A RCS recalibration method for broadband active calibrator

Abstract

The present invention proposes a broadband active calibrator RCS recalibration method. Based on calibration field SAR image data, fine calibration of RCS of all active calibrators under a certain bandwidth can be achieved by carrying out a calibration test. The actual SAR system is used as a calibration signal source, and fine calibration of RCS of active calibrators is achieved by simultaneously arranging active calibrators and corner reflectors in an area, thereby eliminating RCS bandwidth effect. The present invention places all calibrators in one scene to carry out a calibration test, thereby eliminating the measurement stability problem of different batches of tests, and does not need to actually measure the amplitude-frequency curve of each active calibrator, and only needs to carry out a calibration test to achieve fine calibration of bandwidth effect. The present invention places the active calibrator and the corner reflector in one test area, thereby achieving unified calibration of RCS of the active calibrator and RCS of the corner reflector, thereby eliminating the RCS deviation problem caused by different calibration methods of RCS of the active calibrator and RCS of the corner reflector.

Description

Technical Field

[0001] The invention relates to the technical field of active calibrator calibration, and in particular to a broadband active calibrator RCS recalibration method. Background Art

[0002] For SAR radiation calibration, calibrators are divided into two types: active calibrators and passive corner reflectors. The radar cross-section value (RCS) of a passive corner reflector can be calculated from the signal frequency and the size of the corner reflector according to the nominal RCS calculation formula, while the nominal RCS value of an active calibrator is usually calibrated by a dedicated external calibration system through external calibration experiments.

[0003] When the RCS of the active calibrator is calibrated by the external calibration system, it is implemented under the condition of a point frequency signal (generally working at the center frequency point of the active calibrator). Therefore, the nominal RCS value of the active calibrator is the RCS value corresponding to the center frequency point of the active calibrator. The SAR signal is a linear frequency modulation signal with a certain bandwidth. The actual RCS displayed by the active calibrator on the SAR image is the comprehensive RCS within the SAR signal bandwidth. Therefore, there is a deviation between the nominal RCS value of the active calibrator and the actual RCS value, which is called the RCS bandwidth effect.

[0004] As the resolution of spaceborne SAR continues to improve, the SAR signal bandwidth is also getting wider. Here, the characteristic curve of the calibrator RCS changing with frequency is called the amplitude-frequency curve. As the signal bandwidth increases, the fluctuation of this curve will also increase. Under narrowband signals, the RCS bandwidth effect of the active calibrator can be ignored. For wideband SAR, if the RCS bandwidth effect is not considered, there will be the following shortcomings:

[0005] (1) The amplitude-frequency curves of the active calibrator and the passive corner reflector are different, so their RCS bandwidth effects are different. Under broadband conditions, the difference in RCS bandwidth effects between the two will increase, which cannot be ignored for SAR calibration.

[0006] (2) Different active calibrators have different amplitude-frequency curves, so the RCS bandwidth effect of each active calibrator is also different. Under broadband conditions, the RCS bandwidth effect of the active calibrator will be very large and the bandwidth effects of different active calibrators will be inconsistent. For example, for an X-band SAR with a signal bandwidth of 1 GHz, the RCS deviation caused by the RCS bandwidth effect of different active calibrators can reach a maximum of 1 dB, which far exceeds the requirements of SAR radiation calibration for the RCS accuracy and consistency of the calibrator. Summary of the invention

[0007] In view of this, the present invention proposes a broadband active calibrator RCS recalibration method. Based on the calibration field SAR image data, a calibration test is carried out to achieve fine calibration of the RCS of all active calibrators under a certain bandwidth.

[0008] To achieve the above object, the technical solution of the present invention is:

[0009] A broadband active calibrator RCS recalibration method of the present invention comprises the following steps:

[0010] Taking the center position of the distance to the actual imaging coverage area of ​​the calibration field wave position that can be covered by the spaceborne SAR as the center, a flat area is selected within the set range to deploy all the active calibrators to be recalibrated, and at the same time deploy the same number of triangular corner reflectors. The deployment principles are as follows: the deployed corner reflectors have the same size and the number is greater than or equal to 3; all calibrators are deployed along the azimuth direction; active calibrators and corner reflectors are deployed alternately; the interval between calibrators is greater than the size of 50 ground resolution units; the delay time of the active calibrator is set to the lowest non-zero level; the nominal RCS value RCS' of the i-th active calibrator after recalibration ARCi for:

[0011] RCS' ARCi =RCS ARCi -ΔRCS ARCi (dB)

[0012] Where i = 1, 2, 3…N, N is the total number of active calibrators to be recalibrated; RCS ARCi is the initial RCS nominal value of the i-th active calibrator to be recalibrated; ΔRCS ARCi为 The RCS bandwidth effect correction value of the i-th active calibrator, ΔRCS ARCi =P ARCi -P CR -Δ(dB), where P ARCi is the energy value of the i-th active calibrator on the SAR image, where the SAR image is the calibration field SAR image when the SAR satellite passes overhead and has been corrected for relative radiation, P CR is the energy average of all corner reflectors, and Δ is the difference between the set active calibrator RCS value and the calculated corner reflector RCS value.

[0013] Among them, the calculated RCS value of the corner reflector is:

[0014] RCS CR =10lg(4πa 4 / (3λ 2 ))

[0015] Where a is the length of the right-angle side of the corner reflector; λ is the wavelength corresponding to the center frequency of the SAR signal.

[0016] Among them, an external calibration device is used to complete the external calibration experiment of the active calibrator to obtain the initial RCS nominal value of the active calibrator.

[0017] Among them, the determination method of the actual imaging coverage area of ​​the calibration field wave position that can be covered by the spaceborne SAR is:

[0018] The SAR calibration mission planning software is used to determine the wave position that the spaceborne SAR can cover the calibration field, and further determine the actual imaging coverage area of ​​the wave position.

[0019] The RCS value of the active calibrator is uniformly set to be the same as or close to the RCS of the corner reflector.

[0020] Beneficial effects:

[0021] The invention discloses a broadband active calibrator RCS recalibration method based on calibration field SAR image data, takes an actual SAR system as a calibration signal source, and simultaneously arranges an active calibrator and a corner reflector in an area to realize fine calibration of the RCS of the active calibrator, thereby eliminating the RCS bandwidth effect. The invention places all calibrators in one scene to carry out a calibration test, thereby eliminating the measurement stability problem of different batches of tests, and does not need to actually measure the amplitude-frequency curve of each active calibrator, and only needs to carry out a calibration test to realize fine calibration of the bandwidth effect. The invention places the active calibrator and the corner reflector in one test area, thereby realizing unified calibration of the RCS of the active calibrator and the RCS of the corner reflector, thereby eliminating the RCS deviation problem caused by different calibration methods of the RCS of the active calibrator and the RCS of the corner reflector.

[0022] The present invention can carry out recalibration work for spaceborne SARs with different bandwidths respectively when the SAR works at different bandwidths, and can obtain RCS correction values ​​of active calibrators at different bandwidths. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the recalibration process of the present invention.

[0024] Figure 2 This is a schematic diagram of the layout of the calibrator of the present invention. DETAILED DESCRIPTION

[0025] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0026] The present invention provides a method for recalibrating a broadband active calibrator RCS. The recalibration process of the present invention is as follows: Figure 1 As shown, the following steps are included:

[0027] Step 1, determine the initial RCS nominal values ​​of all active calibrators to be recalibrated;

[0028] Specifically, the external calibration device is used to complete the external calibration experiment of the active calibrator to obtain the initial RCS nominal value. The initial RCS nominal value of the i-th active calibrator to be recalibrated is recorded as RCS ARCi ; i = 1, 2, 3 ... N, where N is the total number of active calibrators to be recalibrated;

[0029] Step 2, determining the wave position of the calibration field that can be covered by the spaceborne SAR, and further determining the actual imaging coverage area of ​​the wave position; wherein, this can be achieved by using software such as SAR calibration task planning;

[0030] Step 3: With the distance to the center of the actual imaging coverage area determined in step 2 as the center, select a flat area within the set range, and lay out all the active calibrators to be recalibrated. At the same time, lay out the same number of triangular corner reflectors. The layout principles are as follows:

[0031] The deployed corner reflectors have the same size, and the number is greater than or equal to 3; all calibrators are deployed along the azimuth direction to eliminate the distance-to-space variation effect of the SAR image radiation characteristics; active calibrators and corner reflectors are deployed alternately; the interval between calibrators is greater than the size of 50 ground resolution units, for example, if the ground resolution of a certain satellite-borne SAR is 1 meter, then the interval between calibrators is greater than 50 meters; the delay time of the active calibrators is set to the lowest non-zero level, for example, if the delay time of a certain type of active calibrator is divided into five levels: 0μs, 2μs, 4μs, 6μs, and 8μs, then the delay time of the active calibrators is set to 2μs. In this way, it can be ensured that the distance between the deployment position of the active calibrator and the actual imaging position is minimized. The schematic diagram of the calibrator deployment of the present invention is shown in the figure. Figure 2 shown.

[0032] Step 4: Calculate the RCS value of the corner reflector, recorded as RCS CR :The calculation formula is as follows:

[0033] RCS CR =10lg(4πa 4 / (3λ 2 ))

[0034] Where a is the length of the right-angle side of the corner reflector; λ is the wavelength corresponding to the center frequency of the SAR signal.

[0035] Step 5: Set the RCS value of the active scaler, denoted as RCS ARC ;

[0036] The RCS value of the active calibrator is usually divided into 5dB grades, and the RCS value of the active calibrator is uniformly set to be the same as or close to the RCS of the corner reflector. For example, the RCS of the corner reflector is 47dBm 2, then set the RCS of the active calibrator to 45dBm 2 or 50dBm 2 ;

[0037] Step 6, calculate the difference Δ between the set active calibrator RCS value and the calculated corner reflector RCS value:

[0038] Δ=RCS ARC -RCS CR (dB);

[0039] Step 7, obtaining the calibration field SAR image when the SAR satellite passes overhead, and completing the relative radiation correction of the SAR image;

[0040] Step 8: Extract the point target energy of all calibrators on the SAR image corrected in step 7, where the energy value of the i-th active calibrator is recorded as P ARCi (dB), the energy value of the i-th corner reflector is recorded as P CRi (dB); wherein the point target energy of the calibrator can be extracted using the point target energy extraction software;

[0041] Step 9: Calculate the average energy of all corner reflectors, denoted as P CR (dB);

[0042] Step 10, calculating the RCS bandwidth effect correction value of each active calibrator;

[0043] The calculation formula of the RCS bandwidth effect correction value of the i-th active calibrator is as follows:

[0044] ΔRCS ARCi =P ARCi -P CR -Δ(dB)

[0045] Step 11, calculating the nominal RCS value of each active calibrator after recalibration, and completing the recalibration;

[0046] The calculation formula of the nominal RCS value of the i-th active calibrator after recalibration is as follows:

[0047] RCS' ARCi =RCS ARCi -ΔRCS ARCi (dB).

[0048] 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 broadband active calibrator RCS recalibration method, characterized in that: The steps include: Taking the center position of the distance to the actual imaging coverage area of ​​the calibration field wave position that can be covered by the spaceborne SAR as the center, a flat area is selected within the set range to deploy all the active calibrators to be recalibrated, and at the same time deploy the same number of triangular corner reflectors. The deployment principles are as follows: the deployed corner reflectors have the same size and the number is greater than or equal to 3; all calibrators are deployed along the azimuth direction; active calibrators and corner reflectors are deployed alternately; the interval between calibrators is greater than the size of 50 ground resolution units; the delay time of the active calibrator is set to the lowest non-zero level; the nominal RCS value RCS' of the i-th active calibrator after recalibration ARCi for: RCS’ ARCi =RCS ARCi -ΔRCS ARCi (dB) Where i = 1, 2, 3…N, N is the total number of active calibrators to be recalibrated; RCS ARCi is the initial RCS nominal value of the i-th active calibrator to be recalibrated; ΔRCS ARCi为 The RCS bandwidth effect correction value of the i-th active calibrator, ΔRCS ARCi =P ARCi -P CR -Δ(dB), where P ARCi is the energy value of the i-th active calibrator on the SAR image, where the SAR image is the calibration field SAR image when the SAR satellite passes overhead and has been corrected for relative radiation, P CR is the average energy value of all corner reflectors, Δ is the difference between the set active calibrator RCS value and the calculated corner reflector RCS value; The calculated RCS value of the corner reflector is: RCS CR =10lg(4πa 4 / (3λ 2 )) Where a is the length of the right-angle side of the corner reflector; λ is the wavelength corresponding to the center frequency of the SAR signal.

2. The method according to claim 1, characterized in that The external calibration equipment is used to complete the external calibration experiment of the active calibrator and obtain the initial RCS nominal value of the active calibrator.

3. The method according to claim 1, characterized in that The method for determining the actual imaging coverage area of ​​the calibration field wave position that can be covered by the spaceborne SAR is: The SAR calibration mission planning software is used to determine the wave position that the spaceborne SAR can cover the calibration field, and further determine the actual imaging coverage area of ​​the wave position.

4. The method according to claim 1, characterized in that The RCS value of the active calibrator is uniformly set to be the same as or close to the RCS of the corner reflector.

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