Method for improving swath width of space-borne SAR in medium earth orbit

Abstract

The application discloses a method for improving a mapping band width of a medium earth orbit (MEO) satellite-borne synthetic aperture radar (SAR), and aims at solving the problems of the existing SAR imaging technology, such as difficulty in solving the downward visibility of a MEO satellite-borne SAR system, difficulty in sub-band separation, and too high an analog-to-digital converter (AD) sampling rate, and the like, and improving the mapping band width of the MEO satellite-borne SAR system by combining space-frequency two-dimensional processing in an analog domain and a digital domain on the basis of the existing technology, and avoiding serious range ambiguity of the MEO satellite-borne SAR system.

Description

Technical Field

[0001] This invention belongs to the field of spaceborne radar technology, specifically relating to a method for improving the bandwidth of spaceborne SAR mapping in medium Earth orbit. Background Technology

[0002] Currently, all spaceborne SAR systems in operation are low Earth orbit (LEO) spaceborne SAR systems. They offer high resolution but suffer from weak survivability, long revisit times, and relatively small coverage areas. On the other hand, with technological advancements, the down-look visibility of MEO spaceborne SAR systems can be addressed through proper system configuration, resulting in significant advantages over LEO spaceborne SAR systems in terms of revisit time, survivability, and coverage.

[0003] With the increase in coverage, MEO spaceborne SAR systems face more severe range ambiguity. The traditional method of reducing the system's PRF through azimuth multi-channel technology can no longer meet the requirements of MEO spaceborne SAR systems to resolve range ambiguity, thus making it impossible to obtain SAR products with wide mapping bands and no ambiguity. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for improving the mapping bandwidth of MEO spaceborne SAR. Addressing the problems of insufficient down-look visibility, subband separation difficulties, and excessively high analog-to-digital converter (AD) sampling rates in existing SAR imaging technologies, this invention, based on existing technologies, combines spatial-frequency two-dimensional processing in both the analog and digital domains to avoid severe range ambiguity in the wide mapping band of MEO spaceborne SAR systems, thereby improving the mapping bandwidth of MEO spaceborne SAR systems.

[0005] The technical solution adopted by this invention to solve its technical problem includes the following steps:

[0006] Step 1: Design a system to improve the bandwidth of MEO spaceborne SAR mapping;

[0007] Step 1-1: Based on the system parameters of MEO and LEO spaceborne SAR, and following the principle of power consistency, only consider the power loss caused by the difference in propagation distance, and allocate the power loss to the MEO spaceborne SAR system antenna and average transmit power. Design the MEO spaceborne SAR system elevation linear array, transmitter peak power, and transmit signal time width, and determine the number of antenna array elements, transmit and receive beamforming methods, and peak power and time width parameters.

[0008] Step 1-2: According to the formula for calculating electromagnetic wave propagation loss, L(dB) = 32.45(dB) + 20lg f(MHz) + 20lg R(km), determine the greater propagation loss required by MEO satellite SAR compared to LEO satellite SAR under two-way propagation conditions, where L is the propagation loss in dB, f is the carrier frequency in MHz, and R is the propagation distance in km;

[0009] Steps 1-3: According to ΔL = L MEO -L LEO =20lg(R) MEO / R LEO )Calculate the greater propagation loss required for MEO spaceborne SAR compared to LEO spaceborne SAR, R MEO R is the slant range of the MEO-based SAR. LEO Let ΔL be the slant range of the LEO-based SAR, and then we can calculate it.

[0010] Steps 1-4: Allocate the additional loss ΔL due to electromagnetic wave propagation in the MEO spaceborne SAR system compared to the LEO spaceborne SAR system to the peak power, transmit signal duration, and antenna transceiver gain, thereby obtaining the factors that need to be increased in peak power and transmit signal duration for the MEO spaceborne SAR system; the elevation transceiver antenna adopts a 1-dimensional linear array with a1 elements. During transmission, a1 elements are used for beamforming (DBF), and during reception, the beam is divided into two... The array elements are used to perform DBF beamforming for the far and near areas of the survey band to obtain the total transmit and receive gain.

[0011] Step 2: MEO-based spaceborne SAR range subband separation;

[0012] Step 2-1: Let the transmitted signal be... It consists of m sub-pulses, a m S is the launch guidance vector. m (t) represents the m-th sub-pulse signal, d represents the spacing between antenna elements, and θ m , λ m T m f c，m K m These are the pulse direction, wavelength, time width, carrier frequency, and frequency modulation slope of the m-th sub-pulse, respectively.

[0013]

[0014]

[0015] Step 2-2: The carrier frequencies of the first to M sub-pulses are successively decreased. During transmission, the M range sub-bands from far to near are illuminated in the order of the first to M sub-pulses. By controlling the transmit and receive time window, the echoes of the M range sub-bands are made to overlap in time.

[0016] Steps 2-3: At the receiving front end, the separation of the far-field range sub-band and the near-field range sub-band is achieved through subarray beamforming technology;

[0017] Steps 2-4: In the digital domain, frequency domain filtering is used to separate the sub-bands for each distance.

[0018] Steps 2-5: Imaging and focusing are performed on each range sub-band separately, and the sub-bands are stitched together in the image domain to obtain the synthesized range-wide mapping band;

[0019] Step 3: Reduce the burden on the analog-to-digital converter;

[0020] Step 3-1: Based on prior information from global telemetry and remote sensing, adaptively adjust the bandwidth of each sub-band of the transmitted signal to reduce the combined bandwidth of all sub-bands from the source;

[0021] Step 3-2: At the analog front end, the receiving antenna subarray beamforming technology is used to separate the far and near distance subbands through spatial filtering. All subbands are divided into two parts according to the illumination distance, and the combined bandwidth of each part is reduced to half of the original, further reducing the AD sampling burden.

[0022] Preferably, the R MEO The value is taken as 8000km, R LEO The value is taken as 800km, ΔL=40dB, a1=48.

[0023] The beneficial effects of this invention are as follows:

[0024] This invention solves the problems of insufficient range-downward visibility, difficulty in range subband separation, and excessively high analog-to-digital converter (AD) sampling rate in traditional spaceborne SAR systems. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the transmit beam DBF of the present invention.

[0026] Figure 2 This is a schematic diagram of the receiving beam DBF of the present invention.

[0027] Figure 3 This is a schematic diagram of the distance subband separation method according to an embodiment of the present invention.

[0028] Figure 4 This is a focused image of sub-bands 1 to 8 at distances according to an embodiment of the present invention.

[0029] Figure 5 This is a contour map of the composite images of sub-bands 1 to 8 in an embodiment of the present invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0031] To address the challenges of existing SAR imaging technologies in meeting the requirements for downward-looking visibility, subband separation difficulties, and excessively high analog-to-digital converter (AD) sampling rates in MEO spaceborne SAR systems, this invention aims to propose a method to improve the mapping bandwidth of MEO spaceborne SAR. This method, based on existing technologies, combines spatial-frequency two-dimensional processing in both the analog and digital domains to avoid severe range ambiguity in the wide mapping band of MEO spaceborne SAR systems and thus improve the mapping bandwidth of MEO spaceborne SAR systems.

[0032] A method for improving the bandwidth of spaceborne SAR mapping in medium Earth orbit includes the following steps:

[0033] Step 1: Design a system to improve the bandwidth of MEO spaceborne SAR mapping;

[0034] Step 1-1: Based on the typical system parameters of MEO spaceborne SAR and LEO spaceborne SAR, under the same conditions and in accordance with the principle of power consistency, only the power loss caused by the difference in propagation distance is considered, and the power loss is allocated to the MEO spaceborne SAR system antenna and average transmit power. The elevation linear array, transmitter peak power, and transmit signal time width of the MEO spaceborne SAR system are designed, and the number of antenna array elements, transmit and receive beamforming methods, peak power, time width and other parameters are determined.

[0035] Step 1-2: Using the classic empirical formula for calculating electromagnetic wave propagation loss, L(dB)=32.45(dB)+20lgf(MHz)+20lgR(km), determine the greater propagation loss required by MEO satellite SAR compared to LEO satellite SAR under two-way propagation conditions, where L is the propagation loss in dB, f is the carrier frequency in MHz, and R is the propagation distance in km;

[0036] Steps 1-3: According to ΔL = L MEO -L LEO =20lg(R) MEO / R LEO )Calculate the greater propagation loss required for MEO spaceborne SAR compared to LEO spaceborne SAR, R MEO R represents the slant range of a MEO-based spaceborne SAR, typically taken as 8000 km. LEO The slant range of the LEO-based SAR is typically taken as 800 km. Substituting this value into the calculation, we get ΔL = 40 dB.

[0037] Steps 1-4: The 40dB loss due to electromagnetic wave propagation in the MEO spaceborne SAR system compared to the LEO spaceborne SAR system is allocated as follows: peak power 4dB, transmit signal time width 6dB, antenna transmit / receive gain 30dB. This results in the MEO spaceborne SAR system needing to increase its peak power by approximately 2.5 times and transmit signal time width by approximately 4 times. The elevation-side transmit / receive antenna uses a 1D linear array with 48 elements. During transmission, a 48-element DBF beamforming is used. During reception, it is divided into two 24-element subarrays, with DBF beamforming applied to the near and far areas of the mapping zone respectively. The total transmit / receive gain obtained in this way is approximately 31dB. Figure 1 and Figure 2 As shown.

[0038] Steps 1-5: For subbands with shorter distances, the observation time width can be appropriately reduced to allocate more system power to subbands with longer distances that have greater propagation loss.

[0039] Step 2: MEO-based spaceborne SAR range subband separation;

[0040] Step 2-1: Using the signal pattern of intrapulse multi-frequency sub-pulse (MFSP) and the beamforming technology of the receiving antenna subarray, MEO spaceborne SAR range subband separation is achieved through a space-frequency joint method;

[0041] Step 2-2: Transmit signal is It consists of m sub-pulses, a m S is the launch guidance vector. m (t) represents the m-th sub-pulse signal, d represents the spacing between antenna elements, and θ m , λ m T m f c，m K m These are the pulse direction, wavelength, time width, carrier frequency, and frequency modulation slope of the m-th sub-pulse, respectively.

[0042]

[0043]

[0044] Steps 2-3: The carrier frequencies of the first to M sub-pulses are successively reduced. During transmission, the M range sub-bands from far to near are illuminated in the order of the first to M sub-pulses. By reasonably controlling the transmission and reception time windows, the echoes of the M range sub-bands overlap in time to the greatest extent.

[0045] Steps 2-4: At the receiving front end, the separation of the far-field range sub-band and the near-field range sub-band is achieved through subarray beamforming technology;

[0046] Steps 2-5: In the digital domain, frequency domain filtering is used to further separate the sub-bands at each distance.

[0047] Steps 2-6: Imaging and focusing are performed on each range sub-band separately, and the sub-bands are stitched together in the image domain to obtain the synthesized range-wide mapping band.

[0048] Step 3: Reduce the burden on the analog-to-digital converter (AD);

[0049] Step 3-1: Based on prior information from global telemetry and remote sensing, the bandwidth of each sub-band of the transmitted signal can be adaptively adjusted. For example, for vast areas such as oceans, deserts, mountains, and farmlands, the bandwidth can be appropriately reduced according to application requirements, thereby reducing the combined bandwidth of all sub-bands from the source.

[0050] Step 3-2: At the analog front end, the receiving antenna subarray beamforming technology is used to separate the far and near distance subbands through spatial filtering. All subbands are divided into two parts according to the illumination distance, and the combined bandwidth of each part is reduced to about half of the original, further reducing the AD sampling burden.

[0051] Example:

[0052] To verify the effectiveness of the method of the present invention, the simulation parameters in Table 1 are given here.

[0053] In the simulation, the sub-pulse frequency spacing corresponding to the range sub-bands is 10MHz. Based on the range resolution settings in Table 1, the combined bandwidth of the eight range sub-bands is 400MHz. After performing a depth-of-field (DBF) filter on the elevation-dimensional antenna array, spatial filtering of the near-field (sub-bands 1-4) and far-field (sub-bands 5-8) is achieved. At this point, the combined bandwidth of the near-field and far-field is 200MHz. When performing AD sampling, the sampling frequency only needs to be greater than 200MHz; in the simulation, the AD sampling frequency is set to 240MHz. After separating the near-field and far-field, frequency domain filtering is performed in the digital domain to ultimately achieve the separation of each sub-band. The simulation process is as follows: Figure 3 As shown.

[0054] Table 1 Simulation Data Parameters

[0055]

[0056] Figure 4 The paper demonstrates the focused images of each range self-mapping zone obtained after two-dimensional focusing using the range sub-band separation method proposed in this invention; Figure 5 The method proposed in this invention is used to demonstrate the contour map of the entire survey zone after stitching together.

[0057] In conclusion, the simulation experiments verified the correctness, effectiveness, and reliability of the present invention.

Claims

1. A method for improving the bandwidth of spaceborne SAR mapping in medium Earth orbit, characterized in that, Includes the following steps: Step 1: Design a system to improve the bandwidth of MEO spaceborne SAR mapping; Step 1-1: Based on the parameters of the MEO and LEO spaceborne SAR systems, allocate the power loss to the MEO spaceborne SAR system antenna and average transmit power, design the MEO spaceborne SAR system elevation linear array, transmitter peak power, and transmit signal time width, and determine the number of antenna array elements, transmit and receive beamforming methods, and peak power and time width parameters. Steps 1-2: According to The formula for calculating electromagnetic wave propagation loss determines the greater propagation loss required by MEO-based SAR compared to LEO-based SAR under two-way propagation conditions. In the formula... Propagation loss, expressed in dB. Carrier frequency, in MHz. The distance to be transmitted is measured in km. Steps 1-3: According to The calculation shows that MEO spaceborne SAR requires significantly more propagation loss compared to LEO spaceborne SAR. This represents the slant range of the MEO-based SAR. The slant range of the LEO-based SAR is substituted into the calculation to obtain... ; Steps 1-4: The additional losses due to electromagnetic wave propagation in space experienced by MEO spaceborne SAR systems compared to LEO spaceborne SAR systems. The peak power, transmit signal duration, and antenna transmit / receive gain are allocated to determine the factors that need to be increased for the peak power and transmit signal duration of the MEO spaceborne SAR system; the elevation-oriented transceiver antenna uses a 1D linear array with [number of elements missing]. During launch, Array element DBF performs beamforming, and during reception, it is divided into two. The array elements are used to perform DBF beamforming for the far and near areas of the survey band to obtain the total transmit and receive gain. Step 2: MEO-based spaceborne SAR range subband separation; Step 2-1: Let the transmitted signal be... It consists of M sub-pulses. For the launch guidance vector, For the first m Sub-pulse signal, The spacing between antenna elements. , , , , The first The pulse direction, wavelength, time width, carrier frequency, and frequency modulation slope of each sub-pulse; Step 2-2: The carrier frequencies of the first to M sub-pulses are successively decreased. During transmission, the M range sub-bands from far to near are illuminated in the order of the first to M sub-pulses. By controlling the transmit and receive time window, the echoes of the M range sub-bands are made to overlap in time. Steps 2-3: At the receiving front end, the separation of the far-field range sub-band and the near-field range sub-band is achieved through subarray beamforming technology; Steps 2-4: In the digital domain, frequency domain filtering is used to separate the sub-bands for each distance. Steps 2-5: Imaging and focusing are performed on each range sub-band separately, and the sub-bands are stitched together in the image domain to obtain the synthesized range-wide mapping band; Step 3: Reduce the burden on the analog-to-digital converter; Step 3-1: Based on prior information from global telemetry and remote sensing, adaptively adjust the bandwidth of each sub-band of the transmitted signal to reduce the combined bandwidth of all sub-bands from the source; Step 3-2: At the analog front end, the receiving antenna subarray beamforming technology is used to separate the far and near distance subbands through spatial filtering. All subbands are divided into two parts according to the illumination distance, and the combined bandwidth of each part is reduced to half of the original, further reducing the AD sampling burden.

2. The method for improving the bandwidth of spaceborne SAR mapping in medium Earth orbit according to claim 1, characterized in that, The The value is taken as 8000km. The value is taken as 800km. , =48.

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