittel zur Übertragung von Messdaten schwierig ist und wo die Nutzung eines existierenden SAR-Systems eine kostengünstige Alternative zu solchen periodischen Messungen bieten kann. – Messen des Hochwasserstandes in durch Hochwasser gefährdeten Gebieten, um das Ausmaß der Überschwemmungen durch Nutzung existierender SAR-Systeme aus dem Weltraum schnell und sicher erfassen zu können, wodurch der Katastrophenschutz entsprechende Maßnahmen zum Schutz der Bevölkerung und der Umwelt einleiten kann.
• – Bei anderen Anwendungssituationen, in denen eine sogar noch höhere Empfindlichkeit als diejenige für einen Führungsschienen-Neigungswinkel α = 0° erforderlich ist, kann die Führungsschiene auch in der anderen Richtung gekippt werden (α < 0°).
• – Die Winkelreflektorplatten können nicht nur aus massiven Metallplatten ausgebildet sein, sondern auch aus einer gitterartigen Platte oder einem Metallgitter. Dies könnte dazu beitragen, den Windwiderstand der Platten zu reduzieren und somit die Stabilität des Winkelreflektors unter Windbedingungen zu erhöhen. Zudem wird das Gewicht des Winkelreflektors reduziert.
• – Der Winkelreflektor kann mit Schlitzen oder Löchern versehen sein, so dass sich z. B. Regenwasser und kleine Verunreinigungen nicht in dem Reflektor ansammeln können.
• – Die Größe des Winkelreflektors sollte möglichst an das jeweilige SAR-System angepasst sein, so dass das reflektierte Signal stark genug ist, um eine reproduzierbare Messung zu ermöglichen.
• – Die in 6 gezeigte Anbringungs- und Gestellstruktur kann derart realisiert werden, dass der Winkel α im Feld eingestellt werden kann.

Below are some alternatives for realizing the invention:

• - The angle reflector may be formed of only two vertical plates, which are aligned orthogonal to each other. The third angular surface is then formed by the water surface. Although this approach may allow for a more cost effective and lighter weight design, it is important to note the following: The angle reflector can no longer be tilted towards the SAR instrument, resulting in lower backscatter for identical plate dimensions, depending on the angle of incidence. - The water surface may be during a sweeping movement, eg. B. due to wind, not smooth. In this case, the "averaging" effect caused by the buoy is missing, and the scattering mechanism is no longer ideal (the water surface is not necessarily orthogonal to the two vertical plates).
• Apart from the use for wetland monitoring, the angle reflector device can also be used to observe changes in altitude in other environments. The requirement is that the device can be arranged at a constant height and that the buoy (or the like) is slidably mounted on the surface to be measured. Possible alternative applications are: - Measuring the filling level of gas tanks whose top moves along with the amount of gas contained. - Measuring the fill level of grain silos in remote rural areas, where the use of other means of transmitting measurement data is difficult and where the use of an existing SAR system can provide a cost effective alternative to such periodic measurements. - Measuring flood levels in flood-prone areas in order to quickly and reliably detect the extent of floods by using existing SAR systems from space, thereby enabling civil protection to take appropriate action to protect the population and the environment.
• In other application situations where an even higher sensitivity than that for a guide rail inclination angle α = 0 ° is required, the guide rail can also be tilted in the other direction (α <0 °).
• - The angle reflector plates can be formed not only of solid metal plates, but also of a grid-like plate or a metal grid. This could help to reduce the wind resistance of the plates and thus increase the stability of the angle reflector under wind conditions. In addition, the weight of the angle reflector is reduced.
• - The angle reflector may be provided with slots or holes, so that z. B. rainwater and small impurities can not accumulate in the reflector.
• - The size of the angle reflector should be adapted as far as possible to the respective SAR system, so that the reflected signal is strong enough to allow a reproducible measurement.
• - In the 6 shown mounting and frame structure can be realized such that the angle α can be set in the field.

LIST OF REFERENCE NUMBERS

10

reflector device

12

corner reflector

14

Corner reflector surface

16

Corner reflector surface

18

Corner reflector surface

20

retaining element

22

Handrail

24

float

26

buoy

28

ballast weight

29

Waters (as an example of an object on earth)

30

frame

32

post

34

post

36

guide rail

38

guide element

39

coupling element

40

Double arrow for height change of the water surface

42

water surface

44

Double arrow for movement of the angle reflector

46

SAR device

48

satellite

Claims (10)

Angle reflector device ( 10 ) for SAR applications with - an angle reflector ( 12 ) with at least two orthogonally oriented reflector surfaces for the reflection of radar waves, - a coupling element ( 39 ) for the mechanical coupling of the angle reflector ( 12 ) with a variable in its vertical extension and changing its vertical extent the angle reflector ( 12 ) entraining object ( 29 ) on the ground, on and / or on the coupling element ( 39 ) can additionally slide along, - a stationary frame ( 30 ) at a sharp or obtuse angle to the vertical extent of the object ( 29 ) inclined running guide rail ( 36 ) and - one on the guide rail ( 36 ) and along this slidably guided guide element ( 38 ), which mechanically with the angle reflector ( 12 ), - wherein by means of the guide rail ( 36 ) and the guide element ( 38 ) a change in the vertical extent of the object ( 29 ) in a movement of the angle reflector ( 12 ) along the guide rail ( 36 ) can be implemented. Angle reflector device ( 10 ) according to claim 1, characterized in that the guide element ( 38 ) with the coupling element ( 39 ) is mechanically coupled. Angle reflector device ( 10 ) according to claim 1 or 2, characterized in that the angle reflector ( 12 ) is arranged to change its orientation relative to the direction of incidence of radar waves adjustable. Angle reflector device ( 10 ) according to one of claims 1 to 3, characterized by a holding element ( 20 ), with which the angle reflector ( 12 ), the coupling element ( 39 ) and the guide element ( 38 ) are connected. Angle reflector device ( 10 ) according to one of claims 1 to 4, characterized in that the angle reflector ( 12 ) three in pairs mutually orthogonally arranged reflector surfaces ( 14 . 16 . 18 ) having. Angle reflector device ( 10 ) according to one of claims 1 to 5, characterized in that the reflector surfaces ( 14 . 16 . 18 ) of the angle reflector ( 12 ) are formed solid, perforated or in particular of grid elements and / or that the reflector surfaces ( 14 . 16 . 18 ) are foldable and, if necessary, are automatically erectile again. Angle reflector device ( 10 ) according to one of claims 1 to 6, characterized in that the guide rail ( 36 ) adjustable with respect to their inclination on the frame ( 30 ) is arranged. Angle reflector device ( 10 ) according to one of claims 1 to 7, characterized by the formation of the coupling element ( 39 ) as a floating body ( 24 ) for swimming on and / or in a flowable or free-flowing medium as object ( 29 ) whose vertical extent is variable. Use of an angle reflector device ( 10 ) according to one of claims 1 to 8 for measuring deformations on natural and / or artificial objects ( 29 ) on earth, in particular for measuring the water level of bodies of water, in particular of lakes, or of areas at risk of flooding. Use of an angle reflector device ( 10 ) according to one of claims 1 to 8 for the calibration of SAR devices, in particular of InSAR devices and preferably of DInSAR devices.

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