A high-precision two-dimensional flow field measurement method based on multiple parameters of a sea surface

Abstract

The application discloses a kind of high-precision two-dimensional flow field measurement methods based on marine surface multi-parameter.The along-track baseline in interferometric measurement can provide along-track interference for SAR imaging, and the along-track interference SAR complex image can be used to inverse the radial velocity of sea surface flow field;The cross-track baseline can provide cross-track interference for SAR and altimeter imaging, and the cross-track interference complex image can generate sea surface elevation information, and the height difference between the ocean current region and other sea areas can assist in judging the flow direction and position of the ocean current, and the geostrophic flow field of the ocean can be inverted;Mixed baseline can provide along-track + cross-track baseline multi-angle observation degree of freedom for system imaging, and the system can obtain the multi-angle scattering characteristics of sea surface, which can help to avoid image saturation under complex sea conditions, and the two-dimensional sea surface wind field, sea wave can be inverted by multi-angle interference complex image, and the two-dimensional sea surface flow field can be removed from the ocean background field (sea surface wind, sea wave, sea surface height) to further improve the measurement accuracy of two-dimensional flow field.

Description

Technical Field

[0001] This application relates to the technical field of synthetic aperture radar (SAR) detection technology, and in particular to a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface. Background Technology

[0002] Ocean currents are one of the most fundamental and important factors in marine dynamic environment detection. Understanding and predicting the patterns of global ocean currents can provide important data support for global climate change monitoring and large-scale atmosphere-ocean interaction research. It is also of great significance to fisheries, shipping, sewage discharge and military.

[0003] Existing satellite-based global sea surface current field measurements can be broadly categorized into two types: non-imaging detection, which uses radio ranging or radar backscattering characteristics to measure the radial velocity of ocean currents, and imaging interferometry, which uses the interferometric phase of SAR to invert the sea surface current field. The former is suitable for measuring large and medium-scale current fields, but suffers from insufficient accuracy in measuring nearshore and sub-meso- and small-scale current fields, and generally only obtains one-dimensional current fields. The latter is suitable for measuring small and medium-scale current fields, but the two-dimensional sea surface current field obtained without other ocean parameters has many shortcomings. For example, estimating the Doppler shift caused by the sea surface current field through multi-angle Doppler shifts caused by sea surface wind and waves, and then inverting the two-dimensional sea surface current field, results in a low spatial resolution. Another example is establishing a satellite-ground dual-base SAR observation geometric model with the observation scene center as the geometric center, receiving and processing SAR echo data to obtain the interferometric phase and calculate the two-dimensional current field; however, ground reception can only acquire near-shore current field data. Summary of the Invention

[0004] This application provides a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface. This method has the ability to measure multiple ocean elements, such as Doppler velocity measurement, interferometric altimetry, sea surface wind field measurement, sea surface flow field phase measurement, wave spectrum measurement, and sea surface temperature measurement. By comprehensively measuring multiple parameters of the ocean surface, it overcomes the problem of accuracy degradation under complex sea conditions and improves the measurement accuracy of the two-dimensional flow field of the ocean surface.

[0005] Firstly, a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface is provided. This method is applied to a low-orbit compound-eye SAR satellite system with multiple transmitters and receivers. The satellite system includes a SAR satellite S0 with both transmitting and receiving capabilities and SAR satellites S1 to S2 with receiving capabilities. 10S1 to S4 are distributed on both sides of the flight direction of S0, with a radar line-of-sight projection angle of 90°. S5 to S8 are located on the same operating trajectory as S0. S5 and S6 are located behind the flight trajectory, and S7 and S8 are located in front of the flight trajectory. The distances between S5 and S0, S6 and S0, S7 and S0, and S8 and S0 are all within the range of 100 to 1000 kilometers. S5, S6, S7, and S8 are controlled within the effective range of their corresponding design frequency bands along the flight path baseline. S9, S... 10 On the same zero Doppler plane as S0, only inter-satellite orbital baselines exist; the method includes:

[0006] Radar signals are transmitted from S0, S0~S 10 Simultaneously, radar signals are received to form multiple interferometric pairs, thereby obtaining multiple sea surface flow field vectors that correspond one-to-one with the multiple interferometric pairs. Each interferometric pair determines the corresponding sea surface flow field velocity through inversion, and the sea surface flow field velocity is radially projected onto the sea surface to obtain the sea surface flow field vector.

[0007] The two most orthogonal sea surface flow field vectors are selected from multiple sea surface flow field vectors and used as the results of multi-angle interferometry measurement of the two-dimensional flow field.

[0008] In conjunction with the first aspect, in some implementations of the first aspect, for the k-th interference pair, the system interference phase... satisfy:

[0009]

[0010] v k Let β be the velocity of the sea surface current measured by the k-th interferometric pair. v θ represents the angle between the flow velocity and the projection of the emitted beam onto the ground. i Let β be the radar incident angle of S0. fore,k The motion vector of the sea surface target The angle θ between the line of sight of the radar interferometer phase center of the k-th interferometer pair and the projection of that line of sight onto the sea surface. fore,k Let Δt be the downward viewpoint of the k-th interferometric pair, and Δt be the motion vector of the sea surface target. The duration of exercise;

[0011] Combining the interferometric time baseline and observation angle, based on the system interferometric phase Inversion yields the one-dimensional velocity v of the sea surface flow field. k .

[0012] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes:

[0013] Radar signals are emitted from S0, S0~S 10 Receive radar echo signals to determine S iDoppler frequency shift f when the satellite receives the echo signal di i = 0 to 10, and based on the Doppler frequency shift f di Determine S i The corresponding sea surface velocity v of the satellite di .

[0014] In conjunction with the first aspect, in some implementations of the first aspect, f di =f DC -f Si -f w -f B In the formula, i = 0 to 10, f DC f is the Doppler center frequency of the ocean surface scatterer. Si It is S i The Doppler shift caused by the relative motion between the satellite and the Earth, f w It is the Doppler frequency shift caused by the sea surface wind field, f B It is the Doppler frequency shift caused by Bragg scattering waves;

[0015] According to the Doppler frequency shift f di S i Satellite measured the sea surface velocity. In the formula, θ i Let k be the incident angle of the satellite node. R This represents the radar wave number.

[0016] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes:

[0017] Sea surface temperature provides auxiliary information for determining the direction of ocean currents; the sea surface temperature is obtained by inversion from a passive receiving mode; ocean currents are divided into wind currents, density currents, and compensation currents; the direction of ocean currents is determined by the isothermal gradient lines of the sea surface temperature distribution.

[0018] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes:

[0019] In wind measurement mode, radar signals are emitted from S0, and radar echo signals are simultaneously received from S1 to S4 to obtain multiple sea surface wind vector solutions. These solutions are then synthesized to obtain a unique sea surface wind vector solution, which is used as the sea surface wind field information. The surface drift u caused by the wind field inversion is then calculated from the sea surface wind field. wd .

[0020] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes:

[0021] In altimeter imaging mode, radar signals are emitted by S0, and S9 and S... 10Simultaneously, radar echo signals are received, and an interferometric altimeter is used to detect and invert the data to obtain sea surface height, and the ocean geostrophic current field is also retrieved; among these,

[0022] Elevation and interferometric phase of ground target point P The relation is:

[0023]

[0024] H represents the satellite altitude, B represents the S9 imaging and S... 10 The physical baseline of the imaging radar phase center, θ is the radar side view, and α is the angle between the baseline B and the horizontal direction;

[0025] The geostrophic balance equation is

[0026]

[0027] Where: u f v f These represent the meridional and zonal current velocities, respectively; f is the Coriolis force parameter. ω represents the angular velocity of the Earth's rotation. is the geographic latitude of the target; g is the gravitational acceleration in the ocean, which varies with latitude; h is the average dynamic topography; and x and y represent the latitudinal and longitudinal directions, respectively.

[0028] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes:

[0029] In altimeter imaging mode, S0 is self-generated and self-received to invert the wave spectrum, and then the significant wave height and mean wave period are calculated; among which,

[0030] The sea surface backscattering coefficient can be calculated using radar equations:

[0031]

[0032] In the formula, P r P represents the received echo power. t G represents the radar transmit power. t G r Here, λ represents the gain of the radar transmitting and receiving antenna beam centers, respectively; λ is the radar transmitting wavelength; R is the slant range; δR is the slant range resolution; and φ is the horizontal orientation angle.

[0033] The slant range signal modulation function is

[0034]

[0035] σ 0(R,φ) represents the sea surface backscattering coefficient, which is related to the slant range R and the azimuth angle φ. MEAN(σ 0 (R,φ)) represents the cumulative average sea surface backscattering coefficient, i.e. the sea surface backscattering coefficient under calm sea surface;

[0036] The slant-range modulation function is projected onto a horizontal plane to measure the wave information m(X,φ) propagating along the direction of the beam projection on the sea surface; cross-correlation is performed on the horizontal modulation function m(X,φ), followed by Fourier transform and power spectrum estimation to obtain the modulation spectrum.

[0037]

[0038] F(n,φ) represents the wave direction spectrum, where n is the wave number, φ is the wave propagation direction, and L... y ζ is the footprint width in the azimuth direction, ζ is the sea surface wave steepness; A(θ) is the modulation transfer function;

[0039] The azimuth integral of F(n,φ) yields the omnidirectional wave height spectrum:

[0040]

[0041] The significant wave height is calculated from the wave spectrum:

[0042]

[0043] The velocity contribution u0 of the large-scale wave and the velocity contribution u of the Bragg wave are obtained by inversion from F(n,φ). b .

[0044] In conjunction with the first aspect, in some implementations of the first aspect, the two-dimensional velocity of the sea surface flow field satisfy:

[0045]

[0046] By removing the two-dimensional velocity of the sea surface flow field The velocity contribution u caused by the wind field wd The velocity contribution u0 of large-scale waves and the velocity contribution u of Bragg waves b The sea surface current velocity u is obtained. c .

[0047] Secondly, a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface is provided. This method is applied to a low-Earth orbit compound-eye SAR satellite system with multiple transmitters and receivers. The satellite system includes a SAR satellite S0 with both transmitting and receiving capabilities, and satellites S5 to S8 operating in the same orbit as S0. S5 and S6 are located behind the satellites in their respective orbits, while S7 and S8 are located ahead. The distances between S5 and S0, S6 and S0, S7 and S0, and S8 and S0 are all within the range of 100 to 1000 kilometers. The method includes:

[0048] Radar signals are transmitted by S0, received by S5 and S6 forming interferometric pair 1, and received by S7 and S8 forming interferometric pair 2. The system interference phase is determined by the radar signals received by interferometric pair 1. The system interferometric phase is determined by the radar signal received by interferometer pair 2. By combining the interferometric time baseline and the observation angle inversion, the sea surface flow velocity v1 corresponding to interferometric pair 1 and the sea surface flow velocity v2 corresponding to interferometric pair 2 are obtained as the in-orbit interferometric measurement results of the two-dimensional flow field. Among them, the difference between the vector angle obtained by radially projecting the sea surface flow velocities v1 and v2 onto the sea surface and 90° is less than the preset angle.

[0049] Compared with the prior art, the solution provided in this application has at least the following beneficial technical effects:

[0050] (1) High-precision two-dimensional sea surface flow field measurement. Multiple along-track baselines acquire information such as scattering characteristics, Doppler spectrum, and interferometric phase at different azimuth angles of the sea surface. Through multiple decouplings, the existing sea surface flow field is upgraded from radial to two-dimensional measurement. Sea surface height measurement helps to determine the direction and location of ocean currents. Wave measurement and temperature measurement provide auxiliary information for interferometric measurement, improving the efficiency of interferometric measurement. Based on the two degrees of freedom (range signal bandwidth and azimuth Doppler) sensor configuration of traditional two-dimensional SAR imaging, three additional observation degrees of freedom are added, including the observation degrees of freedom of along-track baselines, the observation degrees of freedom of cross-track baselines, and the multi-angle observation degrees of freedom of along-track + cross-track baselines. Multiple baselines can be achieved in one pass, thereby improving measurement accuracy and acquiring data in a timely manner.

[0051] (2) Comprehensive measurement of multiple parameters of the ocean surface. Based on the system configuration and multiple loads, this invention can simultaneously measure sea surface height, sea surface temperature, sea surface wind field and wave spectrum. Radial sea surface velocity can be measured by Doppler velocimetry, sea surface elevation and geostrophic current can be measured by altimeter and cross-track interferometry, and multi-baseline in-track interferometry provides multi-angle radial sea surface velocity measurement, realizing multi-parameter measurement in a single transit. Attached Figure Description

[0052] Figure 1This is a flowchart of a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface provided by the present invention.

[0053] Figure 2 This is a schematic diagram of the configuration of a low-orbit compound-eye SAR satellite system with one launcher and multiple receivers.

[0054] Figure 3 This is a schematic diagram of the imaging incident angle under different working modes.

[0055] Figure 4 This is a schematic diagram of a multi-receiver node.

[0056] Figure 5 This is a schematic diagram of the phase difference of the transmitted beam.

[0057] Figure 6 This is a schematic diagram of the phase difference of the receiving beam. Detailed Implementation

[0058] The present application will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0059] like Figure 1 As shown, this invention provides a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface. This method can be applied to... Figure 2 The image shows a low-orbit compound-eye SAR satellite system that can launch multiple satellites and receive multiple receivers.

[0060] The low-orbit compound-eye SAR satellite system, which transmits and receives multiple satellites, includes S0 to S10. 10 SAR satellites, of which S0 is a SAR satellite with both transmission and reception capabilities; S5-S8 share the same orbit as S0, distributed several hundred kilometers ahead and behind S0's flight path, respectively; and S5, S6, and S7, S8 are controlled within the effective range of their corresponding design frequency bands along the track baseline. S9, S... 10 S1 to S4 share the same zero Doppler plane as S0, and there is only an inter-orbit baseline between them. S1 to S4 are distributed on both sides of the flight direction of S0, and the projection angle of the radar line of sight is close to 90°.

[0061] Compared to traditional two-dimensional imaging sensor configurations, the aforementioned low-Earth orbit compound-eye SAR satellite system adds three dimensions of observation freedom: along-orbit baseline observation freedom, cross-orbit baseline observation freedom, and along-orbit + cross-orbit baseline multi-angle observation freedom. This aims to address the challenges of measuring all elements of the sea surface and the two-dimensional flow field. The along-orbit baseline primarily measures the two-dimensional sea surface flow field; the cross-orbit baseline primarily measures the ocean surface height, calculating the ocean geostrophic flow field to aid in determining ocean current regions and directions; and the multi-angle observation freedom primarily measures the two-dimensional sea surface wind field, improving the accuracy of inverted flow field retrieval.

[0062] The aforementioned low-Earth orbit compound-eye SAR satellite system, which transmits and receives multiple satellites, has various operating modes, including SAR imaging mode, altimeter imaging mode, wind measurement mode, and passive receiving mode. Among these, the SAR imaging mode is activated by satellite at S0, with S0 to S1... 10 Simultaneous reception; wind measurement mode is S0 transmit, S1~S4 simultaneous reception; altimeter imaging mode is S0 transmit, S0, S9~S 10 Simultaneous reception includes both interferometric altimeter imaging and spectrometer imaging modes; the passive reception mode is S0. This enables the acquisition of multiple parameters of the ocean surface in a single pass. The incident angles of the SAR imaging mode and altimeter imaging mode are complementary, and the imaging area can be completely covered. Figure 3 As shown.

[0063] exist Figure 1 In the high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface shown, when in SAR imaging mode, it is emitted by S0, and S0~S 10 The radial flow field of the sea surface is inverted by receiving the echo to obtain the sea surface scattering characteristics and Doppler spectrum information and calculating the Doppler frequency shift. The specific steps include at least one of the following.

[0064] S101, when in SAR imaging mode, is emitted from S0, S0~S 10 Receive the echo and determine S i Doppler frequency shift f when the satellite receives the echo signal di i = 0 to 10, and based on the Doppler frequency shift f di Determine S i The corresponding sea surface velocity v of the satellite di .

[0065] Assuming the transmission frequency of the satellite signal is f, after long-distance signal propagation, due to the relative motion between the satellite and the target sea area, the actual frequency f of the received echo signal changes. r There is a deviation between the original frequency f and the frequency f; this frequency deviation is called the Doppler frequency shift f. d .

[0066] In reality, the Doppler frequency shift of the ocean surface is affected not only by the relative velocity of the Earth and satellites, but also by sea surface wind fields and Bragg scattering. Therefore, the Doppler center frequency f of the scattering body from the ocean surface is... DC Doppler frequency shift can be obtained. In some embodiments, it is necessary to receive echoes from the same observation area from multiple angles, assuming that the Doppler center frequencies of the sea surface wind field, Bragg scattering, and ocean surface scatterers are consistent, and the difference in the echoes received by different nodes lies in the relative motion velocity of the Earth and the satellite and the angle of incidence.

[0067] f di =f DC -f Si-f w -f B

[0068] In the formula, i = 0 to 10, f Si It is S i The Doppler shift caused by the relative motion between the satellite and the Earth, f w It is the Doppler frequency shift caused by the sea surface wind field, f B It is the Doppler frequency shift caused by Bragg scattering waves.

[0069] Figure 4 This is a schematic diagram of partial node flight. Based on the Doppler frequency shift f... di S i Satellite measured the sea surface velocity v di (The projection of V onto the radar's line of sight, where V is the relative velocity between the satellite and the target sea area, and the radial velocity along the radar's line of sight, with negative values ​​for motion approaching the radar and positive values ​​for motion moving away from the radar) can be expressed as:

[0070]

[0071] In the formula, θ i Let k be the incident angle of the satellite node. R This represents the radar wave number.

[0072] S102, when in SAR imaging mode, is transmitted by S0 and simultaneously received by S5 to S8, and the two-dimensional flow field is obtained by in-orbit interferometry.

[0073] In-orbit interferometry consists of two or more satellites in the same orbit, imaging the same observation area from the same location at different times. The timing of the two imaging sessions is precisely calibrated, and the time difference is used as the in-orbit interferometric time baseline. Each pair forms an interferometric pair. Similarly, when a radar antenna observes the same area twice from the same location, a tiny "system" motion during the two data acquisitions can produce completely different results. The "system" refers to the fact that the surface scattering elements within a single pixel remain constant, but the distance from the antenna to the ocean surface changes. This tiny distance change results in a phase difference between the two interferometric images. The key quantity is the component of the motion in the slant range direction.

[0074] Data is transmitted from S0 and simultaneously received by S5 to S8, and interferometric processing is performed. S0 transmitting and S5 / S6 receiving constitute interferometric pair 1, while S0 transmitting and S7 / S8 receiving constitute interferometric pair 2. Interferometric pair 1 and interferometric pair 2 determine the system interference phase, respectively. and The one-dimensional velocities v1 and v2 of the sea surface flow field are obtained by combining the interferometric time baseline and the observation angle inversion.

[0075] like Figure 5 As shown, the motion vector of the sea surface target It can be represented as

[0076]

[0077] Let be the velocity of the two-dimensional flow field, and D be the vector of the center point of the first and second imaging regions. Therefore, refer to... Figure 6 ,

[0078]

[0079] In the above formula, R1 and R2 are the radar slant ranges when satellite 1 and satellite 2 are imaged in a certain interferometric pair, respectively, and B is the physical baseline of the radar phase center of satellite 1 and satellite 2 imaging.

[0080] The phase of the received beam interference can be expressed as (<A,B> (inner product)

[0081]

[0082] Since R1 >> B,D

[0083]

[0084] θ is the radar side view, and α is the angle between baseline B and the horizontal direction.

[0085] Considering pure in-orbit interference, the received phase is...

[0086]

[0087] Among them, v r =vcosβ fore cosθ fore v r for Projection along the receiving beam direction, β fore The motion vector of the sea surface target The angle θ between the line of sight of the radar interferometer phase center and the projection onto the sea surface. fore From the bottom perspective, sinθ fore =H / R1, where R1 is the radar slant range when imaging satellite 1 in a certain interferometric pair, and H is the satellite altitude.

[0088] Neglecting errors, the system interference phase According to the interference phase of the received beam and the slant range phase difference of the transmitted beam Sure (refer to Figure 5 ,β v The angle between the flow velocity and the projection of the emitted beam onto the ground is represented by d, where d is the distance between the center points of the two imaging regions. λ is the radar wavelength. Therefore:

[0089]

[0090] In some embodiments, considering multiple sets of interference, there are interference pair 1: S0 transmits, S5 and S6 receive; interference pair 2: S0 transmits, S7 and S8 receive.

[0091]

[0092] Interferometric processing requires high coherence between the two images. By calculating the different phase information of each pixel through observation, and combining the interferometric time baseline and the observation angle inversion, the one-dimensional velocity v of the sea surface current field is obtained. k .

[0093] S0 is a side-viewing satellite. S5 and S6 are located behind the flight orbit, about several hundred kilometers away from S0. The vector direction of the sea surface flow velocity v1 obtained by the above method can be determined based on the positional relationship of S0, S5, and S6. The vector of the sea surface flow velocity v1 is radially projected onto the sea surface to obtain vector 1. Similarly, S7 and S8 are located in front of the flight orbit, about several hundred kilometers away from S0. The vector direction of the sea surface flow velocity v2 obtained by the above method can be determined based on the positional relationship of S0, S7, and S8. The vector of the sea surface flow velocity v2 is radially projected onto the sea surface to obtain vector 2. The angle between vector 1 and vector 2 is equal to or approximately equal to 90°. Therefore, the sea surface flow velocities v1 and v2 can reflect the two-dimensional information of the sea surface flow velocity. Through coordinate transformation, the meridional and latitudinal phase sea surface flow information can be obtained. The sea surface flow velocities v1 and v2 can be vector-synthesized to elevate the existing sea surface flow field from radial to two-dimensional.

[0094] S103, when in SAR imaging mode, is emitted from S0, S0~S 10 Simultaneously receive and obtain two-dimensional flow fields through multi-angle interferometry.

[0095] The multi-angle + multi-baseline along-orbit interferometry method enables the system to acquire information such as scattering characteristics and interference phase at different azimuth angles of the sea surface. It can measure the radial sea surface flow velocity at different angles, project it onto the sea surface, and calculate multiple sea surface flow field vector solutions v. k v k The maximum number can be

[0096] In some embodiments, 11 sea surface flow field vector solutions v can be constructed in the following manner. k k = 0, 1, ..., C2 10Interference pair k=0, S0 sends, S0 and S1 receive; Interference pair k=1, S0 sends, S1 and S2 receive; Interference pair k=2, S0 sends, S2 and S3 receive; Interference pair k=3, S0 sends, S3 and S4 receive; Interference pair k=4, S0 sends, S4 and S5 receive; Interference pair k=5, S0 sends, S5 and S6 receive; Interference pair k=6, S0 sends, S6 and S7 receive; Interference pair k=7, S0 sends, S7 and S8 receive; Interference pair k=8, S0 sends, S8 and S9 receive; Interference pair k=9, S0 sends, S9 and S1 receive. 10 Receive; Interference pair k=10, S0 sends, S 10 ,S0 received.

[0097] Multiple sea surface flow field vector solutions v k By combining two sea surface flow vectors that are as close as possible to orthogonal as possible in the range (k = 0, 1, ..., 10), the sea surface flow field can be elevated from radial to two-dimensional.

[0098] By combining multi-angle along-track interferometry and cross-track interferometry, the system can acquire the multi-angle scattering characteristics of the sea surface, which helps to avoid image saturation and improve the accuracy of flow field measurement under complex sea conditions.

[0099] Step 104 provides auxiliary information for determining the direction of sea surface currents based on sea surface temperature. The sea surface temperature is obtained through inversion using a passive receiver mode. Ocean currents are categorized into wind-driven currents, density currents, and compensating currents. Density currents arise because the horizontal distribution of seawater temperature and salinity varies across different sea areas, causing uneven distribution of seawater density and resulting in the tilting of isobaric surfaces. Therefore, the direction of the ocean current can be determined by observing the isothermal gradient lines of the sea surface temperature distribution.

[0100] Step 105, wind measurement mode, S0 transmits, S1~S4 receive simultaneously, and the sea surface wind field is obtained by inversion.

[0101] The sea surface not only radiates electromagnetic energy but also reflects and scatters external electromagnetic energy. Wind fields over the sea surface can be obtained by utilizing these reflection and scattering characteristics. When the incident angle is between 20° and 80°, the sea surface backscattering coefficient σ... 0 It increases with increasing ocean wind speed, decreases with increasing incident angle, and is related to polarization; sea surface winds cause sea surface waves, changing the flatness of the sea surface and inducing the sea surface backscattering coefficient σ. 0 This change allows us to establish the relationship between the backscattering coefficient and wind speed and direction. The backscattering coefficient is measured using instruments, and the wind speed and direction for each wind vector unit are retrieved based on the geophysical model function CMOD5.0. Using a multi-angle receiving method yields multiple sea surface wind vector solutions; vector synthesis is then used to obtain a unique sea surface wind vector solution, which is then used as the sea surface wind field information. The surface drift u caused by the wind field can be retrieved from the sea surface wind field. wd .

[0102] Step 106, altimeter imaging mode, S0 firing, S0, S9, S 10 Simultaneously, the sea surface height can be obtained by receiving and inverting the interferometric altimeter detection data.

[0103] Cross-orbit interferometry consists of two satellites with the same track direction but different cross-orbit directions. Both satellites simultaneously observe the same area, and the distance between them in the cross-orbit direction is precisely measured; this distance forms the cross-orbit interferometric spatial baseline. The two satellites image the same sea area at the same time, obtaining the interferometric phase of the two images. The interferometric phase is a function of the difference in slant range from the radar antenna to the target point during the two imaging sessions. This interferometric phase can be used to calculate sea level elevation information, not only to help determine the direction and location of ocean currents by measuring sea level height, but also to invert the ocean geostrophic current field.

[0104] Assuming the scattering characteristics of the sea surface resolution cells are the same in both images, the phase difference of the cross-track interference obtained from the interference of the two images is:

[0105]

[0106] R1 and R2 are the slant ranges from the radar antenna to the target point during the first and second imaging processes, respectively, and λ is the radar wavelength.

[0107] The elevation and interferometric phase of the ground target point P can be derived. The relation is:

[0108]

[0109] H is the satellite altitude, B is the physical baseline of the radar phase center for the first and second imaging, θ is the radar side view, and α is the angle between baseline B and the horizontal direction.

[0110] The velocity of the geostrophic flow is obtained through the geostrophic flow equilibrium equation under the assumptions that the friction between the sea breeze and the seawater is zero, and the acceleration of the ocean current is zero. The formula is as follows:

[0111]

[0112] Where: u f v f These represent the meridional velocity (eastward) and zonal velocity (northward), respectively. f is the Coriolis force parameter. ω represents the angular velocity of the Earth's rotation. This represents the geographic latitude of the target. g is the gravitational acceleration in the ocean, which varies with latitude. h is the mean dynamic topography, and x and y represent the latitudinal and longitudinal directions, respectively.

[0113] Step 107, altimeter imaging mode, S0 self-transmitting and self-receiving. The spectrometer is an extension of the nadir altimeter measurement. Based on the principle of wave detection by the spectrometer, the wave spectrum is inverted to calculate wave parameters such as significant wave height and average wave period. The spectrometer measures the sea surface backscattering coefficient at a small incident angle of 0° to 10°, and simultaneously measures the sea surface significant wave height and wave spectrum with high precision, which can complement the SAR incident range.

[0114] The sea surface backscattering coefficient can be calculated using radar equations:

[0115]

[0116] In the formula, P r P represents the received echo power. t G represents the radar transmit power. t G r λ represents the gain of the radar transmitting and receiving antenna beam centers, respectively; λ is the radar transmitting wavelength; R is the slant range; δR is the slant range resolution; and φ is the horizontal orientation angle.

[0117] The slant range signal modulation function is

[0118]

[0119] Where, σ 0 (R,φ) represents the sea surface backscattering coefficient, which is related to the slant range R and the azimuth angle φ. MEAN(σ 0 (R,φ)) represents the cumulative average sea surface backscattering coefficient, that is, the sea surface backscattering coefficient under calm sea surface.

[0120] By projecting the slant-range modulation function onto a horizontal plane, the wave information m(X,φ) propagating along the direction of the beam's projection onto the sea surface can be measured. Cross-correlation of the horizontal modulation function m(X,φ), followed by Fourier transform and power spectrum estimation, yields the modulation spectrum.

[0121]

[0122] Where F(n,φ) is the wave direction spectrum, n is the wave number, φ is the wave propagation direction, and L... y ζ represents the footprint width in the azimuth direction, and ζ represents the sea surface wave steepness. The key coefficient A(θ) is the modulation transfer function.

[0123] By performing an azimuth integral on F(n,φ), the omnidirectional wave height spectrum can be obtained:

[0124]

[0125] The significant wave height can be calculated from the wave spectrum:

[0126]

[0127] Based on F(n,φ), the velocity contribution u0 of the large-scale wave and the velocity contribution u of the Bragg wave can be obtained by inversion. b .

[0128] Step 108: Integrate multi-parameter sea surface inversion.

[0129] Provided by the present invention Figure 2 The system configuration shown enables multi-parameter measurement of the sea surface during a single transit, including: multiple radial current velocities measured by the Doppler frequency shift method in step 101 (applicable to large-scale ocean current fields (e.g., within a few hundred kilometers)); multiple radial current velocities measured by the along-orbit interferometry method in step 102 or the multi-angle interferometry method in step 103 (applicable to small-to-medium-scale ocean current fields near the coast (e.g., within 10 kilometers)); the sea surface temperature field provided in step 104 can help determine the direction of the sea surface current field; the sea surface wind field is measured in step 105; and the sea surface height is measured in step 106, which helps determine the direction and location of ocean currents and can also be used to calculate the geostrophic current field. The wave spectrum can be used to invert the current field, combined with step 107.

[0130] The two-dimensional velocity of the sea surface flow field obtained from step 102 or step 103 In addition to sea surface current velocity, it also includes the velocity contribution u0 of large-scale waves and the velocity contribution u of Bragg waves. b The velocity contribution u caused by the wind field wd .

[0131]

[0132] Remove the velocity contribution u caused by the wind field obtained from the inversion in step 105. wd The velocity contribution u0 of the large-scale wave and the velocity contribution u of the Bragg wave are obtained from the inversion in step 106. b The sea surface current velocity u can be obtained. c .

[0133] The isotherms of the sea surface temperature field in step 104 can provide a temperature gradient, which helps correct for flow directions in two-dimensional flow fields that do not match the temperature gradient. The multi-angle interferometry in step 103 can reduce measurement errors of the sea surface flow field under complex sea conditions.

[0134] In summary, this invention provides a high-precision two-dimensional current field measurement method based on multiple ocean surface parameters, primarily targeting comprehensive multi-parameter measurement of the ocean surface and high-precision two-dimensional detection of small-to-medium scale ocean currents under complex sea conditions. This method offers a spaceborne compound-eye SAR system configuration for multi-parameter ocean surface measurement, similar to the compound eyes in the biological visual system. By increasing observational degrees of freedom and coherent processing, it significantly enhances the ability to observe dynamic information, hence the abbreviation "compound-eye SAR system." The system simultaneously possesses the capability to measure multiple ocean elements, including Doppler velocity measurement, interferometric altimetry, sea surface wind field measurement, sea surface current phase measurement, wave spectrum measurement, and sea surface temperature measurement. Firstly, along-orbit and perpendicular-orbit interferometry are the basic modes. SAR and altimeter measurements of sea surface height assist in determining the direction and location of ocean currents; scatterometer measurements of sea surface wind field help remove wind-generated waves, improving the accuracy of ocean current measurements; and spectrometer measurements of wave spectrum and radiometer measurements of sea surface temperature provide auxiliary information for interferometry, providing a basis for determining the direction of ocean currents.

[0135] In interferometry, along-track baselines can provide along-track interferometry for SAR imaging, and along-track interferometric SAR complex images can invert the radial velocity of the sea surface current field. Cross-track baselines can provide cross-track interferometry for SAR and altimeter imaging, and cross-track interferometric complex images can generate sea surface elevation information, measure the height difference between ocean current areas and other sea areas to help determine the direction and location of ocean currents, and invert the ocean geostrophic current field. Hybrid baselines can provide the system imaging with along-track + cross-track baselines for multi-angle observation degrees of freedom. The system can acquire the multi-angle scattering characteristics of the sea surface, which helps to avoid image saturation under complex sea conditions. By inverting the two-dimensional sea surface wind field and waves through multi-angle interferometric complex images, and removing the ocean background field (sea surface wind, waves, and sea surface height) when inverting the two-dimensional sea surface current field, the accuracy of the two-dimensional current field measurement can be further improved.

[0136] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims of the present invention.

Claims

1. A high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface, characterized in that, The method is applied to a low-Earth orbit compound-eye SAR satellite system with multiple transmitters and receivers. This system includes a SAR satellite S0 with both transmitting and receiving capabilities and SAR satellites S1 to S2 with receiving capabilities. 10 S1 to S4 are distributed on both sides of the flight direction of S0, with a radar line-of-sight projection angle of 90°. S5 to S8 are located on the same operating trajectory as S0. S5 and S6 are located behind the flight trajectory, and S7 and S8 are located in front of the flight trajectory. The distances between S5 and S0, S6 and S0, S7 and S0, and S8 and S0 are all within the range of 100 to 1000 kilometers. S5, S6, S7, and S8 are controlled within the effective range of their corresponding design frequency bands along the flight path baseline. S9, S... 10 On the same zero Doppler plane as S0, only inter-satellite orbital baselines exist; the method includes: Radar signals are transmitted from S0, S0~S 10 Simultaneously, radar signals are received to form multiple interferometric pairs, thereby obtaining multiple sea surface flow field vectors that correspond one-to-one with the multiple interferometric pairs. Each interferometric pair determines the corresponding sea surface flow field velocity through inversion, and the sea surface flow field velocity is radially projected onto the sea surface to obtain the sea surface flow field vector. The two most orthogonal sea surface flow field vectors are selected from multiple sea surface flow field vectors and used as the results of multi-angle interferometry measurement of the two-dimensional flow field.

2. The method according to claim 1, characterized in that, For the k-th interference pair, the system interference phase satisfy: v k Let β be the velocity of the sea surface current measured by the k-th interferometric pair. v θ represents the angle between the flow velocity and the projection of the emitted beam onto the ground. i Let β be the radar incident angle of S0. fore,k The motion vector of the sea surface target The angle θ between the line of sight of the radar interferometer phase center of the k-th interferometer pair and the projection of that line of sight onto the sea surface. fore,k Let Δt be the downward viewpoint of the k-th interferometric pair, and Δt be the motion vector of the sea surface target. The duration of exercise; Combining the interferometric time baseline and observation angle, based on the system interferometric phase Inversion yields the one-dimensional velocity v of the sea surface flow field. k .

3. The method according to claim 1 or 2, characterized in that, The method further includes: Radar signals are emitted from S0, S0~S 10 Receive radar echo signals to determine S i Doppler frequency shift f when the satellite receives the echo signal di i = 0 to 10, and based on the Doppler frequency shift f di Determine S i The corresponding sea surface velocity v of the satellite di .

4. The method according to claim 3, characterized in that, f di =f DC -f Si -f w -f B In the formula, i = 0 to 10, f DC f is the Doppler center frequency of the ocean surface scatterer. Si It is S i The Doppler shift caused by the relative motion between the satellite and the Earth, f w It is the Doppler frequency shift caused by the sea surface wind field, f B It is the Doppler frequency shift caused by Bragg scattering waves; According to the Doppler frequency shift f di S i Satellite measured the sea surface velocity. In the formula, θ i Let k be the incident angle of the satellite node. R This represents the radar wave number.

5. The method according to claim 1 or 2, characterized in that, The method further includes: Sea surface temperature provides auxiliary information for determining the direction of ocean currents; the sea surface temperature is obtained by inversion from a passive receiving mode; ocean currents are divided into wind currents, density currents, and compensation currents; the direction of ocean currents is determined by the isothermal gradient lines of the sea surface temperature distribution.

6. The method according to claim 1 or 2, characterized in that, The method further includes: In wind measurement mode, radar signals are emitted from S0, and radar echo signals are simultaneously received from S1 to S4 to obtain multiple sea surface wind vector solutions. These solutions are then synthesized to obtain a unique sea surface wind vector solution, which is used as the sea surface wind field information. The surface drift u caused by the wind field inversion is then calculated from the sea surface wind field. wd .

7. The method according to claim 1 or 2, characterized in that, The method further includes: In altimeter imaging mode, radar signals are emitted by S0, and S9 and S... 10 Simultaneously, radar echo signals are received, and an interferometric altimeter is used to detect and invert the data to obtain sea surface height, and the ocean geostrophic current field is also retrieved; among these, Elevation and interferometric phase of ground target point P The relation is: H represents the satellite altitude, B represents the S9 imaging and S... 10 The physical baseline of the imaging radar phase center, θ is the radar side view, and α is the angle between the baseline B and the horizontal direction; The geostrophic balance equation is Where: u f v f These represent the meridional and zonal current velocities, respectively; f is the Coriolis force parameter. ω represents the angular velocity of the Earth's rotation. is the geographic latitude of the target; g is the gravitational acceleration in the ocean, which varies with latitude; h is the average dynamic topography; and x and y represent the latitudinal and longitudinal directions, respectively.

8. The method according to claim 1 or 2, characterized in that, The method further includes: In altimeter imaging mode, S0 is self-generated and self-received to invert the wave spectrum, and then the significant wave height and mean wave period are calculated; among which, The sea surface backscattering coefficient can be calculated using radar equations: In the formula, P r P represents the received echo power. t G represents the radar transmit power. t G r Here, λ represents the gain of the radar transmitting and receiving antenna beam centers, respectively; λ is the radar transmitting wavelength; R is the slant range; δR is the slant range resolution; and φ is the horizontal orientation angle. The slant range signal modulation function is σ 0 (R,φ) represents the sea surface backscattering coefficient, which is related to the slant range R and the azimuth angle φ. MEAN(σ 0 (R,φ)) represents the cumulative average sea surface backscattering coefficient, i.e. the sea surface backscattering coefficient under calm sea surface; The slant-range modulation function is projected onto a horizontal plane to measure the wave information m(X,φ) propagating along the direction of the beam projection on the sea surface; cross-correlation is performed on the horizontal modulation function m(X,φ), followed by Fourier transform and power spectrum estimation to obtain the modulation spectrum. F(n,φ) represents the wave direction spectrum, where n is the wave number, φ is the wave propagation direction, and L... y ζ is the footprint width in the azimuth direction, ζ is the sea surface wave steepness; A(θ) is the modulation transfer function; The azimuth integral of F(n,φ) yields the omnidirectional wave height spectrum: The significant wave height is calculated from the wave spectrum: The velocity contribution u0 of the large-scale wave and the velocity contribution u of the Bragg wave are obtained by inversion from F(n,φ). b .

9. The method according to claim 1 or 2, characterized in that, The two-dimensional velocity v of the sea surface flow field satisfies: By removing the two-dimensional velocity of the sea surface flow field The velocity contribution u caused by the wind field wd The velocity contribution u0 of large-scale waves and the velocity contribution u of Bragg waves b The sea surface current velocity u is obtained. c .

10. A high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface, characterized in that, The method is applied to a low-Earth orbit compound-eye SAR satellite system with multiple receivers. The system includes a SAR satellite S0 with both transmitting and receiving capabilities, and satellites S5 to S8 operating in the same orbit as S0. S5 and S6 are located behind the satellites in their respective orbits, while S7 and S8 are located ahead. The distances between S5 and S0, S6 and S0, S7 and S0, and S8 and S0 are all within the range of 100 to 1000 kilometers. The method includes: Radar signals are transmitted by S0, received by S5 and S6 forming interferometric pair 1, and received by S7 and S8 forming interferometric pair 2. The system interference phase is determined by the radar signals received by interferometric pair 1. The system interferometric phase is determined by the radar signal received by interferometer pair 2. By combining the interferometric time baseline and the observation angle inversion, the sea surface flow velocity v1 corresponding to interferometric pair 1 and the sea surface flow velocity v2 corresponding to interferometric pair 2 are obtained as the in-orbit interferometric measurement results of the two-dimensional flow field. Among them, the difference between the vector angle obtained by radially projecting the sea surface flow velocities v1 and v2 onto the sea surface and 90° is less than the preset angle.

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