A dual-beam SAR system and method for measuring sea surface height and ocean current velocity

Abstract

The present invention discloses a dual-beam SAR system and method for measuring sea surface height and ocean current velocity. The system comprises three sets of identical X-band antennas Xa, Xb and Xc, three sets of X-band receivers, a digital stand-alone unit, a reference source, an excitation source, an internal calibrator, two servos, two wave controllers, and two sets of position and attitude measurement devices. Along-track and cross-track interferometric baselines are formed, the lengths of which are calculated respectively, and a timing sequence for time-sharing calibration of the three antenna arrays is designed, which is used for calibration of an in-cabin equipment link, a receiving link, and a transmitting link respectively. The dual-beam pointing of the antennas is controlled so that the three arrays collect data from the same target area. Cross-track interferometric data of the front channel or rear channel of the Xa and Xc antennas are collected to calculate the sea surface height of the target area. Along-track interferometric data of the front and rear channels of the Xa and Xb antennas are collected to calculate the ocean current velocity of the target area.

Description

Technical Field

[0001] The present invention belongs to the field of radar technology, and in particular relates to an ocean surveying and mapping technology. Background Art

[0002] Synthetic Aperture Radar (SAR) emits electromagnetic waves toward the ground and simultaneously receives reflected echoes. Using synthetic aperture technology, it achieves two-dimensional, high-resolution imaging of the target or scene. It boasts long-range detection, all-day operation, and minimal weather impact. Interferometric SAR (InSAR) is divided into cross-track InSAR and along-track InSAR. Using antennas at different locations to illuminate and image the same area, the resulting interferometric phase pattern provides information on elevation or target motion.

[0003] Cross-track interferometry measures terrain elevation information. Typical foreign systems include the United States' SRTM and Germany's TanDEM-X / Terra SAR-X dual-satellite system. SRTM carries a main antenna, a retractable antenna mast, and an extravehicular antenna on the space shuttle. There are two main antennas, one for transmitting and receiving C- and X-band signals; the extravehicular antenna is only used for receiving signals. The two are connected by a retractable antenna mast to form an interferometric baseline. SRTM maps the area between 60° north latitude and 56° south latitude, obtaining land elevation information for 80% of the world. Germany uses the Terra SAR-X and Tan DEM-X satellites, which were launched twice in succession, to fly in formation, forming a dual-station interferometric measurement mode. The distance between the two satellites is flexibly adjustable to obtain continuous, high-resolution global DEM data.

[0004] Along-track interferometric ocean current measurement, such as the German EURoPAK project in the early 2000s, used airborne platforms to measure ocean currents and invert coastal underwater topography, achieving relatively successful results. Regarding spaceborne applications, there are currently no in-orbit interferometric SAR (SAR) ocean current measurement satellites. However, several SAR satellite systems, both domestic and international, have conducted experiments using azimuth multi-channel technology. For example, the Tan DEM-X / Terra SAR-X system, the SRTM system, and my country's Gaofen-3 satellite have all conducted experiments with along-track interferometric ocean current measurement.

[0005] With the increasing demand for ocean current measurements in related fields, the international community is developing a new ocean current vector measurement satellite, the Wavemill system. This system uses dual-beam, forward and backward squinting beams to obtain two-dimensional velocity data. While this system has not yet been successfully developed and is still in the design phase, Stephen J. Frasier, in his paper "Dual-Beam Interferometry for Ocean Surface Current Vector Mapping," introduced a dual-beam squinting, along-track interferometry method for obtaining ocean surface current information. This method theoretically demonstrates the feasibility of using along-track interferometry to obtain two-dimensional ocean current velocities.

[0006] Conventional interferometry systems are mostly single systems that can only perform along-track interferometry or off-track interferometry. If both functions are to be achieved, two SAR systems need to be developed, which requires a large number of antennas and corresponding back-end equipment and is costly. More advanced interferometry systems can achieve both along-track and off-track interferometry, but they cannot work simultaneously. The platform configuration needs to be changed when switching functions, and the engineering implementation is relatively complex. It is imperative to develop an ocean mapping SAR system that integrates along-track interferometry and off-track interferometry functions, which can measure sea surface elevation while also obtaining information on the speed of ocean currents. Summary of the Invention

[0007] To address the problems of the prior art, the present invention proposes a dual-beam SAR system and method for measuring sea surface height and ocean current velocity. This system, without changing the platform configuration, can simultaneously form both along-track and off-track interferometric baselines. Conventional SAR and dual-beam SAR echo data can be collected simultaneously in a single pass. To achieve the above objectives, the present invention adopts the following technical solutions.

[0008] The system includes three sets of identical X-band antennas Xa, Xb and Xc, three sets of X-band receivers, one digital single unit, one reference source, one excitation source, one internal calibrator, two servos, two wave control machines and two positions and attitude measurement devices.

[0009] The X-band antenna is used for signal transmission and echo reception. The X-band receiver is used for low-noise amplification, down-conversion, filtering and digital sampling of echo signals. The digital stand-alone unit is used for parameter calculation, timing generation and mode control of the radar system. The reference source is used to provide a 100MHz reference signal and various clocks for the system. The excitation source is used to generate a highly stable, low-phase-noise RF excitation signal. The internal calibrator and antenna jointly complete the calibration of the system's transmitting and receiving links. The servo and beam control machine are used to control the pointing direction of the antenna beam. The position and attitude measurement device is used to compensate for the beam pointing deviation caused by the platform attitude and compensate for motion errors in subsequent data processing.

[0010] In the transmitting state, the excitation source outputs the RF signal, which is driven and amplified, input into the antenna array, and radiates electromagnetic waves in a specific direction; in the receiving state, the antenna unit receives the reflected signal, synthesizes it through the channel, and inputs it into the corresponding receiver to complete the down-conversion, filtering and digital sampling of the echo signal.

[0011] Step 1: Adjust the antenna installation position and spacing to form along-track and off-track interferometry baselines, and calculate their lengths. The Xa and Xb antennas are installed on the left side of the system platform to form an along-track interferometry baseline. The Xc antenna is installed on the right side of the platform to form an off-track interferometry baseline with the Xa antenna. Each set of antennas has two channels, front and back, and each channel operates independently.

[0012] Furthermore, let the platform flight speed be Meters per second, the length of the along-track interference baseline is m, imaging delay is Seconds, using the formula Calculate the length of the along-track interference baseline; assume the platform flight altitude is Meters, the length of the cutting rail interference baseline is meters, the baseline height ratio is , using the formula Calculate the cutting rail interference baseline length.

[0013] Step 2: Design the timing of the time-sharing calibration of the three antenna arrays, including reference calibration, receiving calibration and transmitting calibration, which are used for the calibration of the cabin equipment link, receiving link and transmitting link respectively. The timing includes: the excitation source outputs three calibration time signals, which pass through different links and input into the corresponding receivers; the reference calibration time signal passes through the calibrator and power divider; the receiving calibration time signal passes through the calibrator and calibration link and inputs the antenna array; the transmitting calibration time signal is coupled by the antenna array, passes through the calibration link, and inputs the calibrator and power divider.

[0014] Furthermore, in dual-beam mode, a unified transmit and receive timing is designed for the three antenna arrays. The Xa and Xb antennas transmit signals to the front and rear sides simultaneously, and the front and rear channels of the three antennas receive the echoes from the front and rear sides simultaneously according to the time interval corresponding to the input wave gate.

[0015] Step 3: The digital single machine uniformly controls the two sets of beam control machines and servos, controls the dual-beam pointing of the antenna, and enables the three arrays to collect data from the same target area, including: based on the input squint angle and the attitude angle fed back by the platform, the digital single machine generates the beam position parameters, and the beam control machine controls the antenna beam to point to a specific azimuth direction; based on the ground distance information fed back by the platform, the digital single machine generates command information, and the servo controls the antenna beam to point to a specific distance direction.

[0016] Step 4: Collect cross-track interferometry data from the front or rear channels of the Xa and Xc antennas to calculate the sea surface elevation of the target area, including: using an imaging algorithm to generate two SAR complex images based on the cross-track interferometry data, extracting interference fringes through fine registration, phase filtering, and conjugate multiplication of the complex images, and obtaining the absolute phase difference through phase unwrapping and flat-ground phase recovery. , invert sea surface elevation information.

[0017] Furthermore, let the distances from antennas Xa and Xc to the center of the target area be , the carrier length is Meters, the lower viewing angle is Radians rad, using the formula calculate , using the formula Calculate sea surface elevation.

[0018] Step 5: Collect along-track interferometric data from the front and rear channels of the Xa and Xb antennas to calculate the ocean current velocity in the target area, including: using an imaging algorithm to generate four SAR complex images based on the along-track interferometric data, extracting interference fringes, performing phase noise reduction and phase unwrapping to eliminate blur and obtain the absolute interferometric phase, and calibrating and correcting to eliminate the on-track component in the phase and compensate for the phase offset to obtain the interferometric phase difference. , invert ocean current velocity information.

[0019] Furthermore, let the oblique angle be , the ocean current speed in the line of sight is , using the formula calculate , using the formula Calculate the radial velocity of the ocean current using the formula Calculate the heading speed of an ocean current.

[0020] The beneficial effects of the present invention are as follows: the track-following and track-cutting operations can be performed simultaneously, data can be acquired efficiently, the amount of equipment can be reduced, and there are significant advantages in terms of feasibility, function, cost, weight and link consistency control. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the system block diagram, Figure 2 This is a schematic diagram of the installation of three sets of antennas. Figure 3 This is the flow chart of rail cutting interference processing. Figure 4 is the geometric relationship diagram of the cutting rail interference, Figure 5 This is the flow chart of the along-track interferometric two-dimensional flow field velocity measurement process. Figure 6 This is the timing diagram of the calibration in the system. Figure 7 This is the timing diagram of dual beam operation. Figure 8 It is the transmitting and receiving beam pointing diagram of the dual-beam squint working antenna. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is described in detail below with reference to the embodiments and drawings.

[0023] according to Figure 1 Connect the system devices as shown in the figure. Figure 2 The antenna is installed as shown. During the test, the platform carrying the system was the "Yun-8" aircraft, with a flight altitude of 7500m and a flight speed of 150m / s.

[0024] The typical cutting rail baseline ratio is 5×10^-4. According to the formula, the cutting rail interference baseline length of the system is , that is, the distance between Xa and Xc antennas is 3.75m.

[0025] During imaging processing, time and image resolution are negatively correlated, that is, the longer the exposure time, the smaller the resolution unit and the higher the resolution. There is a certain inverse relationship between the speed measurement sensitivity and the imaging delay, that is, the smaller the imaging delay, the greater the speed measurement sensitivity value.

[0026] In order to ensure good coherence between signals during interference processing, the imaging delay shall not be greater than 5ms. At an incident angle of 45°, if the speed measurement sensitivity value is to be guaranteed to be below 0.03m / s, the imaging delay shall not be less than 2ms.

[0027] Based on the analysis of various indicators, the system takes the imaging delay as 4ms. Under the condition of platform speed of 150m / s, the length of the along-track interferometric baseline is , that is, the distance between Xa and Xb antennas is 1.2m.

[0028] To reduce the amount of equipment and ensure signal consistency, the three antennas use the same internal calibrator. Due to the link characteristics of transmit calibration, simultaneous transmit calibration on the three arrays will cause signal overlap, requiring the design of a time-sharing calibration sequence.

[0029] Using the first 9 frames of signal data, according to Figure 6 As shown, the reference, reception, and transmission calibration of the three arrays and the corresponding receivers are completed in sequence. When the corresponding system is working, the other systems are in a load state and there is no signal input or output.

[0030] When the dual-beam SAR is working, the timing sequence constrains the pilot signal, modulation signal, blanking signal, gate signal, etc., so that the Xa and Xb antennas transmit signals simultaneously.

[0031] According to the time interval corresponding to the input wave gate, the Xa, Xb and Xc antennas receive signals simultaneously. The three sets of antennas and corresponding receivers use the same pulse repetition frequency, fundamentally avoiding the overlap of the transmission and reception windows.

[0032] according to Figure 7The transmission and reception timing of the timing system shown in the figure, where the rectangular pulse part represents the signal transmission window, the curve part represents the signal reception window, and the dotted line part represents non-operation.

[0033] The range of the antenna beam is 0° to 180°, pointing from a parallel ground position on the left side of the system, to a perpendicular ground position, and then to a parallel ground position on the right side of the system. The azimuth range of the antenna beam is -20° to 20°, from the rear oblique area to the front oblique area of the system.

[0034] The antenna transmit and receive beams when the dual-beam system is working are pointed as follows: Figure 8 As shown, the Xa antenna transmits signals obliquely forward, the Xb antenna transmits signals obliquely backward, and the Xc antenna does not transmit signals. The front receiving channels of the three sets of antennas simultaneously receive the front oblique echo, and the rear receiving channels simultaneously receive the rear oblique echo. The oblique angles of the front and rear obliques are equal. When the azimuth is 0°, the system operates in the positive side view mode.

[0035] In sea surface elevation measurement, the small incident angle system is relatively mature. At the same time, the normalized backscatter coefficient of the sea surface decays rapidly with the increase of the incident angle, and it is mainly carried out under small incident angle conditions.

[0036] The incident angle is 5°, which is analogous to the sea surface elevation measurement test of the foreign SWOT system; the incident angle is 10°, which is analogous to the foreign DRIVE / BUSARD airborne sea surface elevation measurement test; the incident angle is 30° and higher, which tests the accuracy and feasibility of sea surface elevation measurement under large incident angle conditions.

[0037] In the measurement of ocean current velocity, information on sea surface wind is required, and experiments on both ocean currents and wind fields are required.

[0038] The dual-beam incident angle range is between 5° and 60°, and the inversion performance at different incident angles is studied; dual-beam illumination tests are carried out in different directions on the same sea area to verify the relationship between ocean current inversion accuracy and wind direction; the ratio of along-track and off-track velocity measurement accuracy is proportional to the incident angle and inversely proportional to the oblique angle.

[0039] Taking all the above into consideration, routes were planned in the north-south and east-west directions in the target sea area, the beam slant angle was set between 10° and 20°, and flight tests were carried out under the conditions of incident angles of 5°, 10° and 30°.

[0040] The steps for processing the cutting rail interference data are as follows: Figure 3 As shown in the figure, the cross-track interference data of the front channel or rear channel of the Xa and Xc antennas are obtained, and two SAR complex images are obtained through the imaging algorithm; after fine alignment, the interference fringes are extracted through phase filtering and complex image conjugate multiplication; and the absolute phase is obtained after phase unwrapping and flat ground phase recovery operations. , and then invert the sea surface elevation information of the target area.

[0041] The geometric relationship of cutting rail interference is as follows Figure 4 As shown,

[0042] The distances from antenna Xa and antenna Xc to the center of the target area are , The unit of carrier length is meter, and the viewing angle is , the unit is radian rad, then the absolute phase difference between the two is , by the absolute phase difference Get accurate downward viewing angle ,pass , and obtain the sea surface elevation information.

[0043] The along-track interferometric two-dimensional flow field velocity measurement process is as follows: Figure 5 As shown, get Under the oblique angle, the along-track interference data of the front and rear channels of the Xa and Xb antennas are processed by the imaging algorithm to produce four SAR images. The interference fringes are extracted after fine registration of the two complex images with the same oblique angle, and then eliminated through phase denoising and phase unwrapping. The blurred absolute interference phase is then calibrated and corrected to remove the track-cutting component in the phase and compensate for the phase offset, and finally the processed interference phase difference is obtained. .Depend on Get the flow field velocity in the direction of sight , according to the geometric relationship, the radial velocity and heading velocity of the ocean current are obtained: , .

Claims

1. A dual-beam SAR method for measuring sea surface height and ocean current velocity, characterized in that: The devices used include: Three identical X-band antennas, Xa, Xb, and Xc, are used for transmitting signals and receiving echoes; Three sets of X-band receivers for low-noise amplification, down-conversion, filtering and digital sampling of echo signals; A digital stand-alone unit for parameter calculation, timing generation, and mode control of radar systems; A reference source, used to provide a 100MHz reference signal and various clocks for the system; A set of excitation sources for generating highly stable, low phase noise RF excitation signals; An internal calibrator, together with the antenna, completes the calibration of the system's transmit and receive links; Two sets of servos and two sets of beam control machines for antenna beam pointing control; Two sets of position and attitude measurement devices are used to compensate for beam pointing deviations caused by the platform attitude and to compensate for motion errors in subsequent data processing; In the transmitting state, the excitation source outputs a radio frequency signal, which is driven and amplified, input into the antenna array, and radiates electromagnetic waves in a specific direction; In the receiving state, the antenna unit receives the reflected signal, synthesizes it through channels, and inputs it into the corresponding receiver to complete the down-conversion, filtering and digital sampling of the echo signal; Step 1: Adjust the installation position and spacing of the antennas to form the along-track and off-track interference baselines, and calculate their lengths respectively; including: installing the Xa and Xb antennas on the left side of the system platform to form the along-track interference baseline; installing the Xc antenna on the right side of the platform to form the off-track interference baseline with the Xa antenna; each set of antennas has two channels, front and back, and each channel works independently; assuming the platform flight speed is V meters per second, the along-track interference baseline length is B AT meters, the imaging delay is τ seconds, using formula B AT =2V×τ to calculate the length of the along-track interference baseline; suppose the platform flight altitude is H meters and the length of the cross-track interference baseline is B CT Meters, the baseline height ratio is k, using formula B CT =H×k to calculate the cutting rail interference baseline length; Step 2: Design the timing for time-sharing calibration of the three antenna arrays, including reference calibration, receive calibration, and transmit calibration, which are used to calibrate the in-cabin equipment link, receive link, and transmit link respectively. Step 3: A single digital machine uniformly controls the two beam controllers and servos, controlling the dual-beam pointing of the antennas so that the three arrays collect data from the same target area. Step 4: Collect cross-track interferometry data from the front and rear channels of the Xa and Xc antennas to calculate the sea surface elevation of the target area; Step 5: Collect along-track interferometry data from the front and rear channels of the Xa and Xb antennas and calculate the ocean current velocity in the target area.

2. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 1, characterized in that: The second step includes: the excitation source outputs three calibration time signals, which pass through different links and are input into corresponding receivers; the reference calibration time signal passes through the scaler and the power divider; the receiving calibration time signal passes through the scaler and the calibration link and is input into the antenna array; During transmit calibration, the signal is coupled by the antenna array, passes through the calibration link, and inputs the calibrator and power divider.

3. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 2, characterized in that: The step 2 also includes: in the dual-beam mode, designing a unified transmit and receive timing of the three antenna arrays, the Xa and Xb antennas transmit signals to the front and rear sides simultaneously, and the front and rear channels of the three antennas receive the echoes from the front and rear sides simultaneously according to the time interval corresponding to the input wave gate.

4. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 1, characterized in that: The step three includes: based on the input squint angle and the attitude angle fed back by the platform, the digital stand-alone unit generates beam position parameters, and the beam control unit controls the antenna beam to point to a specific azimuth direction; based on the ground distance information fed back by the platform, the digital stand-alone unit generates instruction information, and the servo controls the antenna beam to point to a specific distance direction.

5. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 1, characterized in that: The fourth step includes: using an imaging algorithm to generate two SAR complex images based on the cross-track interferometric data, extracting interference fringes through fine registration, phase filtering and complex image conjugate multiplication, obtaining the absolute phase difference Θ through phase unwrapping and flat ground phase recovery, and inverting the sea surface elevation information.

6. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 5, characterized in that: The step 4 also includes: assuming that the distances from the Xa and Xc antennas to the center of the target area are r1 and r2 respectively, the carrier length is λ meters, and the lower viewing angle is Radians rad, using the formula calculate Use the formula Calculate sea surface elevation.

7. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 6, characterized in that: The fifth step includes: using an imaging algorithm to generate four SAR complex images based on the along-track interferometric data, extracting interference fringes, performing phase noise reduction and phase unwrapping to eliminate 2π ambiguity, obtaining the absolute interferometric phase, and calibrating and correcting to eliminate the on-track component in the phase and compensate for the phase offset to obtain the interferometric phase difference θ. AT , invert ocean current velocity information.

8. The dual-beam SAR method for measuring sea surface height and ocean current velocity according to claim 7, characterized in that: The step five also includes: assuming the oblique angle is θ, the ocean current speed in the line of sight is v, and using the formula Calculate v using the formula Calculate the radial velocity of the ocean current using the formula Calculate the heading speed of an ocean current.

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