CN109283495B - X-band radar ocean current inversion method based on cross spectrum analysis - Google Patents

X-band radar ocean current inversion method based on cross spectrum analysis Download PDF

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CN109283495B
CN109283495B CN201811035048.0A CN201811035048A CN109283495B CN 109283495 B CN109283495 B CN 109283495B CN 201811035048 A CN201811035048 A CN 201811035048A CN 109283495 B CN109283495 B CN 109283495B
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CN109283495A (en
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何宜军
陈忠彪
张彪
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Nanjing University of Information Science and Technology
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    • 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
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    • 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
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Abstract

The invention provides an X-band radar ocean current inversion method based on cross spectrum analysis, which comprises the steps of firstly, respectively carrying out cross spectrum analysis on two adjacent images in an X-band radar image to obtain a coherence coefficient spectrum and a phase spectrum; then, respectively averaging the coherence coefficient spectrum and the phase spectrum, determining the dominant wave number according to the peak value of the coherence coefficient spectrum, and eliminating the direction ambiguity of the dominant wave direction by using the average phase; and selecting different wave directions, establishing a model according to the phase velocity obtained by the frequency dispersion relation, and solving by using a least square method to obtain the ocean current vector. The method can solve the problems that the X-band radar has low accuracy in observing the sea surface flow field and a set of coherent X-band radar cannot obtain a vector flow field in the prior art, so as to meet the requirements of business observation of offshore marine environment, marine production and life service and the like.

Description

X-band radar ocean current inversion method based on cross spectrum analysis
Technical Field
The invention belongs to the technical field of ocean remote sensing, and particularly relates to an X-band radar ocean current inversion method based on cross spectrum analysis.
Background
The sea surface flow field is an important ocean dynamic parameter, which not only influences activities such as ocean engineering, ship navigation, ocean cultivation and entertainment, but also is closely related to the occurrence, development and the like of ocean disasters such as ocean oil spill pollution, red tide and the like which are increasingly serious in recent years, so that the ocean current observation has important significance. Current meters and Acoustic Doppler Current Profilers (ADCPs) are widely used for the observation of sea currents, but they can only obtain point or line sea current changes, are difficult and expensive to install and maintain, and may not be used especially in the complex sea areas or extreme sea conditions of interest. Satellite remote sensing can obtain ocean current of a large area, but a satellite-borne radar altimeter can only obtain ground-turn current, a synthetic aperture radar can only measure radial flow velocity, time and space resolution are poor, and a satellite has a long repeated access period, so that the ocean current observation requirement of offshore business cannot be met. The X-band radar has high time and space resolution, can observe the sea surface all day long and all weather, and is widely applied to the observation of ocean currents in recent years.
At present, mainly shipborne navigation X-band radar and coherent X-band radar are used for offshore ocean current observation. The algorithm for inverting the ocean current by using the non-coherent navigation X-band radar image is mainly based on the three-dimensional Fourier transform algorithm proposed by Young et al (1985)[1-4]However, it requires the conversion of the radar image spectrum into a sea wave spectrum using an empirical modulation transfer function, which is associated with the radar system,Sea conditions, installation conditions and other factors are related and difficult to accurately determine, and the form difference of modulation transfer functions obtained by using different experimental data and simulation methods is large[4,5]. The coherent radar can record the strength and phase information of sea echo, obtain the radial velocity of sea from the phase of echo according to Doppler effect, but the radial velocity is related to ocean current, wave motion and the like, so that ocean current is not easy to separate, and more than two sets of radars are required to synchronously observe to obtain vector flow field[2]. In addition, due to the limitation of a radar system, the two radars have the problems of aliasing of time sampling, attenuation of radar images along with distance and direction and the like, and the factors limit the observation accuracy of ocean current. Therefore, the invention provides a simple and feasible method for inverting a sea surface flow field by using an X-band radar, and aims to solve the problems that in the prior art, coherent and non-coherent X-band radars have poor accuracy in observing the sea flow, a vector flow field cannot be obtained by using only one set of coherent radars and the like.
Reference documents:
[1]I.R.Young,W.Rosenthal,and F.Ziemer,"A three-dimensional analysis of marine radar images for the determination of ocean wave directionality and surface currents,"Journal of Geophysical Research,vol.90,pp.1049-1059,1985.
[2] trizna Dennis, Method and apparatus for coherent marine radar measurements of properties of ocean waves and currents, application No.: US20100868912, U.S. patent, filing date: 2010.8.26
[3] Wu stambin et al, an X-band wave-measuring radar ocean current inversion preprocessing method, application No.: CN201310029849.7, china, application date: 2013.1.25
[4]J.C.Nieto-Borge,G.R.Rodriguez,K.Hessner,and P.I.Gonzalez,"Inversion of Marine Radar Images for Surface Wave Analysis,"Journal of Atmospheric and Oceanic Technology,vol.21,pp.1291-1300,2004.
[5]Zhongbiao Chen，Biao Zhang，Yijun He，Zhongfeng Qiu，Perrie William，A New Modulation Transfer Function for Ocean Wave Spectra Retrieval from X-band Marine Radar Imagery，Chinese Journal of Oceanology and Limnology.Vol.33No.5,P.1132-1141,2015.
Disclosure of Invention
The purpose of the invention is as follows: the invention provides a simple and feasible method for inverting a sea surface flow field by using an X-band radar image, and aims to solve the problems that in the prior art, the accuracy of an X-band radar for observing sea current is low, and a set of coherent X-band radar cannot obtain a vector flow field.
The technical scheme is as follows: the invention provides an X-band radar ocean current inversion method based on cross spectrum analysis, which specifically comprises the following steps:
step 1: selecting an X-waveband radar image sequence I, and respectively performing cross spectrum analysis on two adjacent images to obtain a coherence coefficient
Figure GDA0003545122210000021
Figure GDA0003545122210000022
And phase
Figure GDA0003545122210000023
Φij(k,φ)＝arg(Sij),i,j＝1,2,…,n (2)
Wherein Ii(x, y) are intensity values of the radar image, x and y are abscissa and ordinate of the center point of the image, i and j are the sequence of images, n is the number of images, k and
Figure GDA0003545122210000031
forming frequency domain coordinates, K is the wave number,
Figure GDA0003545122210000032
is the wave direction, SijIs an image Ii(x, y) and IjPower spectrum of (x, y), SiiIs Ii(x, y) and its own power spectrum, and arg () represents the argument of a complex number.
Step 2: coherence coefficient according to step 1
Figure GDA0003545122210000033
And phase
Figure GDA0003545122210000034
Performing cross spectrum analysis on all adjacent images to obtain average coherence coefficient
Figure GDA0003545122210000035
Figure GDA0003545122210000036
And average phase
Figure GDA0003545122210000037
Figure GDA0003545122210000038
And step 3: determining the dominant wavenumber k according to the peak value of the coherence coefficient in the step 1mDetermining a real main wave number by using the average phase in the step 2 for eliminating the direction ambiguity of 180 degrees; the 180 degree directional blur is the two symmetrical maxima of the average coherence coefficient in equation 3 about the far point.
And 4, step 4: selecting a main wave direction phimAll wave directions phi of different directions nearbypP is the wave direction phimThe number of all nearby wave directions in different directions is determined, and a model is established;
Figure GDA0003545122210000039
solving equation 5 by using least square method to obtain ocean current vector
Figure GDA00035451222100000310
A value of (d);
wherein, cradarIs wave direction
Figure GDA00035451222100000311
The phase velocity of the directional wave,
Figure GDA00035451222100000312
ctheoryobtaining a theoretical value of the phase velocity according to the dispersion relation theory of sea waves
Figure GDA00035451222100000313
Wherein g is the acceleration of gravity; h is the depth of the water,
Figure GDA00035451222100000314
the time interval between every two adjacent images is the same;
Figure GDA0003545122210000041
is a wave number vector; q ═ 1, 2, 3.
Further, the method for determining the true dominant wave number in step 3 to eliminate the 180-degree directional blur includes: select to satisfy
Figure GDA0003545122210000042
Peak wave number of
Figure GDA0003545122210000043
Is the true dominant wavenumber.
Further, the wave direction phi selected in the step 4pThe wave direction phipSimultaneously satisfies the following requirements:
1，φpthe coefficient of coherence gamma (k, phi)p) > 0.6, wherein the wavenumber k satisfies: k is more than or equal to 0.03 and less than or equal to 0.3 tad/m;
2，|φp-φm|＜30°，
Figure GDA0003545122210000044
is the main wave direction.
Has the advantages that: the method is simple and easy to implement, and can solve the problems that the X-band radar has low accuracy in observing the sea surface flow field and a set of coherent X-band radar cannot obtain a vector flow field in the prior art so as to meet the requirements of business observation, marine production and life service and the like of the offshore marine environment.
Drawings
FIG. 1 is a flow chart of the algorithm of the present invention;
FIG. 2 is a comparison of the wave phase velocity obtained from an X-band radar image in accordance with the present invention and the theoretical phase velocity.
Detailed Description
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the invention and not to limit the invention.
As shown in fig. 1, the present invention provides an X-band radar ocean current inversion method based on cross-spectrum analysis, which specifically includes the following steps:
step 1: selecting an X-waveband radar image sequence I, and respectively performing cross spectrum analysis on two adjacent images to obtain a coherence coefficient
Figure GDA0003545122210000045
Figure GDA0003545122210000046
And phase
Figure GDA0003545122210000047
Figure GDA0003545122210000048
Wherein Ii(x, y) are intensity values of the radar image, x and y are abscissa and ordinate of the center point of the image, i and j are the sequence of images, n is the number of images, k and
Figure GDA0003545122210000051
forming frequency domain coordinates, K is the wave number,
Figure GDA0003545122210000052
is the wave direction, SijIs an image Ii(x, y) and IjPower spectrum of (x, y), SiiIs Ii(x, y) power spectrum with itself, arg () representing the argument of the complex number;
step 2: coherence coefficient according to step 1
Figure GDA0003545122210000053
And phase
Figure GDA0003545122210000054
Performing cross spectrum analysis on all adjacent images to obtain an average coherence coefficient
Figure GDA0003545122210000055
Figure GDA0003545122210000056
And average phase
Figure GDA0003545122210000057
Figure GDA0003545122210000058
And step 3: determining the dominant wavenumber k according to the peak value of the coherence coefficient in the step 1mDetermining a real main wave number by using the average phase in the step 2 for eliminating the direction ambiguity of 180 degrees;
and 4, step 4: selecting a main wave direction phimAll wave directions phi of different directions nearbypP is the wave direction phimThe number of all nearby wave directions in different directions is determined, and a model is established;
Figure GDA0003545122210000059
solving equation 5 by using least square method to obtain ocean current vector
Figure GDA00035451222100000510
A value of (d);
wherein, cradarIs wave direction
Figure GDA00035451222100000511
The phase velocity of the directional wave,
Figure GDA00035451222100000512
ctheoryobtaining a theoretical value of the phase velocity according to the dispersion relation theory of sea waves
Figure GDA00035451222100000513
Wherein g is the acceleration of gravity; h is the depth of the water,
Figure GDA00035451222100000514
the time interval between every two adjacent images is the same;
Figure GDA00035451222100000515
is a wave number vector; q ═ 1, 2, 3.
It should be noted that, in the step 3, determining the true dominant wave number is used to eliminate the directional ambiguity of 180 degrees, and the method for selecting the true dominant wave number is as follows: select to satisfy
Figure GDA0003545122210000061
Peak wave number of
Figure GDA0003545122210000062
Is the true dominant wavenumber.
The wave direction phi selected in the step 5pThe wave direction phipAt the same time satisfyThe following requirements are set forth:
1，φpthe coefficient of coherence gamma (k, phi)p) > 0.6, wherein the wavenumber k satisfies: k is more than or equal to 0.03 and less than or equal to 0.3 tad/m;
2，|φp-φm|＜30°，
Figure GDA0003545122210000063
is the main wave direction.
As shown in fig. 2, the comparison between the wave phase velocity obtained by using an X-band radar image and the theoretical phase velocity shows that the two have better consistency, so that a vector flow field can be obtained by using least square fitting.

Claims (3)

1. An X-band radar ocean current inversion method based on cross spectrum analysis is characterized by comprising the following steps:
step 1: selecting an X-waveband radar image sequence I, and respectively performing cross spectrum analysis on two adjacent images to obtain a coherence coefficient
Figure FDA0003545122200000011
Figure FDA0003545122200000012
And phase
Figure FDA0003545122200000013
Φij(k,φ)＝arg(Sij),i,j＝1,2,…,n (2)
Wherein Ii(x, y) are intensity values of the radar image, x and y are abscissa and ordinate of the center point of the image, i and j are the sequence of images, n is the number of images, k and
Figure FDA0003545122200000014
forming frequency domain coordinates, K is the wave number,
Figure FDA0003545122200000015
is the wave direction, SijIs an image Ii(x, y) and IjPower spectrum of (x, y), SiiIs Ii(x, y) power spectrum with itself, arg () representing the argument of the complex number;
step 2: coherence factor according to step 1
Figure FDA0003545122200000016
And phase
Figure FDA0003545122200000017
Performing cross spectrum analysis on all adjacent images to obtain average coherence coefficient
Figure FDA0003545122200000018
Figure FDA0003545122200000019
And average phase
Figure FDA00035451222000000110
Figure FDA00035451222000000111
And step 3: determining the dominant wavenumber k according to the peak value of the coherence coefficient in the step 1mDetermining a true main wave number by using the average phase in the step 2 for eliminating 180-degree direction ambiguity;
and 4, step 4: selecting a main wave direction phimAll wave directions phi of different directions nearbypP is the wave direction phimThe number of all nearby wave directions in different directions is determined, and a model is established;
Figure FDA0003545122200000021
solving equation 5 by using least square method to obtain ocean current vector
Figure FDA0003545122200000022
A value of (d);
wherein, cradarIs wave direction
Figure FDA0003545122200000023
The phase velocity of the directional wave,
Figure FDA0003545122200000024
ctheoryobtaining a theoretical value of the phase velocity according to the dispersion relation theory of sea waves
Figure FDA0003545122200000025
Wherein g is the acceleration of gravity; h is the depth of the water,
Figure FDA0003545122200000026
the time interval between every two adjacent images is the same;
Figure FDA0003545122200000027
is a wave number vector; q ═ 1, 2, 3, …, p.
2. The method for X-band radar ocean current inversion based on cross-spectral analysis according to claim 1, wherein the true dominant wavenumber determined in step 3 is used for eliminating 180-degree directional ambiguity, and the method for selecting the true dominant wavenumber is as follows: select to satisfy
Figure FDA0003545122200000028
Peak wave number of
Figure FDA0003545122200000029
Is the true dominant wavenumber.
3. The method for X-band radar ocean current inversion based on cross-spectral analysis according to claim 1, wherein the wave direction φ selected in the step 4pThe wave direction phipSimultaneously satisfies the following requirements:
1,φpthe coefficient of coherence gamma (k, phi)p) > 0.6, wherein the wavenumber k satisfies: k is more than or equal to 0.03 and less than or equal to 0.3 rad/m;
2,|φp-φm|＜30°，
Figure FDA00035451222000000210
is the main wave direction.
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