METHOD AND APPARATUS FOR BLURRING EFFECT MITIGATION IN GROUND-BASED RADAR IMAGES

Abstract

In a method for mitigating the blurring effect in a radar image (40) obtained by a ground-based radar system, thereof, a Pulse Repetition Frequency (PRF) value is selected (110) in a radar sensor unit (30) such that radial velocity measurements of the targets of an observed scenario can be made up to a maximum unambiguous velocity v.sub.max, a radial velocity threshold is also selected (101) to discriminate between substantially stationary targets and possible fast-moving targets having radial velocities v.sub.R, j≤v* and v.sub.R, f>v*, respectively. The scenario is conventionally scanned (120) by emitting transmission signals to the targets and receiving corresponding backscattered signals (23) from which raw data (25) are extracted (130), the latter in turn are Doppler-processed (140) so as to discriminate first and second data (31, 32) related to the substantially stationary and to the fast-moving target(s), respectively, according to whether the measured radial velocities (v.sub.R) are lower than the radial velocity threshold (v*) or not, respectively; second data are removed (150) from the Doppler-processed data (27) and radar image (40) is formed from remaining first data, i.e., based on the substantially stationary targets only. The method allows reducing the occurrence of artifacts due to fast-moving objects that are systematically present or that turn up in the scenario at the moment of taking an image thereof, such as truckloads or vehicle in general, as well as crane mobile portion in scenarios like a portion of a mine. (FIG. 11).

Claims

1. A method for mitigating the blurring effect in a ground-based radar image (40) of a scenario (1) including a plurality of targets (2.sub.j, 3), said method comprising the steps of: prearranging (100) a ground-based radar system (10) including a radar antenna unit (20) and a radar sensor unit (30), said radar sensor unit (30) configured to obtain respective range measurements (r) and respective radial velocity measurements (v.sub.R) of said targets (2.sub.j, 3), selecting (110), in said radar sensor unit (30), a Pulse Repetition Frequency (PRF) value such that said radial velocity measurements can be obtained up to a maximum unambiguous velocity (v.sub.max) of said targets (2.sub.j, 3); selecting (101) a radial velocity threshold (v*), said radial velocity threshold (v*) defining, among said targets (2j, 3): a plurality of substantially stationary targets (2.sub.j) having radial velocities (v.sub.R, j) lower than or equal to said radial velocity threshold (v*); at least one fast-moving target (3) having a radial velocity (v.sub.R, f) higher than said radial velocity threshold (v*); scanning (120) said scenario (1) by said radar system (10), including steps of causing said radar antenna unit (20) to emit transmission signals (22) and of receiving backscattered signals (23) from respective said targets (2.sub.j, 3) in response to said transmission signals (22); extracting (130) raw data (25) from said backscattered signals (23), said raw data (25) related to said targets (2.sub.j, 3) including said substantially stationary targets (2.sub.j) and said fast-moving target (3); Doppler-processing (140) said raw data (25), obtaining Doppler-processed data (27) containing said radial velocity measurements (v.sub.R), in order to discriminate: first data (32) that are related to said substantially stationary targets (2.sub.j), and second data (33) that are related to said fast-moving target (3), according to whether said radial velocity measurements (v.sub.R) are lower or not lower than said radial velocity threshold (v*), respectively; removing (150) said second data (33) from said Doppler-processed data (27), obtaining said first data (32) alone, which are related to said substantially stationary targets (2.sub.j); forming (160) a radar image (40) of said scenario (1) from said first data (32) said radar image (40) representing only said substantially stationary targets (2.sub.j).

2. A method according to claim 1, wherein said Pulse Repetition Frequency value of said radar sensor unit (30) is selected in such a way that said maximum unambiguous velocity (v.sub.max) is higher than said radial velocity (v.sub.R, f) of said fast-moving target (3).

3. A method according to claim 1, wherein said step of scanning (120) said scenario (1) comprises a step of mechanically changing the position of said radar antenna unit (20) along a trajectory (γ) at a predetermined scan speed (v.sub.s), said radar antenna unit having a predetermined field-of view angle (±θ.sub.FOV).

4. A method according to claim 3, wherein said radial velocity threshold (v*) is set equal to a value selected between: said scan speed (v.sub.s); a maximum relative radial velocity that can be measured between the sensor and a substantially stationary target, i.e., v.sub.s.Math.sin(θ.sub.FOV).

5. A method according to claim 3, wherein said Pulse Repetition Frequency (PRF) is set at a value such that said maximum unambiguous velocity (v.sub.max) is higher than radial velocity v.sub.R, f of said fast-moving target (3).

6. A method according to claim 1, wherein said radar antenna unit (20) comprises a plurality of antennas (21.sub.i), and said step of scanning (120) said scenario (1) is carried out by an electronic scan between said antennas (21).

7. A method according to claim 1, wherein said step of scanning (120) said scenario (1) comprises a step of mechanically changing the orientation of said radar antenna unit (20) within a predetermined angle (a) including said scenario (1).

8. A method according to claim 1, comprising steps of: obtaining (170) position and speed measurements of said fast-moving target (3); forming (175) a track (35) of said fast-moving target (3) by iterating in the time said step of obtaining (170) said position and speed measurements.

9. A method according to claim 8, comprising steps of: defining (102) a plurality of hazard levels (37); associating (180) said track (35) to a hazard level (37) based on said position and/or speed measurements; providing (185) an alarm signal responsive to said hazard level (37).

10. A ground-based radar system (10) comprising: a radar antenna unit (20) and a radar sensor unit (30), said radar sensor unit (30) configured to obtain respective range measurements (r) and respective radial velocity measurements (v.sub.R) of targets (2.sub.j, 3) that are present within a scenario (1); said radar sensor unit (30) configured with a Pulse Repetition Frequency (PRF) such that said radial velocity measurements can be obtained up to a maximum unambiguous velocity (v.sub.max) said radar sensor unit (30) including a scan means for causing said radar antenna unit (20) to emit transmission signals (22) and to receive backscattered signals (23) in response to said transmission signals (22) from respective said targets (2.sub.j, 3), in such a way to scan said scenario (1); a computation means for: extracting raw data (25) from said backscattered signals (23), said raw data (25) related to said targets (2.sub.j, 3); Doppler-processing said raw data (25), obtaining Doppler-processed data (27) containing said radial velocity measurements (v.sub.R); discriminating first data (32) that are related to substantially stationary targets (2.sub.j), and second data (33) that are related to fast-moving target (3), according to whether said radial velocity measurements (v.sub.R) are lower or not lower than a predetermined radial velocity threshold (v*), respectively; removing said second data (33) from said Doppler-processed data (27), so as to obtain said first data (32) alone, which are related to said substantially stationary targets (2.sub.j); forming a radar image (40) of said scenario (1) from said first data (32), said radar image (40) representing only said substantially stationary targets (2.sub.j).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention will be now shown with the description of its exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings, in which:

[0067] FIG. 1 schematically shows a ground-based radar system and a scenario including a plurality of targets;

[0068] FIG. 2 schematically shows a ground-based, synthetic aperture radar system and a scenario including a plurality of targets;

[0069] FIG. 3 is a detail of a single target in the scenario and with the system of FIG. 2, showing the relationship between the scan speed of the radar antenna unit and the relative radial velocity of that target, in which the velocity vectors are represented in a reference system integral to the radar antenna unit;

[0070] FIG. 4-6 are GB-SAR images showing blurring effects, i.e., artifacts caused by moving objects in that are present in the scenarios;

[0071] FIG. 7 schematically shows a ground-based radar system configured to change the orientation of its radar antenna unit within a predetermined angle including a scenario including a plurality of targets;

[0072] FIG. 8 schematically shows a ground-based radar system comprising a plurality of antennas and configured to perform a step of scanning a scenario;

[0073] FIG. 9 shows a detail of the radar system of FIG. 8, in which the antennas are actuated sequentially;

[0074] FIG. 10 shows a detail of the radar system of FIG. 8, in which the antennas are actuated simultaneously;

[0075] FIG. 11 shows a flowchart of the method according to the invention;

[0076] FIG. 12 is a range-velocity map of a scenario including both substantially stationary and fast-moving objects, obtained by a GB-SAR system;

[0077] FIG. 13 is an azimuth-range map of the same scenario as in FIG. 12, obtained by operating the GB-SAR system according to the prior art;

[0078] FIG. 14 is an azimuth-range map of the same scenario as in FIG. 12, obtained by operating the GB-SAR system according to the method of the invention;

[0079] FIG. 15 show a flowchart of the method according to another aspect of the invention, in which a track of a fast-moving target is obtained;

[0080] FIG. 16 show a flowchart of the method according to the same aspect of the invention, in which the track of a fast-moving target is associated to a hazard level to provide an alarm signal;

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0081] With reference to the annexed drawings, a method is described for mitigating the blurring effect in ground-based radar images of a scenario 1 including a plurality of targets 2.sub.j, 3, as shown in FIG. 1, schematically depicting a GB radar system 10 including a radar antenna unit 20 and a radar sensor unit 30.

[0082] The description encompasses both radar systems in which scenario 1 is scanned mechanically, i.e., by moving a single antenna 21 of radar antenna unit 20, FIGS. 2 and 7, and radar systems in which scenario 1 is scanned electronically by actuating multiple antennas 21.sub.1, 21.sub.2, . . . 21.sub.m of radar antenna unit 20, FIGS. 8-10.

[0083] More in detail, FIGS. 2 and 7 schematically show mechanically scanned SAR and RAR systems 10, respectively.

[0084] With reference to FIG. 7, GB radar system 10 can be a GB-RAR system in which step 120 of scanning scenario 1 is actuated mechanically by changing the orientation of radar antenna unit 20 within a predetermined angle a, so as to include scenario 1.

[0085] In electronically scanned radar system 10 of FIG. 8, antennas 21 can be actuated in transmission and reception according to a time program, in particular they can be actuated sequentially, FIG. 9, or simultaneously, FIG. 10. In the former case, antennas 21i, i=1 . . . m can be sequentially caused to emit transmission signals 22 and to receive backscattered signals 23 from targets 2.sub.j, 3 each during a fraction of a predetermined scan period. Backscattered signals 23 are combined and processed as well known to a skilled person. In the latter case, the antennas 21i i=1 . . . m emit and receive at the same time.

[0086] In the method, an image 40 of scenario 1, including a plurality of targets 2.sub.1, 2.sub.2, . . . 2.sub.n and 3, is basically obtained by performing steps 100 to 160 schematically depicted in the block diagram of FIG. 11 after selecting a radial velocity threshold v*, step 101, which is the limit velocity above which the contribution of moving objects to image 40 can be suppressed by the method.

[0087] More in detail, the selected value of radial velocity threshold v* makes it possible to distinguish, among targets 2.sub.j and 3, a plurality of substantially stationary targets 2.sub.j, the radial velocity v.sub.R, j of which is lower than or equal to selected radial velocity threshold v*, and one or more fast-moving targets 3, the radial velocity v.sub.R, f of which is higher than selected radial velocity threshold v*.

[0088] Substantially stationary targets 2.sub.j include therefore the targets to be monitored in scenario 1, for instance orographic or structural elements whose possible displacement are the object that must be investigated by GB radar system 10.

[0089] For the sake of simplicity, only one fast-moving target 3 is shown in FIGS. 1, 2, 7 and 10 and taken into consideration in the description, the extension to a plurality of fast-moving targets being obvious for a skilled person. Fast-moving targets 3 are elements that can be accidentally or systematically present in the scenario, for instance, vehicles in a mine, or other mechanical means used to displace objects such as rocks or construction materials in a building site, or plants moving under the action of the wind, and the like.

[0090] Still with reference to FIG. 11, after a step 100 of conventionally prearranging radar system 10 in a suitable position with respect to scenario 1, a step 110 is carried out of selecting a Pulse Repetition Frequency value, i.e., the number of pulses the radar antenna unit will transmit in one time-unit by, such that the radial velocity measurements can be obtained up to a maximum unambiguous velocity v.sub.max=λ/4×(PRF) of the targets 2 ,3.

[0091] Subsequently, radar system 10 is operated by performing a step 120 of scanning scenario 1 by radar antenna unit 20 of GB radar system 10 is performed mechanically, FIG. 2 or 7, or electronically, FIGS. 8-10.

[0092] By step 120 of scanning scenario 1, radar antenna unit 20 is conventionally caused to emit transmission signals 22 and to receive backscattered signals 23 from targets 2.sub.j, 3 in response to transmission signals 22. In this connection, the main tasks of radar sensor unit 30 consists in generating transmission signals 22 to radar antenna unit 20 according to some parameters depending upon the specific application, and to convert backscattered signals 23, received from antenna unit 20, into digital data.

[0093] This way, measurements of the range r.sub.j and of the radial velocity v.sub.R, j and v.sub.R, f are obtained for each target 2.sub.j and 3 up to a maximum unambiguous velocity v.sub.max depending upon selected Pulse Repetition Frequency (PRF) value according to the relationship v.sub.max=λ/4×(PRF), as anticipated.

[0094] A step 130 follows of extracting raw data 25 related to targets 2.sub.j, 3 from backscattered signals 23, which are subsequently Doppler-processed in step 140, thus obtaining Doppler-processed data 27 containing, in particular, radial velocity measurements v.sub.R.

[0095] Doppler processing 140 enables distinguishing first data 32 related to stationary targets 2.sub.j, and second data 33 related to fast-moving target 3, according to whether the measured radial velocity v.sub.R are lower or not lower than radial velocity threshold v*, respectively. This way, second data 33 can be removed from Doppler-processed data in a step 150, thus obtaining first data 32 only. First data 32 relate to stationary targets 2j including those of interest, and are subjected to further processing.

[0096] In fact, a step 160 of forming a radar image 40 of scenario 1 from first data 32 is carried out. Step 160 of forming a radar image 40 can be of different complexity levels. In some embodiments, it can merely consist of a step of associating coordinates of an image file to pixels of a display device. In other embodiments, it can even encompass complicated data processing procedures such as range-migration or time-domain back-projection algorithms, unless these steps have been already performed when Doppler-processing raw data, in particular, by radar sensor device 30.

[0097] The contribution of fast-moving target 3 is excluded from radar image 40, and only the contributions of substantially stationary targets 2.sub.j are present. These are the contribution of all substantially stationary targets 2.sub.j moving at a radial velocity v.sub.R, j lower than radial velocity threshold v*. Therefore, stationary targets 2.sub.j include the targets of interest along with possible targets moving at a radial velocity lower than previously defined threshold v*.

[0098] Therefore, the lower preliminary selected radial velocity threshold v*, the lower the amount of contributions of moving objects in the radar image 40.

[0099] In particular, Pulse Repetition Frequency (PRF) can be set at a value such that maximum unambiguous velocity v.sub.max=λ/4×(PRF) higher than radial velocity v.sub.R, f of fast-moving target 3.

[0100] As anticipated, with reference to FIG. 2, the GB radar system can be a GB-SAR system in which step 120 of scanning scenario 1 is actuated mechanically by changing the position of radar antenna unit 20 at a scan speed vs along a trajectory γ. In this case, radial velocity threshold v* can be set equal to scan speed v.sub.s.

[0101] The advantages provided by the method according to the invention, in the case of a GB-SAR system, can be readily understood when considering FIGS. 12-14.

[0102] FIG. 12 show GB-SAR Doppler-processed data in the form of a range-velocity map of a scenario in which a crane is moving. Signals 94 related to stationary targets can be recognized at radial speed 0 m/s, along with signals 95 from the moving crane, displayed as elliptical traces between ranges of 320 m and 370 m, and signals from windblown vegetation mainly present between radial speeds of −0.25 m/s and +0.25 m/s.

[0103] FIGS. 13 and 14 are azimuth-range maps of the same scenario as in FIG. 12, obtained by operating the GB-SAR system according to the prior art and according to the method of the invention, respectively. The effectiveness of the method of the invention can be assessed by comparing FIGS. 13 and 14. In FIG. 13, artifacts 96 due to the moving crane can be recognized between ranges of 320 m and 370 m. As shown in FIG. 14 shows, most of artifacts 96 are suppressed, thanks to the method according to the invention. The only residual artefacts 96 are in a band about a range of 345 m, corresponding to the part of the jib of the crane that is most close to the mast, which turns about the mast at a peripheral velocity lower than radial velocity threshold v*.

[0104] With reference to FIG. 15, in another aspect of the invention, the method can also comprise a step 170 of obtaining range and radial velocity measurements also of at least one fast-moving target 3, followed by a step 175 of forming a track 35 of fast-moving target 3, by iterating in the time step 170 of obtaining the measurements pertaining fast-moving target 3.

[0105] In particular, as shown in FIG. 16, in an embodiment, the method can also comprise a preliminary step 102 of defining a plurality of hazard levels 37 in connection with possible consequences of the displacement of fast-moving target 3 in scenario 1, in particular if the scenario also includes operators 1 or in any case living beings, or valued goods. Once track 35 has been formed as described above, a step 180 is provided of associating track 35 to one of said hazard levels 37 based on the position and/or speed values, possibly followed by a step 185 of providing an alarm signal responsive to the hazard level 37.

[0106] The foregoing description exemplary embodiments and specific examples of the invention will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt such embodiments for various applications without further research and without parting from the invention, and, accordingly, it is to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment and to the examples. The means and the materials to perform the various functions described herein could have a different nature without, for this reason, departing from the scope of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.

BIBLIOGRAPHIC REFERENCES

[0107] 1) Andreas Jungner, “Ground-Based Synthetic Aperture Radar Data Processing for Deformation Measurement”, Master's of Science Thesis in Geodesy No. 3116, TRITA-GIT EX 09-11 Division of Geodesy Royal Institute of Technology (KTH) Stockholm, Sweden, May 2009

[0108] 2) Cuenca, Marc Lort, “Contribution to ground-based and UAV SAR systems for Earth observation”, thesis submitted to the Universitat Politecnica de Catalunya, Barcelona, Spain, 2017

[0109] 3) Marc Lort et al, “Impact of wind-induced scatterers motion on GB-sar imaging”, IEEE journal of selected topics in applied Earth observations and remote sensing, vol. 11, No. 10, 1° ottobre 2018, pages 3757-3768.