AU2016228476B2

AU2016228476B2 - Method for calculating a confidence echo signal that is exempt from multipath propagation effects and for determining a distance and/or a direction to an echo source and device and vehicle

Info

Publication number: AU2016228476B2
Application number: AU2016228476A
Authority: AU
Inventors: Aneta Nikolovska
Original assignee: Atlas Elektronik GmbH
Current assignee: Atlas Elektronik GmbH
Priority date: 2015-03-06
Filing date: 2016-02-09
Publication date: 2018-09-13
Anticipated expiration: 2036-02-09
Also published as: EP3265839B1; IL253456B; IL253456A0; KR20170103885A; AU2016228476A1; EP3265839A1; WO2016141918A1; DE102015103315A1; KR101948459B1

Abstract

The invention relates to a method for calculating a confidence echo signal V(t) that is exempt from multipath propagation effects, in particular a confidence underwater echo signal, said method comprising the following steps: - determining a time dependent measurement echo signal M(t), which has a superimposition of the confidence echo signal V(t) and a multipath echo signal S(t), such that in particular the measurement echo signal has an interference, - calculating an envelope curve U(t) of the measurement echo signal M(t), - calculating a period T of the envelope curve U(t) and using the measurement echo signal during an initial time period, which comprises a time span a*T, wherein a is a confidence factor less than or equal to 1 and T is the calculated period, such that the confidence echo signal V(t) that is exempt from multipath propagation effects is given for [t=0 to a*T].

Description

METHOD FOR CALCULATING A CONFIDENCE ECHO SIGNAL THAT IS FREE OF

MULTIPATH PROPAGATION EFFECTS AND FOR DETERMINING A DISTANCE AND/OR A DIRECTION TO AN ECHO SOURCE, AS WELL AS A DEVICE AND VEHICLE

The invention relates to a method for calculating a confidence echo signal V(t) - in particular, a confidence underwater echo signal - free of multipath propagation effects and a method for calculating a distance and/or a direction to an echo source, wherein the method for determining a confidence echo free of multipath propagation effects also includes a device and a vehicle .

Underwater sound waves propagating in the sea are disturbed, in particular, by inhomogeneities and multipath propagation effects. In particular, for example, an underwater sound wave can be superimposed by a component which is backscattered at the sea surface (and/or the seabed). These effects result in, for example, a source position detection being encumbered with direction and distance errors. This means that measured underwater echo signals can include instances of interference which can be cleaned up only with difficulty (computationally time-consuming) or not at all.

In particular, when surveying objects below sea level such as, for example, the foundations of off-shore wind turbines or sunken archaeological sites, an accurate survey is required so that a realistic image of the scanned area can be obtained.

The object of the invention is to improve the state of the art.

The problem is solved by a method for calculating a confidence echo signal V(t) free of multipath propagation effects - in particular, a confidence underwater echo signal - said method having the following steps:

- Determining a time-dependent measurement echo signal M(t) which has a superimposition of the confidence echo signal

V(t) and a multipath echo signal S(t) such that, in particular, the measurement echo signal M(t) has an interference,

- Calculating an envelope curve U (t) of the measurement echo signal M(t),

- Calculating a period T of the envelope curve U(t), and

- Using the measurement echo signal during an initial time period which comprises a time span a*T, where a is a confidence factor less than or equal to 1 and T is the calculated period, such that the confidence echo signal V(t) which is free of multipath propagation effects is given for [t=0 to a*T] .

A signal can thus be formed from which interference arising from multipath propagation effects can be eliminated. On the basis of this signal, a further processing step can then be carried out which, for example, determines a direction (synonymous with bearing) or the distance to an echo source.

It will be useful to clarify certain terms:

A 'multipath propagation effect' is, in particular, an interference with or a superimposition onto a water echo wave, which occurs due to the water echo source itself. This occurs, in particular, due to a reflected portion of the underwater echo wave being superimposed due to the (under) water echo wave itself. In this way, a component of the water echo wave reflected at the water surface can interfere with the directly emitted part of the water echo wave.

A 'confidence echo signal V(t)' is, in particular, a measurement signal or a processed signal which is effectively 'superimposition-free' or 'interference-free.' Consequently, it is a cleaned signal, at least as far as multipath propagation effects are concerned, which is available for further processing .

By 'determining a time-dependent measurement echo signal M(t)' is meant, in particular, the conversion of a pressure change into an electrical signal by means of an underwater transducer. The electrical signal can thereby be processed, and thus signalconditioned. Hydrophones such as piezoceramic elements can, in particular, be used as acoustic transducers. The measurement echo signal has, in particular, an amplitude which changes over time.

The 'confidence echo signal V(t) ' is, in particular, that part of the (measurement echo) signal in which it can be assumed that there are no or only minor effects arising from multipath propagation effects.

Here, a core concept of the invention is that an interfering multipath signal generally requires more time to reach the receiving acoustic transducer, so that these effects impinge temporally after the arrival of the confidence echo signal. In particular, due to the periodicity of the envelope curve, a time value can be determined by which it can be ruled out that multipath propagation effects are already present.

A 'measurement echo signal S (t) ' is, in particular, an echo signal which is not sent directly from an echo source to the acoustic transducer, but which is, for example, reflected at water layers or at the sea surface, and thus leads to certain interference or beat effects.

The 'envelope curve U(t)' is, strictly speaking, mathematically a curve which touches at one point every curve (measured acoustic frequency) of an array of curves (received (total) measurement echo signals). Due to the interference, a slight increase or decrease occurs in the respective maximum amplitudes of adjacent oscillations of the measurement echo signal, such that a periodic pattern forms. The approximated course of this pattern is, in the present case, called the envelope curve and is described mathematically by the function U (t) . A 'period Τ' can be assigned to this envelope curve U(t) . This period T can be a constant, or even an averaged, value. The core of the inventive concept here is that, in the interval between t = 0 and t = T, no, or only minor, influences on the measurement echo signal S(t) arise.

An 'initial time period' resulting from this is effectively a confidence period by means of which the measurement signal is not, or only slightly, disturbed.

In order to be quite sure that the measurement signal is not influenced by any interference from the measurement echo signal S (t) , a 'confidence factor' is introduced which has a value of <1. The maximum time period of the initial time period thus comprises the time value T, which corresponds to the 'calculated period. ' The start of the initial time period, and thus the definition for t=0, is, in particular, set on the basis of a threshold amplitude, which uses, for example, upwards of 5% of a maximum signal of the measurement signal.

In a further embodiment, the confidence factor has a value between 0.1 and 0.7 - in particular, between 0.2 and 0.5, or, in particular, between 0.25 and 0.4. In particular, for values around approximately 0.3, it appears that there are virtually no multipath propagation effects, so that this signal can be rated as a confidence echo signal V(t).

process. This mathematical as 'fitting.' Fitting, or

To be able to determine as quickly and simply as possible the period of the envelope curve U(t), it can be calculated by means of a mathematical adjustment adjustment is generally known mathematical adjustment, is also classified under the heading of regression calculation and refers to a mathematical optimization method used for determining (estimating) a series of measurement data, the unknown parameters of a geometric-physical model, or the parameters of a given function. In particular, it can be assumed of the envelope curve in the present case that it is a periodic function such as, for example, a sine function. The period T can be determined with the aid of this fitted sine function .

In a further embodiment, the time-dependent measurement echo signal M(t) is determined using a hydrophone.

Here, the hydrophone is, in particular, an acoustic transducer which converts pressure differences at the hydrophone into electrical signals. Hydrophones of this kind can, for example, be realized with piezoceramic elements.

To determine distance and direction (synonymous with bearing), the time-dependent measurement signal can be determined by means of further hydrophones, wherein all of the hydrophones together form, in particular, a sonar antenna. Sonar antennas of this kind can, for example, take the form of linear antennas or be converted accordingly into a signal corresponding to the linear antenna, or be converted mathematically or computationally in such a way that signals of a linear antenna are obtained.

In a further aspect of the invention, the problem is solved by a method for determining a distance and/or a direction to an echo source, wherein the method includes a method for determining a confidence echo signal V(t) free of multipath propagation effects in accordance with one of the preceding claims, wherein the method has the following additional steps:

- Determining an echo source direction by means of the confidence echo signal V (t) free of multipath propagation effects,

- Successive mathematical curving of the sonar antenna by means of the confidence sound signal V(t) free of multipath propagation effects in the determined echo source direction,

- Determining a signal maximum and the associated radius of curvature, and

- Determining distance using the determined radius of curvature .

In this way, an object can, in particular, be surveyed underwater. Furthermore, the direct path to an underwater object can, for example, be determined for the purposes of maintaining a foundation of a wind turbine or drilling tower, or searching for a wreck. This reduces the time needed for locating the object and increases, for example, the effective working time for divers.

It will be useful to clarify certain terms:

The 'distance' is, in particular, a distance from the antenna to the object being located. In particular, the center of the antenna is used here as a starting point. Other points can also be used, but then they need to be determined computationally.

The 'direction' is also referred to as the bearing. A usual indication is from the aforementioned starting point from an angle - in particular, in the case of a linear antenna - to an orthogonal of the linear antenna through the starting point. This means that, when the object is directly ahead, the bearing will, for example, be 0.

'Determining an echo source direction' is effected, in particular, by a mathematical pivoting of a (virtual) linear antenna about the starting point. Here, the signals at the hydrophones are added together in each case for individual bearing angles, or, for example, the integrated amplitudes of the hydrophones are in each case added together, and it can be assumed that the maximum signal is present at the angle (at the bearing) when an orthogonal to the linear antenna points directly to the echo source.

The mathematical rotation is here, in particular, carried out in steps - for example, in each case by 1°. In the present case, the point of rotation is once again, in particular, the starting point for which the distance to an echo object is then subsequently determined.

Instead of the amplitude, it is, of course, also possible to calculate the area below the mathematical measurement echo signal, this also being a measure of the energy. Further derived variables of the amplitude or of the amplitude curve can also be used.

'Successive mathematical curving' means that, in the case of the (virtual) linear antenna being directed at the echo source (bearing 0°, for example), the individual hydrophones of the (virtual) linear antenna are time-shifted with respect to each other in such a way that they correspond to an arc segment which can be described by a radius. This radius (of curvature) then corresponds to the distance to the echo object. In successive mathematical curving, the totalized amplitude signal present for each individual curvature is, in particular, once again determined, wherein, once again, the maximum signal - the correct curvature - effectively represents an excellent estimate of the distance (radius of curvature).

In a further aspect, the problem is solved by a device which is set up in such a way that one of the previously described methods can be carried out. In particular, such a device comprises a sonar. Not only sonars with linear antennas, but also with other bases, such as, for example, cylindrical bases, are possible here. In the case of the sonars, it is possible to use both towed and side-scan sonars or the like. The actual implementation can then be realized computationally, e.g., by FPGA or by a software code, which is converted accordingly by a computer .

In a further aspect, the problem is solved by a vehicle - in particular, a watercraft - which is equipped with the previously described device. Of course, helicopters or the like can also be provided here, provided they have, for example, a sonar antenna.

The invention is explained in more detail below with reference to an exemplary embodiment. The figures show:

Figure 1 a schematic representation of a multipath propagation effect, and

Figure 2 a highly schematic representation of a time signal at a single hydrophone and the associated mathematical envelope curve.

Let object 101 be, in the present case, an AUV (Autonomous Underwater Vehicle), which is to be monitored with the flank-array sonar antenna 103. The flank-array sonar antenna has a multiplicity of hydrophones.

The propeller of the AUV 101 here radiates underwater echo signals as direct sound waves 105. These sound waves 105 are reflected, in particular, at the water surface 109, so that underwater sound waves 107 reflected at the water surface are superimposed on the direct sound wave 105. The superimposition of the waves is then measured electronically and processed by the flank-array sonar antenna 103 and the associated hydrophones .

Due to the superimposition of the direct underwater sound waves 105 and of the sound waves 107 reflected at the water surface, a superimposition (interference) of the signals occurs, which is, for example, measured by a single hydrophone of the flank-array sonar antenna 103. Such a single hydrophone signal is shown, in particular, in Figure 2.

Due to the interference, a periodic structure is, in each case, formed at the amplitude maxima and minima. This periodic structure is adjusted by an envelope curve 223 which has been fitted mathematically. In the present case, it is assumed that the fit function is a harmonic sine function. On the basis of the parameters found for this sine function, the period T of the fitted envelope curve 223 is determined.

In the present case, it is assumed that, up to a period T, a signal is essentially unaffected by the sound waves 107 reflected at the water surface. The time t=0 is set when the measured threshold amplitude A_TH lies at 3% of the first maximum. After this, the time course of the measurement signal 221 is used up to a confidence amplitude A_v at time point t=T, so that a confidence sound signal V(t) is present for the time interval t=0 to t=T.

In the present case, the confidence amplitude A_v is determined for all hydrophones, for the sake of simplicity. The flank-array sonar antenna 103 is then mathematically rotated about its center. In addition, the sum of the confidence amplitudes A_v for all hydrophones is determined for each angle of rotation (step width 0.1°) . At the maximum value which results for the sum of the confidence amplitudes A_v determined across all hydrophones, it is assumed that the mathematically rotated flank-array sonar antenna 103 is oriented orthogonally to the AUV 101.

In this position, the mathematically rotated flank-array sonar antenna 103 is now mathematically curved. As with the mathematical rotation, this curving is also achieved by the individual signals of the hydrophones being evaluated on a timeshifted basis. Curving then takes place step by step (successively) . In the present case, it is, of course, also possible to set first a small and then a large radius of curvature, so as to approximate an optimal result more quickly. This, too, is understood as successive mathematical curving.

Once again, the sum of the resulting confidence amplitudes A_v of the individual hydrophones of the flank-array sonar antenna 103 is determined, and it is assumed that, with a maximum value, the curvature of the flank-array sonar antenna 103 lies on a circular arc with a radius of curvature, so that the radius of curvature corresponds to the distance to the AUV 101, since it is assumed that the underwater sound waves propagate essentially spherically.

of reference numbers

Object / AUV

Flank-array sonar antenna

Direct sound waves

Sound waves reflected at the water surface

Surface of the water

Threshold amplitude

Amplitude

Confidence amplitude

Period

Time

Measurement signal

Fitted envelope curve

Claims

Claims :

1. Method for calculating a confidence echo signal V(t) free of multipath propagation effects - in particular, a confidence underwater echo signal - said method having the following steps :

-Determining a time-dependent measurement echo signal M(t) which has a superimposition of the confidence echo signal V(t) and a multipath echo signal S (t) , such that, in particular, the measurement echo signal M(t) has an interference,

-Calculating an envelope curve U (t) of the measurement echo signal M(t),

- Calculating a period T of the envelope curve U(t), and

- Using the measurement echo signal during an initial time period which comprises a time span a*T, where a is a confidence factor less than or equal to 1 and T is the calculated period, such that the confidence echo signal V(t) which is free of multipath propagation effects is given for [t=0 to a*T].
2. Method according to Claim 1, characterized in that the confidence factor has a value between 0.1 and 0.7 - in particular, between 0.2 and 0.5, or, in particular, between 0.25 and 0.4.
3. Method according to one of the preceding claims, characterized in that the envelope curve is produced by a mathematical fit procedure.
4. Method according to one of the preceding claims, characterized in that the time-dependent measurement echo signal M(t) is determined by means of a hydrophone.

2016228476 10 Aug 2018
5. Method according to Claim 4, characterized in that the timedependent measurement echo signal M(t) is determined by means of further hydrophones, wherein all of the hydrophones together form, in particular, a sonar antenna.
6. Method for determining a distance and/or a direction to an echo source, wherein the method includes a method for determining a confidence echo signal V(t) free of multipath propagation effects in accordance with one of the preceding claims, wherein the method has the following additional steps:

-Determining an echo source direction by means of the confidence echo signal V(t) free of multipath propagation effects,

-Successive mathematical curving of the sonar antenna by means of the confidence sound signal V(t) free of multipath propagation effects in the determined echo source direction,

-Determining a signal maximum and the associated radius of curvature, and

-Determining distance using the determined radius of curvature.
7. Device - in particular, a sonar system - which is set up in such a way that a method according to any one claims 1 to 4 can be carried out.
8. Vehicle - in particular, a watercraft - which is equipped with a device according to Claim 7.