Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
  • G01S13/88—Radar or analogous systems specially adapted for specific applications
  • G01S13/885—Radar or analogous systems specially adapted for specific applications for ground probing
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
  • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
  • G01S7/411—Identification of targets based on measurements of radar reflectivity
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
  • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
  • G01S7/414—Discriminating targets with respect to background clutter

Definitions

• the invention relates to the field of ground penetrating radars or ground penetrating radars which cover all the techniques for detecting, locating or identifying underground targets by means of a radiofrequency system.
• Underground targets are, for example, pipes of different diameter and nature (steel, PVC, cement, concrete, etc.) which can be buried at different depths.
• ground penetrating radars One objective of ground penetrating radars is to locate such objects with precision in order to be able to correctly map a subsoil, for example for safety requirements during work.
• a general problem targeted by the invention relates to the location of underground targets with precision regardless of the dielectric media traversed by the radar signals.
• a first type of compatible localization method for ground-penetrating radars relates to so-called migration techniques derived from imagery. These methods are based on solving the wave equation by interpolation in the frequency domain.
• the document [1] describes a radar imaging method for systems with one transmitting antenna and one receiving antenna (of the SISO type for "Single Input Single Output” in English).
• Documents [4] and [6] give examples of radar imaging methods applied to systems having several transmit and receive antennas, that is to say antenna arrays (of the MIMO type for " Multiple Input Multiple Output ”in English).
• a drawback of imaging or migration methods is that they generally only use the phase information of the signals picked up.
• the amplitude information is not used neither to determine the position of a target nor to estimate the transmission losses of the signals in the medium traversed.
• these methods aim to construct an image of an area and require post-processing to determine the position of a target.
• Document [5] describes another method based on an antenna array to virtually create a plane wave. This solution also has the drawback of requiring transmission preprocessing.
• Document [2] describes a MIMO-type radar technique which exploits the amplitude of the signals picked up but does not consider the influence of transmission losses during the propagation of signals in the ground. This simplification has the effect of limiting the location precision, in particular in the case of heterogeneous media and areas comprising several nearby targets to be detected.
• the present invention aims to remedy the drawbacks of the aforementioned techniques of the state of the art by proposing a location method which takes into account the dielectric losses in the ground in order to improve the precision of detection and location of targets. .
• the subject of the invention is a method for detecting at least one target buried in an area of the ground, using a ground penetrating radar, the radar comprising at least one transmitting antenna and at least one receiving antenna, the method comprising the steps of:
• - define a first hypothesis H0 corresponding to an absence of target in the ground area, the channel matrix under this first hypothesis being modeled by noise, - define a second hypothesis H1 corresponding to the presence of at least one target in the ground area, the channel matrix under this second hypothesis being modeled by a propagation channel model depending at least on the characteristics of the transmitting antennas and receiving antennas and signal propagation losses in the ground,
• the step of performing a test of the likelihood ratio between the first hypothesis H0 and the second hypothesis H1 comprises the sub-steps of:
• the step of determining at least one estimator of the maximum likelihood of the position and of the radar equivalent area of a target, under the second hypothesis H1 is carried out by searching for at least an extremum of a function of three variables depending on the position of a target in the ground area, on the radar equivalent area of a target and on a signal transmission loss coefficient in the ground area.
• the search for at least one extremum of said function of three variables is carried out by:
• the likelihood of the channel matrix under the second hypothesis H1 is determined by an extremum of said function.
• the likelihood of the channel matrix under the first hypothesis H0 is determined at least by calculating the energy of the channel matrix.
• a target position is sought in a two or three-dimensional grid having a defined initial pitch, the method being iterated over several iterations by reducing the area and the initial pitch at each iteration.
• the method according to the invention comprises the steps of:
• the temporal windowing is defined so as to filter the contributions of the signals corresponding to a distance interval between the radar and the ground area.
• the process is repeated for several different filters so as to cover the entire area of the ground.
• the method according to the invention further comprises a step of canceling the contribution of a target detected in the measurements of signals acquired, for example by means of a cancellation algorithm. echo, the method being iterated over several successive iterations, canceling on each new iteration the echo of the target detected in the previous iteration.
• the invention also relates to a ground penetrating radar comprising at least one transmitting antenna and at least one receiving antenna and a device for detecting at least one target buried in an area of the ground configured to execute the steps of the detection method according to the invention.
• the invention also relates to a computer program comprising instructions for the execution of the method according to the invention, when the program is executed by a processor as well as a recording medium readable by a processor on which is recorded a program comprising instructions for the execution of the method according to the invention, when the program is executed by a processor.
• Figure 1 is an illustrative diagram of the operation of a ground penetrating radar
• FIG. 2 represents a general flowchart describing the steps of a target detection method according to one embodiment of the invention
• FIG. 3 represents a flowchart describing an embodiment of the target detection step of the method according to the invention
• FIG. 4 represents an illustrative diagram of an example of iterative operation of the invention
• FIG. 5 represents a flowchart describing the steps of an alternative embodiment of the method according to the invention
• FIG. 6 represents a diagram of a temporal response of a propagation channel to illustrate an alternative embodiment of the invention
• FIG. 7 represents a flowchart describing the steps of another variant embodiment of the method according to the invention.
• FIG. 8 is a diagram of an example of a ground penetrating radar capable of carrying out the invention.
• FIG. 1 shows a simplified diagram of the operating principle of an RDR ground penetrating radar.
• a radar comprises one or more transmitting antennas and one or more receiving antennas.
• the radar has five transmitting antennas and four receiving antennas. Without departing from the scope of the invention, the number of transmitting and receiving antennas can take any non-zero integer value.
• An area of the ground broken down into a grid G is analyzed by ground penetrating radar RDR by performing a sequence of transmissions and signal acquisitions for each pair consisting of a transmitting antenna and a receiving antenna.
• an acquisition sequence consists in transmitting a radar signal from the first antenna Tx1 in transmission, this signal is reflected on a possible target P located in the zone G then back-propagated towards each reception antenna Rx1, Rx2, Rx3 , Rx4. This sequence is repeated for each transmitting antenna Tx2, Tx3, Tx4, Tx5.
• the target search area can be a two or three dimensional area.
• the radar can be mobile, for example on board a vehicle, to cover different areas of the ground.
• the invention is implemented by a radar of the type of Figure 1.
• Figure 2 illustrates the main steps of the target detection method according to the invention.
• the first step 201 consists in acquiring the signals during a transmission-reception sequence as described above.
• the acquisition sequence performs a frequency sweep according to a set of predefined frequencies.
• the transmission-reception sequences on all the antennas are repeated for several frequencies defining a frequency band to be analyzed.
• the acquisition sequence can be sequential on transmission as described above but can also be carried out by simultaneously transmitting several orthogonal signals on all the transmission antennas so as to directly carry out a simultaneous acquisition of the various echoes of each transmitting antenna for a given frequency.
• the scanning in successive frequencies can be replaced by the use of a waveform of the OFDM (Orthogonal Frequency Division Multiplexing) type which makes it possible to code in transmission the signals by distribution on orthogonal frequency channels in the form multiple sub-carriers.
• OFDM Orthogonal Frequency Division Multiplexing
• the width of the channels or sub-bands and their spacing can be configured. These parameters can vary for each sub-band according to the signal-to-noise ratio specific to this sub-band.
• a step 202 of estimating channel matrices is performed.
• Signal acquisition is triggered synchronously with transmission.
• the signals are transposed to baseband and then demodulated via OFDM demodulation.
• a channel estimation is carried out on each subcarrier, for example by correlating the symbols received with the known symbols transmitted on this subcarrier.
• the channel estimation is performed on several OFDM symbols by consistently averaging the result to reduce the impact of thermal noise or interference.
• a channel estimation is carried out for each propagation channel defined by a link between a transmission antenna Txi and a reception antenna Rxj but also for each frequency sub-band.
• the complete frequency response is obtained by concatenation of the responses of each sub-band.
• a set of channel matrices H r (f) is obtained, for several frequencies, each matrix having a dimension equal to the number of reception antennas by the number d 'transmitting antennas.
• K is the number of targets for which an echo is received.
• n (f) denotes the noise matrix for the frequency f.
• the invention is in particular based on a particular modeling of the channel matrix as detailed below, which takes into account the dielectric losses in the medium of which the soil is made.
• the noise-free channel matrix H (f, p k ) can be expressed using the following relation, where is the term-by-term multiplication operator between two matrices.
• U (f, p k , L) is the propagation delay matrix whose elements (i, j) are expressed using the following relation:
• v is the speed of propagation of the wave in the medium crossed by the supposedly homogeneous wave.
• the medium traversed is partly made up of air and soil layers or only soil layers if the transmitting antennas are positioned directly against the ground surface.
• L is a loss coefficient which depends on the medium crossed by the wave. Depending on the medium (earth, sand, rock) of which the soil is made, the emitted wave undergoes more or less significant attenuations that should be taken into account to correctly model the channel matrix.
• the matrix A (f, p k , L) represents the matrix of the amplitudes of the echoes reflected on the target. This matrix integrates the losses in the medium, the antenna gains and the radar equivalent area of the target.
• Y (/, p fc ) models the radar equivalent surface of the target at the position p k .
• G t (f, p k ) models the transmission antenna pattern in the direction of the position of the target.
• G r (f, p k ) models the reception antenna pattern in the direction of the position of the target.
• the two matrices G t (f, p k ) and G r (f, p k ) are known and are determined from the characteristics of the antennas.
• B 0 (f, p k ) and B 1 (f, p k , L) are two matrices which together model the propagation losses of spherical waves in the ground.
• the invention differs in particular from the methods of the state of the art by taking into account the loss coefficient L in the middle of the soil.
• An objective of the detection method by radar processing, according to the invention is to determine the position of a target having knowledge of the frequency response of the monitored area.
• the third step 203 of the method according to the invention consists in carrying out a target detection test from the modeling of the propagation channel.
• H 0 corresponds to a hypothesis of the absence of a target
• Hi corresponds to an hypothesis of the presence of a target.
• the channel matrix can be expressed under the two assumptions using the following relationships:
• H 0 ) denotes the probability that the received channel matrix corresponds to the hypothesis H 0 .
• H 1 ) denotes the probability that the received channel matrix corresponds to the hypothesis H 1 .
• the sign oc means "proportional to”.
• the Neyman-Pearson criterion is used which was developed to maximize the probability of detection while ensuring a given probability of false alarm. This criterion consists in comparing the ratio of the likelihoods of the channel matrices, under each of the hypotheses, with a predetermined threshold l. This is a so-called LRT test for "Likelihood Ratio Test".
• a target is detected when the following relationship is verified:
• Relation (11) is also written in the form:
• FIG. 3 details the steps necessary for carrying out the detection test 203.
• the first step 301 consists in estimating the unknown parameters, that is to say the position of the target and its radar equivalent surface, as well as the loss coefficient L, in the sense of maximum likelihood.
• the observation zone or space is discretized in the form of a grid G having a predefined pitch (see Figure 1).
• a detection is declared for any cell of the grid validating the test under the H-i hypothesis.
• the maximum likelihood estimator of the parameters y and p is obtained by finding the minimum of the following function:
• step 301 consists in finding the minimum of a function g (y, p k , L, /), the function g being defined from relation (14).
• the function g can have several local minima in the space defined by the three parameters (Y, p k , L).
• a first variant of the invention consists in searching for several local minima (for example the N preponderant minima, with N a predefined integer) which will each correspond to a potential target.
• Another variant of the invention consists in being limited to the search for a single local minimum (the most important), the method described in Figures 2 and 3 being applied for the search for a single target.
• the search for one or more local minima can be carried out by an optimization function available to those skilled in the art, for example one of the digital resolution functions described in one of the references [7], [8], [9].
• the estimate of y (f) is calculated using relation (18) for a set of assumptions of position p k and of loss coefficient L.
• positions P k vary over all the positions of the grid G superimposed on the zone to be analyzed. These positions can be defined in two or three dimensions.
• the values of the coefficient L can be chosen from among several hypotheses, taking into account the a priori knowledge of the composition of the soil.
• Step 301 can be refined by successive iterations by redefining, at each iteration, a new grid G2 of finer pitch around the estimate of the position p k obtained in the previous iteration as illustrated in figure 4.
• step 301 one (or more) estimate is obtained which is then used to carry out a detection test via equation (13).
• a step 302 the likelihood of the channel matrix received under the Hi hypothesis is calculated. This calculation is in fact directly given by the value of the estimate of
• a step 303 the likelihood of the channel matrix received is calculated under the hypothesis H 0 . This calculation is carried out by calculating the term which corresponds to the sum or the average, over all the frequencies, of the squared modulus of the channel matrix. It corresponds to the energy in frequency of the channel matrix.
• the norm operator of a complex matrix is defined by the trace of the product of the matrix and the conjugated transpose matrix:
• a step 304 the likelihood test of equation (13) is carried out by comparing the difference between the terms calculated respectively in steps 302 and 303, with a detection threshold l.
• the speed v of propagation of the wave in the media crossed can be estimated by an external method or else can be considered as an additional variable of the minimization problem.
• Figure 5 shows schematically an alternative embodiment of the invention suitable for the detection of multiple targets.
• the target detection method described in Figures 2 and 3 is applied to search for a single target (step 501), that is to say the target having the preponderant echo.
• an echo cancellation algorithm is applied to the signals received (or directly to the channel matrix H r (f)) in order to cancel the contribution of the reflections of the signal on the target detected in step 501.
• One possible algorithm is the algorithm known by the acronym CLEAN, for example described in reference [3].
• the iterations are stopped via a stop criterion 503 which can consist of a fixed number of iterations or of a criterion depending on the radar equivalent area or the energy of the echoes of the detected targets.
• the criterion can also be defined from a difference in energy or radar equivalent area between two targets detected in two successive iterations.
• the image of the analyzed area is supplied as the input of an optimization algorithm 504 to improve the accuracy of the image.
• the optimization algorithm is, for example, chosen from those described in references [7], [8] or [9].
• the algorithm can be based on the stochastic gradient method.
• the basic assumption for the detection of a target via a test under two assumptions as described previously, is that the noise statistic corresponds to an additive Gaussian white noise. In the case of several targets, this assumption is no longer true, which leads to bias in the detection algorithm.
• This variant consists in applying a filter in the form of temporal windowing to the channel matrices in order to separately treat different sub-zones of the zone of the ground to be analyzed and thus to overcome the problems of dynamics.
• h a vector consisting of the estimate of the frequency propagation channel for a pair of transmit and receive antennas
• this vector is converted into the time domain via an inverse Fourier transform IFFT (step 701).
• a filter is applied in the time domain (step 702) to the time response of the channel.
• any type of window can be used, for example a Flanning window, or even a Blackman-Flarris window.
• the result is converted (step 703) into the frequency domain via a direct Fourier transform then the new frequency vector obtained is used to constitute the channel matrix H r (f) at the input of the detection method according to one of the variants described above.
• the process described in FIG. 7 is applied for each propagation channel connecting one of the transmitting antennas to one of the receiving antennas.
• the detection method in its entirety, is iterated for several different time windows, for example by varying the windows over the entire area to be analyzed by circularly permuting the filter w of p / 2 positions at each iteration. where p is equal to the number of 1.
• FIG. 6 shows schematically, on a diagram, an example of the temporal response 601 of a propagation channel between a transmitting antenna and a receiving antenna. On the same diagram, several time windows have been shown staggered and corresponding to several iterations of the method described above.
• FIG. 8 is a diagram of an example of a ground penetrating radar configured to implement the invention.
• the radar consists of an EM transmitter and a REC receiver, each provided with at least one antenna A1, A2.
• the EM transmitter comprises a generator of a sequence of GEN bits or symbols, an OFDM MOD, a frequency converter RF1 to transpose the signal to a carrier frequency and an amplifier AMP.
• the receiver REC comprises a frequency converter RF2 for transposing the received signal into baseband, an OFDM DEMOD demodulator and a processing unit consisting of a channel estimation module EST which determines the channel matrices from of the demodulated signals and of the sequence emitted by the generator GEN, an integrator INT which carries out an integration (average) and a concatenation of the frequency sub-bands and a target detection module DET which implements the processing steps described above for perform any one of the embodiments of the invention.
• a channel estimation module EST which determines the channel matrices from of the demodulated signals and of the sequence emitted by the generator GEN
• an integrator INT which carries out an integration (average) and a concatenation of the frequency sub-bands
• a target detection module DET which implements the processing steps described above for perform any one of the embodiments of the invention.
• the EST channel estimation, INT integration and DET detection modules can be produced in software and / or hardware form, in particular by using one or more processor (s) and one or more memory (s).
• the processor can be a generic processor, a specific processor, an integrated circuit specific to an application (also known under the English name of ASIC for “Application-Specific Integrated Circuit”) or an array of programmable gates in situ (also known as the English name of FPGA for “Field-Programmable Gâte Array”).

Landscapes

• Engineering & Computer Science (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Physics & Mathematics (AREA)
• Computer Networks & Wireless Communication (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Radar Systems Or Details Thereof (AREA)
• Geophysics And Detection Of Objects (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=73013559

Family Applications (1)

Country Status (4)

Families Citing this family (4)

Family Cites Families (1)

• 2020
  • 2020-06-30 FR FR2006870A patent/FR3111994B1/fr active Active
• 2021
  • 2021-06-22 US US18/012,965 patent/US20230266461A1/en active Pending
  • 2021-06-22 EP EP21733471.3A patent/EP4172655A1/de active Pending
  • 2021-06-22 WO PCT/EP2021/067038 patent/WO2022002700A1/fr unknown

Also Published As

Similar Documents

Legal Events