A method and system for removing 180 degree wave direction ambiguity from sea wave directional spectra measured by a spaceborne spectrometer

Abstract

The application discloses a 180-degree wave direction ambiguity removing method and system for a spaceborne wave spectrometer sea wave direction spectrum, and belongs to the field of microwave remote sensing. The direction and scanning time of an incident beam are controlled through an electric scanning detection mode of the spaceborne wave spectrometer, so that the beam direction is unchanged within a time interval of delta t, that is, the observation direction of the sea surface is unchanged, so as to ensure the correlation of the sea surface, and then to ensure that two wave signals for cross spectrum calculation have sufficient correlation. The 180-degree wave direction ambiguity can be removed by using the cross spectrum method, so that the inversion precision of the spaceborne wave spectrometer sea wave spectrum and sea wave parameters is improved, and then the application of the spaceborne wave spectrometer sea wave spectrum data is promoted. The application provides the 180-degree wave direction ambiguity removing method for the spaceborne wave spectrometer sea wave direction spectrum. Since the electric scanning detection mode increases the correlation of the wave signals detected at different times, the cross spectrum method can remove the 180-degree wave direction ambiguity in the sea wave direction spectrum.

Description

Technical Field

[0001] This invention relates to the field of microwave remote sensing, and more specifically, to a method and system for removing 180° wave direction ambiguity from a star carrier spectrometer. Background Technology

[0002] To meet the needs of global marine scientific research and applications, such as early warning of hazardous sea conditions and environmental forecasting, my country and France jointly developed the China-France Ocean Satellite CFOSAT, which was launched in 2018. Currently, the SWIM satellite's onboard carrier wave spectrometer can acquire large-area, long-term global wave spectrum information. Research and application of the wave spectrometer are of great significance for interpreting marine phenomena, studying marine applications, exploring marine development, and forecasting marine information.

[0003] The wave direction spectrum is a Fourier transform of the autocorrelation function of the wave surface function, which can characterize the distribution of wave energy in various wave directions and wave numbers within the detection range. We obtain the wave spectrum by performing a Fourier transform of the autocorrelation function on the echo signals received by SWIM during sea surface detection, and then extract the wave direction spectrum from the wave spectrum. The wave spectrum obtained by the Fourier transform of the autocorrelation function is symmetrically distributed in the wave number domain, so the resulting two-dimensional wave spectrum has a 180° wave direction ambiguity problem, that is, it will produce two symmetrical wave components with wave directions that differ by 180°.

[0004] When using the cross-spectral method to remove wave direction ambiguity, it is necessary to calculate the cross-spectrum between two wave signals with a time interval of Δt, and then determine the propagation direction of the waves. However, as can be seen from the above SWIM sea surface detection process, due to the flight of the satellite and the rotation of the radar detection antenna, the radar footprint overlap rate is low and the signal coherence is weak when the same beam is used to detect the sea surface, resulting in low accuracy in extracting wave direction information. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method and system for removing 180° wave direction ambiguity in the wave direction spectrum detected by a star carrier spectrometer, thereby solving the problem of 180° wave direction ambiguity in the wave direction spectrum detected by existing star carrier spectrometers.

[0006] To achieve the above objectives, this invention provides a method for removing 180° wave direction ambiguity from a star carrier spectrometer, comprising the following steps:

[0007] S1. Using the detection method of satellite carrier spectrometer electrical scanning, the beam direction is kept constant within the time interval Δt, and the sea surface is detected at times t and t+Δt respectively, and wave signals containing sea wave wave information are extracted;

[0008] S2. Extract the wave direction spectrum from the wave signal at time t to obtain the wave direction observation azimuth angle and the wave direction fuzzy observation azimuth angle that are 180° out of phase;

[0009] S3. Perform cross-spectral calculation on the wave signals at times t and t+Δt;

[0010] S4. Extract wave direction based on the characteristics of the imaginary part of the cross spectrum, and remove the 180° wave direction ambiguity from the wave direction spectrum.

[0011] Preferably, S3 includes: calculating the cross spectrum using the average periodogram method, and combining the fluctuation signals m at time t and time t+Δt. 1 (x,φ) and m 2 The signal (x, φ) is divided into several segments. The cross spectrum of each segment is calculated, and the average of the segments is taken as the cross spectrum. The asymptotically consistent unbiased estimate is given by x, where x is the ground distance, φ is the observation azimuth, and k is the wave number.

[0012] The cross spectrum is:

[0013]

[0014] in, Let be the cross spectrum of the i-th wave signal, u = 1, 2, ..., M, q = 1, 2, ..., Q. Let be the discretized wave signals at times t and t+Δt, respectively, where dx is the size of the sea surface element and Q is the length of the Fourier transform.

[0015] The imaginary part of the cross spectrum is:

[0016]

[0017]

[0018] Where ω is the angular frequency.

[0019] When the imaginary part of the cross spectrum is positive, the wave direction observation azimuth is the wave direction;

[0020] When the imaginary part of the cross spectrum is negative, the azimuth angle of the wave direction fuzzy observation is the wave direction.

[0021] The present invention also provides a system for removing 180° wave direction ambiguity from a star carrier spectrometer, comprising: a computer-readable storage medium and a processor;

[0022] The computer-readable storage medium is used to store executable instructions;

[0023] The processor is used to read the executable instructions stored in the computer-readable storage medium and execute the above-described method for removing 180° wave direction ambiguity from the star carrier spectrometer wave direction spectrum.

[0024] Compared with existing technologies, the technical solutions conceived in this invention require ensuring the correlation of radar echo signals to extract wave direction information more accurately. Therefore, this invention provides a scanning method distinct from mechanical scanning: electronic scanning. Electronic scanning allows for flexible control of the incident beam direction and scanning time, ensuring the beam direction remains unchanged within a time interval Δt, i.e., the observation direction of the sea surface remains constant, thus guaranteeing the correlation of the sea surface and ensuring sufficient correlation between the two wave signals used for cross-spectral calculation. Using the cross-spectral method to remove wave direction ambiguity can improve the inversion accuracy of wave spectra and wave parameters from satellite carrier spectrometers, thereby promoting the application of satellite carrier spectrometer wave spectrum data. This invention proposes a method for removing 180° wave direction ambiguity from the wave direction spectrum of satellite carrier spectrometers. Since the electronic scanning detection method increases the correlation of wave signals detected at different times, the cross-spectral method can remove 180° wave direction ambiguity from the wave direction spectrum. Attached Figure Description

[0025] Figure 1 This is a flowchart of 180° wave direction blur removal using the star carrier spectrometer provided in this embodiment of the invention;

[0026] Figure 2(a) shows the real part of the cross spectrum under wind and wave conditions;

[0027] Figure 2(b) shows the imaginary part of the cross spectrum under wind and wave conditions;

[0028] Figure 3(a) shows the real part of the cross spectrum under swell conditions;

[0029] Figure 3(b) shows the imaginary part of the cross spectrum under swell conditions. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0031] This invention discloses a method for removing 180° wave direction ambiguity from a satellite carrier spectrometer. By using an electrical scanning method to maintain the observation direction of the sea surface, sufficient correlation is ensured between the two wave signals used for cross-spectral calculation. For wave signals containing wave information with a time interval of Δt, the average periodogram method is used for cross-spectral calculation, and the spectral distribution characteristics of the real and imaginary parts of the cross-spectrum are analyzed. Utilizing the antisymmetric property of the imaginary part of the cross-spectrum, wave direction ambiguity in the real part is removed, thus determining the wave propagation direction.

[0032] like Figure 1 As shown, this invention provides a method for removing 180° wave direction ambiguity from a star carrier spectrometer, the method comprising the following steps:

[0033] S1. Using the detection method of satellite carrier spectrometer electrical scanning, the beam direction is kept constant within the time interval Δt, and the sea surface is detected at times t and t+Δt respectively, and wave signals containing sea wave wave information are extracted;

[0034] S2. Extract the wave direction spectrum from the wave signal at time t to obtain the wave direction observation azimuth angle and the wave direction fuzzy observation azimuth angle that are 180° out of phase;

[0035] S3. Perform cross-spectral calculation on the wave signals at times t and t+Δt;

[0036] S4. Extract wave direction based on the characteristics of the imaginary part of the cross spectrum, and remove the 180° wave direction ambiguity from the wave direction spectrum.

[0037] Specifically, in step S1, the beam direction is kept constant within the time interval Δt using an electronic scanning detection method. The sea surface is detected at times t and t+Δt, and wave signals containing wave wave information are extracted. At time t, the wave surface function of the sea surface can be expressed as follows:

[0038]

[0039] At time t+Δt, the wave surface function of the sea surface can be expressed as follows:

[0040]

[0041] Among them, a mn ω represents amplitude. mn ε represents angular frequency. mn Let ξ be the initial phase of the wavefront function, x and y be the coordinates of a point on the horizontal plane, ξ(x,y,t) represent the vertical height of the sea surface at point (x,y) at time t, and k be the initial phase of the wavefront function. m k n These represent the wave numbers of different wave components in the x and y directions, respectively.

[0042]

[0043] Where F(k) mx ,k ny ) is the wavelength spectrum F(k x ,k y dk is obtained by discretizing the wavenumber. x dk y The wavenumber interval is the discretized value.

[0044] In sea surface detection, traditional mechanical scanning relies on rotating the radar antenna to scan the beam. Therefore, when scanning the same incident beam, the time intervals differ significantly, by about 220 ms. Within this time interval, the rotation of the radar antenna alters the observation azimuth of the sea surface, resulting in weak correlation between the wave signals detected at two different times. The effectiveness of cross-spectral analysis in removing wave direction ambiguity depends on the correlation of the sea surface signals; the stronger the correlation, the better the wave direction ambiguity removal. Electronic scanning, unlike traditional mechanical scanning, uses electronic control to alter the beam direction, making it controllable. Therefore, by ensuring the beam direction is the same at different times, the same sea surface can be detected, enhancing the correlation of the wave signals detected at different times.

[0045] Specifically, within the incident angle range of the wave spectrometer, the normalized backscattering cross section σ0 of the sea surface can be expressed by a quasi-specular scattering model as follows:

[0046]

[0047] Where θ is the incident angle, φ is the observation azimuth angle, ρ is the Fresnel reflection coefficient when incident perpendicularly, and P(ξ) x ,ξ y ) is the joint slope probability density function of the wave surface function ξ. In a wave spectrometer, ξ x =tanθ, ξ y =0 represents the slopes in the distance and azimuth directions on the sea surface, respectively.

[0048] The radar wave signal is a weighted average of the azimuth direction, which can be expressed by the following formula:

[0049]

[0050]

[0051] in, This represents the backscattering coefficient of the radar without long-wave tilt modulation. Indicates the gain in the antenna azimuth direction. This is the antenna azimuth angle.

[0052] Step S2. Extract the wave direction spectrum from the wave signal at time t to obtain the wave direction observation azimuth and the wave direction ambiguity observation azimuth, which are 180° out of phase. At time t, the wave direction spectrum can be expressed as:

[0053]

[0054] Where x is the ground distance, φ is the observation azimuth angle, k is the wave number, and ζ is the ground distance interval.

[0055] Since the wave signal is a real signal, its autocorrelation function undergoes a Fourier transform to become a real even function, meaning the wave direction spectrum is 180° symmetrical. The peak values ​​of the wave direction spectrum correspond to two observation azimuths that differ by 180°, namely the wave direction observation azimuth and the wave direction ambiguity observation azimuth.

[0056] Step S3. Perform cross-spectral calculation on the wave signals at times t and t+Δt.

[0057] Specifically, the cross spectrum is calculated using the average periodogram method, which combines the two wave signals m at time t and t+Δt. 1 (x,φ) and m 2 The signal (x, φ) is divided into several segments. The cross spectrum of each segment is calculated, and the average result of all segments is taken as the cross spectrum. The asymptotically consistent unbiased estimate. Using the average periodogram method can reduce the variance of the fluctuation spectrum.

[0058] The specific processing procedure is as follows:

[0059] Assume the two wave signals at time t and t+Δt are m 1 (x,φ) and m 2 Discretize (x,φ) and divide it into M segments. Process each segment using the cross-spectral method. Then the cross-spectral spectrum of the i-th segment is:

[0060]

[0061] i = 1, 2, ..., M, q = 1, 2, ..., m

[0062] Where dx is the size of the sea surface element, and Q is the length of the Fourier transform. These are the discretized wave signals at times t and t+Δt, respectively.

[0063] Finally, the M cross spectra obtained are averaged to obtain the cross spectrum for that period.

[0064]

[0065] Step S4. Extract wave direction based on the characteristics of the imaginary part of the cross spectrum and remove the 180° wave direction ambiguity from the wave direction spectrum.

[0066] To simplify the derivation, two one-dimensional, single-frequency cosine wave signals m are used. 1 (x) and m 2 Let (x) represent the wave surface function of the sea surface observed at the same location at adjacent times (t=0, t=Δt), then m 1 (x) and m 2 (x) are respectively:

[0067] m 1(x)=m0cos(k0x)

[0068] m 2 (x)=m0cos(k0x-ωΔt)

[0069] Where m0 is a constant representing the amplitude, k0 is the wave number, ω is the angular frequency, and ωΔt is the phase difference between the signals. 1 (x) and m 2 (x) The cross-spectral results of the two signals are as follows:

[0070]

[0071] δ(k) is the Dirac function, based on its properties:

[0072] δ(k)=0,(k≠0)

[0073]

[0074] Taking the real and imaginary parts of the cross spectrum separately, we can obtain the real and imaginary part spectra of the cross spectrum as follows:

[0075]

[0076]

[0077]

[0078] As can be seen from the above equation, the real part of the cross spectrum has two peaks at k = k0 and k = -k0, exhibiting a symmetrical distribution in the wavenumber domain, while the imaginary part of the cross spectrum exhibits an antisymmetric distribution. Based on the antisymmetric characteristic of the imaginary part of the cross spectrum, The signal propagates in the positive direction. The time signal propagates in the negative direction. Extending this to the two-dimensional wave direction spectrum, when the imaginary part of the cross spectrum is positive, the wave direction observation azimuth is the wave direction; when the imaginary part of the cross spectrum is negative, the wave direction fuzzy observation azimuth is the wave direction.

[0079] Figures 2(a) and 2(b) show the real parts of the cross spectrum of a 10° beam with a wave direction of 0° under wind and wave conditions. and the virtual part Spectral results. The wave spectrum uses the EL spectrum, with the horizontal axis representing wave number and the vertical axis representing the observation azimuth. According to the color scale in the figure, yellow represents positive values, and the higher the brightness, the larger the spectral value; blue represents negative values, and the darker the color, the smaller the spectral value. It can be seen that the wave direction corresponding to the positive values ​​of the imaginary part of the cross spectrum is consistent with the wave propagation direction.

[0080] As shown in Figure 2(a), the real part of the cross spectrum is symmetrical about 180°, and the peak value appears near 0° and 180°. Therefore, the actual direction of wave propagation cannot be determined based on the real part of the cross spectrum.

[0081] As shown in Figure 2(b), the imaginary part of the cross spectrum has positive values ​​near 180° and 0°, but it is mainly concentrated near 0°. Therefore, it can be determined that the wave direction is near 0°.

[0082] Figures 3(a) and 3(b) show the real parts of the cross spectrum of a 10° beam with a wave direction of 0° under swell conditions. and the virtual part Spectral results. The surge spectrum uses the DV spectrum, and the meanings of the horizontal and vertical axes and color codes are consistent with those in Figures 2(a) and 2(b). It can be seen that the wave direction corresponding to the positive value of the imaginary part of the cross spectrum is consistent with the wave propagation direction.

[0083] As shown in Figures 3(a) and 3(b), the real values ​​of the cross spectrum are symmetrical about 180°. The positive values ​​of the imaginary part are mainly concentrated around 0°, which indicates that the wave direction is 0°.

[0084] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for removing 180° wave direction ambiguity from a star carrier spectrometer, characterized in that, Includes the following steps: S1. The beam is controlled at time intervals using an electronically scanned detection method. The pointer inside remains unchanged, at t and t+ respectively. It constantly detects the sea surface and extracts wave signals containing information about ocean wave fluctuations; At time t, the wave surface function of the sea surface can be expressed as follows: At any given time, the wave surface function of the sea surface can be expressed as follows: in, Indicates amplitude, Represents angular frequency. Let be the initial phase of the wavefront function. x , y Let these be the coordinates of a point on the horizontal plane. This indicates at time t. Vertical height above sea level , They represent , Wave numbers of wave components in different directions; in, High-frequency spectrum Obtained by wavenumber discretization The discretized wavenumber interval; S2. Extract the wave direction spectrum from the wave signal at time t to obtain the wave direction observation azimuth angle and the wave direction fuzzy observation azimuth angle that are 180° out of phase; S3. For t and t+ The cross spectrum of the fluctuating signal at a given time is calculated; specifically, this includes calculating the cross spectrum using the average periodogram method. Time and Fluctuation signal at time and The signal is divided into several segments, and the cross spectrum of each segment is calculated. Finally, the average of all segments is taken as the cross spectrum. The asymptotically consistent unbiased estimate of, where x It is the distance from the ground. To observe the azimuth angle, k Wavenumber; Cross spectrum: in, For the first i Cross spectrum of segmented wave signals , , ( , ( They are respectively and Time of the first i Discretized wave signals The dimensions are the surface area of ​​the sea. Q The length of the Fourier transform; S4. Extract wave direction based on the characteristics of the imaginary part of the cross spectrum, and remove the 180° wave direction ambiguity from the wave direction spectrum.

2. The method according to claim 1, characterized in that, The imaginary part of the cross spectrum is: in, ω is the angular frequency.

3. The method according to claim 2, characterized in that, When the imaginary part of the cross spectrum is positive, the wave direction observation azimuth is the wave direction; When the imaginary part of the cross spectrum is negative, the azimuth angle of the wave direction fuzzy observation is the wave direction.

4. A system for removing 180° wave direction ambiguity from a star carrier spectrometer, characterized in that, include: Computer-readable storage media and processors; The computer-readable storage medium is used to store executable instructions; The processor is used to read executable instructions stored in the computer-readable storage medium and execute the 180° wave direction ambiguity removal method of the star carrier spectrometer according to any one of claims 1 to 3.

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