Method and system for locating a transmitter by multiple geographically remote receiving stations using a known object path

Abstract

The present invention relates to a method for locating a transmitter, the method being implemented by a processing unit of a processing station of a location system.

Description

[Technical Field]

[0001] The present invention relates to the field of location of objects, and more particularly of objects transmitting radio frequency waves or signals.

[0002] More generally, the present invention relates to a system that includes multiple sensors and is dedicated to receiving, analyzing, and using signals, using for this purpose the diversity of characteristics of one and the same signal sensed at different geographic locations.

[0003] The present invention more particularly relates to synchronizing receiving stations that are geographically remote from the location of a system associated with a transmitter. [Background technology]

[0004] One of the main methods used to remotely and accurately locate an object transmitting or receiving a signal requires precise measurements of one of the signals received by multiple stations located at multiple geographic locations and the separation in the receiving dates of the same portion of this signal. In particular, location is performed by calculating the object's trajectory by utilizing TDOA (Time Difference of Arrival) and FDOA (Frequency Difference of Arrival) measurements, either jointly or separately.

[0005] To measure the TDOA and FDOA parts of the signal, signals dated on a local time base are transmitted by each station and then compared, for example by time and frequency correlation in a common processing unit, to extract therefrom the time-domain and frequency-domain separations that are the TDOA and DOA. Another means of measuring FDOA and TDOA consists, for example, of comparing the time or frequency associated with the reception of distinguishable components of the signal received simultaneously by each station. Documents EP 1 701 177 (A2) and US 2009 / 189851 (A1) describe such techniques.

[0006] For reliable measurements, it is important that the local time bases of each station are all synchronized with each other.

[0007] To do this, each station receives signals from one or more satellite positioning systems known as GNSS (Global Navigation Satellite Systems), for example GPS, GLONASS, Galileo or BeiDu. In particular, these GNSS systems make it possible to derive an absolute universal time as well as a highly stable frequency reference to which the base of the local time is slaved.

[0008] One problem is that sometimes access to the GNSS system is scrambled, degraded, or even unavailable (after malfunctions or malicious acts), which leads to this loss of universal time. Even if stations are equipped with very stable local clocks, they will drift from the loss of GNSS, quickly resulting in a sufficient separation that more or less no longer guarantees the required measurement accuracy.

[0009] Therefore, to ensure continuity in location, a certain resilience to this loss is necessary, since it is necessary to guarantee sufficient operational security of the location system, especially when there are critical applications, such as the monitoring of the orbits of satellite constellations in low Earth orbit, in order to meet the regulatory requirements applicable to such operators, and in particular with regard to the risk of collisions (combinations) between space objects.

[0010] To be resilient, the system's synchronization with GNSS systems can support several constellations (e.g., GPS, GLONASS, Galileo, BeiDu, etc.), but the signals used are concentrated in the same frequency band, causing a common vulnerability to interference and scrambling.

[0011] To mitigate the loss of universal time, provisions can be made to distribute a common time base for each station over long distances via cable links, e.g., optical fiber, to all receiving stations, with each local time base using this common time base.

[0012] However, such a solution based on its own means would be too burdensome, and the use of public networks, which requires traversing many layers of different protocols and hardware, would not yield good performance, especially with regard to differential accuracy (significant time jitter at the end).

[0013] It is also possible to slave the local clock to a low frequency (LF) time-based radio broadcast such as DCF77, but the accuracy of these systems is insufficient for the present requirements.

[0014] It is also necessary to have another means to accurately reset the time bases of the stations relative to each other, and at least to control their differential drift. Summary of the Invention

[0015] The present invention provides a method that allows very accurate estimation of the relative drift of local clocks at remote stations, such that they are synchronized, in order to precisely locate transmitters. As a function of the type of local clock used, the reliability is very large, extending out to 10 days or more, which corresponds to the time during which ephemerides of celestial objects can be used accurately in the absence of absolute clocks.

[0016] To this end, the present invention provides, according to a first aspect, a method for determining the location of a transmitter, the method being implemented in a processing unit of a processing station of a location system, the method comprising the steps of: - receiving signals acquired by geographically remote receiving stations, said signals being dated on the local time base of each receiving station, corresponding to signals from a transmitter to be located, and from at least one known object; - based on said dated signal, the measured TDOA for the transmitter and the object to be located, and / or determining a measured FDOA for the transmitter and the known object to be located; - based on known ephemeris for the known object and the geographical location of the receiving station, Theoretical TDOA for the known object, and / or A theoretical FDOA is determined; - determining a residual error affecting the measured TDOA and / or FDOA by taking the difference between the measured TDOA and / or FDOA and the theoretical TDOA and / or FDOA for the known object; - determining the number of data for local time base correction based on residuals affecting the measured TDOA and / or FDOA.

[0017] The method according to the first aspect of the invention can be completed by the following features, taken alone or in any combination thereof that is technically possible: The method comprises a step of determining, based on the measured TDOA and / or FDOA for a plurality of objects, an indication of the reliability of the measured time or frequency domain separation, which indication of reliability makes it possible to determine whether the ephemeris of the known object can be used to correct the local clock, and in particular whether the known object is maneuvering. The method includes a step of determining the position of the object or known object based on measurements made by the station. The position is used to determine a measure of reliability, which advantageously consists of comparing with each other a number of TDOAs and / or FDOAs measured for a number of known but different objects to check the consistency of the residuals, and determining all objects that can be used to correct the local clock. The method includes the step of determining the location of the transmitter using the measured TDOA and / or FDOA once the receiving station has corrected its local time base. - said receiving stations are synchronized with each other, preferably intermittently, by signals coming from a satellite positioning system;

[0018] According to a second aspect, the present invention provides a processing station of a system for locating a transmitter to be located, comprising a processing unit configured to implement a method according to the first aspect of the invention.

[0019] According to a third aspect, the present invention provides a system for locating a transmitter to be located, said locating system comprising a processing station according to the second aspect of the invention, said receiving station comprising: a local clock configured to provide a local time base; and a first receiver configured to acquire a signal from the object to be located; The first receiver is further configured to acquire signals from celestial or artificial objects, the behavior of which is predictable or known, and the acquired signals are dated according to a local time base.

[0020] Each receiving station advantageously further comprises a second receiver configured to acquire signals from a satellite positioning system, said second receiver further configured to demodulate the signals acquired by said second receiver, extract therefrom an absolute time base and correct said local time base of each receiving station.

[0021] The present invention provides, according to a fourth aspect, a location system comprising at least two receiving stations according to the third aspect and a processing station according to the second aspect of the invention.

[0022] Advantageously, in said location system, the processing station is constituted by one of the receiving stations.

[0023] According to a fifth aspect, the present invention provides a computer program product which, when executed by a computer, performs the location method according to the first aspect of the present invention.

[0024] The principle of the invention therefore consists in mutually resetting the time bases of several receiving stations by receiving, via radio frequency means, signals from natural or artificial celestial objects, which have common visibility and location and which are known or predictable independently of the GNSS constellation. In particular, these may be celestial bodies, the trajectories of which are estimated by the system itself. These objects are here referred to by the expression "known objects".

[0025] To do this, the remote receiving station is equipped with a local time base and provides the date by counting the ticks of a clock signal oscillating at a very stable frequency.

[0026] They collect, via antennas and radio frequency devices, the signals of the transmitter to be located, GNSS signals, if available, and signals of other known objects with common visibility from at least two receiving stations, preferably three for better accuracy. After coherent transformation and synchronous sampling of the signals of the target to be located and of signals from a reference source in the time and frequency domains, the digitized signal sample sequences are dated according to a local time base.

[0027] The receiving stations communicate with a processing unit, which may be co-located with one of the receiving stations, if applicable. They transmit these dated signal sequences for the tracked transmitter (to be located) as well as for the reference object. If there is sufficient time and frequency overlap between the sequences, the processing unit can search for the time and frequency separation that provides the maximum correlation between signals from one and the same object or transmitter for each pair of stations. These time and frequency separations (TDOA and FDOA) are then calculated in each receiving station's respective local time base. If, according to a celestial mechanism, the ephemeris of a known object is known in an absolute time base (or, alternatively, the ephemeris of the processing station), the processing unit also calculates its theoretical TDOA (and, if applicable, FDOA) for the same pair of stations.

[0028] Once the time bases of a pair of stations are perfectly aligned and calibrated, they are slaved to absolute time, and then the theoretical and measured TDOA and FDOA must match to the nearest irreducible error, among which are thermal receiver noise and short-term clock jitter, which are statistically zero and have only short-term effects.

[0029] In the medium to long term, the residual error of TDOA / FDOA, i.e., the separation between the measured and theoretical values, if it is not zero, is formed by the following contributions and offsets: - local time base drift, - bias between timestamps and the local time base, -Diffusion of atmospheric and ionospheric propagation. If these offsets can be considered to be specific to each station, independent of the object from which they are received, the residual error between the measured and theoretical TDOA / FDOA therefore provides an estimate of the difference in these offsets between each pair of stations, and whatever the reference object, its ephemeris is known. The processing unit therefore calculates these values ​​during the normal calibration phase.

[0030] If there are multiple known objects and their ephemerides are known, the estimation will be more accurate. However, when locating an object with no known ephemeris, a correction related to this estimation of timestamp separation can be applied, i.e., a residual can be subtracted, since it is unique and the corrected TDOA / FDOA measurement will necessarily match the theoretical calculation, i.e., the ephemeris that can be extracted.

[0031] If a GNSS signal is lost, the time base of each receiving station will slowly drift, and the remaining date separation of several days will remain low enough to allow the correct use of ephemeris expressed in absolute time bases. Known objects, i.e., stars or geostationary satellites that do not experience any manipulation, whose ephemeris is predictable by celestial mechanisms, collect signals and use them to subsequently correct the time-stamping separation between the time bases of receiving stations.

[0032] To ensure good operational security, the system is programmed to collect several known objects in sequence. It then checks whether all objects provide a consistent set of date separation corrections. If certain objects provide incorrect corrections, they must be (temporarily) set aside from the list of standards. In the case of geostationary satellites, this means, among other things, that an operation has taken place in the meantime. [Brief explanation of the drawings]

[0033] Other characteristics, objects and advantages of the present invention will become apparent from the following description, which is purely illustrative and non-limiting and which must be read with reference to the accompanying drawings, in which: [Figure 1] FIG. 1 is a diagram illustrating a locating system according to the present invention. [Figure 2] FIG. 2 shows a diagram of a receiving station of a location determining system according to the present invention. [Figure 3] FIG. 3 illustrates a method for determining location according to the present invention.

[0034] In all figures, similar elements have the same reference numbers. DETAILED DESCRIPTION OF THE INVENTION

[0035] With reference to FIG. 1, a system 1 for locating a transmitter 3 of a radio frequency signal includes at least two receiving stations 2a, 2b, 2c.

[0036] The receiving stations 2a, 2b, 2c are geographically separated, isolated, and distant, and are in a link with a processing station 6 that is used to process signals from these receiving stations 2a, 2b, 2c, determine one signal received by each of these stations, and the time and frequency offsets associated with the same signal, and based on these offsets, infer the trajectory, and therefore the location, of the transmitter 3. These time and frequency offsets are the TDOA and FDOA described in the introduction. Although the processing station 6 is separate from the receiving stations in FIG. 1, in certain embodiments, one of the receiving stations may also be a processing station.

[0037] The transmitter 3 to be located is, for example, a satellite, but can be any object as long as it transmits a radio frequency signal that can be received by at least two receiving stations.

[0038] In FIG. 1, three stations are shown, two stations would be sufficient, but the more stations there are the better the accuracy of the calculations.

[0039] With reference to FIG. 2, each receiving station 2x (x=a or b or c) includes a first receiver 21x configured to acquire signals from a transmitter 3 to be located, and a second receiver 23x configured to acquire signals from one or more satellites of one or more GNSS constellations 5.

[0040] Furthermore, the first receiver 21x is configured to acquire signals from a known object 4. In particular, since processing of the known object serves to assess the characteristic parameters of imperfections inherent in the receiving station and that reduce the accuracy of the localization of the transmitter 3, it is consequently essential that the processing of receiving signals coming from the known object 4 is subject to the same degradation and does not go via a dedicated receiving line.

[0041] Signals from natural or artificial celestial bodies make it possible to mitigate the lack of GNSS signals, as will be further explained below.

[0042] These celestial or artificial objects are, for example, stars or other geostationary satellites. In the following description, the expression "known objects" is used to refer to these objects. These known objects have the advantage that their ephemeris is known and their trajectories can be reliably calculated, and therefore their TDOA and / or FDOA can be predicted. In particular, because TDOA and FDOA are predictable, it is possible to assess errors in the calculation of the TDOA and / or FDOA of a transmitter to be located by comparing values ​​calculated based on measurements and values ​​obtained from predictions.

[0043] Furthermore, the signal emitted by a radiating object is noise related to its equivalent temperature, and is therefore detectable if its temperature is relatively high relative to the 3 Kelvin cosmic radiation (Sun, Moon, quasars, etc.). Ground stations, being relatively close compared to the distance from these objects, will virtually see this object from the same angle, and therefore will be subjected to the same thermal noise, although sometimes slightly shifted due to differences in the stations' separation from each other.

[0044] Therefore, the correlation of two signals is greatest when these two signals are rearranged: the autocorrelation function of broadband white noise is in fact a "Dirac" pulse at time 0. It will be appreciated that noise therefore has correlation properties (the Fourier transform of the autocorrelation function of noise gives its spectrum by definition).

[0045] Returning to Figure 2, each receiving station 2a, 2b, 2c further includes a local clock h2x configured to provide a local time base tlocal2x. Furthermore, the receivers 21x, 23x are time-aligned to this clock. The term "local clock" is understood to mean an oscillator that provides a stable frequency signal, on whose rising or falling edge it is possible to trigger and time-align the acquisition sampling by each receiver. Furthermore, counting of clock edges provides a general time-stamping of each acquisition sample.

[0046] If GNSS signals are available, the local time base is synchronized to an absolute time base resulting from demodulation of the GNSS signals. In this regard, the second receiver 23x is configured to acquire signals from the satellite positioning system 5 and is further configured to demodulate the acquired signals to extract the absolute time base therefrom in order to correct each local time base of each receiving station.

[0047] The receiving stations 2x are then synchronized to each other by signals from the satellite positioning system using this absolute time base. Note that when GNSS signals are unavailable, the local time bases, which are no longer slaved to absolute time, will drift weakly but independently for all receiving stations such that the separation in synchronization increases with time.

[0048] Each receiving station 2a, 2b, 2c also includes a receiving antenna A1x connected to the respective receiver 21x, 23x. Furthermore, each receiving station includes a communication interface (not shown) for communicating with the processing station 6.

[0049] Regarding acquisition, each receiver consists of a conventional radio frequency receiving unit, including a frequency converter slaved to a frequency reference, and a multi-channel digitizing line derived from an analog-to-digital converter slaved to a frequency reference, which is well known to those skilled in the art and will not be described in further detail here.

[0050] This is relevant in cases where GNSS signals are unavailable and, as a result, the local time base is no longer reliable and provides incorrect dates, slowly drifting as soon as GNSS signals become unavailable.

[0051] In this particular situation, a method for locating the transmitter 3 will be described below in relation to Figure 3. Such a method is implemented in the processing unit 7 of the processing station 6.

[0052] At least two receiving stations 2a, 2b, 2c proceed to acquire (step E1) and time-stamp (step E2) the parts of the signal from the transmitter 3 to be located and at least one known object 4. In particular, for the transmitter they acquire a signal Semetteur_x and for the object they acquire a signal Sobjet_x. These signals are dated using the local time base of each receiving station 2a, 2b, 2c.

[0053] These signals are transmitted to a processing station 6 (step E3), which, after receiving them (step REC), for example correlates the signals from several stations pairwise in order to be able to compare identical parts of the signals in order to deduce therefrom TDOAij and / or FDOAij, i.e. the time-domain and frequency-domain separation of identical signal parts determined for two stations i, j (the indices i and j denote stations a, b, c).

[0054] One of the expected purposes of using an object with known ephemeris is to be able to correct the local time base of the receiving station as soon as the TDOA of the known object, or else the FDOA of the known object, is used.

[0055] In terms of TDOAij, it is the difference in propagation time it takes for the same part of transmitter 3's signal to reach station i and then to reach station j. Of course, these time separations are measured with respect to a local time base which is imprecise, assuming the absence of GNSS signals.

[0056] Thus, based on the timestamps of the received signals from at least two receiving stations 2a, 2b, 2c, the processing station determines the transmitter to be located and the measured time separation TDOA_objet_ corresponding to the received signals associated with the known object(s). ij MES ,TDOA_emetteur ij MES (Step DET1). Of course, a similar process is possible based on the FDOA.

[0057] Next, based on the known ephemeris and the absolute time base determined relative to at least one known object, a theoretical time and / or frequency separation TDOA_object_ ij TH,FDOA_objet_ ij TH is determined relative to a known object (DET2).

[0058] The time domain error that affects TDOA is RES_TDOA, which is calculated by taking the difference between the measured separation and the theoretical value. ij (or TDOA residual) is determined (step DET3) and makes it possible to correct the time base of the receiving station (step E4). Similarly, the frequency error (or FDOA residual) can be calculated based on the measured FDOA value and the theoretical value.

[0059] According to one embodiment, data for local time-based corrections is determined based on residual errors affecting the measured TDOA and / or FDOA (step DET5).

[0060] These correction data are then transmitted to each receiving station (step TRANS) to reset the date of their local clocks.

[0061] According to one embodiment, the processing station keeps the residuals and performs a time-stamping correction of the signal portions received from each station (step CONS).

[0062] Finally, once the receiving stations have corrected their local time bases, location of the transmitter 3 using the measured TDOA and / or FDOA (step LOC1) is performed.

[0063] In terms of TDOA, this gives the following expression: TDOA_objet_ ij MES =TDOA ij reel +CorrNoise+Δ ErrGNSS ij +Δ ErrTshort ij +Δ BiasCal where: -TDOA ij reel: The actual physical value you are trying to measure. CorrNoise: Correlated noise, typically AWGN (Additive White Gaussian Noise) (white Gaussian noise with zero mean and predictable energy, determined by the channel). -Δ BiasCal : Errors caused by offsets in the physical devices of the station and that can be calibrated (propagation time through the equipment, uncertainty in the actual geographical position of the receiver). These are considered to be very stable on the scale of weeks and are therefore considered known since they are estimated by the calibration process. -Δ ErrGNSS ij : The difference in time stamping error obtained by using GNSS signals (typically low and tending to be of the AWGN type). -Δ ErrTshort ij : The difference in short-term time stamping error (which is not compensated for by the GPS correction process and is therefore short-term clock jitter).

[0064] When a known object is tracked by a receiving station (nominal rating), its position, and therefore the actual TDOA ij reel The value is the nearest error of the propagator ERR_PROPAG_TDOA ij is known, and the theoretical TDOA ij TH By eliminating terms that are assumed to be known, we therefore define the TDOA residual as: RES_TDOA ij =TDOA ij Mes-TDOA ij TH=ERR_PROPAG_TDOA ij +CorrNoise +Δ ErrGNSS ij +Δ ErrTshort ij .(Here, Δ BiasCal is known and believed to have been eliminated).

[0065] Most of these terms are estimated in the noise category that can be approached by low zero-mean noise and are of negligible magnitude compared to the drift that is expected to be present. Once the station is in nominal mode (startup phase finished, first slaved to GNSS, then continuously slaved to GNSS, etc.), REF_FREQ i (Hence the term Δ ErrTshort ij ) drift and the associated time base is REF_FREQ in non-slave mode. i The tracking of known objects relies only on the characteristics of the known objects, which are selected to be of very good quality. In nominal mode (i.e., when time-domain synchronization using the GNSS constellation is operational), tracking of known objects therefore makes it possible to estimate the system's short-term time-stamping error (caused by short-term jitter in the local clock), which can also be used to correct the receiving station's local clock. In one implementation of the invention, processing loops using known objects are active and are used even when GNSS synchronization is active and operational. These loops are then used only to correct the short-term jitter of the local clock.

[0066] When GNSS synchronization is no longer possible, the present invention compensates for both medium-term and short-term drifts in the local clock.

[0067] As explained, the acquisition of time domain errors is based on tracking known objects, and therefore the reliability of the measurements thereon is crucial.

[0068] In particular, if these are geostationary satellites, they may be in a maneuvering phase so that their orbits are later, in the case of ephemeris, unpredictable.The method for locating the transmitter to be located assumes the previous (and, if applicable, concurrent) position of the reference artificial object (step LOC2).

[0069] The localization method therefore comprises a step of determining an indicator of the reliability of the measured time separation based on the measured TDOA and / or FDOA of the known object (step DET4). The indicator of reliability is intended to determine whether the ephemeris of the known object can be used for local clock correction. This indicator of reliability makes it possible, in particular, to determine whether the known object is maneuvering or not.

[0070] Of course, known objects are not relevant to these concepts of reliability if they are celestial bodies (e.g., the Sun): specifically, these known objects are classified and easily identifiable, and highly accurate ephemerides are available.

[0071] However, the classified natural objects may not be in permanent visibility (e.g., when using the Sun as a known object, its visibility is of course subject to day / night alternations) due to the high reliability of the reset measurements (independent of propagation errors), and the use of artificial objects (e.g., stationary objects) in unchanging visibility can be systematically used in relative resetting (subject to propagation errors) in the phase of visibility of natural objects.

[0072] Additionally and advantageously, the reliability of measurements originating from known objects consists in comparing with each other multiple TDOA residuals obtained for multiple known but different objects to check the alignment of these time domain residuals, since they are not intended to be dependent on known objects. If some of these objects are too far from the others, it can be concluded that their ephemerides are unreliable and they can be removed from the list of reference objects that can be used to maintain synchronization between stations.

Claims

1. 1. A method for locating a transmitter, the method being implemented in a processing unit of a processing station of a location system, the method comprising: receiving the acquired signal by a geographically remote receiving station; the signals are dated in the local time base of each receiving station, correspond to signals from a transmitter to be located, and are from at least one known object; Steps and Based on said dated signal, the measured TDOA for the transmitter and the object to be located (TDOA_objet_ijMES, TDOA_emetteurijMES), and / or the measured FDOA for the transmitter and the known object to be located (FDOA_objet_ijMES, FDOA_emetteurijMES); determining a based on the known ephemeris for the known object and the geographic location of the receiving station; a theoretical TDOA (TDOA_objet_ijTH) and / or a theoretical FDOA (FDOA_objet_ijTH) for the known object; determining a determining a residual error contributing to the measured TDOA and / or FDOA by taking the difference between the measured TDOA and / or FDOA and the theoretical TDOA and / or FDOA for the known object; Each receiving station corrects its local clock, and the processing station correcting the timestamp of the portion of the signal from each receiving station; Steps and Once the receiving stations have corrected their local time bases and the processing station has corrected the timestamps of the portions of the signal from each receiving station, determining the location of the transmitter using the measured TDOA and / or FDOA; Steps and Including, The method further comprises: determining a measure of reliability of the measured time domain or frequency domain separation based on the measured TDOA and / or FDOA for a plurality of objects; the reliability indicator allows determining whether the ephemeris of the known object can be used to correct the local clock and to determine whether the known object is operational; method.

2. The method comprises: determining the number of data points for the local time base correction based on a residual error that contributes to the measured TDOA and / or FDOA; transmitting data relating to the determined correction of the local time base to each receiving station, each receiving station correcting its local clock in this way; The method of claim 1 , comprising:

3. The method comprises: and retaining the determined local time base correction; the processing station similarly corrects the timestamps of the portions of the signal received from each receiving station; The method of claim 1.

4. The method comprises: determining a position of the known object based on measurements made by each receiving station; 4. The method according to any one of claims 1 to 3.

5. the location is used to determine the reliability index; The method of claim 1.

6. The step of determining a measure of reliability comprises: comparing a plurality of TDOAs and / or FDOAs measured for a plurality of known but different objects to each other to check the consistency of the residual errors; and determining all objects that can be used to correct the local clock; The method of claim 1.

7. the receiving stations are intermittently synchronized with one another by signals coming from a satellite positioning system; 7. The method according to any one of claims 1 to 6.

8. 1. A system for locating a transmitter to be located, the system comprising: A processing unit configured to perform the method according to any one of claims 1 to 7; at least two receiving stations; Each of the at least two receiving stations comprises: a local clock configured to provide a local time base; a first receiver configured to acquire a signal from the object to be located; the first receiver is further configured to acquire a signal from a known object, the acquired signal being dated according to the local time base; system.

9. Each receiving station further includes a second receiver configured to acquire signals from the satellite positioning system; the second receiver is further configured to demodulate the signal acquired by the second receiver, extract an absolute time base therefrom, and correct the local time base of each receiving station; The system of claim 8.

10. the processing station is comprised of one of the receiving stations; 10. The system according to claim 8 or 9.

11. A computer program comprising code instructions, When the code instructions are executed by a computer: The method according to any one of claims 1 to 7, wherein the computer is configured to implement the method. Computer program.

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