Passive satellite-based sar antenna range two-way pattern measurement method

Abstract

The application provides a SAR antenna distance two-way pattern measurement method based on a passive satellite, which can arrange a calibration reference target on the ground in a mapping bandwidth of a satellite-borne SAR antenna distance two-way pattern measurement and realize high-frequency autonomous calibration without manual participation. The application takes a low-cost calibration satellite carrying a passive calibrator as a reference target, solves the satellite-borne SAR antenna distance two-way pattern measurement problem through reasonable orbit design, arranges the calibration reference target on the ground in the mapping bandwidth of the satellite-borne SAR antenna distance two-way pattern measurement, and the method can realize high-frequency autonomous calibration without manual participation.

Description

Technical Field

[0001] This invention relates to the field of radar earth observation technology, specifically to a method for measuring the range-direction two-way radiation pattern of a SAR antenna based on a passive satellite. Background Technology

[0002] Synthetic Aperture Radar (SAR) technology is an active remote sensing technology capable of acquiring high-resolution microwave remote sensing images. It is unaffected by lighting and weather conditions, enabling all-weather, 24 / 7 Earth observation. Spaceborne SAR requires rigorous on-orbit radiometric calibration to ensure its relative and absolute radiometric accuracy, allowing for the quantitative application of ground feature backscattering information converted from SAR image grayscale. With the increasing demands for quantitative applications, radiometric calibration plays a crucial role in improving the radiometric accuracy of spaceborne SAR images. Among the various aspects, the SAR antenna pattern is a major source of error affecting the radiometric accuracy of SAR images.

[0003] Initially, spaceborne SAR range pattern measurement employed a point-target-based method, utilizing active or passive calibrators with known radar cross section (RCS) arranged along the SAR mapping strip as reference targets. Currently, spaceborne SAR range pattern measurement largely adopts a distributed-target-based method, using large-area, stable tropical rainforests (such as the Amazon rainforest) with known backscattering coefficients as calibration reference targets. In recent times, spaceborne SAR antenna patterns can be measured using receivers mounted on calibration satellites, but only single-pass antenna patterns can be measured. Various new SAR systems present new demands and challenges for SAR antenna pattern measurement, and existing technical solutions have the following shortcomings:

[0004] Ground-based calibration reference target placement is difficult. Antenna pattern measurement schemes for new SAR systems used in ocean, polar, lunar, and Mars deep space exploration are difficult to implement on the ground. Calibration frequency is low. Traditional land-based calibration missions often take several months, and the calibration frequency depends on the satellite revisit frequency. Summary of the Invention

[0005] In view of this, the present invention proposes a two-way SAR antenna range pattern measurement method based on passive satellites, which can deploy the mapping bandwidth and calibration reference target of the spaceborne SAR antenna range pattern measurement on the ground, and achieve high-frequency autonomous calibration without human intervention.

[0006] To achieve the above objectives, this invention provides a method for measuring the range-direction two-way radiation pattern of a SAR antenna based on a passive satellite, comprising the following steps:

[0007] Step 1: Calculate the SAR satellite's space velocity based on its orbital altitude;

[0008] Step 2: Determine the orbital altitude of the calibration satellite; calculate the space velocity of the calibration satellite based on its orbital parameters;

[0009] Step 3: Calculate the coverage area and movement speed of the SAR mapping strip at the calibration satellite orbital altitude;

[0010] Step 4: Calculate the on-orbit velocity component and cross-orbit velocity component of the calibration satellite, and calculate the orbital inclination of the calibration satellite;

[0011] Step 5: Image each calibration satellite within the mapping zone and measure the passive calibrator response from the resulting images to obtain the response value of each calibrator;

[0012] Step 6: Using the three-dimensional coordinates and antenna pointing information of the SAR satellite and the calibration satellite, establish the relative geometric relationship between the two satellites, and calculate the elevation angle and slant range corresponding to the imaging time of the calibration satellite;

[0013] Step 7: Use the obtained slope distance to perform normalized distance correction on the response values ​​of each scaler, and obtain the corrected response values;

[0014] Step 8: Reconstruct the two-way range antenna pattern based on the beam incidence angle and the corrected response value.

[0015] In step 3, it is assumed that the range beam center of the spaceborne SAR points to θ0 at 30°, the range beamwidth is 4°, and the azimuth transmit beamwidth is θ. w If the angle is 2°, then the range mapping bandwidth W of the spaceborne SAR relative to the calibration satellite is... r for:

[0016] W r =(H SAR -H CAL )*(tan(θ0+2°)-tan(θ0-2°))

[0017] The width of the azimuth mapping zone is: The speed at which the surveying strip moves is:

[0018]

[0019] Where r0 = (H SAR -HCAL) / cos(θ0-2°) is the shortest distance from the calibration satellite to the SAR antenna when the calibration satellite is within the mapping zone, θ i R is the beam incidence angle corresponding to the i-th calibration satellite; e H is the Earth's radius. SARH represents the orbital altitude of the SAR satellite. CAL To calibrate the satellite's orbital altitude, V CAL To calibrate the satellite's space velocity, V SAR For SAR satellite space velocity.

[0020] In step 4, the method for calculating the orbital inclination of the calibration satellite is as follows:

[0021] Let the velocity component V of the calibration satellite follow the orbit. CAL_A The time-orbit component V of the azimuth width of the mapping zone is intersected with the velocity of the calibration satellite. CAL_R The time taken to measure the distance to the width of the survey strip is equal;

[0022] Calibration satellite orbital inclination i CAL =i SAR +arccos(V CAL_A / V CAL ), Among them, i SAR Let be the orbital inclination of the SAR satellite; from this, the orbital inclination of the calibration satellite can be calculated.

[0023] In step 8, the sampled values ​​of the SAR antenna's two-way range pattern are obtained from the radar equations:

[0024]

[0025] Where P i The scattering intensity of the point target carried by the i-th calibration satellite; g is the radar transmit / receive gain, λ is the signal wavelength, and τ is the signal wavelength. P f is the pulse width. s f is the sampling frequency of the SAR receiver. PRF The transmit pulse repetition frequency is V, the velocity of the SAR satellite relative to the calibration satellite is ρ. α R represents the azimuth resolution unit size. i Let G be the distance between the i-th calibration satellite and the SAR satellite. 2 (θ i ) represents the antenna pattern value, θ i Let σ be the beam incident angle corresponding to the i-th calibration satellite. i The RCS of the point target carried by the i-th calibration satellite.

[0026] Among them, the range pattern of the spaceborne SAR antenna is obtained by curve fitting.

[0027] Beneficial effects:

[0028] 1. This invention uses a low-cost calibration satellite equipped with a passive calibrator as a reference target. Through reasonable orbit design, it solves the problem of measuring the range pattern of a spaceborne SAR antenna in two-way path. It deploys the mapping bandwidth and calibration reference target on the ground to solve the problem of measuring the range pattern of a spaceborne SAR antenna in two-way path. Furthermore, the method of this invention can achieve high-frequency autonomous calibration without human intervention.

[0029] 2. In the method of this invention, the calibration satellite is in space, which can reduce the impact of background noise and other factors on measurement accuracy.

[0030] 3. This invention obtains the range pattern of a spaceborne SAR antenna by curve fitting. The curve fitting method can be linear fitting, polynomial fitting, or other methods, and has strong adaptability. Attached Figure Description

[0031] Figure 1 This is a flowchart of the SAR antenna range-direction two-way pattern measurement method based on a calibrated satellite according to the present invention.

[0032] Figure 2 This is a schematic diagram of the orbits of the SAR satellite and calibration satellite of this invention. Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0034] The flowchart of the method for measuring the range-direction two-way radiation pattern of a SAR antenna based on a passive satellite according to the present invention is as follows: Figure 1 As shown, it includes the following steps.

[0035] Step 1: Calculate the SAR satellite's space velocity based on its orbital altitude. The calculation formula can be expressed as formula (1):

[0036]

[0037] Where R e =6371km is the Earth's radius, μ = 3.986 × 10 14 m 3 / s 2 H is the Earth's gravitational constant. SAR This represents the orbital altitude of the SAR satellite.

[0038] Step 2: Determine the orbital altitude H of the calibration satellite CAL ; Calculate the space velocity V of the calibration satellite based on its orbital parameters. CAL The schematic diagram of the SAR satellite and calibration satellite orbits of this invention is shown below. Figure 2 As shown.

[0039] Step 3: Calculate the coverage area and movement speed of the SAR mapping strip at the calibration satellite orbital altitude, as detailed below:

[0040] Assuming the range beam center of the spaceborne SAR is θ0 = 30°, the range beamwidth is 4°, and the azimuth transmit beamwidth is θ w If the angle is 2°, then its range-direction mapping bandwidth W relative to the calibration satellite can be calculated. r For: W r =(H SAR -HCAL)*(tan(θ0+2°)-tan(θ0-2°)), the width of the azimuth mapping zone is The speed at which the surveying strip moves is:

[0041]

[0042] Where r0 = (H SAR -H CAL θ / cos(θ0-2°) is the shortest distance from the calibration satellite to the SAR antenna when the satellite is within the mapping zone, where θ i Let ...

[0043] Step 4: Calculate the along-orbit velocity component (azimuth velocity component along the mapping zone) and the cross-orbit velocity component (range velocity component along the mapping zone) of the calibration satellite, and calculate the orbital inclination of the calibration satellite. The specific calculation method for the orbital inclination of the calibration satellite is as follows:

[0044] Let the velocity component V of the calibration satellite follow the orbit. CAL_A (Azimuth velocity component) The time of passage through the azimuth width of the mapping strip and the orbital component of the calibration satellite velocity V CAL_R The time taken for the velocity component (along the range direction) to pass through the range width of the survey strip is equal, and the calculation formula can be expressed as formula (3):

[0045]

[0046] Calibration satellite orbital inclination i CAL =i SAR +arccos(V CAL_A / V CAL ), Among them, i SAR Let be the orbital inclination of the SAR satellite; from this, the orbital inclination of the calibration satellite can be calculated, and multiple calibration satellites equipped with passive reflectors can be placed within the mapping zone on this orbit.

[0047] Step 5: Since there is relative motion between the calibration satellite and the SAR satellite, the calibration satellites within the mapping zone are imaged, and the passive calibrator response is measured from the resulting images to obtain the response value of each calibrator.

[0048] Step 6: Using the three-dimensional coordinates and antenna pointing information of the SAR satellite and the calibration satellite, establish the relative geometric relationship between the two satellites, and calculate the elevation angle and slant range corresponding to the imaging time of the calibration satellite.

[0049] Step 7: Use the obtained slant range to perform normalized distance correction on the response values ​​of each calibrator to obtain the corrected response values, which is to compensate for the difference in sampling values ​​caused by the difference in distance between the calibration satellite and the onboard SAR satellite.

[0050] Step 8: Reconstruct the two-way range-direction antenna pattern based on the beam incidence angle and the corrected response value. Specifically: According to the radar equations, the sampled values ​​of the SAR antenna's two-way range-direction pattern can be obtained as follows:

[0051]

[0052] Where P i The scattering intensity of the point target carried by the i-th calibration satellite; g is the radar transmit / receive gain, λ is the signal wavelength, and τ is the signal wavelength. P f is the pulse width. s f is the sampling frequency of the SAR receiver. PRF The transmit pulse repetition frequency is V, the velocity of the SAR satellite relative to the calibration satellite is ρ. α R represents the azimuth resolution unit size. i Let G be the distance between the i-th calibration satellite and the SAR satellite. 2 (θ i ) represents the antenna pattern value, θ i Let σ be the beam incident angle corresponding to the i-th calibration satellite. i Let RCS be the point target mounted on the i-th calibration satellite. The range pattern of the spaceborne SAR antenna is obtained through curve fitting. The curve fitting method can be linear fitting, polynomial fitting, or other methods.

[0053] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for measuring the range-direction two-way radiation pattern of a SAR antenna based on a passive satellite, characterized in that, Includes the following steps: Step 1: Calculate the SAR satellite's space velocity based on its orbital altitude; Step 2: Determine the orbital altitude of the calibration satellite; calculate the space velocity of the calibration satellite based on its orbital parameters; Step 3: Calculate the coverage area and movement speed of the SAR mapping strip at the calibration satellite orbital altitude; Step 4: Calculate the on-orbit velocity component and cross-orbit velocity component of the calibration satellite, and calculate the orbital inclination of the calibration satellite; Step 5: Image each calibration satellite within the mapping zone and measure the passive calibrator response from the resulting images to obtain the response value of each calibrator; Step 6: Using the three-dimensional coordinates and antenna pointing information of the SAR satellite and the calibration satellite, establish the relative geometric relationship between the two satellites, and calculate the elevation angle and slant range corresponding to the imaging time of the calibration satellite; Step 7: Use the obtained slope distance to perform normalized distance correction on the response values ​​of each scaler, and obtain the corrected response values; Step 8: Reconstruct the two-way range antenna pattern based on the beam incidence angle and the corrected response value; In step 3, it is assumed that the range beam center of the spaceborne SAR points to The range beamwidth is 4°, and the azimuth beamwidth is 30°. If the angle is 2°, then the range mapping bandwidth of the spaceborne SAR relative to the calibration satellite is... for: The width of the azimuth mapping zone is: The moving speed of the surveying strip is: in The shortest distance from the calibration satellite to the SAR antenna when the calibration satellite is within the mapping zone. Benefit is the first i The beam incidence angle corresponding to each calibration satellite; For the Earth's radius, This represents the orbital altitude of the SAR satellite. To calibrate the satellite's orbital altitude, To calibrate satellite space velocity, For SAR satellite space velocity; In step 4, the specific method for calculating the orbital inclination angle of the calibration satellite is as follows: Calibration satellite velocity along orbit component The time and orbital components of the azimuth width of the mapping zone are intersected by the velocity of the calibration satellite. The time taken to measure the distance to the width of the survey strip is equal; Calibration satellite orbital inclination , ,in, Let the SAR satellite's orbital inclination be used; from this, the orbital inclination of the calibration satellite can be calculated. In step 8, the sampled values ​​of the SAR antenna's two-way range pattern are obtained from the radar equations: in For the first i The scattering intensity of point targets carried by each calibration satellite; , For radar transmit and receive gain, For the signal wavelength, The pulse width. The sampling frequency of the SAR receiver. The repetition frequency of the transmitted pulse. The velocity of the SAR satellite relative to the calibration satellite. This refers to the size of the azimuth resolution unit; For the first i The distance between calibration satellites and SAR satellites These are the antenna radiation pattern values. Benefit is the first i The beam incidence angle corresponding to each calibration satellite For the first i RCS of point targets carried by a calibration satellite.

2. The method as described in claim 1, characterized in that, The range pattern of the spaceborne SAR antenna was obtained by curve fitting.

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