System and method for sensing avalanches, landslides and rockfalls

Abstract

The invention relates to a system for determining a path of a movement of masses in a terrain (101), comprising a radar device, i.e. a device having at least one transmitting antenna for transmitting primary radio waves and at least one receiving antenna for generating reception signals caused by secondary radio waves, which are reflected by the terrain as a result of the primary radio waves. The system also comprises an evaluation unit which is designed to determine, on the basis of the reception signals, whether there is a movement-of-masses activity at a specific position, which is represented by a position coordinate value, e.g. range and azimuth. This results in movement-of-masses activities for various points (51) when there is a movement of masses. The evaluation unit is designed to determine a path (55) of the movement of masses from a temporal development of the positions of the points (51) for which a movement-of-masses activity has been ascertained. To determine the path (55), a cluster (52) of points can be formed by means of a clustering method.

Description

[0001] SYSTEM AND METHOD FOR DETECTING AVALANCHES, LANDSLIDES AND ROCKFALLS

[0002] The invention relates to a system and a method for detecting a path of mass movements in the terrain, in particular snow avalanches or earth mass movements (landslides, rockfalls).

[0003] There is a need to record the processes, particularly the paths and spatial extent of mass movements (snow avalanches, landslides, rockfalls and the like) in order to better understand such mass movements, to make forecasts and also to be able to warn those affected of dangers in good time.

[0004] According to the state of the art, the recording of mass movement processes is primarily done visually, through photographs or possibly film recordings with which the avalanche or rockfall is recorded during or after its descent and, if necessary, measured. The disadvantage of this is that whether such recording is even possible depends on the time of day and the weather. Very often, visibility is poor during such events, for example due to weather conditions and / or the time of day, so that the path and spatial extent of snow avalanches, rockfalls, or similar events can only be determined hours or even days later. Furthermore, statements about the dynamics (speed, temporal order of sequential sub-events, etc.) are often not possible or only possible to a limited extent.

[0005] In Ash Matthew et al., “Two-dimensional radar imaging of flowing avalanches'" Cold Regions Science and Technology 102, pp. 41-51 (2014), an approach is discussed according to which an avalanche is observed using a radar device. Using recorded radar images, an avalanche path is subsequently measured manually by noting the spatial coordinates of the radar signal maxima in images taken every second and mapping them onto a geographical coordinate system. This manual procedure relies on the competence of experts who have to interpret the radar images, exclude artifacts, and manually evaluate them. It would not be suitable for everyday applications.

[0006] It is an object of the present invention to remedy this situation and to provide a system and a method for determining the path of an avalanche, which overcomes disadvantages of the prior art and in particular enables the path to be detected independently of weather conditions and time of day.

[0007] According to one aspect of the invention, a system for determining a path of a mass movement in a terrain comprises a radar device, i.e. a device with at least one transmitting antenna for transmitting primary radio waves and at least one receiving antenna for generating received signals which are caused by secondary radio waves reflected from the terrain due to the primary radio waves. Furthermore, it has an evaluation unit which is configured to determine, based on the received signals, whether or not mass movement activity is present at a specific position - represented by a position coordinate value, e.g., range and azimuth. This results in mass movement activities for different position coordinate values ​​in the presence of a mass movement. The evaluation unit is configured to determine, from a temporal development of the position coordinate values ​​(i.e.,To determine a mass movement path (ultimately the positions) for which mass movement activity has been detected, a path is determined. In contrast to the state of the art, a path is not determined retrospectively based on the situation caused by the mass movement and models; instead, an observation of the mass movement is used as it occurs, i.e., in real time, and its temporal development is taken into account.

[0008] In contrast to the aforementioned prior art according to Ash Matthew et al., the proposed approach is not based on a subsequent manual evaluation of the recorded data; rather, the evaluation unit is capable of determining a path. That the evaluation unit is 'equipped' to determine the path means that it contains the means; in particular, it can imply that it is configured to do so and that it has all the means to determine the path. In particular, it can mean that the evaluation unit can determine the path automatically, in particular without manual intervention by a person.

[0009] In contrast to the prior art, the path is not determined based on recorded image data, but rather based on a radar signal generated during the process. This does have the apparent disadvantage that the terrain must be continuously monitored with a radar device, because it is generally not possible to predict, at least not accurately, when a mass movement event will occur. However, this apparent disadvantage is often not serious. It is already known to monitor an area with a radar system in order to detect possible avalanches, rockfalls, or the like in real time and, for example, to trigger road closures. Therefore, in many cases, constant monitoring of the terrain is already provided, and the corresponding radar device can be used as the radar device of the system according to the invention. In contrast to the apparent disadvantage mentioned, the procedure according to the invention has important advantages.

[0010] The determination of a path using a radar signal can, for example, be done in real time. The path can therefore be determined very quickly and can, for example, even be incorporated into terrain monitoring and the corresponding measures to be taken - i.e. a side effect can be that the determination of the path supplements and improves terrain monitoring. For example, the path can be used as an additional criterion for distinguishing between signals triggered by genuine mass movements in the terrain and those triggered by other events (e.g. vehicles moving in the terrain) or artifacts. A mass movement is only present if a determined path - and possibly the temporal development of the detected movement in the terrain - corresponds to a realistic pattern for mass movements. This can, for example, make it easier to rule out false alarms.

[0011] A further advantage is that the path can be determined reliably regardless of the weather and visibility conditions.

[0012] Furthermore, it enables insights into the dynamics of the mass movement event (namely, its speed versus time), which would not have been possible with purely optical path determination. By enabling real-time path determination, it can not only provide insights but also improve terrain monitoring.

[0013] Another advantage is that the path can be determined automatically, without requiring a specialist to invest time, for example, in analyzing image data or performing calculations. At least if a radar device is already available—for example, for avalanche or rockfall warnings—the invention is therefore also economically advantageous.

[0014] Determining a path from the temporal evolution of radar signals (so-called radar tracking) is already known, for example, for civil or military air traffic control systems. The path of an aircraft is extracted from the detected radar signals to simplify tracking for the monitors. The principle assumes that the radar signal generates a point for each object to be monitored (i.e., the object has a unique coordinate and a unique speed). This requirement is not met for mass movements in terrain, because mass movements have a significant spatial extent, and values ​​such as speed, direction, etc. are not identical across the entire spatial extent. It would be pointless and, moreover, complex—and, due to difficult assignments, not easily possible—to apply the well-known tracking principle to all points that can be detected in the radar signal during a mass movement.According to the invention, instead of attempting to capture a cluster of very many paths, a path characteristic of the entire mass movement is determined. This proves to be a sensible approach and, through comparison with previously identified paths and / or with an equation of motion that takes into account information about the terrain, enables meaningful discrimination.

[0015] The radar device is in particular stationary, i.e. generally both the transmitting antenna(s) and the receiving antenna(s) are stationary. A "stationary" radar device is understood here to be a radar device that can be mounted and operated in a stationary arrangement, i.e. one that is fixed in place relative to the ground, and does not have and / or require any moving antennas relative to the ground - this is in contrast to radar devices mounted on aircraft, satellites or in a motor vehicle, for example, or to radar devices that have a carriage on which the antennas are moved. As mobile radar devices, these devices are only functional when the antennas are moved relative to the terrain to be monitored. The radar device often does not have any moving parts at all, although the use of auxiliary devices with moving parts (e.g. a fan, a hard disk or the like) is of course not excluded.It is also not impossible for the entire radar antenna array to be rotatable around a vertical axis, for example, if the area to be monitored is not always the same. For example, it might be conceivable that different areas need to be monitored in summer and winter. It is also conceivable that a different area needs to be monitored at night than during the day.

[0016] The radar device can, in particular, be an FMCW (Frequency Modulated Continuous Wave) radar device. This means that the radar device can be configured to transmit the transmission signal at a modulated transmission frequency, in particular as a sequence of frequency ramps ("chirps").

[0017] The radar device has, in particular, a plurality of receiving antennas and / or a plurality of transmitting antennas. These are mounted or can be mounted in a distributed manner, in particular in a horizontal direction. In addition to a resolution in "range", which is generally obtained directly (in an FMCW radar from a frequency shift between the transmitted and received signals), the position can therefore also be resolved in azimuth. Since a high position resolution is not required to determine the path, it can be sufficient in practice to have two receiving antennas arranged next to one another (relatively close for a large unambiguous range), along with a single transmitting antenna. In the simplest case, for example, the presence of mass movement activity at a specific position can be deduced directly from the radar signal.For example, it can be checked whether the radar signal deviates significantly from a background value—for example, an average value. If this is the case, it can be concluded that movement can be detected at that location.

[0018] Alternatively or additionally, it is possible that each position coordinate is assigned a movement value (in particular a speed value, e.g. a speed or a Doppler shift) determined quantitatively, e.g. by means of Fourier transformation, and this speed value is evaluated - for example, depending on the terrain, the presence of a mass movement can be inferred if a significant movement in the direction of the radar device is detected.

[0019] By extracting potentially relevant movement from a radar signal, a suitable differentiation method (to eliminate background, fluctuations, etc.) results in a cloud of points with (possible) mass movement activity, with each point having a position coordinate. Optionally, each point can also be assigned a quantitative movement value (Doppler shift, velocity).

[0020] The differentiation method can, for example, involve determining a long-term average for each position coordinate (e.g., per range bin, i.e., per range value range, or per range-azimuth bin). This is done, for example, by applying a low-pass filter with a long time constant of, for example, at least one or two minutes, or even five minutes or more, for example, 10-15 minutes, to the radar signal. This long-term average is subtracted from the current signal, for example, divided by a variance that is also continuously determined, and the resulting value is compared with a threshold. If the value is above the threshold, the corresponding position (point) is taken into account. If a mass movement event occurs, the aforementioned cloud of points is created where the mass movement event occurred.

[0021] In one group of embodiments, it can be provided, in particular, that the determination of the long-term average is interrupted as soon as, and where, the discrimination described below actually detects a mass movement event. This means that the background is essentially frozen at the location of the mass movement event: the long-term average determined before the mass movement event is used for the differentiation process. This takes into account the fact that certain mass movement events, such as snow avalanches, can take longer than a few seconds to pass a specific location. Without stopping the determination at such a location, the mass movement event could therefore falsify the determination of the long-term average and ultimately the differentiation.

[0022] The resulting points can then be grouped after the differentiation process, in particular using a clustering method. In one group of embodiments, this is done without making any assumptions about the number of clusters.

[0023] For the clustering process, for example, the means and algorithms known for cluster analysis and group assignment can be used, whereby the vectors to be grouped can include the position coordinates, and whereby, for example, the active clusters determined in past time periods can also be included, which can be particularly relevant when several simultaneously active paths are close to each other and / or approaching each other.

[0024] Additionally or alternatively, the determined velocity values ​​(e.g. in the form of the Doppler shift value) can optionally also be incorporated into the clustering process, e.g. by including these velocity values ​​in the vectors to be grouped in addition to the position coordinates.

[0025] Cluster analysis results in a number of clusters (in the case of an avalanche, often a single cluster; but two, three, or more clusters can also result). Furthermore, cluster analysis can also result in points that cannot be assigned to any cluster. These points are not considered, for example, in the subsequent determination of the path.

[0026] In embodiments where a clustering method is applied, a path and, if applicable, an extent of the mass movement event is then determined per cluster.

[0027] To determine the path, a standardized distribution of points within the cluster can be assumed – if several clusters are identified, this applies to each cluster, resulting in a separate path for each cluster – for example, a Gaussian normal distribution. This directly results in a local center of the cluster, and a path is derived from the temporal development of this local center. Additionally or alternatively, it is also possible to use a front of the cluster or at least take it into account to determine the path. A front of mass movement for a specific cluster is determined, for example, by considering those points in the cluster that are at the front in terms of the slope, e.g., closest to the radar device.

[0028] To determine the path, the temporal evolution is taken into account, ie, if necessary, the center of the cluster, the front or another geometric feature is tracked as a function of time.

[0029] In embodiments, it may be provided that, in addition to the actual radar signal, further criteria are applied in order to conclude whether or not a mass movement is occurring.

[0030] For example, in some embodiments, the aforementioned temporal development is subjected to discrimination, taking into account knowledge of the terrain. For example, events can be discarded very quickly if the direction of movement resulting from the temporal development is inconsistent with the physical conditions—e.g., mass movements for which an uphill or sideways direction is determined instead of downhill. This approach allows the sensitivity of the measurement to be increased, for example, by setting a threshold value that is not too high in the differentiation process, since possible artifacts or signals originating from other movements (e.g., passing cars) can be excluded very quickly and reliably through discrimination.

[0031] An example of a suitable discrimination method is the application of a linear filter, such as a Kalman filter, with the equations of motion applicable to mass movement in the terrain as a constraint. The filter parameters can therefore include assumptions about the dynamics of the mass movement, i.e., both the speed and direction, and in some embodiments, for example, also the size (spatial extent). A simplified model of the terrain can serve as a basis for this, in which the terrain is approximated by inclined planes, with the geometric properties of the planes serving as parameters.

[0032] In addition to the path, the extent of the mass movement can also be determined—that is, the entire, approximately determined contour image, i.e., the outline. The extent can also be determined directly from the model parameters of the standardized distribution. For example, when applying the Gaussian normal distribution, it can be determined from the path and the standard deviation by assuming an interval of plus or minus the standard deviation or a certain multiple of the standard deviation around the path in the horizontal direction.

[0033] In embodiments, a detected path can be associated with a path already identified at an earlier point in time, if such a path exists. Thus, determining the path enables very short-term forecasts regarding the further course of the mass movement. Determining the path can then also be incorporated into the monitoring of the area and the triggering of any necessary measures. For example, it can be provided that a road leading through the monitored area, or lying below the monitored area, is only closed if the detected mass movement is associated with a path that does not exclude a threat to the road, or if it is not associated with a known path. If, however, it is associated with a path that does not pose a threat to the road, it can remain open. In this way, the inventive method can also be used to avoid false alarms.If a path is detected that cannot be assigned to a previously saved path, it can be saved again and, together with the previously saved paths, serve as a reference for future events.

[0034] This text describes a system comprising a radar device and an evaluation unit configured to perform specific evaluation steps. The fact that the system or evaluation unit is "configured" to perform certain steps means that such steps are not only possible, but that the system or evaluation unit also has the means to perform them—such means may include hardware and / or software. For example, the evaluation unit is programmed to determine the path of the mass movement and to output this information as needed—or continuously—via a display or interface.

[0035] In addition to a system, the present invention also provides a method for determining a path. The method can, in particular, include the steps for which the system described in this text is configured.

[0036] The subject matter of the invention is explained in more detail below with reference to the accompanying drawings. In the drawings, like reference numerals designate like or similar elements. The drawings are schematic and not to scale. They show:

[0037] Fig. 1 A system for determining a path of mass movement positioned relative to a terrain;

[0038] Fig. 2 is a diagram of elements of a radar device as part of the system;

[0039] Fig. 3 shows an illustration of steps of the evaluation process; and Figs. 4a-4f show a chronological sequence with points for which the received signals indicate movement, with further steps of the evaluation process being explained based on the sequence.

[0040] Figure 1 shows a radar device 1 of the type according to the invention, which is positioned relative to a terrain 101 in which avalanches, rockfall events, and / or other mass movements are to be expected. The radar device is stationary and provided with a suitable frame 111, by means of which it can be permanently set up and anchored in the terrain. In Fig. 1, two avalanche areas 102 are schematically shown in the area covered by the radar device, in which avalanches are to be expected. In the event of an avalanche in one of the two avalanche areas, a road 104 passing underneath could be endangered (analogously, rockfall events or other mass movements could also be relevant). Also indicated is a signaling system 105, by which the road can be closed if a dangerous event is detected.A typical distance between the radar device and the surveillance area is between a fraction of a kilometer and several kilometers, e.g. 0.5-5 km.

[0041] The radar device has a transmitting antenna 4 and at least two receiving antennas 6, the two receiving antennas being spaced apart from each other horizontally. The spacing of the receiving antennas 6 from each other can correspond to a value between approximately half a wavelength and one wavelength, ie, at 17 GHz, between just under 0.9 cm and just under 1.8 cm.

[0042] As an alternative to an arrangement with at least two receiving antennas, an arrangement with only one receiving antenna but at least two transmitting antennas can also be used. For azimuth resolution, it is important that phase differences can be detected depending on the azimuth angle (and thus the horizontal position of the elements that reflect the radar waves).

[0043] The radar device is connected to an evaluation unit 3, in which, among other things, a path through the terrain is determined based on the received radar signals. The evaluation unit can be integrated into the antenna or arranged separately from it; it can also comprise elements integrated into the antenna as well as separate elements. The hardware of the evaluation unit can comprise dedicated elements, e.g., at least one FPGA and / or a dedicated graphics processor, and / or it can also be partially formed by a general-purpose computer. The term "evaluation unit" therefore does not imply that the elements of this evaluation unit are physically connected.

[0044] The evaluation unit may also have a communication unit for connection to a network and / or a separate operator computer 7.

[0045] Figure 2 shows a diagram of elements of the control and evaluation unit together with a transmitting antenna 4 and a receiving antenna 6. A clock generator OSC clocks a numerically controlled oscillator (DDS) which, with the aid of a control signal 29, generates a frequency ramp which, in turn, serves as a reference for a downstream phase-locked loop with a high-frequency oscillator (PLL) and generates a frequency-modulated, phase-stable transmit signal, for example in a frequency band in the gigahertz range.

[0046] The frequency-modulated transmission signal has a radio frequency suitable for the intended application and, where applicable, legally approved. For example, it operates in a frequency band around 17 GHz. The transmission signal is amplified appropriately, e.g., by a power amplifier (PA), and fed to the transmitting antenna, which transmits the corresponding primary radio waves 11.

[0047] The transmitting antenna 4 generates primary radio waves 11 based on the transmitted signal, which are reflected back from the terrain 101, including any moving objects, so that the resulting secondary radio waves 12 can be detected by the receiving antennas 6.

[0048] The secondary radio waves 12 reflected back from the terrain generate a received signal in each of the receiving antennas 6, which, after suitable amplification (LNA), is mixed with the transmitted signal (mixer 24). Generally, each receiving antenna 6 is assigned its own mixer 24, which is located in the immediate vicinity of the receiving antenna.

[0049] As is known, the output of mixer 24 produces a mixed signal that includes signal components with the sum of the frequencies of the transmitted signal and the received signal, as well as signal components with the difference frequency. A low-pass filter filters out the high-frequency components, so that only signal components with the difference frequency are further processed. This filtered mixed signal ("intermediate frequency signal") is meaningful because the frequency difference between the transmitted and received signals depends on the propagation time (i.e., the time period between transmission and reception) due to the frequency ramps ("chirps"), as well as on the Doppler shift. This circumstance underlies the operating principle of FMCW (Frequency Modulated Continuous Wave) radar systems and is described in the literature.In addition to the aforementioned low-pass filter, a high-pass filter can also be applied to the mixed signal to filter out very low-frequency signal components, particularly those originating from reflections close to the transmitting antenna. Such low-frequency signal components are often comparatively high in energy and hardly meaningful.

[0050] The functionalities of the low-pass filter and the optional high-pass filter are implemented in a band-pass filter 25 in the embodiment of Fig. 2; however, it is also possible for the low-pass filter and high-pass filter to be present as separate, downstream elements.

[0051] The resulting intermediate frequency signal, possibly high-pass filtered, is subjected to analog-to-digital conversion (ADC) and then subjected to subsequent analysis. This may include a first Fourier transform per chirp to determine a range resolution, and possibly a second Fourier transform across the chirps to determine a Doppler frequency shift and a phase, as well as a comparison of the corresponding results per receiving antenna for an azimuth resolution.

[0052] The resulting signal 41, resolved in range and azimuth, is, as illustrated in Figure 3, fed into the determination 42 of a long-term average per range-azimuth-bin, for example, with a time constant between 10 and 15 minutes. For the aforementioned differentiation process, this long-term average is subtracted from the signal (subtraction 43), unless a mass movement event has been assigned to the relevant coordinates (see the following steps). If, however, a mass movement event has been detected, the long-term average from before the mass movement event is used instead of the current long-term average (44; dashed line).

[0053] The result is compared with a threshold (comparison 46) – possibly after division by the variance or another value representing expected fluctuations. If it is greater than the threshold, it is considered; otherwise, it is not. The result of this evaluation for all range-azimuth bins, in the presence of a mass movement, is a cloud 48 of points 51, each of which is assigned to a position in the terrain 101 based on the range-azimuth coordinates. Figures 4a-4f show an example of a temporal sequence during a mass movement event.

[0054] The determined points 51 are then subjected to cluster analysis, which may result in no cluster, one cluster 52, or multiple clusters. In addition to the position coordinates, the measured Doppler shifts can also be included in the cluster analysis.

[0055] In the illustrated embodiment, a single cluster 52 of points is detected, which is represented by a dotted outline. As indicated in Fig. 4c, singular points 61 can also be measured that cannot be assigned to any cluster and, for example, disappear again. These are discarded and have no influence on the determination of the path and outline.

[0056] A standardized distribution is assumed for the cluster, for example, a Gaussian normal distribution. This results in a cluster center. The temporal evolution of this center results in a path 55. Other approaches to determining the cluster center are also possible, for example, by finding the vertical (and possibly also the horizontal) line that divides the cluster into two equal parts, determining the arithmetic mean of the positions of the points, etc. In such cases, the temporal evolution of the center can also yield the path.

[0057] As an alternative to the cluster center, another characteristic of the cluster can be used to determine the path. For example, based on the direction of movement (see also below), a front of the mass movement can be determined (in Fig. 4a-4f, at the bottom of the point cloud), and a center of this front can define the path. The front can be formed by the points that are forward in relation to the slope, i.e., generally closest to the radar device.

[0058] Other approaches can also be found, for example using image processing.

[0059] In a further step, some embodiments discriminate between real mass movement events and artifacts. This is done, for example, using a linear filter, e.g., a Kalman filter. Taking the equations of motion into account, it is checked whether the determined path (including the speed of travel; 'speed of travel' here is the speed at which the front or center of the mass movement moves across the terrain; the speed of travel does not have to be identical to the speed resulting from the measured Doppler shift) corresponds to a realistic path. The equations of motion, which in turn can be incorporated into the constraints for the filter function used, incorporate the previously known information about the terrain conditions stored in the evaluation unit 7. This information can, for example,include a simplified model of the terrain in which the terrain is modeled in sections using inclined planes. The parameters of this model are determined in advance, for example, based on cartographic data or newly taken measurements. They are stored permanently in the evaluation unit, explicitly as terrain parameters (e.g., in the form of coordinates and inclination of the inclined planes) and / or implicitly as filter parameters.

[0060] If a measurement proves plausible during discrimination, ie, consistent with a mass movement, it is pursued further to develop path 55, as illustrated in Figures 4a-4f. Otherwise, it is discarded. This discrimination process can be relatively rapid, ie, it can be determined within a few seconds whether or not an effective mass movement is present.

[0061] As already mentioned above, the information regarding whether a mass movement has been verified as plausible or not is also used to determine the long-term average for background subtraction. The long-term average is, for example, frozen at the last determined value before the onset of the mass movement.

[0062] In addition to the path, an outline 56 can also be determined in parallel, which records the spatial extent of the mass movement. Assuming a standardized distribution, a lateral extent - and thus, taking the course into account, the entire outline - can result naturally from the assumption of a certain width resulting from the standardized distribution, for example from the path and, in the case of a Gaussian normal distribution, the parameter o ("standard deviation"), which determines the width perpendicular to the path. A determined path 55 and / or, if applicable, a determined outline 56 can be compared with previously stored paths / outlines from past events. If a match is found, this information can, for example, be incorporated into the functioning of a warning function. For example, in Fig. 1, avalanches in an avalanche zone (in Fig.1 shown on the left) may pose a danger to road 104, while avalanche paths in another avalanche area (in Fig. 1 on the right) do not pose a danger due to the topography.

[0063] If a recorded path does not correspond to a previously stored path, it can be re-saved and used in forecasts related to future mass movement events.

[0064] Under certain circumstances, it may also be possible to delete a path that has already been saved from the database after a certain period of time if it does not correspond to a path that was actually measured during that period.

Claims

PATENT CLAIMS 1. A system for determining a path (55) of a mass movement in a terrain (101), comprising a radar device (1) with at least one transmitting antenna (4) for emitting primary radio waves and at least one receiving antenna (6) for generating received signals which are caused by secondary radio waves (12) reflected from the terrain (101) as a result of the primary radio waves (11), as well as an evaluation unit which is configured to infer the presence or absence of a mass movement activity from the received signals as a function of at least one position coordinate, from which mass movement activities arise for various positions represented by position coordinate values, wherein the evaluation unit is further configured to determine a path (55) of the mass movement from a temporal development of the positions for which a mass movement activity was detected.

2. The system according to claim 1, wherein the evaluation unit is configured to determine a long-term average of the received signal for each position, and wherein determining the mass movement activity includes a comparison between the received signal and the long-term average.

3. The system of claim 2, wherein the comparison includes checking whether or not a difference between the received signal and the long-term average satisfies a threshold condition. System according to claim 2 or 3, wherein measurements of the received signal that take place during a detected mass movement activity are not taken into account when determining the long-term average. System according to one of the preceding claims, wherein the evaluation unit is configured to determine those positions for which the received signals indicate a Movement, to a clustering process and to determine the path per cluster (52). System according to claim 5, wherein for the clustering process Vectors comprise the position coordinates of the positions for which mass movement activity was detected. System according to claim 5 or 6, wherein the evaluation unit is configured to assume a standardized distribution, for example a Gaussian normal distribution, for a cluster (52) resulting from the clustering method. System according to one of the preceding claims, wherein the evaluation unit is configured to perform a discrimination step for determining the path (55), in which the temporal development of the positions is compared with at least one equation of motion. System according to claim 8, wherein a Kalman filter is used for the discrimination step.

10. The system according to claim 8 or 9, wherein the discrimination step is performed in real time and is carried out during a suspected mass movement event, so that discrimination can be completed before the suspected mass movement event ends.

11. The system according to any one of the preceding claims, wherein the evaluation unit is configured to determine, in addition to the path, an outline (56) of the mass movement.

12. System according to one of the preceding claims, wherein the evaluation unit has a database with stored possible paths, and wherein the evaluation unit is arranged to compare the path (55) of the mass movement with the possible paths.

13. System according to claim 12, which is arranged to store the path (55) of the mass movement in the database with stored possible paths if it cannot be assigned to any stored possible path.

14. System according to one of the preceding claims, which comprises a plurality of Receiving antennas (4) and / or a plurality of transmitting antennas (6), which is why the position coordinate values ​​depend on range and azimuth.

15. System according to one of the preceding claims, wherein the at least one transmitting antenna (4) and the at least one receiving antenna (6) are stationary. Method for determining a path (55) of a mass movement in a terrain (101), wherein primary radio waves (11) are emitted and emitted due to the primary radio waves (11), and wherein secondary radio waves (12) reflected due to the primary radio waves (11) cause a received signal, wherein this received signal is evaluated as a function of at least one position coordinate in order to conclude the presence or absence of a mass movement activity, from which mass movement activities result for different positions represented by position coordinate values, and wherein from a temporal development of the positions for which a Mass movement activity has been detected, a path of mass movement is determined.

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