The invention relates to a method and a system for mapping a target region. Furthermore, the invention relates generally to computer program product and a computer readable medium for executing the method.
Approximately 70% of the earth surface is covered by ocean. Mapping the ocean-floor in deep water poses great challenges, both from a technical and an economic perspective. This may be a reason why only as much as 20% of the ocean floor has actually been explored and mapped in detail.
Mapping of a submerged surface and/or detecting of structures that are present in/on this surface may be performed by a floating platform (e.g. a vessel) or an underwater platform carrying a sonar transceiver. Such a platform may transmit acoustic signals in a predetermined direction away from the platform and towards the intended target region, and then receive and process the acoustic response signal.
Pulse-based mapping systems rely on the round-trip time for the acoustic signals propagating through the water. The instantaneous location of the platform, the direction of transmission and reception of the acoustic pulses, and the round trip time for pulses returning from the submerged surface are recorded. Processing of the received sonar pulses and associated spatial and timing relations may yield a geometrical map of the surface area that was covered by the signal transmissions.
The roundtrip time for pulsed systems increases as function of propagation distance. This renders acoustic imaging increasingly more difficult for surfaces that are located at greater depths. One known approach to mitigate this is to use a surface vessel with a continuous wave or frequency modulated multi-beam acoustic array. The associated operational costs are typically high, while the achievable imaging resolution remains relatively low. Another known approach involves bringing a subsea vehicle (e.g. a towed or autonomous underwater vehicle) close to the seabed. The operational costs are also high in this case, due to the required presence of operators and a support vessel, and the inherent complexity in the coordination of the equipment.
It would be desirable to provide a system and method for acoustic mapping of an underwater surface, for which the obtainable mapping resolution is relatively high but for which the operational costs may be significantly reduced.
According to a first aspect of the invention, there is provided a method for mapping a target region of space in accordance with claim 1. This target region is composed of multiple volume elements, and at least some of these elements accommodate objects or local non-uniformities that cause an incoming signal to scatter. The method involves the following actions:
The method further includes:
A traditional beamforming process relies on the assumption that signal waves with a macroscopically flat wavefront hit the physical detection array, and manipulates the phase or time-delay of each receiver element such that all elements are in phase for a particular one- or two-dimensional direction or for a particular point in three-dimensional space. In a physical array, adding the (appropriately delayed) signals from all receiver elements together yields a total signal optimized for that direction or three-dimensional (3D) point. However, the imposing of phase coherence ignores any temporal information carried by the incoming signal.
By contrast, the method according to the first aspect does utilize the temporal information in the signal, thereby establishing a synthetic array that is able to correlate on particular ranges within the target region, while suppressing sources with different pathlengths to a high extent. By using a relatively long and non-repetitive signal with a large number of bits in the PRN sequence, and simultaneously moving at least one of the transmitter and the receiver, it becomes possible to obtain strong signal correlation values only for selected ranges. This cannot be achieved with traditional pulsed systems, which repeatedly transmit relatively short and similar signal pulses. The proposed method allows acquisition hardware to remain relatively simple and low-cost, and can be used in a wide range of applications, for instance in acoustic mapping of subsea surfaces, objects, and soil layers, in seismic imaging (i.e. mapping the subsurface of the earth), in seismic cone penetration testing, in detection of unexploded ordinance (UXO), boulders, or buried cables or pipes, in medical acoustic imaging, in radar mapping, or in ground penetrating radar (GPR). Note that the terms “map” and “mapping” are used here in a general sense, as referring to a surjective correspondence between an input set (domain) of (mutually adjacent) spatial coordinates and an output set (codomain) of calculated correlation strength values for each such coordinate, yielding a two-dimensional or three-dimensional image.
The binary sequence of PRN values may for instance be a sequence of PRN bits modulated onto the carrier signal using binary phase-shift keying.
In embodiments, the binary sequence of PRN values in the probing signal is unique and has a total duration at least as long as a total time required by the at least one of the first and second platforms to complete the respective trajectory. In one embodiment, the binary sequence of PRN values in the probing signal is unique and has the total duration of the method for mapping a target region (60).
By modulating using a PRN code that is as long as possible in comparison to the total duration of the trajectory, the correlation value between the generated test signals and the actually received response signal can be maximized, while false positive correlation contributions from unwanted clutter are maximally suppressed. However, in alternative embodiments, PRN codes with a shorter duration may be used, for instance a PRN code that lasts for only an Nth part of the trajectory traversal time (N Σ) but which is re-transmitted N times in an consecutive manner, i.e, a pulsed signal with sequential PRN code and that does not repeat itself could be transmitted. In this regard, the sequential PRN code should be continuous, and not necessarily the signal. In this way, pulsed signals could be used as the probing signal, as long as the content of the pulsed signal is pseudorandom and sequential. Then the transmission of the probing signal and the reception of the received signal need to be timed by using an estimate of the local depth. By timing the reception and the transmission the transmitter and receiver could be in the same ASV or platform, without the receiver being disturbed.
In embodiments, the target region is a target area of a submerged surface. In this case, the signal transmitter may be an acoustic transmitter provided on a first waterborne platform, the signal receiver may be an acoustic receiver provided on a second waterborne platform, and the probing signal may be a continuous acoustic signal comprising the binary sequence of PRN values modulated onto an acoustic carrier signal.
The term “waterborne platform” is used herein to refer broadly to a vehicle or non-propelled body that carries the functional components for executing the mapping method, and which is adapted to be conveyed on/across water. For instance, the first and second platforms may be moved along respective trajectories across a surface of the water and relative to the submerged surface. This submerged surface generally lies at a distance below the water surface and forms an water-soil interface between the body of water above it and the layer of earth below it.
In embodiments, the moving involves moving at least one of the first and second platforms along a corresponding closed curve, while continuously transmitting the probing signal and receiving the response signal over the entire extent of the closed curve. One or both of the closed curves may be a simple closed curve, such as a circular trajectory. For instance, one of the transmitter and receiver may be moving along a circular trajectory around the other one of the transmitter and receiver, whereas this other one is held at an (essentially) stationary position. Alternatively, both the transmitter and the receiver may be co-rotating at different instantaneous locations along circular trajectories around a common centre.
In alternative embodiments, at least one of the first and second platforms may be moving along a linear trajectory. For instance, one of the transmitter and receiver may be moving along a linear trajectory relative to the other one of the transmitter and receiver, whereas this other one is held at an (essentially) stationary position. Alternatively, both the transmitter and the receiver may be moving side by side in essentially linear trajectories, for instance mutually parallel trajectories.
In yet alternative embodiments, both the transmitter and the receiver may be provided on the same vehicle or platform, such that transmitter and receiver are jointly movable along the same trajectory during signal transmission and reception.
According to embodiments, the transforming of the probing signal for each selected portion of the target region includes:
According to a further embodiment, the transforming of the probing signal includes shifting instantaneous values of the probing signal in time, in such a way that the respective test signal associated with the respective portion of the target region is a sequence of amplitude values defined by p(tR)=w(tR−Δt). Here, tR represents the time instance at which the receiver receives the response signal. At represents the total time delay time based on the signal propagation speed in the signal carrying medium and the propagation distance from the instantaneous position of the signal transmitter at the time that the probing signal was transmitted, via the selected portion where the probing signal was scattered, to the instantaneous position of the signal receiver at the time of receipt. Furthermore, p(tR) represents the instantaneous value of test signal p at time instant tR, and w(tR−Δt) represents the instantaneous value of the probing signal w at earlier time instant tR−Δt.
In an embodiment, the correlating includes calculating, for each selected portion of the target region, a correlation strength between, on the one hand the response signal, and on the other hand the test signal corresponding with the selected portion, in order to determine the map of correlation strengths associated with respective selected portions of the target region.
In a further embodiment, the correlation strength for the respective portion of the target region is calculated using a discrete correlation operation defined by
J(ξ, η, ζ)=Σt p(t)·r(t)=Σt
In an embodiment, the method further includes:
In a further embodiment, the second platform includes a further signal receiver in proximity of the signal receiver. The further signal receiver is configured to receive a further response signal composed of a plurality of further signal components that result from scattering of the probing signal by distinct portions of the target region, and to cooperate with the signal receiver by dynamically adjusting a phase difference between the response signal and the further response signal received. In this case, the method may further include dynamically adjusting the phase difference as a function of instantaneous position of the second platform relative to the target region, in order to supress components in the response signal and the further response signal originating from potential scatter sources in a region coinciding with the false positive correlation values in the PSF image.
By adjusting relative phase shifts between similar signals received by multiple closely-spaced receivers in a post-processing stage, it becomes possible to adjust the dual/multi-receiver directivity profile such that certain spatial signal contributions and hence certain contributions to the calculated correlation metric are suppressed (referred to as “nulling”). Methods for creating nulls by adding and phase shifting wave-like signals from multiple receivers per se are considered known, and will not be explained in detail here. Using just two receiver transducers may already suffice, as the signal contribution that needs to be suppressed at any one time during the signal transmission corresponds to a small local patch around a point on the leakage curve of false positive correlation values in the PSF image. If the two receiver transducers are mounted perpendicular to the direction of movement of the second platform, depending on the phase shift between the two transducers, adding both received signals (in post processing after applying the phase shift) will generate nulls in an elevation plane. By dynamically changing the phase shift along the trajectory of the second platform, the null may be dynamically steered to coincide with a location on the leakage curve that is received at that time instance. As the portion of the target region that corresponds to the correct position of the scatter target and that yields the true correlation strength comes from a significantly different direction (e.g. from the centre of the synthetic array), the signal contribution from this portion will be much less suppressed.
In embodiments, the probing signal is defined by ww(t)=A sin(2πωt+δ) S(t). In this equation, t represents time, A represents an amplitude of the carrier signal, ω represents an angular frequency of the carrier signal, δ represents a fixed phase shift of the carrier signal in a range 0≤ō<2π, and S(t) represents the binary sequence of PRN values as function of time. In case the sequence of PRN values is a sequence of PRN bits modulated onto the carrier signal, S(t) may represent a band-pass filtered representation of the PRN bit sequence.
In an embodiment, the signal transmitter is configured to emit the probing signal within a primary emission beam having a substantially uniform spatial gain profile, at least within a solid angle that covers the target area during the emitting of the probing signal.
According to a second aspect of the invention, and in accordance with the advantages and effects described herein above, there is provided a system for mapping a target region of space that comprises a plurality of portions with signal scatterers. The system for mapping the target region is advantageously a system which allows the mapping according to the method of the first aspect of the present invention. Accordingly, advantageously it allows providing a probing signal comprising a binary sequence of pseudo-random-noise values modulating a carrier signal. The system includes a processing device that is configured to:
The processing device may be mounted on-board one of the platforms that carries the signal transmitter or receiver, on-board a nearby vehicle or vessel, or be part of a remote processing centre or a cloud computing facility.
In an embodiment, the system further includes the signal transmitter and the signal receiver. At least one of the signal transmitter and receiver may then be adapted to be moved along a respective trajectory relative to the target region while transmitting the probing signal or receiving the response signal respectively.
The signal transmitter may be adapted to transmit the probing signal within a predominantly conical intensity distribution that is directed predominantly vertically downwards into a signal carrying medium and towards an expected location of the target region. Alternatively or in addition, the signal receiver may be an omni-directional receiver, adapted to simultaneously receive multiple signal components that are reflected by distinct portions in the target region.
According to a third aspect of the invention, there is provided a computer program product configured to provide instructions to carry out processing steps in a method according to the first aspect, when loaded on a computer arrangement.
According to a fourth aspect of the invention, there is provided a computer readable medium, comprising a computer program product according to the third aspect.
Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts. In the drawings, like numerals designate like elements. Multiple instances of an element may each include separate letters appended to the reference number (for instance “72a” and “72b”). The reference number may be used without an appended letter (e.g. “72”) to generally refer to an unspecified instance or to all instances of that element.
The figures are meant for illustrative purposes only, and do not serve as restriction of the scope or the protection as laid down by the claims.
The following is a description of certain embodiments of the invention, given by way of example only and with reference to the figures. Cartesian and spherical coordinates will be used in the next figures to describe spatial relations for the exemplary embodiments. When the term “position” is used herein without modifier, this may refer broadly to linear position (i.e. “location”), to angular position (i.e. “orientation” or “attitude”), or a combination thereof (e.g. to 6DOF position). The specific terms “location” and “orientation” are explicitly used when a distinction is intended. It should be understood that the directional definitions and preferred orientations presented herein merely serve to elucidate geometrical relations for specific embodiments and are not otherwise intended to limit the scope of the invention or claims.
This exemplary system 10 comprises a first waterborne platform 20, a second waterborne platform 40, and a vessel 90 with a processing device 92. The first and second platforms 20, 40 are located at or near the water surface 16. At least one of the first and second platforms 20, 40 is moveable across the water surface 16 and relative to the other platform 40, 20, as well as relative to the submerged surface 18. In this example, the first and second platforms 20, 40 are autonomous surface vehicles (ASVs), which are floating, un-crewed, and relatively small. These ASVs 20, 40 are used to image a top layer of the submerged surface 18, with a spatial resolution that is sufficient to observe possible wrecks or other objects on/in this surface 18. Depending on the depth of the surface 18, the spatial resolution of the images may for instance be in a range of 0.1 metres to 10 metres. The ASVs 20, 40 may be remotely controlled, or may be pre-programmed to perform their measurements autonomously.
The first ASV 20 is provided with an acoustic transmitter, and the second ASV 40 is provided with an acoustic receiver. The ASVs 20, 40 are synchronized to a common time-base, for instance at the microsecond level in the present example which relies on transmitting and receiving acoustic signals. The ASVs 20, 40 are configured to determine their instantaneous locations QT, QR, and optionally also their orientations (attitude).
During an acquisition survey, at least one of the ASVs 20, 40 travels along a determined trajectory 34, 54 across the surface 16 of the water 12, while the first ASV 20 emits a continuous acoustic probing signal 62 towards a wide target area 60 of the submerged surface 18, and while the second ASV 40 continuously receives a response signal 76 originating from scatter sources located on distinct portions of the submerged surface 18. In this example, the moving ASV (20, 40, or both) traverses a closed trajectory 34, 54 to obtain a sufficiently good geometry and coverage for each location within the target area 60. In this example, the trajectory 34 of the first ASV 20 is predominantly a single stationary location QT, whereas the second ASV 40 moves along a closed circular trajectory 54 with a large radius R around the first ASV 20.
The separation of the emitter and the receiver on the distinct ASVs 20, 40 allows isotropic transmission and measurement of continuous acoustic signals having relatively large signal powers, while avoiding saturation of the receiver (as opposed to pulsed systems and monostatic systems).
The continuous probing signal 62 includes a long sequence of pseudo random noise (PRN) bits, which are modulated onto an acoustic carrier wave signal (for instance as phase or frequency modulation). If the PRN code does not repeat itself for the duration of the survey, then it may be considered fully (pseudo) random at the time-scale of the entire survey. The relative motion of the ASVs 20, 40 during the transmission and subsequent receipt of the acoustic signals 62, 76 allows the system 10 to act as a synthetic aperture sonar, which relies on high precision positioning to determine the instantaneous positions QR, QT of the ASVs 20, 40 at each instance t. The acquired data may require relatively complex processing steps, but such processing may be executed elsewhere and/or off-line.
Accurate positioning and timing measurements during transmission and reception of the continuous acoustic signals 62, 76 ensure that the response signal 76, which contains reflections from the target area 60 at all times during the survey, can be correlated with the probing signal 62 for selected individual voxels in a three-dimensional volume containing the target area 60, potentially at depths of several kilometres and with a resolution in the order of 0.1 meter to about 20 metres.
In general, post processing of the measurement data may be performed on-board the ASVs 20, 40, or at any other location after all data of the survey has been gathered. In the illustrated example, the data processing may be performed by processor 92 on-board the vessel 90 (which may be nearby or remote). This vessel 90 is provided with a data receiver, configured to communicate with and receive the measurement data from the ASVs 20, 40. In alternative embodiments, the acoustic signals 62, 76 (which may have a relatively low bandwidth) and position and timing measurements may be uploaded via satellite to a remote processing station or to the cloud for automated processing.
The acoustic transmitter 26 is adapted to transmit the probing signal 62 in a substantially uniform conical intensity distribution 64 downwards through the water 12 and towards an expected location of the target area 60. Temporal characteristics of this exemplary probing signal 62 will be discussed with reference to
In the example illustrated in
The acoustic receiver 46 is configured to receive the acoustic response signal 76, which is composed of (among other things) acoustic signal components that are scattered by distinct portions of the submerged surface 18 within the target area 60. The second positioning device 48 is configured to dynamically/repeatedly determine instantaneous (global) positions QR of the second platform 40 and/or its acoustic receiver 46 during receipt of the response signal 76. The second timer device 50 is configured to dynamically/repeatedly sample time values tR associated with selected instances during reception of the response signal 76. The timer devices 30, 50 are time-synchronized to a common time base, with an accuracy in the order of microseconds or less.
In this example, the acoustic receiver 46 is an omni-directional receiver, adapted to receive the acoustic response signal 76, which is composed of multiple signal components from multiple scattering sources located across the submerged surface 18. This receiver 46 is configured to detect and sample both the amplitude and the phase of the response signal 76. The second platform 40 is configured to continuously collect the response signal 76 while moving along its trajectory 54 around the first platform 20.
Each of the first and second platforms 20, 40 receives GNSS signals from a GNS system (e.g. Global Positioning System satellites), to allow each platform 20, 40 to determine accurate positioning information. The positioning devices 28, 48 may be GNSS positioning devices that include both real-time kinematic (RTK) data relative to the receiver, and precise point positioning data based on GPS and GLONASS orbit and clock corrections (G2). Each of the positioning devices 28, 48 may also include an inertial measurement unit (IMU) for determining an indication of pitch, yaw, and roll.
Each of the first and second platforms 20, 40 may further include a communication system which is configured to report platform status, to allow receiving external control commands, and to transmit acquired measurement data to the external vessel 90 or station.
The processing device 92 is configured to collect the probing signal 62, the response signal 76, the sets of position measurements and timing measurements from the first and second platforms 20, 40, and to process these to determine a correlation metric 80. This correlation metric 80 will be used to construct a map 84 of the target area 60 of the submerged surface 18, as will be explained below with reference to
In the example of
The probing signal 62 transmitted by the first platform 20 propagates via different paths 62a, 62b, 62c towards the submerged surface 18, where these signal portions 62 impinge on distinct portions 72a, 72b, 72c of the surface 18. These signal portions 62 are randomly scattered in all directions due to the irregular structure of the surface 18. The resulting scattered signal components 74a, 74b, 74c travel along distinct paths back towards the water surface 16. Some of these scattered components 74 are then picked up by the receiver 46 at the second ASV 40. The actual response signal 76 received by the receiver 46 will be composed of a large number of such components 74, though, and these components 74 are difficult to distinguish.
In this example, the first ASV 20 remains (quasi) stationary at position QT, while transmitting the probing signal 62 towards the target area 60. During this transmission, the first ASV 20 records transmission times tT corresponding with specific amplitude instances w(tT) of the probing signal 62, and also repeatedly determines its instantaneous location QT to verify whether the assumption of stationarity is still complied with, or whether positional correction is needed. The ASV 20 may temporarily store this time and location data for future processing or for transmission to a remote processing device 92.
Meanwhile, the second ASV 40 moves along various second locations QR of its circular trajectory 54, while continuously receiving the resulting response signal 76 from the various portions 72 of the submerged surface 18. During this reception, the second ASV 40 records reception times tR corresponding with specific amplitude instances r(tR) of the response signal 76, and also repeatedly determines its instantaneous location QR. This time and position data may be temporarily stored by the second ASV 40, or transmitted to the processing device 92.
Here, S(t) is a low-pass filtered version of the binary pseudo-random code 70, A is an initial amplitude of the probing signal 62, and w is the frequency of the carrier wave 68.
Data processing in the exemplary method is based on predicting the phase of a signal that the receiver 46 is expected to receive from a scatter source that is presumed to be present at a particular location at the submerged surface 18, and correlating this predicted signal with the actually received response signal 76 for the duration of the survey. If the putative scatter source is indeed present at that location, and if the prediction is correct, then the correlation operation is expected to yield a relatively high correlation metric value J. The magnitude of this correlation metric value J is expected to contain a large portion of the scatter strength of the putative scatter source, as well as (random) components from other scatter sources. The considerable length of the PRN-coded probing signal 62 will help suppress signal contributions from other scatter sources in the target area 60, as these other sources correspond to different pathlengths, which are expected to change continuously throughout the duration of the survey.
Suppose a portion 72a of a submerged surface 18 around the position (0, 0, z) is to be imaged. The first platform 20 with transmitter 26 moves along a determined trajectory 34 while continuously emitting the probing signal 62 of the form w(t) in equation (1). In the example of
Consider a particular three-dimensional point Pk=(ξ, η, ζ) Σ within the field of view of transmit-receive system 10. Assume that a scatter source present in point Pk reflects a portion of the probing signal 62 forward to the receiver 46 without changing the form of the signal, and assuming that the transmitter 26 and the receiver 46 remain stationary. One then expects to receive a delayed version of the time representation w(t) of the probing signal 62. If one of the platforms 20, 40 moves, then the signal traveling from the transmitter 26, via sample point Pk at area portion, to the receiver 46, experiences a propagation delay Δt that is proportional to the length of the path QT→*Pk→*QR from the transmitter 26, to the point Pk, to the receiver 46. If the transmitter 26 and receiver 46 are kept stationary, then this pathlength remains constant, but if one or both of the transmitter 26 and receiver 46 move, then the pathlength and corresponding propagation delay Δt change continuously.
The time representation w(t) of the probing signal 62 contains a known non-repeating PRN coding sequence 70. This coding sequence 70 is known in advance at both the transmitter 26 and the receiver 46. The received sequence of bits depends on the travel path, and is entirely predictable (within the accuracy with which the sound velocity and trajectories of the platforms 20, 40 are known) for each point Pk in the subspace being imaged. The sequence is not unique for each point Pk in the subspace, as will be discussed in more detail below.
When at least one of the platforms 20, 40 moves, the propagation pathlength will change continuously in time, which will cause the distinct coding bits in the probing sequence 70 of w(t) to experience different delays, depending on the coordinates of the imaged point Pk.
The image value for a point Pk may be calculated by a correlation operation J between, on the one hand the actually received bit sequence in the received signal 76, and on the other hand a test signal pattern p(t) based on the bit sequence 70 in the probing signal 62 that is predicted to be received after traversing the path QT→*Pk→*QR. This correlation may be expressed as
wherein R={ri}0N
If a scatter source is present at point Pk, the operation defined by (eq.2) will return a value close to Nbcp, with cp being a characteristic reflectivity of this scatter source. If no scatter source is present at Pk, then the operation (eq.2) returns a lower value that approaches √{square root over (Nb)}.
The exemplary imaging algorithm may be formally summarized by Table 1 below:
The reception time tR for an (instantaneous) portion of the scattered probing signal depends on: the transmission time tT, the position QT of the transmitter 26 at time tT, the position of the receiver 46 at time tR, the position Pk=(ξ, η, ζ) of the selected voxel for which the correlation strength J is calculated, and the velocity vs of sound in the water 12. The latter typically varies as a function of vertical distance z i.e. vs=vs(z). Combining these contributions yields:
Moving known quantities in the above equation to the right yields:
Expanding vector components yields:
wherein B has been defined by
The quantity B above is a known constant for a transmitter 26 located at a position QT that remains stationary for all times tT. For generic trajectories of the receiver 46, equation (eq.5) needs to be numerically solved for each moment of time tR that the receiver 46 receives an instantaneous portion of the response signal 76.
Consider a single bit, being part of the transmitted probing signal 66, which is sent out by the transmitter 26 at location QT and at time instance tT. This bit will arrive at the scatter location Pk at time tP, and (after being scattered) subsequently arrives at the receiver 46 at location QR and at time tR. The arrival times are defined by:
Eq.(7) describes a formulaic dependency, whereas eq.(8) contains the unknown and changing travel time Δt⬆ on both sides of the equation, rendering eq.(8) a non-linear equation that may need to be solved for every sample of the signal at the reception side when both platforms are continuously moving.
The following algorithm may be used to obtain a predicted test pattern p(t). This algorithm relies on the introduction of a virtual transmitter/receiver-pair at the point Pk being imaged. Consider a virtual receiver Vx placed at point Pk and construct a function
representing the delays each instantaneous value of the transmitted signal w(t) undergoes before arriving at Pk. Then the message b(t) received by this virtual receiver Vx may be described by
Here, {tilde over (w)} represents a zero-padded version of the transmitted signal w(t), and TV is a duration T of the message plus some arbitrary extra listening time interval.
Construct a function
representing the delays each instantaneous value of b(t) undergoes starting at static Pk before arriving at moving QR. Now allow the virtual receiver to re-transmit the message b(t) it has received at 14, such that the broadcast from Vx is subsequently received as the response signal r(t) by the receiver 46 at location QR
Here, {tilde over (b)} represents a zero-padded version of the signal b(t) broadcast by the virtual transmitter Vx, and TR represents the duration TV of the virtual broadcast plus some arbitrary extra listening time interval.
Platform 20 with transmitter 26 and platform 40 with receiver 46 are both assumed to be moving. In practice, measurements of the instantaneous locations QT and QR for the traveling platforms 20, 40 take place repetitively, yielding ordered time-stamped lists of coordinates:
The time stamps in eq.(11) are not necessarily coinciding and their lengths may be different (MT≠MR), but these time stamps nevertheless comply with tT
In the present algorithm, the following two linear interpolators are first defined:
Here, the interpolator function ITx represents a coordinate vector expression, with an x-component defined by the linear equation x=xT+(t−tT)·(xT+1−xT)/(tT+1−tT), and with y- and z-components involving similar expressions based on yT and zT, respectively. The interpolator function IRx involves similar expressions for receiver coordinates xR, yR, and zR.
Consider a signal transmission of duration TD sampled at a sufficient rate, w={w(ti)}i=0N
Consider a virtual broadcaster located at the point Pk. Introduce a sufficiently finely sampled time base {tj}j=0N
A further zero-padded linear interpolator Ib may be introduced for this broadcast:
Finally, a finely sampled time base {tr}r=0N
The proposed algorithm involves repeatedly calculating the nested interpolation operations in eq. (12) through (17) for each time sample tr
In the method in
The point spread function (PSF) for the presented surface mapping algorithm describes the image corresponding to a perfect point scatter source. For a transmitter 26 that is assumed stationary at QT=(0, 0, zsurface=0) and a receiver 46 that travels along a circular trajectory 54 described by QR(t)=(R·cos (φt), R·sin (φt), Zsurface=0) around the transmitter 26, one arrives at a circular synthetic aperture model. Here, R represents the radius and φ the polar angle of the circular trajectory 54.
An experimentally determined PSF image 82a for such configuration with R=1000 metres and a 100 kbit probing message transmitted at 1 kHz bitrate is shown in
The leakage curve of ambiguity points 80c, which is circular in case the scatter source is assumed to be located at the centre voxel Pk=(0, 0, ζ) as shown in
Consider the first platform 20 with transmitter 26 and second platform 40 with receiver 46, with each platform traversing its own closed loop trajectory 34, 54. Consider a point Pk=(x,y,z) for which an image is being computed. A signal transmitted at any moment of time tT traverses the path QT (tT)→Pk→QR(tR) in the time Δtf=Δt⬇+ΔtT, so tR=tT+Δtf. From geometrical considerations, it follows that Pk ϵQ={q Δ| t(QT→q→QR)=Δtf}. Q is called the locus of points for which the time of flight Δtf is the same. Intersection of this 3D surface with the plane ζ=z gives an ellipse-like 2D curve, designated as FT. Consider the moment of time tT+Δt, where Δt is the time resolution available for the system. Consider the same ellipse-like curve as before, computed for this moment of time. This curve FT+Δt will intersect FT in two points: P=(x,y,z) or the point being imaged, and a further point {tilde over (P)} that is designated herein as the “ghost point”. The leakage curve is formed by the union of the ghost points 180c. To obtain the leakage curve for given trajectories of the transmitter 26 and the receiver 46 and the point Pk being imaged is therefore tantamount to computing the locations of the second intersection points.
The method may be enhanced by increasing the correlation sensitivity for a potential scatter source at the central surface portion 72a, based on suppression of the ambiguity using steerable nulling of received response signals. For this purpose, the second ASV 40 may include at least one further acoustic receiver 47, which is configured to receive a further response signal (see
The present invention may be embodied in other specific forms. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. It will be apparent to the person skilled in the art that alternative embodiments of the invention can be conceived and reduced to practice. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope, to the extent permitted by national law.
The example of
In the example, the emitter 26 and receiver 46 were located on distinct ASVs 20, 40, but this is not essential. In alternative embodiments, both transmitter and receiver may be provided on the same vehicle or platform, provided the receiver is designed so as not to saturate by the transmitter signal, for instance by adequate signal isolation and/or a very high dynamic range of the receiver. In another embodiment using a single vessel the transmitter will interrupt the transmission of the continuous probing signal during certain periods of time and those periods of time will be used by the receiver to receive the response signal.
In a further example, migration could be used to reduce interference between transmitter and receiver. This would be applicable both when the receiver and the transmitter are in different AVs and when the receiver and the transmitter are in the same AV. Moreover, a transducer could be used such that the transducer would transmit the probing signal and received the reflected signal. By using migration, high correlation gain through with the received signal (wide bean) is obtained, as compared to physical beamforminrg with a narrow beam system. This allows the system to be more robust against interferences.
In another example, a geometric mean could be used to cancel out leakage. To avoid defocusing, data obtained from two or more different trajectories could be used to calculate two or more different maps, by means of one or more extra sensor(s) in the water volume. The data from the two different trajectories could be obtained by using two different AVs wherein each one moves following a different trajectory than the other one. For instance, one AV moves forming a circular trajectory having a radius d2 and the other AV would move following another circular trajectory having a radius d2 wherein d1 and d2 are different. Alternatively, the data from the two different trajectories could be obtained by using a single AV that moves first following one trajectory than then following another trajectory. For instance, the AV may move first forming a circular trajectory having a radius d1 and after would move following another circular trajectory having a radius d2 wherein d1 and d2 are different.
In another example, the duty cycle of continuous PRN code may be 100% as compared to the usual duty cycle of 1% to 10% for pulse based systems. This allows to obtain a better image of the seabed as more energy is concentrated in the water column.
In a further example, a waveform representation may be used for the probing signal such that is represented as complex signals comprising an imaginary and a real part wherein time delays will be represented as signal shifts. This allows to apply baseband processing to the received signals thereby providing a simpler system.
In the above examples, the system and method were employed in acoustic mapping/imaging of subsea surfaces, for instance at a depth of 100 metres of more. It should, however be understood that the method may also be employed at shallower depths. Alternatively or in addition, the method may also be applicable to sub-bottom imaging, although the achievable depth range is expected to be smaller in that case. In that case, soundwaves may be used to penetrate the seabed.
The received signals may be weighed such that e.g. strong specular and much weaker diffuse reflections are balanced in the correlation so that signals received from all points on the trajectory are included properly to avoid that the specular reflections are dominant.
In a further example, multiple carriers with possibly overlapping bandwidths may be used to modulate the probing signal in order to solve the water column problem.
In yet alternative embodiments, the method can be employed in acoustic imaging of subsea objects, in detection of unexploded ordinance, boulders, buried cables or pipes, in seismic imaging (i.e. earth subsurface mapping), in seismic cone penetration testing, in medical acoustic imaging, in radar mapping, or in ground penetrating radar.
Number | Date | Country | Kind |
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2028276 | May 2021 | NL | national |
The present application is a National Stage entry of International Application No. PCT/NL2022/050279, filed on May 20, 2022, which claims priority to Netherlands Application Number 2028276, filed on May 21, 2021, both of which are incorporated by reference in their entireties.
Filing Document | Filing Date | Country | Kind |
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PCT/NL2022/050279 | 5/20/2022 | WO |