A method for measuring SAR radiation calibration constant based on calibration satellite

Abstract

The present invention discloses a SAR radiation calibration constant measurement method based on a calibration satellite, comprising the following steps: first, determining the orbital parameters of the calibration satellite; second, calculating the distance between the calibration satellite and the SAR satellite in the mapping zone, and selecting the calibration satellite; third, imaging the selected calibration satellites by using the SAR satellite, and acquiring the SAR images of the calibration satellites; fourth, extracting the energy of all effective calibration satellites according to the SAR images of the calibration satellites; fifth, measuring the calibration constant by using the energy of the effective calibration satellites; and sixth, converting the calibration constant, thereby completing the measurement of the SAR satellite calibration constant. The present invention can use the calibration satellite equipped with a passive calibrator as a reference target, increase the calibration constant measurement frequency, realize unattended calibration, reduce the calibration time to the order of magnitude of the orbital period, and realize the rapid and continuous execution of the calibration task.

Description

Technical Field

[0001] The invention belongs to the technical field of radar earth observation, and in particular relates to a SAR radiation calibration constant measurement method based on a calibration satellite. Background Art

[0002] Synthetic Aperture Radar (SAR) technology is an active remote sensing technology that can obtain high-resolution microwave remote sensing images. It is not limited by lighting and climate conditions and can achieve all-day and all-weather ground observation. Spaceborne SAR requires strict on-orbit radiation calibration to convert the grayscale of SAR images into quantitative application of ground object backscatter information. With the increasing demand for quantitative applications, radiation calibration technology plays a vital role in improving the radiation accuracy of spaceborne SAR images.

[0003] The measurement of absolute radiometric calibration constants of spaceborne SAR relies on the ground providing targets with precisely known radar cross-sections (RCS) as standard reference sources. Commonly used standard reference targets include point targets and distributed targets, so the measurement techniques are divided into point target-based and distributed target-based measurement techniques. Point targets (passive corner reflectors and active calibrators) can provide high-precision and high-stability reference values ​​of radar cross-sections, which is a test method currently widely used internationally: a group of accurately calibrated calibrators are arranged in a certain way within the surveying band, and the original data of the calibration field is imaged after the radar passes overhead, and precise radiation correction is performed during the imaging process. The image position of each calibrator is determined on the image, the point target response energy is extracted, and the calibration constant is calculated. However, various new SAR systems currently bring new demands and challenges to traditional land calibration technology.

[0004] The existing technical solutions have the following deficiencies:

[0005] (1) Low calibration frequency. Emergency observation satellites and micro-nano SAR satellites (with short lifespans) for sudden disasters and wars require calibration to be completed within a few days. Traditional calibration technologies often take several months and are difficult to meet timeliness requirements.

[0006] (2) The deployment of calibrators on the ground requires a lot of manpower and material resources. The radiation calibration constant measurement scheme for a new type of SAR system, such as deep space exploration around the moon and Mars, is difficult to implement on the ground. Summary of the invention

[0007] In view of this, the present invention provides a SAR radiation calibration constant measurement method based on a calibration satellite, which can use a calibration satellite equipped with a passive calibrator as a reference target, increase the calibration constant measurement frequency, realize unattended calibration, reduce the calibration time to the order of orbital period, and realize rapid and continuous execution of calibration tasks.

[0008] The technical solution for implementing the present invention is as follows:

[0009] A SAR radiation calibration constant measurement method based on a calibration satellite comprises the following steps:

[0010] Step 1: Determine the calibration satellite orbit parameters;

[0011] Step 2: Calculate the distance between the calibration satellite and the SAR satellite within the surveying band, and select the calibration satellite;

[0012] Step 3: Use SAR satellites to image the selected calibration satellites to obtain SAR images of the calibration satellites;

[0013] Step 4: Extract the energy of all effective calibration satellites based on the SAR images of each calibration satellite;

[0014] Step 5: Use the energy of the effective calibration satellite to measure the calibration constant;

[0015] Step 6: Calibration constant conversion;

[0016] Since then, the measurement of SAR satellite calibration constants has been completed.

[0017] Furthermore, the specific process of step 2 is:

[0018] The distance between each calibration satellite and the SAR satellite in the surveying band is calculated, the radar noise equivalent backscatter coefficient (NESZ) is calculated according to the calibration satellite orbit parameters, and the calibration satellite that meets the signal-to-noise ratio greater than 35dB is selected.

[0019] Furthermore, in step 4, the energy of all effective calibration satellites in each calibration satellite SAR image is extracted using an integration method.

[0020] Furthermore, step five is specifically as follows:

[0021] The calibration constants of all valid calibration satellites are measured in sequence, and the statistical average value of the calibration constants measured by all valid calibration satellites is calculated, that is, the calibration constant.

[0022] Furthermore, the calibration constant conversion is specifically:

[0023] K T =K+3·logR CAL -3·logR T

[0024] K T is the converted calibration constant, K is the measurement result of the calibration constant, R CAL is the average distance between each calibration satellite and the SAR satellite, R Tis the distance from the SAR satellite to the ground observation point.

[0025] Beneficial effects:

[0026] 1. The method of the present invention can solve the problem that the calibration reference target is difficult to be arranged on the ground in the measurement of the radiation calibration constant of satellite-borne SAR.

[0027] 2. The method of the present invention can realize high-frequency autonomous calibration without manual intervention.

[0028] 3. In the method of the present invention, the calibration satellite is in space, which can reduce the influence of factors such as background noise on the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The present invention is a flow chart of the SAR radiation calibration constant measurement method based on the calibration satellite.

[0030] Figure 2 is the noise equivalent backscatter coefficient. DETAILED DESCRIPTION

[0031] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0032] like Figure 1 As shown, the method of the present invention comprises the following steps.

[0033] Step 1: Determine the calibration satellite orbit parameters. To increase the calibration frequency, the calibration satellite orbit can be selected to have the same orbit type, appropriate orbit height difference, and similar orbit inclination as the SAR satellite. The two form relative motion, and multiple calibration satellites can be arranged on the calibration satellite orbit. For example, the SAR satellite adopts a sun-synchronous orbit with an orbital altitude of 755km, an orbital inclination of 98.4°, a descending node local time of 6:00am, a right-side viewing, range beam width of 2.288°, an azimuth transmission beam width of 0.32°, and a beam center pointing (downward viewing angle) of 31.5°. The calibration satellite orbit altitude can be selected to be 500km, the orbital inclination is 97.4°, the orbit type is a sun-synchronous orbit, and the descending node local time is 6:00am.

[0034] Step 2: Calculate the distance between the calibration satellite and the SAR satellite in the mapping band and select the appropriate calibration satellite. According to the orbit information of the calibration satellite and the SAR satellite in step 1, calculate the relative distance between each calibration satellite and the SAR satellite in the mapping band, and calculate the noise equivalent backscatter coefficient. The calculation formula can be expressed as formula (1):

[0035]

[0036] R is the effective distance, k is the Boltzmann constant, T0=158.45K is the system noise temperature, Loss=10dB is the system loss, v is the SAR satellite speed, G=48.5dB is the antenna gain, is the average power, ρr=c / 2B is the distance resolution, and the NESZ curve with the range is as follows: Figure 2 As shown in the figure, when the distance between the calibration satellite and the SAR satellite is 300-400km, the value of NESZ is in the range of (-33dB, -37dB). Considering the calibration payload beam width and signal-to-noise ratio > 35dB, a suitable calibration satellite is selected.

[0037] Step 3: Calibration satellite imaging: The SAR satellite performs imaging processing on each calibration satellite in the surveying band, obtains the SAR images of each calibration satellite, and performs relative radiation correction processing on the SAR images.

[0038] Step 4: Extract point target energy. Read in SAR data and use the integral method to extract the energy of all valid calibration satellites in the image. There are n point targets (calibration satellites) in the read SAR image. Take the i-th point target as the center, extract the data of a rectangular window of appropriate size, interpolate the rectangular window by a certain multiple, and use the integral method to extract the energy of the valid calibration satellite.

[0039] Step 5: Measure the calibration constant. Measure the calibration constants of all n valid point targets (calibration satellites) in step 2, K i is the calibration constant corresponding to the i-th point target (calibration satellite). The measurement result is then normalized and corrected to obtain the calibration constant. The calculation formula can be expressed as formula (2):

[0040]

[0041] where ε i is the energy of the i-th point target (calibration satellite), σ i is the RCS value of the i-th point target (calibration satellite), and the calibration constant K measured by all n valid point targets (calibration satellites) is i The statistical average value of is taken as the measurement result K of the calibration constant.

[0042] Step 6: Calibration constant conversion. According to the calibration constant measurement result K in step 6, the calibration constant conversion is performed. From the radar equation, it can be seen that the conversion formula is shown in formula (3):

[0043] K T =K+3·logR CAL -3·logR T (3)

[0044] KT is the calibration constant after conversion, R CAL is the average distance between each calibration satellite and the SAR satellite, R T is the distance from the SAR satellite to the ground observation point.

[0045] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A SAR radiation calibration constant measurement method based on a calibration satellite, characterized in that: The following steps are involved: Step 1: Determine the calibration satellite orbit parameters; Step 2: Calculate the distance between the calibration satellite and the SAR satellite within the surveying band and select the calibration satellite. The specific process is as follows: Calculate the distance between each calibration satellite and the SAR satellite in the surveying band, and calculate the radar noise equivalent backscatter coefficient according to the calibration satellite orbit parameters. The calculation formula is: R is the effective distance, k is the Boltzmann constant, T0=158.45K is the system noise temperature, Loss=10dB is the system loss, v is the SAR satellite speed, G=48.5dB is the antenna gain, is the average power, ρ r =c / 2B is the range resolution; select the calibration satellite with a signal-to-noise ratio greater than 35dB; Step 3: Use SAR satellites to image the selected calibration satellites, obtain SAR images of each calibration satellite, and perform relative radiation correction processing on the SAR images; Step 4: Extract the energy of all effective calibration satellites based on the SAR images of each calibration satellite; Among them, the SAR data is read in, and the energy of all effective calibration satellites in the SAR image of each calibration satellite is extracted by the integration method. Specifically, there are n point targets in the read SAR image, and the rectangular window data is extracted with the i-th point target as the center, and the rectangular window is interpolated by a certain multiple, and the energy of the effective calibration satellite is extracted by the integration method; Step 5: Use the energy of the effective calibration satellite to measure the calibration constant; measure the calibration constants of all effective calibration satellites in turn, and calculate the statistical average value of the calibration constants measured by all effective calibration satellites, that is, the calibration constant; Step 6: Calibration constant conversion, specifically: K T =K+3·logR CAL -3·logR T K T is the converted calibration constant, K is the measurement result of the calibration constant, R CAL is the average distance between each calibration satellite and the SAR satellite, R T is the distance from the SAR satellite to the ground observation point; Since then, the measurement of SAR satellite calibration constants has been completed.

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