A method for adaptive beam position design of high-orbit SAR

Abstract

The present invention discloses a high-orbit SAR adaptive beam position design method: Step 1, determining the imaging mode, beam position, polarization mode, satellite position at the current orbital time, satellite attitude, antenna installation azimuth angle, antenna installation pitch angle, and antenna imaging beam position scanning angle; Step 2, determining the transmit pulse width according to the imaging mode and setting the maximum duty cycle and minimum duty cycle; Step 3, determining the maximum available PRF, image ambiguity, and system sensitivity at the current orbital time based on the maximum and minimum duty cycles; Step 4, determining whether to update the PRF based on the image ambiguity and system sensitivity; Step 5, calculating the radar echo start time and echo window length; Step 6, entering the next orbital time and returning to Step 1. The method of the present invention can adaptively select different PRFs according to different orbital positions, thereby achieving adaptive adjustment of radar parameters.

Description

Technical Field

[0001] The present invention belongs to the field of radar technology, and specifically relates to a method for adaptive beam position design of high-orbit SAR (SAR). The method is applicable to the system beam position design of highly elliptical orbit SAR (SAR) and satellite-borne, missile-borne, and airborne high-squint SAR (SAR). Background Art

[0002] Countries with strong research foundations in spaceborne SAR include the United States, Germany, Canada, Italy, and Japan. Typical low-Earth orbit (LEO) spaceborne SAR systems currently in orbit include the United States' Lacrosse SAR satellite, Canada's Radarsat-1 and Radarsat-2, the German Space Agency (DLR)'s TerraSAR X / TanDEM-X, Italy's Cosmo-Skymed, the European Space Agency's Envisat, ERS1 / 2, and Sentinel-1, and Japan's ALOS and ALOS-2. In addition to these countries, Russia, Israel, South Korea, and India are also actively developing spaceborne SAR technology. my country has also successfully launched several synthetic aperture radar satellites.

[0003] System beamform design is a crucial aspect of spaceborne SAR (SAR). Beamform design is closely linked to image quality and system performance, and therefore forms a crucial component of spaceborne SAR top-level design. Currently, beamform design for low-Earth orbit (LEO) spaceborne SAR primarily selects the pulse repetition frequency based on transmit pulse obstruction constraints, as well as range and azimuth ambiguity. As SAR resolution increases, the factors considered in beamform design also increase accordingly.

[0004] The Nanjing Institute of Electronic Technology addresses the issue that the high speed of low-orbit spaceborne SAR (SAR) and the large fluctuations in orbital altitude and ground elevation make the beam position parameters designed for a fixed observation location unsuitable for global applications. This study analyzed the constraints imposed on the beam position parameters by satellite-ground geometry, system performance constraints, and variations in satellite orbital altitude and ground elevation. The study then proposed an adaptive calculation method for global beam position parameters for spaceborne SAR. However, this method focuses on the beam position calculation method based on variations in satellite orbital altitude and ground elevation, not on the need to adjust radar parameters and beam position due to variations in orbital velocity. Furthermore, the method does not provide criteria or specific criteria for beam position variations.

[0005] Yu Ze and his colleagues at the Beijing University of Aeronautics and Astronautics proposed a beamline design method for spaceborne phased array synthetic aperture radars. This method analyzes the factors constraining beamline design, particularly the ambiguity performance of phased array systems, and clarifies the relationship between ambiguity performance and system parameters. However, this method does not address the need for radar parameter and beamline adjustments due to orbital velocity variations and range migration effects, nor does it provide criteria or specific criteria for beamline changes. Summary of the Invention

[0006] The present invention proposes a high-orbit SAR adaptive beam position design method to solve the problem that radar parameters need to be adjusted due to orbital velocity changes and range migration effects.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A high-orbit SAR adaptive beam position design method specifically includes the following steps:

[0009] Step 1: Determine the imaging mode, beam position, polarization mode, satellite position at the current orbital moment, satellite attitude, antenna installation azimuth angle, antenna installation elevation angle, and antenna imaging beam position scanning angle;

[0010] Step 2: Determine the transmit pulse width according to the imaging mode and set the maximum duty cycle and the minimum duty cycle;

[0011] Step 3: Determine the maximum available PRF, image ambiguity, and system sensitivity at the current orbital moment based on the maximum duty cycle and the minimum duty cycle;

[0012] Step 4: Compare the image blur and system sensitivity obtained in step 3 with the image blur and system sensitivity corresponding to the maximum available PRF at the previous orbit. If the image blur and system sensitivity corresponding to the maximum available PRF at the previous orbit decrease by more than a preset threshold compared to the current image blur and system sensitivity, update the system imaging PRF according to the maximum available PRF obtained in step 31. Otherwise, maintain the system imaging PRF consistent with the maximum available PRF at the previous orbit.

[0013] Step 5: Determine the minimum and maximum angles of incidence of the observation scene based on the satellite attitude, antenna installation azimuth and pitch angle, and antenna imaging wave position scanning angle at the current orbital time, and calculate the minimum and maximum slant ranges of the observation scene based on the satellite position at the current orbital time; finally, calculate the radar echo start time and echo window length based on the minimum and maximum angles of incidence, minimum and maximum slant ranges of the observation scene, and return a calculation success flag;

[0014] Step 6: Enter the next orbital moment and return to step 1.

[0015] Furthermore, in step 2, the maximum duty cycle is 15% and the minimum duty cycle is 4%.

[0016] Furthermore, step 3 specifically includes the following sub-steps:

[0017] Step 31, determining the maximum available PRF based on the maximum duty cycle set in step 2;

[0018] Step 32, calculating the image blur based on the maximum available PRF obtained in step 31 and the polarization mode determined in step 1;

[0019] In step 33, determine whether the image blur and system sensitivity simultaneously meet the imaging index requirement of better than -20 dB. If not, reduce the current maximum duty cycle by 1%, and repeat steps 31 and 32 until the maximum duty cycle is less than the minimum duty cycle set in step 2. Then return the calculation failure flag, save the maximum available PRF, image blur, and system sensitivity at the current orbit time, and then proceed to step 4. If both the image blur and system sensitivity meet the imaging index requirements, proceed directly to step 4.

[0020] Furthermore, the operation of step 31 is to determine the maximum available PRF under the maximum duty cycle constraint set in step 2 while satisfying the following two equations:

[0021]

[0022] in:

[0023] R min —Minimum slant range within synthetic aperture time;

[0024] R max —maximum slant range within synthetic aperture time;

[0025] c—speed of light, 3×10 8 m / s;

[0026] H L —Distance from the platform to the sub-satellite point;

[0027] T p — pulse width;

[0028] T g —The time width of the guard band before and after the transmit pulse;

[0029] T w-nadir —Sub-satellite echo duration;

[0030] m,n—natural numbers, and m≠n.

[0031] Conventional low-orbit SAR has relatively fixed orbital parameters throughout the entire orbital range, so its beam position design is relatively simple, as long as it meets the emission shielding requirements and ambiguity indicators. However, the orbital parameters of high-orbit SAR vary over time throughout the entire orbital range, and the beam position design is also affected by the orbital altitude, orbital velocity changes, range migration effects, etc., requiring adaptive beam position design to meet system requirements. Compared to conventional low-orbit SAR berth design methods, the method of the present invention comprehensively considers the impact of the high-orbit SAR orbital altitude, orbital velocity changes, range migration effects, ground surface height, etc. on the beam position design, and can adaptively select different PRFs according to different orbital positions, thereby achieving adaptive adjustment of radar parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the maximum available PRF calculation result obtained by the high-orbit SAR adaptive beam position design method of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be described in further detail below with reference to the accompanying drawings.

[0034] The high-orbit SAR adaptive beam position design method provided by the present invention specifically includes the following steps:

[0035] Step 1: Determine the imaging mode, beam position, polarization mode, satellite position at the current orbital moment, satellite attitude, antenna installation azimuth angle, antenna installation elevation angle, and antenna imaging beam position scanning angle. These parameters are determined by the satellite system parameters and are known information.

[0036] Step 2: Determine the transmit pulse width Tp according to the imaging mode and set the maximum duty cycle η max % and minimum duty cycle η min % (In this embodiment, the maximum duty cycle is determined by the high-orbit 20m SAR satellite system to be η max %, and the minimum duty cycle is 15% and 4% respectively);

[0037] Step 3: Determine the maximum available PRF, image ambiguity, and system sensitivity at the current orbital moment based on the maximum duty cycle and minimum duty cycle set in step 2.

[0038] Step 3 specifically includes the following sub-steps:

[0039] Step 31 : Determine the maximum available PRF (Pulse repetition frequency) based on the maximum duty cycle set in step 2 .

[0040] Specifically, the PRF is determined according to the criteria for selecting PRF for high-orbit SAR. The selection criteria are as follows:

[0041] (1) The upper limit of PRF selection must satisfy the range ambiguity, and the lower limit must satisfy the azimuth ambiguity.

[0042] (2) PRF can adapt to the echo window changes caused by range migration.

[0043] According to the above criteria, the impact of range migration caused by the equivalent squint angle on the echo delay must be considered in the high-orbit SAR beam position design. Based on this, in the present invention, the selection of PRF must satisfy the following two equations:

[0044]

[0045] in:

[0046] R min —Minimum slant range within synthetic aperture time;

[0047] R max —maximum slant range within synthetic aperture time;

[0048] c—speed of light, 3×10 8 m / s;

[0049] H L —Distance from the platform to the sub-satellite point;

[0050] T p — pulse width;

[0051] T g —The time width of the guard band before and after the transmit pulse;

[0052] T w-nadir —Sub-satellite echo duration;

[0053] m,n—natural numbers, and m≠n;

[0054] While satisfying the above two equations, determine the maximum available PRF under the maximum duty cycle constraint set in step 2. This operation can ensure the highest system sensitivity (meeting the general indicator requirement of different mode edge values ​​≤ -22dB to -28dB);

[0055] Step 32, calculating the image blur based on the maximum available PRF obtained in step 31 and the polarization mode determined in step 1;

[0056] Step 33: Determine whether the image ambiguity and system sensitivity simultaneously meet the imaging index requirement of better than -20 dB. If not, reduce the current maximum duty cycle by 1%, and repeat steps 31 and 32 until the maximum duty cycle is less than the minimum duty cycle set in step 2. This indicates that an available system imaging PRF cannot be searched. At this time, a calculation failure flag is returned, and the maximum available PRF, image ambiguity, and system sensitivity at the current orbit time are saved, thereby completing the radar parameter adaptive calculation, and then proceeding to step 4. If both the image ambiguity and system sensitivity meet the imaging index requirements, proceed directly to step 4.

[0057] Step 4: Compare the image ambiguity and system sensitivity obtained in step 3 with the image ambiguity and system sensitivity corresponding to the maximum available PRF at the previous orbital moment (when step 4 is performed for the first time, the values ​​at the previous orbital moment are the initial values ​​stored in the satellite system). If the image ambiguity and system sensitivity corresponding to the maximum available PRF at the previous orbital moment decrease by more than a preset threshold (1 dB in this embodiment) compared to the current image ambiguity and system sensitivity, then update the system imaging PRF (i.e., the maximum available PRF) according to the maximum available PRF obtained in step 31. Otherwise, maintain the system imaging PRF consistent with the maximum available PRF at the previous orbital moment.

[0058] The design idea of ​​the above scheme is as follows: if the maximum available PRF at the previous orbital moment is used at the current moment, and the calculated image ambiguity and system sensitivity index decrease by less than or equal to 1 dB, the satellite can accept it, and the system imaging PRF is not updated. If the image ambiguity and system sensitivity index decrease by more than 1 dB, the satellite cannot accept it, and the system imaging PRF is updated to the maximum available PRF obtained in step 31.

[0059] Step 5: Determine the minimum and maximum angles of incidence of the observation scene based on the satellite attitude, antenna installation azimuth and pitch angle, and antenna imaging wave position scanning angle at the current orbital time, and calculate the minimum and maximum slant ranges of the observation scene based on the satellite position at the current orbital time; finally, calculate the radar echo start time and echo window length based on the minimum and maximum angles of incidence, minimum and maximum slant ranges of the observation scene, and return a calculation success flag.

[0060] Step 6: Enter the next orbital moment and return to step 1. This step calculates the radar parameters for the next orbital moment (mainly including the maximum available PRF of the system imaging, image ambiguity, and system sensitivity), thereby realizing the radar parameter calculation for each orbital moment and ultimately completing the high-orbit SAR adaptive beam position design.

[0061] In order to verify the effectiveness of the method of the present invention, the following parameters are selected for simulation experiments.

[0062] Satellite orbit altitude 42164km, orbit inclination 20°, eccentricity 0, perigee argument 88°, true perigee 180°, right side observation, radar antenna downward viewing angle 3°, strip imaging mode, Figure 1 The maximum available PRF calculation results obtained by using the high-orbit SAR adaptive beam position design method of the present invention are given. Figure 1 It can be seen that the PRF value of the high-orbit SAR system can be obtained by using the wave position adaptive calculation, and different wave position maps can be obtained by selecting different orbital positions.

Claims

1. A high-orbit SAR adaptive beam position design method, characterized by: The specific steps include: Step 1: Determine the imaging mode, beam position, polarization mode, satellite position at the current orbital moment, satellite attitude, antenna installation azimuth angle, antenna installation elevation angle, and antenna imaging beam position scanning angle; Step 2: Determine the transmit pulse width according to the imaging mode and set the maximum duty cycle and the minimum duty cycle; Step 3: Determine the maximum available PRF, image ambiguity, and system sensitivity at the current orbital moment based on the maximum duty cycle and the minimum duty cycle. This step specifically includes the following sub-steps: Step 31, determining the maximum available PRF based on the maximum duty cycle set in step 2; Step 32, calculating the image blur based on the maximum available PRF obtained in step 31 and the polarization mode determined in step 1; Step 33: Determine whether both the image blur and system sensitivity meet the imaging index requirement of better than -20 dB. If not, reduce the current maximum duty cycle by 1%, and repeat steps 31 and 32 until the maximum duty cycle is less than the minimum duty cycle set in step 2. Then, return the calculation failure flag, save the maximum available PRF, image blur, and system sensitivity at the current orbital moment, and then proceed to step 4. If both the image blur and system sensitivity meet the imaging index requirements, proceed directly to step 4. Step 4: Compare the image blur and system sensitivity obtained in step 3 with the image blur and system sensitivity corresponding to the maximum available PRF at the previous orbit. If the image blur and system sensitivity corresponding to the maximum available PRF at the previous orbit decrease by more than a preset threshold compared to the current image blur and system sensitivity, update the system imaging PRF according to the maximum available PRF obtained in step 31. Otherwise, maintain the system imaging PRF consistent with the maximum available PRF at the previous orbit. Step 5: Determine the minimum and maximum angles of incidence of the observation scene based on the satellite attitude, antenna installation azimuth and pitch angle, and antenna imaging wave position scanning angle at the current orbital time, and calculate the minimum and maximum slant ranges of the observation scene based on the satellite position at the current orbital time; finally, calculate the radar echo start time and echo window length based on the minimum and maximum angles of incidence, minimum and maximum slant ranges of the observation scene, and return a calculation success flag; Step 6: Enter the next orbital moment and return to step 1.

2. The high-orbit SAR adaptive beam position design method according to claim 1, characterized in that: In step 2, the maximum duty cycle is 15% and the minimum duty cycle is 4%.

3. The high-orbit SAR adaptive beam position design method according to claim 1, wherein: The operation of step 31 is to determine the maximum available PRF under the maximum duty cycle constraint set in step 2 while satisfying the following two equations: in: —Minimum slant range within synthetic aperture time; —maximum slant range within synthetic aperture time; c—speed of light, 3×10 8 m / s; —Distance from the platform to the sub-satellite point; — pulse width; —The time width of the guard band before and after the transmit pulse; —Sub-satellite echo duration; , —natural numbers, and ≠ .

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