High-precision two-dimensional flow field measurement method based on multiple parameters of ocean surface

Abstract

The invention discloses a high-precision two-dimensional flow field measurement method based on multiple parameters of an ocean surface. In interference measurement, an along-track baseline can provide along-track interference for SAR imaging, and along-track interference SAR complex images can invert the radial velocity of a sea surface flow field; a cross-rail baseline can provide cross-rail interference for SAR and altimeter imaging, a cross-rail interference complex image can generate sea surface elevation information, the height difference between an ocean current area and other sea areas is measured to assist in judging the flow direction and position of ocean current, and an ocean earth transfer flow field is inverted; the mixed baseline can provide multi-angle observation freedom degrees of along-track and cross-track baselines for system imaging, the system can obtain multi-angle scattering characteristics of the sea surface, image saturation can be avoided under complex sea conditions, a two-dimensional sea surface wind field and sea waves can be inverted through multi-angle interference complex images, and the system has a wide application prospect. When the sea surface two-dimensional flow field is inverted, the sea background field (sea surface wind, sea wave and sea surface height) is removed, so that the measurement precision of the two-dimensional flow field can be further improved.

Description

Technical Field

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

[0002] Ocean currents are one of the most basic and important factors in the detection of the ocean dynamic environment. Mastering and predicting the laws 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 affairs.

[0003] The existing global sea surface flow field measurements based on satellite platforms can be roughly divided into two categories. One is non-imaging detection, which uses radio ranging or radar backscattering characteristics to measure the radial velocity of ocean currents, and the other is imaging interferometry, which uses the interference phase of SAR to invert the sea surface flow field. The former is suitable for measuring large and medium-scale flow fields, but has insufficient accuracy in measuring nearshore flow fields and sub-medium and small-scale flow fields, and can generally only measure one-dimensional flow fields; the latter is suitable for measuring small and medium-scale flow fields, but the two-dimensional sea surface flow field measured without other ocean parameter input has many shortcomings. For example, the Doppler frequency shift caused by the sea surface wind field and waves at multiple angles is estimated, and then the two-dimensional sea surface flow field is inverted. The spatial resolution of the two-dimensional sea surface flow field obtained by this method is low. For another example, by establishing a satellite-ground bistatic SAR observation geometric model with the center of the observation scene as the geometric center, receiving SAR echo data for processing, obtaining the interference phase, and calculating the two-dimensional flow field, but ground reception can only obtain flow field data near the coast. Summary of the invention

[0004] The present application provides a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface. The method simultaneously has the ability to measure multiple ocean elements such as Doppler velocity measurement, interferometric height measurement, sea surface wind field measurement, sea surface flow field phase measurement, wave spectrum measurement, and sea surface temperature measurement. Through the comprehensive measurement of multiple parameters of the ocean surface, the problem of decreased accuracy under complex sea conditions is overcome, and the measurement accuracy of the two-dimensional flow field on the ocean surface is improved.

[0005] In the first aspect, a high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface is provided, wherein the method is applied to a one-transmit-multiple-receive low-orbit compound-eye SAR satellite system, wherein the satellite system comprises a SAR satellite S0 with both a transmitting and receiving function and SAR satellites S1 to S2 with a receiving function. 10, S1~S4 are distributed on both sides of the flight direction of S0, and the projection angle of the radar line of sight direction corresponds to 90°. S5~S8 are located in the same orbit as S0. S5 and S6 are located behind the flight orbit, and S7 and S8 are located in front of the flight orbit. The distances between S5 and S0, between S6 and S0, between S7 and S0, and between S8 and S0 are all within the range of 100 to 1000 kilometers. S5, S6, S7, S8 are controlled within the effective range of the corresponding design frequency band along the track baseline. S9, S 10 The zero Doppler plane is the same as S0, and only the cross-track baseline exists between the satellites; the method comprises:

[0006] S0 sends the radar signal, S0~S 10 receiving radar signals simultaneously to form a plurality of interference pairs, and then obtaining a plurality of sea surface flow field vectors corresponding to the plurality of interference pairs, wherein each interference pair determines the corresponding sea surface flow field velocity by 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 a plurality of sea surface flow field vectors as the multi-angle interferometric measurement results of the two-dimensional flow field.

[0008] In combination with the first aspect, in some implementations of the first aspect, for the kth interference pair, the system interference phase satisfy:

[0009]

[0010] v k is the sea surface flow velocity measured by the kth interferometer pair, β v represents the angle between the flow field velocity and the projection of the transmitting beam on the ground, θ i is the radar incident angle of S0, β fore,k is the sea surface target motion vector The angle between the kth interferometer radar interference phase center line of sight and the projection on the sea surface, θ fore,k is the lower viewing angle of the kth interference pair, Δt is the motion vector of the sea surface target Duration of exercise;

[0011] Combined with the interference time baseline and observation angle, according to the system interference phase Inversion, we get the one-dimensional velocity v of the sea surface flow field k .

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

[0013] S0 sends a radar signal, S0~S 10 Receive radar echo signal and determine S iThe Doppler frequency shift f when the satellite receives the echo signal di , i = 0 ~ 10, and according to the Doppler frequency shift f di , determine S i The sea surface distance velocity v corresponding to the satellite di .

[0014] In combination with the first aspect, in some implementations of the first aspect, f di =f DC -f Si -f w -f B , where i = 0 to 10, f DC is the Doppler center frequency of the ocean surface scatterer, f Si YesS i The Doppler frequency shift caused by the relative motion between the satellite and the Earth, f w is the Doppler frequency shift caused by the sea surface wind field, f B is the Doppler frequency shift produced by the Bragg scattered wave;

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

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

[0017] The sea surface temperature is used to provide auxiliary information for judging the flow direction of the sea surface flow field; the sea surface temperature is inverted by the passive receiving mode; the ocean currents are divided into wind currents, density currents and compensation currents; the direction of the ocean currents is judged by the isothermal gradient lines of the sea surface temperature distribution.

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

[0019] In the wind measurement mode, S0 sends a radar signal, and S1~S4 simultaneously receive radar echo signals to obtain multiple sea surface wind vector solutions. The only sea surface wind vector solution is obtained through vector synthesis and used as the sea surface wind field information; the surface drift u caused by the sea surface wind field inversion wd .

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

[0021] In the altimeter imaging mode, S0 sends radar signals, S9, S 10At the same time, the radar echo signal is received, the interferometric altimeter detection is inverted to obtain the sea surface height, and the ocean geostrophic flow field is inverted; among them,

[0022] The height of the ground target point P and the interferometric phase The relationship is:

[0023]

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

[0025] The geostrophic flow balance equation is

[0026]

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

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

[0029] In the altimeter imaging mode, S0 is self-transmitting and self-receiving to invert the wave spectrum, and then calculate the significant wave height and average wave period;

[0030] The sea surface backscatter coefficient can be calculated using the radar equation:

[0031]

[0032] Where P r Indicates the received echo power, P t Represents radar transmission power, G t , G r are the gains of the radar transmitting antenna 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 azimuth angle on the horizontal plane;

[0033] The modulation function of the slant range signal is:

[0034]

[0035] σ 0(R, φ) represents the sea surface backscatter 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 backscatter coefficient, that is, the sea surface backscatter coefficient under calm sea surface;

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

[0037]

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

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

[0040]

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

[0042]

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

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

[0045]

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

[0047] In a second aspect, a high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface is provided, the method being applied to a one-transmit-multiple-receive low-orbit compound-eye SAR satellite system, the satellite system comprising a SAR satellite S0 having both transmitting and receiving functions, and satellites S5 to S8 in the same orbit as S0, S5 and S6 being located behind the flight orbit, S7 and S8 being located in front of the flight orbit, and the distances between S5 and S0, between S6 and S0, between S7 and S0, and between S8 and S0 are all within a range of 100 to 1000 kilometers; the method comprising:

[0048] S0 sends the radar signal, S5 and S6 receive the radar signal and form interference pair 1, S7 and S8 receive the radar signal and form interference pair 2, and the radar signal received by interference pair 1 determines the system interference phase. The system interference phase is determined by the radar signal received by interference pair 2 Combined with the interference time baseline and observation angle inversion, the sea surface flow field velocity v1 corresponding to interference pair 1 and the sea surface flow field velocity v2 corresponding to interference pair 2 are obtained as the along-track interferometric measurement results of the two-dimensional flow field, wherein the difference between the vector angle obtained by radially projecting the sea surface flow field velocity v1 and the sea surface flow field velocity v2 to the sea surface and 90° is less than the preset angle.

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

[0050] (1) High-precision two-dimensional sea surface flow field measurement. Multiple along-track baselines obtain information such as scattering characteristics, Doppler spectra, and interference 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; the sea surface height is measured to assist in determining the direction and position of the ocean current; wave measurement and temperature measurement provide auxiliary information for interferometric measurement, improving the efficiency of interferometric measurement; on the basis of the two degrees of freedom (range signal bandwidth, azimuth Doppler) sensor configuration of traditional two-dimensional SAR imaging, three dimensions of 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 realized in one transit, thereby improving measurement accuracy and obtaining data in a timely manner.

[0051] (2) Comprehensive measurement of multiple parameters on the ocean surface. Based on the system configuration and multiple loads, the present invention can simultaneously measure the sea surface height, sea surface temperature, sea surface wind field, and wave spectrum. The radial sea surface velocity can be measured by Doppler velocity measurement. The altimeter and cross-track interferometry can measure the sea surface elevation and geostrophic flow. Multi-baseline along-track interferometry provides multi-angle radial sea surface velocity measurement, realizing one-time multi-parameter measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1The present invention provides a flow chart of a high-precision two-dimensional flow field measurement method based on multiple parameters of the ocean surface.

[0053] Figure 2 This is a schematic diagram of the configuration of a one-launch, multiple-receiver low-orbit compound-eye SAR satellite system.

[0054] Figure 3 It is a schematic diagram of imaging incident angles in different working modes.

[0055] Figure 4 This is a schematic diagram of some nodes in a one-transmit-multiple-receive system.

[0056] Figure 5 It is a schematic diagram of the phase difference of the transmitting beam.

[0057] Figure 6 It is a schematic diagram of the phase difference of the receiving beam. DETAILED DESCRIPTION

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

[0059] like Figure 1 As shown, the present invention provides a high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface. This method can be applied to Figure 2 The single-launch, multi-receiver low-orbit compound-eye SAR satellite system shown.

[0060] The one-transmit-many-receive low-orbit compound-eye SAR satellite system includes S0~S 10 SAR satellites, among which S0 is a SAR satellite with both transmitting and receiving functions, S5~S8 are in the same orbit as S0, and are respectively distributed hundreds of kilometers ahead and behind the flight direction of S0. S5, S6, S7, and S8 are controlled within the effective range of the corresponding design frequency band along the track baseline, and S9, S 10 In the same zero Doppler plane as S0, there is only a cross-track baseline between the satellites. S1~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 with the sensor configuration of traditional two-dimensional imaging, the above-mentioned single-launch, multi-receiver low-orbit compound-eye SAR satellite system has added three dimensions of observation freedom, including the observation freedom of the along-track baseline, the observation freedom of the cross-track baseline, and the multi-angle observation freedom of the along-track + cross-track baseline, aiming to solve the difficulties of measuring all elements of the sea surface and two-dimensional flow fields. Among them, the along-track baseline mainly measures the two-dimensional sea surface flow field; the cross-track baseline mainly measures the height of the ocean surface, calculates the ocean geostrophic flow field, and assists in determining the ocean current area and direction; the multi-angle observation freedom mainly measures the two-dimensional sea surface wind field to improve the accuracy of the inverted flow field.

[0062] The above-mentioned one-launch-multiple-receive low-orbit compound-eye SAR satellite system has multiple working modes, including SAR imaging mode, altimeter imaging mode, wind measurement mode, and passive receiving mode. Among them, the SAR imaging mode is S0 launch, S0~S 10 Receive simultaneously; in wind measurement mode, S0 sends, and S1~S4 receive simultaneously; in altimeter imaging mode, S0 sends, and S0, S9~S 10 Simultaneous reception includes interferometric altimeter imaging and spectrometer imaging modes; the passive reception mode is S0 reception. Acquisition of multiple parameters of the ocean surface is achieved in one pass. The incident angles of the SAR imaging mode and the altimeter imaging mode are complementary, and the imaging area can be fully covered, such as Figure 3 shown.

[0063] exist Figure 1 In the high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface, when in SAR imaging mode, S0 emits 10 The echo is received to obtain the sea surface scattering characteristics, Doppler spectrum information, and Doppler frequency shift to invert the sea surface radial flow field. The specific steps include at least one of the following.

[0064] S101, when in SAR imaging mode, sent by S0, S0~S 10 Receive the echo and determine S i The Doppler frequency shift f when the satellite receives the echo signal di , i = 0 ~ 10, and according to the Doppler frequency shift f di , determine S i The sea surface distance velocity v corresponding to the satellite di .

[0065] Assuming that the corresponding transmission frequency of the satellite signal is f, after the signal propagates over a long distance, due to the relative motion between the satellite and the target sea area, the actual frequency f when the satellite receives the echo signal r There is a deviation from the original frequency f, and this frequency deviation is the Doppler frequency shift f d .

[0066] In fact, the Doppler frequency shift of the ocean surface is not only affected by the relative speed of the earth and the satellite, but also by the sea surface wind field and Bragg scattering. Therefore, the Doppler center frequency f of the scatterer on 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 at multiple angles. It is assumed that the Doppler center frequencies of the sea surface wind field, Bragg scattering and ocean surface scatterers are consistent, and the difference in receiving echoes at different nodes lies in the relative motion speed of the earth and the satellite and the incident angle.

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

[0068] Where i = 0 to 10, f Si YesS i The Doppler frequency shift caused by the relative motion between the satellite and the Earth, f w is the Doppler frequency shift caused by the sea surface wind field, f B It is the Doppler frequency shift produced by the Bragg scattered wave.

[0069] Figure 4 This is a schematic diagram of the flight of some nodes. According to the Doppler frequency shift f di , S i The satellite measured the sea surface distance velocity v di (V is the projection of the radar line of sight, V is the relative speed between the satellite and the target sea area, and the radial speed along the radar line of sight is negative when approaching the radar and positive when moving away from the radar) can be expressed as:

[0070]

[0071] In the formula, θ i is the satellite node incident angle, k R is the radar wave number.

[0072] S102, when in SAR imaging mode, S0 sends, S5-S8 receive simultaneously, and along-track interferometry is performed to obtain a two-dimensional flow field.

[0073] Along-track interferometry consists of two or more satellites in the same orbit. They image the same observation area at the same position but at different times. The time of the two images is accurately calibrated, and the time difference is the along-track interferometry time baseline. Two satellites form an interferometry pair. The radar antenna observes the same area twice at the same position. The slight "system" movement during the two data acquisitions will bring completely different effects. The "system" refers to the surface scattering element within a pixel unit remains constant, but the distance from the antenna to the ocean surface area changes. This slight distance change will bring about a phase difference between the two interferometric images. The key quantity is the component of the motion in the slant range direction.

[0074] S0 sends, S5~S8 receive data and perform interference processing at the same time. S0 sends, S5, S6 receive to form interference pair 1, S0 sends, S7, S8 receive to form interference pair 2. The interference phase of the system is determined by interference pair 1 and interference pair 2 respectively. and The one-dimensional velocities v1 and v2 of the sea surface flow field are obtained by combining the interference time baseline and the observation angle inversion.

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

[0076]

[0077] is the two-dimensional flow field velocity, and D is the center point vector of the first imaging and second imaging areas. 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, and B is the physical baseline of the phase center of the imaging radar of satellite 1 and satellite 2.

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

[0081]

[0082] Since, R1>>B,D

[0083]

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

[0085] Considering pure along-track interference, the received phase is

[0086]

[0087] Among them, v r =vcosβ fore cosθ fore , v r for Projection in the receive beam direction, β fore is the sea surface target motion vector The angle between the radar interference phase center line of sight and the projection on the sea surface, θ fore is the lower viewing angle, sinθ fore =H / R1, R1 is the radar slant range when satellite 1 is imaged in a certain interferometric alignment, and H is the satellite altitude.

[0088] Ignoring the error, the system interference phase According to the received beam interference phase and the transmit beam slant range phase difference Sure (refer to Figure 5 , β v It represents the angle between the flow field velocity and the projection of the transmitting beam on the ground, d is the distance between the center points of the two imaging areas, λ is the radar wavelength). Therefore:

[0089]

[0090] In some embodiments, taking into account multiple groups of interference, there is interference pair 1: S0 transmits, S5, S6 receive; interference pair 2: S0 transmits, S7, S8 receive.

[0091]

[0092] Interference processing requires that the coherence of the two images before and after is very high. The different phase information of each pixel is obtained by calculation and observation. The one-dimensional velocity v of the sea surface flow field is obtained by combining the interference time baseline and the observation angle inversion. k .

[0093] S0 is a side-on-looking satellite, and S5 and S6 are located behind the flight track, with a distance of several hundred kilometers between them and S0. The vector direction of the sea surface flow field velocity v1 obtained by the above method can be determined based on the positional relationship among S0, S5 and S6. The vector of the sea surface flow field velocity v1 is radially projected onto the sea surface to obtain vector 1. Similarly, S7 and S8 are located in front of the flight track, with a distance of several hundred kilometers between them and S0. The vector direction of the sea surface flow field velocity v2 obtained by the above method can be determined based on the positional relationship among S0, S7 and S8. The vector of the sea surface flow field 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 field velocities v1 and v2 can reflect the two-dimensional information of the sea surface flow field velocity. Through coordinate transformation, the sea surface flow field information in the longitude and latitude phases can be obtained. The sea surface flow field velocities v1 and v2 can be synthesized by vectors to enhance the existing sea surface flow field from radial to two-dimensional.

[0094] S103, when in SAR imaging mode, is sent by S0, S0~S 10 Simultaneous reception and multi-angle interferometry are used to obtain the two-dimensional flow field.

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

[0096] In some embodiments, 11 sea surface flow field vector solutions v can be constructed in the following way k ,k=0,1,…,C2 10:Interference pair k=0, S0 sends, S0, S1 receives; Interference pair k=1, S0 sends, S1, S2 receives; Interference pair k=2, S0 sends, S2, S3 receives; Interference pair k=3, S0 sends, S3, S4 receives; Interference pair k=4, S0 sends, S4, S5 receives; Interference pair k=5, S0 sends, S5, S6 receives; Interference pair k=6, S0 sends, S6, S7 receives; Interference pair k=7, S0 sends, S7, S8 receives; Interference pair k=8, S0 sends, S8, S9 receives; Interference pair k=9, S0 sends, S9, S10 receives 10 Receive; Interference pair k = 10, S0 transmit, S 10 ,S0 received.

[0097] Solve multiple sea surface flow vectors v k By using two sea surface flow field vectors (k=0,1,…,10) that are as close to orthogonal as possible, the sea surface flow field can be improved from radial to two-dimensional through vector synthesis.

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

[0099] Step 104, providing auxiliary information for judging the flow direction of the sea surface flow field based on the sea surface temperature. The sea surface temperature is obtained by inversion of the passive receiving mode. Ocean currents are divided into wind currents, density currents and compensation currents. The density current is caused by the uneven distribution of seawater density in the horizontal direction due to the different horizontal distribution of seawater temperature and salinity in different sea areas, which causes the isobaric surface to tilt. Therefore, the direction of the ocean current can be judged by the isothermal gradient lines of the sea surface temperature distribution.

[0100] Step 105, wind measurement mode, S0 sends, S1-S4 receive simultaneously, and invert to obtain the sea surface wind field.

[0101] The sea surface not only radiates electromagnetic energy, but also reflects and scatters external electromagnetic energy. The wind field on the sea surface can be obtained by using the reflection and scattering characteristics. When the incident angle is between 20° and 80°, the sea surface backscatter coefficient σ 0 It increases with the increase of ocean wind speed, decreases with the increase of incident angle, and is related to polarization; the sea surface wind causes sea surface wind waves, changes the flatness of the sea surface, and causes the sea surface backscattering coefficient σ 0 Change, thus the relationship between the backscatter coefficient and wind speed and wind direction can be established. The backscatter coefficient is measured by the instrument, and the wind speed and wind direction of each wind vector unit are inverted based on the geophysical model function CMOD5.0. The multi-angle receiving method will obtain multiple sea surface wind vector solutions, and the unique sea surface wind vector solution is obtained through vector synthesis and used as the sea surface wind field information. The surface drift u caused by the wind field can be inverted from the sea surface wind field wd .

[0102] Step 106, altimeter imaging mode, S0 sends, S0, S9, S 10 At the same time, the sea surface height can be obtained by receiving and inverting the interferometric altimeter detection.

[0103] Cross-track interferometry consists of two satellites with the same track direction and different cross-track directions. The two satellites observe the same area at the same time and accurately measure the distance between the two satellites in the cross-track direction. This is the cross-track interferometry spatial baseline. Two satellites observe and image the same sea area at the same time to obtain the interference phase of the two images. The interference phase is a function of the difference in slant distance from the radar antenna to the target point during the two images. The sea level elevation information can be calculated. It can not only assist in determining the direction and position of ocean currents by measuring the sea surface height, but also invert the ocean geostrophic flow field.

[0104] Assuming that the scattering characteristics of the sea surface resolution unit are the same during the two imagings, the cross-track interference phase difference obtained by interfering the two images is:

[0105]

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

[0107] The height of the ground target point P and the interferometric phase can be derived The relationship is:

[0108]

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

[0110] The velocity of the geostrophic current is obtained by assuming that the friction between the sea breeze and the sea water is zero and the acceleration of the ocean current is zero. The formula is:

[0111]

[0112] Where: u f 、v f They represent the meridional velocity (eastward) and the zonal velocity (northward). f is the Coriolis force parameter, ω represents the angular velocity of the Earth's rotation. is the geographical latitude of the target. g is the gravitational acceleration in the ocean, which varies with latitude. h is the average dynamic terrain, and x and y represent the latitude and longitude directions respectively.

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

[0114] The sea surface backscatter coefficient can be calculated using the radar equation:

[0115]

[0116] Where P r Represents the received echo power, P t Represents radar transmission power, G t , G r are the gains of the radar transmitting antenna 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 azimuth angle on the horizontal plane.

[0117] The modulation function of the slant range signal is:

[0118]

[0119] Among them, σ 0 (R, φ) represents the sea surface backscatter 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 backscatter coefficient, that is, the sea surface backscatter coefficient under calm sea surface.

[0120] The slant range modulation function is projected onto the horizontal plane to measure the wave information m(X,φ) propagating along the projection direction of the beam on the sea surface. The horizontal modulation function m(X,φ) is cross-correlated, and then Fourier transform and power spectrum estimation are performed to obtain the modulation spectrum:

[0121]

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

[0123] By integrating F(n,φ) in azimuth, we can get the omnidirectional wave height spectrum:

[0124]

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

[0126]

[0127] According to F(n,φ), the velocity contribution u0 of large-scale waves and the velocity contribution u of Bragg waves can be inverted. b .

[0128] Step 108, comprehensive sea surface multi-parameter inversion.

[0129] The present invention provides Figure 2 The system configuration shown realizes a single-pass sea surface multi-parameter measurement, including: multiple sea surface radial velocities measured by the Doppler frequency shift method in step 101 (applicable to large-scale ocean flow fields (e.g., a range of several hundred kilometers)); multiple sea surface radial velocities measured by the along-track interference method in step 102 or the multi-angle interference method in step 103 (applicable to small and medium-sized nearshore ocean flow fields (e.g., within 10 kilometers)); the sea surface temperature field provided in step 104 can assist in determining the direction of the sea surface flow field; step 105 measures the sea surface wind field; step 106 measures the sea surface height, which, on the one hand, assists in determining the direction and position of the ocean current, and on the other hand, can calculate the geostrophic flow field, and the wave spectrum can be used to invert the flow field, combined with step 107.

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

[0131]

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

[0133] The sea surface temperature field isotherms in step 104 can give the temperature gradient, and assist in correcting the flow direction of the two-dimensional flow field that does not conform to the temperature gradient. The multi-angle interferometric measurement in step 103 can reduce the sea surface flow field measurement error under complex sea conditions.

[0134] In summary, the present invention provides a high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface, which is mainly aimed at the comprehensive measurement of multi-parameters of the ocean surface under complex sea conditions and the two-dimensional high-precision detection of small and medium-scale ocean currents. The method provides a spatial configuration of a satellite-borne compound eye SAR system for multi-parameter measurement of the ocean surface, which is similar to the compound eye of the visual system in biology. By increasing the degree of observation freedom and coherent processing, the ability to observe dynamic information is greatly improved, so it is referred to as a compound eye SAR system. The system also has the ability to measure multiple ocean elements such as Doppler velocity measurement, interferometric height measurement, measurement of sea surface wind field, measurement of sea surface flow field phase, measurement of wave spectrum, and measurement of sea surface temperature. First, along-track and vertical track interferometric measurement is the basic mode. SAR and altimeter measure sea surface height to assist in determining the direction and position of ocean currents; scatterometer measures sea surface wind field to help remove wind-induced waves and improve the accuracy of ocean current measurement; spectrometer measures wave spectrum and radiometer measures sea surface temperature to provide auxiliary information for interferometric measurement, providing a basis for determining the direction of ocean currents.

[0135] In interferometric measurement, the along-track baseline can provide along-track interference for SAR imaging, and the along-track interference SAR complex image can invert the radial velocity of the 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, measure the height difference between the ocean current area and other sea areas to assist in determining the direction and position of the ocean current, and invert the ocean geostrophic flow field; the mixed baseline can provide the system imaging with along-track + cross-track baseline multi-angle observation freedom. The system can obtain the multi-angle scattering characteristics of the sea surface, which helps to avoid image saturation under complex sea conditions, and invert the two-dimensional sea surface wind field and waves through multi-angle interference complex images. When inverting the two-dimensional sea surface flow field, removing the ocean background field (sea surface wind, waves, sea surface height) can further improve the measurement accuracy of the two-dimensional flow field.

[0136] Although the present invention is disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may 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 shall be based on the scope defined by the claims of the present invention.

Claims

1. A high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface, characterized in that: The method is applied to a one-transmit-multiple-receive low-orbit compound-eye SAR satellite system, wherein the satellite system comprises a SAR satellite S0 with both a transmitting and receiving function and SAR satellites S1 to S2 with a receiving function. 10 , S1~S4 are distributed on both sides of the flight direction of S0, and the projection angle of the radar line of sight direction corresponds to 90°. S5~S8 are located in the same orbit as S0. S5 and S6 are located behind the flight orbit, and S7 and S8 are located in front of the flight orbit. The distances between S5 and S0, between S6 and S0, between S7 and S0, and between S8 and S0 are all within the range of 100 to 1000 kilometers. S5, S6, S7, S8 are controlled within the effective range of the corresponding design frequency band along the track baseline. S9, S 10 The zero Doppler plane is the same as S0, and only the cross-track baseline exists between the satellites; the method comprises: S0 sends the radar signal, S0~S 10 receiving radar signals simultaneously to form a plurality of interference pairs, and then obtaining a plurality of sea surface flow field vectors corresponding to the plurality of interference pairs one by one, wherein each interference pair determines the corresponding sea surface flow field velocity by 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 a plurality of sea surface flow field vectors as the multi-angle interferometric measurement results of the two-dimensional flow field.

2. The method according to claim 1, characterized in that For the kth interference pair, the system interference phase satisfy: v k is the sea surface flow velocity measured by the kth interferometer pair, β v represents the angle between the flow field velocity and the projection of the transmitting beam on the ground, θ i is the radar incident angle of S0, β fore,k is the sea surface target motion vector The angle between the kth interferometer radar interference phase center line of sight and the projection on the sea surface, θ fore,k is the lower viewing angle of the kth interference pair, Δt is the motion vector of the sea surface target Duration of exercise; Combined with the interference time baseline and observation angle, according to the system interference phase Inversion, we get 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 comprises: S0 sends radar signal, S0~S 10 Receive radar echo signal and determine S i The Doppler frequency shift f when the satellite receives the echo signal di , i = 0 ~ 10, and according to the Doppler frequency shift f di , determine S i The sea surface distance velocity v corresponding to the satellite di .

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

5. The method according to claim 1 or 2, characterized in that: The method further comprises: The sea surface temperature is used to provide auxiliary information for judging the flow direction of the sea surface flow field; the sea surface temperature is inverted by the passive receiving mode; the ocean currents are divided into wind currents, density currents and compensation currents; the direction of the ocean currents is judged 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 comprises: In the wind measurement mode, S0 sends a radar signal, and S1~S4 simultaneously receive radar echo signals to obtain multiple sea surface wind vector solutions. The only sea surface wind vector solution is obtained through vector synthesis and used as the sea surface wind field information; the surface drift u caused by the sea surface wind field inversion wd .

7. The method according to claim 1 or 2, characterized in that: The method further comprises: In the altimeter imaging mode, S0 sends radar signals, S9, S 10 At the same time, the radar echo signal is received, the interferometric altimeter detection is inverted to obtain the sea surface height, and the ocean geostrophic flow field is inverted; among them, The height of the ground target point P and the interferometric phase The relationship is: H is the satellite altitude, B is the S9 image and S 10 The physical baseline of the imaging radar phase center, θ is the radar side angle of view, and α is the angle between the baseline B and the horizontal direction; The geostrophic flow balance equation is Where: u f 、v f represent the longitudinal velocity and latitudinal velocity respectively; f is the Coriolis force parameter, ω represents the angular velocity of the Earth's rotation. is the geographical latitude of the target; g is the gravitational acceleration in the ocean, which varies with latitude; h is the average dynamic terrain, and x and y represent the latitude and longitude directions respectively.

8. The method according to claim 1 or 2, characterized in that: The method further comprises: In the altimeter imaging mode, S0 is self-transmitting and self-receiving to invert the wave spectrum, and then calculate the significant wave height and average wave period; The sea surface backscatter coefficient can be calculated using the radar equation: Where P r Represents the received echo power, P t Represents radar transmission power, G t , G r are the gains of the radar transmitting antenna 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 azimuth angle on the horizontal plane; The modulation function of the slant range signal is: σ 0 (R, φ) represents the sea surface backscatter 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 backscatter coefficient, that is, the sea surface backscatter coefficient under calm sea surface; The slant range modulation function is projected onto the horizontal plane to measure the wave information m(X,φ) propagating along the projection direction of the beam on the sea surface; the horizontal modulation function m(X,φ) is cross-correlated, and then Fourier transform and power spectrum estimation are performed to obtain the modulation spectrum F(n,φ) is the wave directional spectrum, n is the wave number, φ is the propagation direction of the wave, L y is the footprint width in azimuth, ζ is the sea surface wave steepness; A(θ) is the modulation transfer function; The azimuth integration of F(n,φ) yields the omnidirectional wave height spectrum: The significant wave height is calculated from the wave spectrum: According to the inversion of F(n,φ), the velocity contribution u0 of the large-scale wave and the velocity contribution u of the Bragg wave are obtained. 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 in wd , the velocity contribution u0 of large-scale waves and the velocity contribution u of Bragg waves b , and the sea surface velocity u is obtained c .

10. A high-precision two-dimensional flow field measurement method based on multi-parameters of the ocean surface, characterized in that: The method is applied to a one-transmit-multiple-receive low-orbit compound-eye SAR satellite system, the satellite system comprising a SAR satellite S0 with both transmitting and receiving functions, and satellites S5 to S8 in the same orbit as S0, S5 and S6 are located behind the flight orbit, S7 and S8 are located in front of the flight orbit, and the distances between S5 and S0, between S6 and S0, between S7 and S0, and between S8 and S0 are all within a range of 100 to 1000 kilometers; the method comprises: S0 sends the radar signal, S5 and S6 receive the radar signal and form interference pair 1, S7 and S8 receive the radar signal and form interference pair 2, and the radar signal received by interference pair 1 determines the system interference phase. The system interference phase is determined by the radar signal received by interference pair 2 Combined with the interference time baseline and observation angle inversion, the sea surface flow field velocity v1 corresponding to interference pair 1 and the sea surface flow field velocity v2 corresponding to interference pair 2 are obtained as the along-track interferometric measurement results of the two-dimensional flow field, wherein the difference between the vector angle obtained by radially projecting the sea surface flow field velocity v1 and the sea surface flow field velocity v2 to the sea surface and 90° is less than the preset angle.

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