Patent Description:
Avalanche monitoring is of critical importance for safety of people and infrastructure. Nowadays several technologies are used to detect the onset of avalanches. Radar is chief among contender technologies as it provides persistent remote monitoring in all weather conditions. Avalanche monitoring comprises several different specific tasks such as: <NUM>/<NUM> monitoring of critical infrastructures, such as roads running nearby exposed mountain slopes, prediction of imminent avalanches, and confirmation of occurred avalanches after triggering blasting. Regarding the blasting scenario, there exist several cases and locations in which avalanches are likely to form after significant accumulation of new snow or after rapid increase of air temperature. In such cases, operators can trigger the avalanche via controlled blasting to reduce risk to people and infrastructure. It is then necessary to have a confirmation that the blasting actually generated an avalanche. To achieve this, a dedicated and punctual observation of a specific area is required, and it has to be performed independent of weather and visibility conditions.

Currently this task is performed using different technologies, among which mechanical systems located at specific points of a hillside, namely wires torn by a falling avalanche. These mechanical trip-wires have the major disadvantage that after every incident, new wires need to be strung again and this operation can endanger technicians. Another solution is to use copters to monitor the predefined area whilst blasting an avalanche, but this solution can only be used under good weather conditions. An alternative solution to solve the aforementioned drawbacks is the use of active radars, as they (i) are unaffected by low visibility; (ii) can offer reliable monitoring of small areas from a safe distance; and (iii) do fulfill all the requirements of a warning system. Commercial active radar solutions dealing with this specific task are already available, namely <NPL>, <NPL>, and <NPL>. They are all using Doppler radar FMCW, ground based, and operating at X-band (the frequency band from <NUM>-<NUM> offers both a good snow reflectivity and low propagation losses due to rain/fog/clouds). In Kogelnig et al. , the active radar is installed on the opposite side of the valley facing the avalanche path to be monitored. As the beam width of the antenna only allows a width of <NUM> at <NUM> range, it is important to choose the most important spot of the avalanche track. This spot can be targeted in advance if this system is used in collaboration with avalanche towers that trigger avalanches. The radar is used then as confirmation that an avalanche has been triggered. A similar design is proposed by Koschuch: a coherent pulse Doppler radar, targeting only a specific area where avalanches often happen or where avalanches can endanger infrastructure or people. In Meier et al. , the active avalanche radar is also connected to a road closure system, and it is currently deployed on the road to Zermatt, Switzerland. This radar performs a live monitoring of the mountain slope: if an avalanche is detected with the radar, traffic lights on the roads tum red automatically and the road closes with barriers. A second radar is used to see if the avalanche reaches a lower altitude in order to reopen the road immediately if the avalanche did not reach the road. The system is also integrated with webcams to check if the road is free from debris. The active avalanche radar of Meier et al is based on <CIT> referring to an active Doppler radar using a transmitter and a receiver, as well as a plurality of radars. The hillside is divided in range sectors, which it allows the radar to know exactly where the avalanche occurred.

<CIT> describes a passive radar method in which Synthetic Aperture Radar (SAR) images are generated using GPS signals.

Thus, there is a need to provide an alternative to the use of radars to confirm the occurrence of an avalanche.

This need is addressed by methods and apparatuses in accordance with the independent claims. Potentially advantageous embodiments are addressed by the dependent claims.

According to a first aspect of the present disclosure it is provided a passive radar avalanche warning method. The method includes illuminating an area of a potential avalanche with at least one third-party transmitter, obtaining a first signal corresponding to a direct arrival from the third-party transmitter and a second signal corresponding to a reflection from the area of the potential avalanche, determining a Doppler shift between the first and the second signal, and indicating an avalanche if the Doppler shift exceeds a predetermined threshold. The skilled person having benefit from the present disclosure will appreciate that the expression avalanche also includes phenomena like rockslides. The area of a potential avalanche can include critical infrastructures, such as mountain slopes, for example.

The present disclosure proposes to use a passive radar system for avalanche detection and warning. Passive radar systems refer to radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals. More particularly, the present disclosure proposes to use a passive bistatic radar system comprising at least one third-party transmitter and a receiver that are separated.

Generally, the third-party transmitter could be configured to transmit or broadcast any kind of RF signals, including analog or digital video/television signals, analog or digital audio signals, radio signals of cellular communication systems, etc. In some example implementations, the third-party transmitter comprises a satellite-based transmitter. According to the invention, the satellite-based transmitter comprises a geostationary satellite configured to broadcast a digital television signal, such as Digital Video Broadcasting (DVB), DVB-S, DVB-S2 and/or DVB-SH, for example. These satellites have a global coverage of the earth, especially DVB-S2.

In some example implementations, determining the Doppler shift comprises determining a range-Doppler map based on comparing the first and the second signal. A range-Doppler map may comprise a plurality of range bins and a plurality of Doppler bins associated with each range bin, for example. Visualizing a signal in the range-Doppler domain can help to understand connections among targets. From a range-Doppler map, one can see how far away the targets are and how quickly they are approaching or receding, distinguish among targets moving at various speeds at various ranges. If a target platform is stationary, a range-Doppler map shows a response from stationary targets at zero Doppler. For targets that are moving relative to the receiver, the range-Doppler map shows a response at nonzero Doppler values. Range information can be obtained by determining a phase difference between the first and the second signal, for example. Doppler shifts can be determined via a Fourier transform (fast Fourier transform, FFT) of the first and/or the second signal, for example.

In some example implementations, comparing the first and the second signal comprises cross-correlating the first and the second signal as a measure of similarity of the two signals as a function of the displacement of one relative to the other.

Geostationary satellites broadcasting digital television signals (DVB-S, DVB-S2 and/or DVB-SH) may suffer relatively low power density at earth's surface. To cope with weak signal power, some example implementations propose using a first directive (directional) antenna pointing towards the third-party transmitter for receiving the first signal and a second directive antenna pointing towards the area of the potential avalanche for receiving the second signal.

In some example implementations, a directive antenna pointing towards the area of the potential avalanche is used for receiving the second signal and wherein the first signal is obtained from the second signal by demodulating and re-modulating a digital signal contained in the second signal. This may be done if digital signals are broadcast. Here, it is also possible to operate the proposed passive radar with a single channel, requiring only a single antenna to be pointed towards the target. In this case, the clean replica of the transmitted signal for the cross-correlation in can be retrieved by demodulating and re-modulating the digital signal.

In some example implementations, the method may further include purposely triggering the avalanche, and confirming the avalanche if the measured Doppler shift exceeds the predetermined threshold. In this way it can be confirmed that the avalanche has been triggered as desired.

According to a further aspect of the present disclosure it is also provided a computer program having a program code for performing the method of any one of the previous implementations when the computer program is executed on a programmable hardware device.

According to a further aspect of the present disclosure it is also provided a passive radar avalanche warning system. The proposed passive radar avalanche warning system comprises at least one third-party transmitter for illuminating an area of a potential avalanche, at least one receiver configured to obtain a first signal corresponding to a direct arrival from the third-party transmitter and a second signal corresponding to a reflection from the area of the potential avalanche, and a processor configured to determine a Doppler shift between the first and the second signal and to indicate an avalanche if the Doppler shift exceeds a predetermined threshold.

The present disclosure relates to an avalanche warning system based on passive bi-/multi-static radar preferably utilizing satellite-borne illuminations of opportunity. Avalanches occur frequently throughout the world. On the surface of most commercial ski-slopes, and industrial open-cast mines, there are known "break-points" that are monitored to observe earth-subsidence. Once subsidence is detected, the deployed sensor may trigger an avalanche warning. Existing contender technologies are mostly active radars that are dependent on EM spectrum allocation and are expensive. The system proposed here is an entirely passive "green technology". It does not require a license to use the EM spectrum and it can function in all weather conditions, irrespective of time-of-day or season. Illumination may be provided by GEO satellites and, unlike contender technologies, the illuminating signal may be globally available.

An objective of this disclosure is to bring an alternative to the use of active radars to confirm the occurrence of an avalanche by exploiting a passive radar system. Instead of using a proper dedicated waveform, the proposed system utilizes robust and stable pre-existing signals, called illuminators of opportunity (IOs). An idea of this disclosure is to use satellite-based illuminators to get illumination at any time and at any place.

<FIG> illustrates an embodiment of the proposed passive radar avalanche warning system <NUM>.

The example passive radar avalanche warning system <NUM> comprises at least one third-party transmitter <NUM> for illuminating an area <NUM> of a potential avalanche, a first receive antenna <NUM>-<NUM> configured to receive a first signal <NUM>-<NUM> corresponding to a direct arrival from the third-party transmitter <NUM> and a second receive antenna <NUM>-<NUM> configured to receive a second signal <NUM>-<NUM> corresponding to a reflection from the area <NUM> of the potential avalanche. The passive radar avalanche warning system <NUM> further comprises a processor <NUM> configured to determine a Doppler shift between the first and the second signals <NUM>-<NUM>, <NUM>-<NUM> and to indicate an avalanche if the Doppler shift exceeds a predetermined threshold.

The skilled person having benefit from the present disclosure will appreciate that also one or more further receive antennas/channels configured to receive signals corresponding to reflections from one or more further areas of potential avalanches can be used. In this way, a spatial observation area may be increased. Each receive channel may comprise respective RF front-end circuitry to receive, amplify, down-convert, and convert the signals from analog to digital domain. Resulting digital baseband signals may then be fed into digital processing circuitry <NUM>.

In principle, various third-party transmitters or illuminators <NUM> can be used for the proposed passive bi- or multi-static radar avalanche warning system <NUM><NUM>: e.g.,.

An overview of further potential satellite IOs can be found in <NPL>.

As illustrated in <FIG>, the passive radar avalanche warning system <NUM> may use two receive channels, each one connected to a separate antenna <NUM>-<NUM>, <NUM>-<NUM>. The antenna <NUM>-<NUM> connected to the reference channel may be directed towards the satellite <NUM> to establish a clear signal. In the case of GNSS illumination, an omnidirectional antenna may be considered. The second antenna <NUM>-<NUM> may be directed to a specific area of interest <NUM> of the mountain slope.

A principle of the proposed passive radar avalanche warning system is to cross-correlate the direct and reflected signals <NUM>-<NUM>, <NUM>-<NUM> received from the two channels: the direct signal <NUM>-<NUM> from the transmitter <NUM> and the signal <NUM>-<NUM> reflected by the target <NUM>. After reception, down-conversion and digitization, the two signals <NUM>-<NUM>, <NUM>-<NUM> may be crosscorrelated, thus generating a range-Doppler map. In some implementations, the exploited signals may be time continuous. Therefore, a segmentation of the signal stream may be needed. Each segment may be referred to as coherent processing interval (CPI), and it may result in one corresponding single range-Doppler map.

Example processing steps are illustrated in <FIG>.

The example passive radar avalanche warning method <NUM> includes illuminating a geographic area <NUM> of a potential avalanche with at least one third-party transmitter <NUM>. Method <NUM> further includes receiving <NUM>-<NUM> a first signal <NUM>-<NUM> corresponding to a direct (line-of-sight) arrival from the third-party transmitter <NUM> and receiving <NUM>-<NUM> a second signal <NUM>-<NUM> corresponding to a reflection from the area <NUM> of the potential avalanche. Down-conversion of signals <NUM>-<NUM>, <NUM>-<NUM> may be performed coherently, e.g., with a single local oscillator controlling respective mixers. Method <NUM> further includes cross-correlating <NUM> the first and the second signals <NUM>-<NUM>, <NUM>-<NUM>. Target detection <NUM> can be done based on the cross-correlation <NUM>. Target detection <NUM> may include determining a Doppler shift between the first and the second signal <NUM>-<NUM>, <NUM>-<NUM> and indicating an avalanche if the Doppler shift exceeds a predetermined threshold.

A signal after cross-correlation <NUM> is normally referred to as range compressed since the cross-correlation concentrates (or compresses) the target signal returns in short time intervals. Cross-correlation, which may be equivalent to a so-called matched filter, can be replaced, in some alternative implementations, by other range compression filters, such as reciprocal or convolutional filters. In some implementations, the cross-correlation can be also efficiently performed in the frequency domain.

The expression of the cross-ambiguity function is: <MAT> where ssurv is a complex signal received by the surveillance channel <NUM>-<NUM> and sref the reference signal, * denotes the complex conjugate, τ is the Time Difference of Arrival (TDOA), fD the potential Doppler shift. The temporal integration may be limited by the coherent time interval. The ambiguity function (<NUM>) corresponds to the response of a matched filter shifted in Doppler. In some implementations, digital data may be used, so the ambiguity function may be sampled over N samples, yielding <MAT>.

The receiver including antennas <NUM>-<NUM>, <NUM>-<NUM> is stationary and the transmitter <NUM> is also assumed stationary during the CPI. Hence, the range-Doppler map resulting from the processing in <FIG> would yield zero Doppler responses for stationary targets <NUM>, whereas non-zero responses for a moving target <NUM>. Moving targets, such as avalanches, can then be easily detected in the range-Doppler map as echoes that are removed from the zero-Doppler. Detection can be carried out by simple two-dimensional CA-CFAR, with a detection threshold set to guarantee given performance in terms of probability of detection and probability of false alarm, for a given target model. Constant false alarm rate (CFAR) detection refers to an adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference. In simple CFAR detection schemes, the threshold level may be calculated by estimating the level of the noise floor around the bin or cell under test (CUT). This can be found by taking a block of bins/cells around the CUT and calculating the average power level. To avoid corrupting this estimate with power from the CUT itself, cells immediately adjacent to the CUT may be ignored (and referred to as "guard cells"). A target may be declared present in the CUT if it is both greater than all its adjacent cells and greater than the local average power level. The estimate of the local power level may sometimes be increased slightly to allow for the limited sample size. This approach is called a cell-averaging CFAR (CA-CFAR).

If digital signals are broadcast by transmitter <NUM>, it may also be possible to operate the passive radar system <NUM> with only a single receive channel, requiring only a single receive antenna to be pointed towards the target geographic area <NUM>. In this case, clean replica of the transmitted signal for the cross-correlation in (<NUM>) may be be retrieved by demodulating and re-modulating the digital signal. This approach may work as long as the direct signal is received with an adequate signal to noise ratio from the antenna pointing towards the geographic area <NUM>. This might however not be the case for all satellite IOs, due to limited signal power density at the earth surface. The advantages of using passive radar instead of active radars are the following:.

A proof of concept experiment has been carried out with the satellite based passive radar system SABBIA developed by Fraunhofer FHR, see <NPL> and <NPL>.

Not having the possibility of conducting an experiment with a controlled avalanche, a small controlled landslide was considered instead. The passive radar system has been deployed as in <FIG>, namely with one reference antenna <NUM>-<NUM> pointing at the satellite IO (Astra <NUM>. 2E has been considered for this case), and the surveillance antenna <NUM>-<NUM> pointing at the controlled landslide. The aim of this experiment was to investigate whether the passive radar system could detect the movement of these rocks. Data has been recorded from the two receive channels, down-converted to base band and processed according to (<NUM>) and to the processing sketched in <FIG>.

<FIG> shows a range-Doppler map <NUM> obtained few seconds before the landslide occurred. It can be clearly seen that the returns are occurring at zero-Doppler only, meaning that there is no movement detected by the system. On the other hand, <FIG> shows a Range Doppler map <NUM> obtained during the landslide. The landslide is clearly detected, namely there is a non-zero Doppler shift due to the movement of the rocks along the slope.

To summarize, the present disclosure relates to a passive radar exploiting satellite-based illuminators of opportunity for avalanche and landslide detection, specifically, for confirmation of occurred avalanches after triggering blasting. The passive radar exploiting satellite based illuminators of opportunity may be defined as an electronic system comprising the following hardware and software sub-systems:.

Examples may also cover program storage devices, such as digital data storage media, which are machine-, processor- or computer-readable and encode and/or contain machine-executable, processorexecutable or computer-executable programs and instructions.

Claim 1:
A passive radar avalanche warning method, comprising
illuminating an area (<NUM>) of a potential avalanche with at least one third-party satellite-based transmitter (<NUM>), wherein the satellite-based transmitter (<NUM>) comprises a geostationary satellite configured to broadcast a digital television signal;
obtaining a first signal (<NUM>-<NUM>) corresponding to a direct arrival of the digital television signal from the third-party satellite-based transmitter (<NUM>) and a second signal (<NUM>-<NUM>) corresponding to a reflection of the digital television signal from the area (<NUM>) of the potential avalanche;
determining a Doppler shift between the first and the second signal (<NUM>-<NUM>; <NUM>-<NUM>); and
indicating an avalanche if the Doppler shift exceeds a predetermined threshold.