Oil tank level determination by satellite radar

Description

[0001] The invention relates to a method for determining a fill level of a cylindrical oil tank with a shell wall and a floating roof and to a radar system for a satellite for determining a fill level of a cylindrical oil tank with a shell wall and a floating roof.

[0002] The following information does not necessarily refer to a specific prior art document, but reflects generally known information: Large quantities of crude oil or refined end products are typically stored in large cylindrical tanks. To prevent the formation of a gas pocket containing flammable and environmentally harmful vapors between the roof of the tank and the oil stored therein when the fill level changes, such oil tanks typically have a floating roof. This floating roof rises and falls along with the fill level of the oil tank, as the floating roof rests directly on the surface of the oil or end product, preventing the aforementioned vapors from accumulating between the roof and the oil or end product. Therefore, by observing the height of the floating roof, conclusions can be drawn about the fill level of the respective oil tank.Knowing the fill level of an oil tank can be of considerable economic and strategic importance.

[0003] In principle, it is known in the prior art to generate information about oil tanks based on optical information, i.e., images based on wavelengths in the visible range. For example, US 2014 / 0363084 A1 relates to a system for monitoring an oil tank, comprising an extraction module and an analysis module. The extraction module extracts information from image data, and the analysis module performs operations based on the extracted information. A disadvantage of obtaining information using such optical information is that image capture is highly dependent on external conditions, such as solar radiation, clouds above the oil tanks, dust in the air, and the like.

[0004] The paper titled "Oil Storage Estimation with Time-Series L-Band Sar Imagery," published by the 2018 "International Geoscience and Remote Sensing" (IGARSS) Symposium, with authors Takuma Anahara and Masanobu Shimada, further demonstrates a method for determining a change in the position of an oil tank's floating roof between different points in time using radar technology. This allows the volume and maximum capacity of the oil tank in question to be known. However, this requires a series of radar images from a synthetic aperture radar (SAR) device, which reduces the efficiency of the method. Furthermore, only a single point on the floating roof is tracked, which can reduce accuracy.

[0005] Furthermore, US 10,672,139 B2 relates to a method for measuring a liquid volume and / or a corresponding filling rate of floating roof tanks, which comprises selecting an area of ​​interest (AOI) to be monitored at the Earth's surface, collecting geographic coordinates of tanks within the AOI, and downloading and preprocessing time series of SAR reference images covering the AOI. The method further comprises projecting geographic coordinates of the tanks onto the time series of images to determine pixel coordinates of the tanks, and determining tank dimensions by processing the images at the pixel coordinates.The method further comprises downloading and preprocessing at least one new SAR image over the AOI and registering it on the reference images, and for each tank of the AOI, detecting a bright spot in the new SAR image corresponding to the roof of the tank and converting its pixel coordinates into liquid volume and / or fill rate information.

[0006] WO 2019 / 098280 A1 further relates to a system, a method, a program, and a recording medium storing the program for estimating the position or the amount of position change of a rectangular structure in a structure. A SAR intensity image including a target area is viewed at multiple times.

[0007] CN 111540007 A also relates to a method for estimating the storage capacity of an oil tank using SAR images, the steps being as follows: Step 1: Identifying the oil tank from the SAR image, wherein the oil tank is a floating roof oil tank; Step 2: Using threshold segmentation to identify the tank top, oil tank bottom, and oil tank floating roof; Step 3: Determining azimuth and angle of incidence during satellite observation; Step 4: Determining an actual height of the oil tank; Step 5: Calculating the storage capacity of the oil tank; Step 6: Determining the height of the floating roof of the oil tank; Step 7: Calculating a storage rate and storage volume of the oil tank.

[0008] Finally, CN 110276796 A relates to a storage estimation method for a floating-roof oil tank based on a high-resolution SAR image and belongs to the technical field of SAR remote sensing image processing and analysis. CN 110276796 A utilizes the dihedral reflection characteristics of a high-resolution SAR remote sensing image and the geometric characteristics of an image map, and calculates the height H between the bottom and top of the near-end oil tank to directly measure the height of the floating top surface of the oil tank and the top of the inner wall of the far-end oil tank h on the SAR image. The distance between the top of the proximal oil tank and the top of the far-end oil tank formed by secondary reflection of the image intensity peak point is used as the diameter of the oil tank to finally obtain the oil storage volume of the oil tank.

[0009] The object of the invention is to determine the fill level of such a large, in particular cylindrical, tank for crude oil or for a refined end product precisely, efficiently and independently of the position of the sun.

[0010] The invention is based on the features of the independent claims. Advantageous developments and refinements are the subject of the dependent claims.

[0011] A first aspect of the invention relates to a method for determining a fill level of a cylindrical oil tank with a shell wall and a floating roof, comprising the steps: Emitting microwaves from a satellite in orbit or stationary in an Earth orbit from an oblique view in the direction of the cylindrical oil tank located on the Earth's surface, receiving microwaves reflected from the cylindrical oil tank and generating a radar image using a synthetic aperture method, determining a plurality of point scatterers from the radar image by image processing and / or signal processing such that the radar image only contains binary information, wherein the point scatterers are created by reflections of the microwaves striking the cylindrical oil tank and appear on the radar image as color-contrasting points, generating and adjusting at least two semicircles using a nonlinear optimization method or a Hough transformation such that the point scatterers lie on or as close as possible to the semicircles,wherein the semicircles comprise at least the following: a first semicircle associated with a contact line of an outer side of the shell wall with the ground, and a second semicircle associated with a contact line of an inner side of the shell wall with an upper end face of the floating roof, determining a diameter of the oil tank and a height of the floating roof relative to the ground from the respectively adjusted first semicircle and second semicircle, and determining the fill level of the oil tank from the determined diameter of the oil tank and the height of the floating roof.

[0012] While the shell wall of the cylindrical oil tank has a circular cross-section and extends over a certain height, the ground (in particular a foundation on the ground) forms a lower end face of the cylindrical oil tank, and the top side of the floating roof forms an upper end face.

[0013] The fill level of a cylindrical oil tank indicates, in particular, how much crude oil or a final product thereof the oil tank is currently filled with. As explained above, the floating roof rises and falls along with the fill level of the oil tank. Since the thickness of the floating roof is relatively small compared to the height of the shell wall of the cylindrical oil tank and typically also small compared to the average fill level of the oil tank, this thickness is preferably neglected and the height of the surface of the front side of the floating roof is also assumed to be the fill level. Alternatively, an a priori known value for the thickness of the floating roof can be subtracted from the determined height of the floating roof, although this value can also be estimated.

[0014] The satellite, in particular, has a corresponding transmitting antenna and a receiving antenna, whereby the receiving antenna and the transmitting antenna can also be integrated into a common radar antenna. When the satellite is in Earth orbit, its trajectory passes over a series of areas that can be observed by the satellite. A stationary satellite, on the other hand, is a geostationary satellite, which rotates in time with the Earth's rotation.

[0015] Microwaves are a form of electromagnetic radiation in the range particularly between 300 MHz and 300 GHz, i.e. in the range with wavelengths between 1 mm and 1 m. These can also be synonymously referred to as radar beams.

[0016] The term "oblique view" means that the satellite is not exactly positioned above the oil tank, meaning that the oil tank is detected by the satellite with a deviation from the nadir. In other words, the position of the satellite relative to the observed cylindrical oil tank is such that a normal at the location of the observed cylindrical oil tank to the local surface of the Earth's reference ellipsoid forms a non-zero angle with an imaginary line connecting the observed cylindrical oil tank and the satellite.

[0017] The position of the cylindrical oil tank to be observed can be known through prior knowledge. In particular, a number of maps exist that indicate the position of oil tanks, for example, on refinery grounds. Many such oil tanks can be found, for example, on the "OpenStreetMap" map. Alternatively, the position of the oil tanks can be determined in a first pass of radar-based satellite imaging using a synthetic aperture method, preferably through image recognition or other automated artificial intelligence methods. Thus, if the position of the cylindrical oil tank to be observed is known, a microwave antenna can be aligned together with the typically known position of the satellite so that the cylindrical oil tank to be observed is illuminated by the satellite's microwaves.

[0018] The determination of the plurality of point scatterers from the radar image is carried out by image processing and / or signal processing. The locations on the cylindrical oil tank where double reflections occur appear in the radar image as particularly conspicuous in color, particularly as particularly bright spots, ideally points. The higher the resolution of the radar image, the more likely these bright spots are to be ideal points. By examining the particularly prominent spots and filtering out the rest (often referred to as "clutter" in the literature), the noise component of the radar image is reduced, in particular, because other multiple reflections and speckle noise are filtered out. According to the invention, the determination of the plurality of point scatterers from the radar image is carried out by image processing and / or signal processing such that the radar image only contains binary information, particularly black and white information.In the ideal case, therefore, a processed radar image remains, which essentially only shows point scatter from reflections on the cylindrical oil tank.

[0019] In radar images derived from synthetic aperture radar observations, locations with near-point scattering are typically called "coherent scatterers." Such point scatterers typically dominate a resolution-determined region of the radar image (pixel of the image). Such a pixel, which exhibits a large number of statistically identical point scatterers that are randomly distributed and none of which appears dominant, is also referred to as "clutter." Various methods are known for detecting such point scatterers in a single high-resolution synthetic aperture radar (SAR) image.An overview of such known methods can be found in the paper titled "A New Detection Algorithm for Coherent Scatterers in SAR Data," published in "IEEE Transactions on Geoscience and Remote Sensing" in 2015, with the authors: Maria J. Sanjuan-Ferrer, Irena Hajnsek, Konstantinos P. Papathanassiou, and Alberto Moreira. One of these approaches can be used to detect such point scatterers as described above and below. The exact distance (the so-called "range") between the radar sensor on the satellite and each of the detected point scatterers can then be determined with sub-pixel accuracy (i.e., a higher accuracy than the range resolution of the SAR radar sensor itself). This is also described in the above paper, which is hereby incorporated. This advantageously improves the accuracy in determining the height of the floating roof, as a more precise distance determination is possible.

[0020] The semicircles are adjusted based on the following parameters in particular: location of their center, radius or diameter, and apparent offset of the semicircles from one another. Since the satellite is aimed at the oil tank to be observed from an oblique view, horizontally distant objects on the Earth's surface can be detected as being further away by detecting the microwaves' travel time accordingly. This is because the microwaves have a longer travel time the further the object is horizontally from the nadir as seen from the satellite. However, if reflections occur from an object area at a certain height above the Earth's surface, the microwaves will have a shorter travel time and will therefore be detected as being closer to the nadir from the satellite's perspective. This is reflected in the offset of the point scatterers lying (approximately) on the semicircles relative to one another.Furthermore, the semicircles are preferably adjusted only up to predetermined restrictions, wherein the restrictions in particular have a maximum value for the radius or diameter, and the height of the oil tank or the floating roof.

[0021] The reason why only semicircles are adjusted instead of full circles is that a cylindrical oil tank, especially a metallic one, leads to total internal reflections of the microwaves. In the shadow of the microwaves on the cylindrical oil tank, no reflections are therefore detected. In principle, three semicircles can be identified by the satellite, since double reflections occur at these contact lines. A first semicircle results at the transition from the outer wall of the cylindrical oil tank's lateral surface to the floor on the half-side of the oil tank facing the satellite. A second semicircle from a second double reflection in the satellite's field of view results on the illuminated inner wall of the lateral surface at the transition to the upper side of the front surface of the floating roof.A third semicircle from a third double reflection results along the outside of the shell wall of the oil tank where a maintenance walkway runs in the area of ​​the upper edge of the shell wall, but again on the side facing the satellite. However, detecting the first and second semicircles is sufficient to determine the current fill level of the oil tank. This is because the distance between the satellite and the ground is then known through the lower semicircle on the ground. Furthermore, detecting the second semicircle on the front of the floating roof determines the relative height of the floating roof to the ground. The cross-sectional area of ​​the oil tank is known from the radius or diameter of the lower semicircle or this second semicircle on the floating roof. In principle, it is irrelevant whether the diameter or a radius of the oil tank is determined; both quantities can be converted into one another without difficulty.The distance of the floating roof from the ground also provides a height, and from the height and the cross-sectional area, the volume of oil or the end product with which the oil tank is filled can be calculated.

[0022] Since the observed tank has a cylindrical shape, the observable transitions between the outer wall and the ground, between the maintenance corridor and the outer wall, and between the inner wall and the upper end of the floating roof form semicircles, which appear elliptical in the radar image due to the oblique view. However, this does not limit the calculation possibilities, since the ratios of the extensions of the ellipse axes can be calculated using the angle of incidence of the microwaves relative to the Earth's surface.

[0023] It is advantageously assumed that the thickness of the shell wall of the cylindrical oil tank and the thickness of the floating roof are negligible to a first approximation. If the method according to the invention is repeated over several time intervals, primarily to track the change in the fill level of the observed cylindrical oil tank, the thickness of the floating roof is eliminated anyway by subtracting the respective measured heights, since the thickness remains constant over time.

[0024] It is therefore an advantageous effect of the invention that, by using a synthetic aperture radar (SAR) method, observation can be carried out regardless of the position of the sun and the degree of cloud cover above an oil tank. Furthermore, the achievable accuracy is advantageously not dependent on environmental factors in the area around the oil tank, but only on the spatial resolution of the radar sensor, the selected image geometry and the quality of the algorithm used to extract information from the sensor data. The advantageous high efficiency of the method lies in the fact that only a single radar image from a SAR radar device is required to determine the current fill level and the maximum capacity of an oil tank with a floating roof. The use of a large number of point scatterers (coherent scatterers)The use of coherent scatterers instead of a single point scatterer (particularly from double reflections between the floating roof and the walls of such a tank) further enables a highly accurate and robust method for determining the current height of an oil tank's floating roof, as well as the diameter of the shell wall, compared to the state of the art. Furthermore, the advantageous use of information from the point scatterers allows the height of the floating roof to be determined with sub-pixel accuracy. Furthermore, if a large number of radar images are generated for different points in time, the proposed method can synergistically combine their information content to improve the accuracy of all radar images and the overall robustness of the method, instead of only generating individual, technically independent radar images.

[0025] According to an advantageous embodiment, at least three semicircles are generated and adjusted so that the point scatterers lie on or as close as possible to the semicircles, wherein the semicircles have a third semicircle which is assigned to a contact line of the outer side of the shell wall with a maintenance passage in the region of the upper edge of the shell wall, wherein an overall height of the oil tank is determined, wherein a maximum possible fill level of the oil tank is determined from the determined diameter and the overall height of the oil tank.If the relative position and extent of each of the three semicircles mentioned above are known, the maximum capacity of the cylindrical oil tank is also known (at least approximately), as is the current fill level. This is because not only the height of the floating roof relative to the outer walls of the cylindrical oil tank is known, but also the maximum height of the floating roof, which correlates with the height of the maintenance walkway at the upper edge of the shell wall. As an approximation, the height of the maintenance walkway can be directly assumed as the maximum height of the floating roof; or, a predetermined constant amount is added or subtracted from the height of the maintenance walkway to determine the maximum height of the floating roof.

[0026] According to a further advantageous embodiment, at least one of the following restrictions is applied when adjusting the semicircles: all semicircles have the same radius; the centers of all semicircles lie on a common line perpendicular to the Earth's surface; only the first semicircle and the third semicircle lie on a common side of the mantle wall;

[0027] According to a further advantageous embodiment, a single radar image is generated using a synthetic aperture method. This advantageously increases efficiency compared to systems that require multiple radar images.

[0028] According to the invention, a nonlinear optimization method or a Hough transform is used to generate and adapt the semicircles. A first possibility for adapting the semicircles consists in selecting, from a very large number of possible semicircles, one that best matches the local point scatterers. Another possibility is the application of a nonlinear optimization method, in particular with a deterministic approach, for example a fastest descent method for minimizing a cost function or a quadratic optimization with such a cost function, where the cost function specifies the sum of the distances between the point scatterers and a respective semicircle to be adapted. Other methods of nonlinear optimization are also possible, in particular an evolutionary algorithm or genetic algorithms.A Hough transformation is a well-known method for recognizing given geometric shapes, which is carried out here in particular with the semicircles to be adapted.

[0029] According to a further advantageous embodiment, the method steps according to one of the preceding claims are carried out repeatedly at at least two different times in order to track a change in the fill level of the oil tank.

[0030] According to a further advantageous embodiment, when adjusting the semicircles, all point scatterers remaining at the same location over the two points in time are assigned to the contact line of the outside of the shell wall with the ground or the contact line of the outside of the shell wall with the maintenance passage in the region of the upper edge of the shell wall, and all point scatterers whose location changes over time are assigned to the contact line of the inside of the shell wall of the oil tank with the upper end face of the floating roof, wherein each of the point scatterers considered is checked for interferometric coherence between the two points in time in order to rule out confusion between the point scatterers.

[0031] The point scatterers under consideration are checked for their interferometric coherence between the two points in time, as different point scatterers could appear at the same location in the radar image. In addition to separating the point scatterers that remain at the same location over the two points in time from the moving point scatterers in the radar image, this check for interferometric coherence is advantageously carried out to ensure that the respective remaining point scatterer under consideration is one and the same point scatterer over the two points in time. High interferometric coherence of a point scatterer assumed to remain indicates no movement of the same point scatterer, while low interferometric coherence indicates a change in the point scatterer. The separation between the point scatterers that remain at the same location and the point scatterers that have moved ultimately enables the individual point scatterers to be assigned to the respective tangency lines.The assignment of the point scatterers remaining in their respective locations to the contact line between the outer side of the shell wall and the ground or to the contact line between the outer side of the shell wall and the maintenance corridor allows the determination of the tank diameter and the maximum fill level of this tank. Furthermore, based on the diameter and by assigning the point scatterers that have moved to the contact line between the inner side of the shell wall and the floating roof to the second semicircle for determining the current height of the floating roof, the current fill level of the floating roof can be determined. Advantageously, the change in height is determined in a first step and the initial absolute height of the floating roof is determined in a second step.

[0032] According to a further advantageous embodiment, the change in the fill level of the oil tank is determined by solving an optimization problem, wherein the optimization problem comprises the search for the maximum related number of point scatterers over the two points in time, wherein point scatterers remaining at the same location and point scatterers whose location changes over time are separated from each other, and the change in the fill level of the oil tank is determined from the change in height of the point scatterers whose location changes over time.

[0033] This is preferably done by analyzing a cross-correlation of the moving point scatterers in the processed radar images for different points in time (and, in general, the movements of the point scatterers) along a range axis (the so-called "range axis"). This approach advantageously utilizes not only the respective double reflections themselves, but also all physical features and conditions of the floating roof. The change in the fill level can therefore also be determined with sub-pixel accuracy, since the position of each individual point scatterer with respect to its distance (the so-called "range") can be determined with sub-pixel accuracy, as described above.

[0034] According to a further advantageous embodiment, the method steps explained above or below are repeatedly performed at more than two different points in time. By repeatedly equating displacements of the floating roof recorded at non-consecutive points in time with the sum of all recorded displacements of the floating roof between the points in time, an overdetermined system of equations is generated to determine a displacement of the floating roof over a specified period of time. To solve the overdetermined system of equations, a method is preferably used that is particularly advantageously robust against so-called "outliers" in order to avoid unnecessarily distorting the overall result.By using such redundant information, the noise component in the data used can be further reduced and the accuracy of the process can be advantageously increased in order to determine semicircles that correspond as closely as possible to the reality of the oil tank.

[0035] Once the changes in the fill level of the oil tank have been determined from the changes in height of the floating roof, it is preferable to determine the initial height of the floating roof in order to determine an absolute measure of the volume of stored oil in the oil tank at each point in time, rather than just the relative changes in level. The estimate of the initial absolute height of the floating roof is preferably determined using several, in particular all, radar images from the different points in time, in particular by determining the respective semicircle from the respective double reflection in each of the radar images, and then using the change in the height of the floating roof determined for the respective previous point in time to determine the estimate of the initial height of the floating roof. If the center of the bottom of the tank and the radius orIf the diameter and maximum height of the floating roof are known, a Hough transformation no longer necessarily needs to be performed to explicitly determine the second semicircle at the transition from the inside of the shell wall to the top of the floating roof. Instead, a binary template can be created that has a semicircle with the radius of the oil tank, and a cross-correlation can be established for all of its possible heights. By using the multitude of radar images from the respective points in time, in particular all of these radar images, to determine the initial height of the floating roof, the estimate of this initial height can be advantageously made more accurate and more robust to errors.

[0036] If the process steps are repeated at least two different times to track a change in the oil tank level, the following procedure is preferred: Detecting point scatterers for each of the radar images, determining point scatterers remaining in the same location compared to moving point scatterers, determining a radius or diameter of the oil tank and a height of the oil tank using the point scatterers detected as remaining in place, in particular using a Hough transformation, determining an estimate of a change in height of the floating roof from pairwise combined radar images of a plurality, in particular all, considered points in time, and determining an estimate of the initial height of the floating roof by adjusting the second semicircle in a plurality, in particular all, radar images of the considered points in time.

[0037] A further aspect of the invention relates to a radar system for a satellite for determining the fill level of a cylindrical oil tank having a casing wall and a floating roof, comprising a radar unit designed to emit microwaves from an oblique view in the direction of a cylindrical oil tank located on the Earth's surface and to receive microwaves reflected from the cylindrical oil tank, and comprising a computing unit designed to generate a radar image from the received microwaves using a synthetic aperture method, as well as to determine a plurality of point scatterers from the radar image by image processing and / or signal processing such that the radar image only contains binary information, wherein the point scatterers are created by reflections of the microwaves striking the cylindrical oil tank and appear on the radar image as color-contrasting points,and is designed to generate and adjust at least two semicircles using a method of nonlinear optimization or a Hough transformation such that the point scatterers lie on or as close as possible to the semicircles, wherein the semicircles have at least the following: a first semicircle associated with a contact line of the outer side of the shell wall with the ground, and a second semicircle associated with a contact line of an inner side of the shell wall of the oil tank with the upper end face of the floating roof, wherein the computing unit is further designed to determine a diameter of the oil tank and a height of the floating roof relative to the ground from the respectively adjusted first semicircle and second semicircle, and to determine the fill level of the oil tank from the determined diameter of the oil tank and the height of the floating roof.

[0038] Equivalently, the computing unit can be located on the satellite or on a ground station.

[0039] Advantages and preferred developments of the proposed radar system result from an analogous and analogous transfer of the statements made above in connection with the proposed method.

[0040] Further advantages, features, and details will become apparent from the following description, which – where appropriate with reference to the drawings – describes at least one embodiment in detail. Identical, similar, and / or functionally equivalent parts are provided with the same reference numerals.

[0041] They show: Fig. 1 shows a method for determining a fill level of a cylindrical oil tank according to an embodiment of the invention, Fig. 2 shows an exemplary cylindrical oil tank, Fig. 3 shows a radar system when carrying out the method according to the embodiment of the invention, and Fig. 4 shows an example from a real application with point scatterers.

[0042] The representations in the figures are schematic and not to scale.

[0043] Fig. 1 shows a method for determining a fill level of a cylindrical oil tank 100 with a shell wall 102 and a floating roof 104. Such an oil tank 100 is shown by way of example in the Fig. 2 Furthermore, the method is carried out on a satellite 1 with a radar system 2, which is Fig. 3 The procedure explained below can therefore also be carried out using the Fig. 3 can be reproduced. In a first step of the method, microwaves S1 are transmitted from a satellite 1 orbiting in Earth orbit from an oblique view in the direction of the cylindrical oil tank 100 located on the Earth's surface. Oblique view means that the satellite 1 does not have the oil tank 100 below it from its nadir, but rather is at an angle to this nadir in the direction of the oil tank 100. In a second step of the method, microwaves reflected from the cylindrical oil tank 100 are received S2. A single radar image, which can be displayed pictorially, is generated from the detected reflected microwaves. This is done using a synthetic aperture radar (SAR) method.Finally, a plurality of point scatterers is determined S3 from the radar image. The point scatterers are created by reflections of the microwaves striking the cylindrical oil tank 100 and appear as brightly contrasting points on the radar image. The subsequent generation and adjustment S4 of three semicircles is then carried out such that the point scatterers lie on or as close as possible to the semicircles. The semicircles have the following: a first semicircle 11 associated with a contact line between an outer side of the shell wall 102 and the ground, a second semicircle 12 associated with a contact line between an inner side of the shell wall 102 and an upper end face of the floating roof 104, and a third semicircle 13 associated with a contact line between the outer side of the shell wall 102 and a maintenance passage in the region of the upper edge of the shell wall 102.Finally, the determination S5 of a diameter of the oil tank 100 and a height of the floating roof 104 relative to the ground is carried out from the respectively adjusted first semicircle 11 and second semicircle 12, and the determination S6 of the fill level of the oil tank 100 is carried out from the determined diameter and the height of the floating roof 104 and the determination of a maximum possible fill level of the oil tank 100 is carried out from the determined diameter and the total height of the oil tank 100 from the respectively adjusted first semicircle 11 and third semicircle 13.When fitting the semicircles, all of the following restrictions are applied: All semicircles have the same radius; the centers of all semicircles lie on a common straight line perpendicular to the Earth's surface; and only the first semicircle 11 and the third semicircle 13 lie on a common side of the mantle wall 102, while the second semicircle 12 lies on an opposite side of the mantle wall 102. These restrictions are part of a nonlinear constrained optimization, in which the cost function is calculated from the sum of the distances between the individual point scatterers and the semicircles to be fitted. The restrictions transformed as a component of costs are added to the cost function, so that minimizing the cost function automatically leads to compliance with these restrictions. The nonlinear optimization algorithm converges in such a way that the point scatterers are automatically assigned to the correct semicircle.Point spreader.

[0044] Fig. 2 shows such an oil tank 100. The height of the floating roof 104 correlates with the oil level in the oil tank 100. The shell wall 102 has a circular cross-section. A maintenance passage is located at the upper end of the shell wall 102.

[0045] Fig. 3 shows a radar system 2 on a satellite 1. The radar system 2 is used to determine a fill level of a cylindrical oil tank 100 as shown in Fig. 2 The radar system 2 has a radar unit 3 designed to transmit microwaves from an oblique view toward the cylindrical oil tank 100 located on the Earth's surface and to receive microwaves reflected by the cylindrical oil tank 100. Since the satellite 1 is oriented obliquely with respect to the cylindrical oil tank 100, the semicircles actually present on the oil tank 100 appear elliptical in the radar image. The reason the three semicircles mentioned above are considered specifically is that the microwave radiation, due to their oblique angle of incidence caused by the oblique view, travels back towards the satellite 1 through double reflections. The double reflections occur at exactly the locations of the three imaginary semicircles mentioned above.In the case of the first semicircle 11 and the third semicircle 13, the incoming microwaves are each first reflected by the outer surface of the shell wall 102 of the cylindrical oil tank 100. In the case of the first semicircle 11, the microwaves are then reflected by the ground back to satellite 1. In the case of the third semicircle 13, after being reflected by the outer wall of the shell wall 102, the microwaves are reflected back to satellite 1 at a maintenance walkway in the region of the upper edge of the shell wall 102. In the case of the second semicircle 12, the incoming microwaves are first reflected by the inner side of the shell wall 102, and this reflection is then in turn reflected by the upper end surface of the floating roof 104 of the cylindrical oil tank 100 in the direction of satellite 1.A computing unit 5 of the radar system 2 is used to generate a radar image using a synthetic aperture method from the received microwaves, as well as to determine a plurality of point scatterers from the radar image. The point scatterers are created by reflections of the microwaves striking the cylindrical oil tank 100 and appear on the radar image as color-contrasting points.The computing unit 5 further serves to generate and adjust the three semicircles so that the point scatterers lie on or as close as possible to the semicircles, wherein the semicircles have the following: a first semicircle 11 associated with a contact line between the outer side of the shell wall 102 and the ground, a second semicircle 12 associated with a contact line between an inner side of the shell wall 102 of the oil tank 100 and the upper end face of the floating roof 104, and a third semicircle associated with a contact line between the outer side of the shell wall 102 and a maintenance passage in the region of the upper edge of the shell wall 102. The first semicircle 11 is at the level of the earth's surface and therefore does not appear distorted in the radar image. However, the third semicircle 13 appears closer to the satellite 1 in the radar image from the perspective of satellite 1 than the first semicircle 11.This is because the microwaves are reflected at a shorter distance from satellite 1 and, from the perspective of satellite 1, therefore have a shorter travel time. Therefore, the microwaves reflected at the upper edge of the shell wall 102 of the oil tank 100 are detected as being at a shorter horizontal distance from satellite 1. If this two-dimensional image of the point scatterers, or the semicircles placed on them, is rotated by another ninety degrees, the final image is created in the . Fig. 3 , whose three semicircles 11, 12, 13 produce a quasi-spatial image of the oil tank 100, but from the opposite perspective of satellite 1. This apparent distortion of the point scatterers or the semicircles aligned with them is also called the "layover effect." Finally, the computing unit 5 determines a diameter of the oil tank 100, a height of the floating roof 104 relative to the ground, and a height of the oil tank 100 from the geometric dimensions of the semicircles, which appear as ellipses in the radar image. From these geometric dimensions of the oil tank 100 in its current configuration, the computing unit 5 finally determines the current fill level of the oil tank 100 and the capacity of the oil tank 100.

[0046] Fig. 4shows an example from real application, where the left of the images shows an amplitude image, the middle of the images the point scatterers ("static coherent scatterers"), and the right of the images the detected first 11 and third 13 semicircles each as thick black lines.

[0047] Although the invention has been illustrated and explained in detail by means of preferred embodiments, the invention is not limited by the disclosed examples, and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention. It is therefore clear that a multitude of possible variations exist. It is also clear that the embodiments mentioned by way of example are truly only examples and should not be construed as limiting the scope, possible applications, or configuration of the invention in any way.Rather, the preceding description and the description of the figures enable the person skilled in the art to implement the exemplary embodiments in concrete terms, whereby the person skilled in the art, with knowledge of the disclosed inventive concept, can make various changes, for example with regard to the function or the arrangement of individual elements mentioned in an exemplary embodiment, without departing from the scope of protection defined by the claims. List of reference symbols

[0048] 1Satellite 2Radar system 3Radar unit 5CPU 11First semicircle 12Second semicircle 13Third semicircle 100Oil tank 102Shell wall 104Floating roof S1Send S2Receive S3Detect S4Adapt S5Detect S6Detect

Claims

1. A method for the determination of a fill level of a cylindrical oil tank (100) with a shell wall (102) and a floating roof (104), comprising the steps: - Transmission (S1) from an oblique viewpoint of microwaves, from a satellite (1) that is orbiting or stationary in an earth orbit, in the direction of the cylindrical oil tank (100) located on the earth's surface, - Reception (S2) of microwaves reflected from the cylindrical oil tank (100), and generation of a radar image using a synthetic aperture method, characterised by the steps: - Determination (S3) of a multiplicity of point scatterers from the radar image by means of image processing and / or signal processing, such that the radar image then has only binary information, wherein the point scatterers are produced by reflections of microwaves striking the cylindrical oil tank (100), and appearing on the radar image as points that stand out in colour, - Generation and adjustment (S4) of at least two semicircles using a method of non-linear optimisation, or a Hough transformation, such that the point scatterers lie on, or as close as possible to, the semicircles, wherein the semicircles have at least the following: a first semicircle (11) assigned to a line of contact between an outer face of the shell wall (102) and the ground, and a second semicircle (12) assigned to a line of contact between an inner face of the shell wall (102) and an upper end face of the floating roof (104), - Determination (S5) of a diameter of the oil tank (100), and a height of the floating roof (104) relative to the ground, from the respectively adjusted first semicircle (11) and second semicircle (12), and - Determination (S6) of the fill level of the oil tank (100) from the diameter of the oil tank (100) and the height of the floating roof (104) determined.

2. The method in accordance with Claim 1, wherein at least three semicircles are generated and adjusted such that the point scatterers are on, or as close as possible to, the semicircles, wherein the semicircles have a third semicircle (13), which is assigned to a line of contact between the outer face of the shell wall (102) and a maintenance passage in the region of the upper edge of the shell wall (102), wherein a total height of the oil tank (100) is determined, wherein a maximum possible fill level of the oil tank (100) is determined from the diameter and the total height of the oil tank (100) that have been determined.

3. The method in accordance with one of the preceding claims, wherein at least one of the following restrictions is applied when adjusting the semicircles: - all semicircles have the same radius; - the centres of all semicircles lie on a common straight line perpendicular to the earth's surface; - the first semicircle (11) and the third semicircle (13) lie on a common face of the shell wall (102).

4. The method in accordance with one of the preceding claims, wherein a single radar image is generated using a synthetic aperture method.

5. The method in accordance with one of the preceding claims, wherein the method steps in accordance with one of the preceding claims are executed repeatedly at at least two different times, so as to track an alteration in the fill level of the oil tank (100).

6. The method in accordance with Claim 5, wherein when adjusting the semicircles, all point scatterers remaining at the same location over the two points in time are assigned to the line of contact between the outer face of the shell wall (102) and the ground, or to the line of contact between the outer face of the shell wall (102) and the maintenance passage in the region of the upper edge of the shell wall (102), and all point scatterers whose location varies over time are assigned to the line of contact between the inner face of the shell wall (102) of the oil tank (100) and the upper end face of the floating roof (104), wherein each of the point scatterers under consideration is checked for interferometric coherence between the two points in time, so as to rule out any confusion between the point scatterers.

7. The method in accordance with Claim 5, wherein the alteration in the filling level of the oil tank (100) is determined by solving an optimisation problem, wherein the optimisation problem comprises the search for the maximum number of related point scatterers over the two points in time, wherein point scatterers remaining at the same location, and point scatterers whose location alters over time, are separated from one another, and the change in the fill level of the oil tank (100) is determined from the change in height of the point scatterers whose locations alter over time.

8. The method in accordance with one of the Claims 5 to 7, wherein the method steps in accordance with one of the Claims 1 to 4 are executed repeatedly at more than two different points in time, wherein by the multiple comparison of displacements of the floating roof (104) detected at points in time not considered consecutively, with the sum of all displacements of the floating roof (104) recorded between the points in time considered, an over-determined system of equations is generated, so as to determine a displacement of the floating roof (104) over a prescribed period of time.

9. A radar system (2) for a satellite (1) for purposes of determining a filling level of a cylindrical oil tank (100) with a shell wall (102) and a floating roof (104), comprising a radar unit (3), which is designed to emit microwaves from an oblique viewpoint in the direction of a cylindrical oil tank (100) located on the earth's surface, and to receive microwaves reflected from the cylindrical oil tank (100), and having a computing unit (5) that is designed to generate a radar image from the received microwaves using a synthetic aperture method, wherein the computing unit (5) is characterised by the fact that it is designed to determine a multiplicity of point scatterers from the radar image by image processing and / or signal processing, such that the radar image then has only binary information, wherein the point scatterers are produced by reflections of the microwaves striking the cylindrical oil tank (100) and appearing on the radar image as points that stand out in colour, and for purposes of generating and adjusting at least two semicircles a method of non-linear optimisation or a Hough transformation is used, such that the point scatterers lie on, or as close as possible to, the semicircles, wherein the semicircles comprise at least the following: a first semicircle (11) assigned to a line of contact between the outer face of the shell wall (102) and the ground, and a second semicircle (12) assigned to a line of contact between an inner face of the shell wall (102) of the oil tank (100) and the upper end face of the floating roof (104), wherein the computing unit (5) is furthermore designed to determine a diameter of the oil tank (100) and a height of the floating roof (104) relative to the ground from the respectively adjusted first semicircle (11) and second semicircle (12), and to determine the fill level of the oil tank (100) from the diameter and the height of the floating roof (104) determined.