CN110488281B - Large-bandwidth DBF-SAR dispersion correction method - Google Patents
Large-bandwidth DBF-SAR dispersion correction method Download PDFInfo
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- CN110488281B CN110488281B CN201910678665.0A CN201910678665A CN110488281B CN 110488281 B CN110488281 B CN 110488281B CN 201910678665 A CN201910678665 A CN 201910678665A CN 110488281 B CN110488281 B CN 110488281B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
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Abstract
The embodiment of the invention discloses a large-bandwidth DBF-SAR dispersion correction method, which comprises the following steps: transmitting a linear frequency modulation signal, and receiving an echo in a multi-channel manner in a pitching manner; forming a plurality of sub-beams pointing to different directions by using a multi-path beam former, and realizing high-gain coverage reception of pulse ground width by using the multi-sub-beams; the weighting vector of the multi-path beam former uses the equivalent wavelength of the high-gain narrow frequency band corresponding to each weighting vector so as to effectively reduce the influence caused by wavelength change; each sub-beam former synthesizes signals, and a band-pass filter is used for taking out a corresponding high-gain narrow frequency band; and recombining the high-gain narrow-band signals output by the sub-beam formers into a full-bandwidth high-gain broadband signal for subsequent imaging processing.
Description
Technical Field
The embodiment of the application relates to the field of communication, in particular to a large-bandwidth DBF-SAR dispersion correction method.
Background
The invention relates to correcting the influence of large bandwidth Digital beam-forming (DBF) satellite-borne Synthetic Aperture Radar (SAR) frequency dispersion on the signal-to-noise ratio of a system. According to application requirements, the space-borne SAR needs to realize high-resolution wide-width imaging, and in order to ensure that a system has higher receiving gain so as to ensure that the signal-to-noise ratio of the system meets design requirements, a pitching DBF receiving technology is considered, and the receiving geometry of the DBF technology is shown in figure 1.
The DBF technique uses multi-channel reception from the pitch direction, and the received radio frequency signal is subjected to low-noise amplification, down-conversion and digitization to obtain a baseband digital signal of each channel, as shown in fig. 2. The system has high data rate, and in order to ensure that data can be downloaded smoothly, DBF synthesis of multi-channel data is finished on a satellite in real time, and one path of data is synthesized and then downloaded to the ground. The real-time requirements for DBF synthesis are therefore high. Due to the scanning reception principle of the DBF technology, the echo delay difference of each channel is time-varying, and the delay relationship is shown in fig. 3. If the time delay processing is performed by a digital interpolation method, the number of interpolation filters is very large, which occupies a great amount of satellite digital resources, so that the DBF processing of the satellite-borne SAR cannot use a real-time delay method.
Because the echo delay difference of each channel is generally very small, under the condition of narrow band, the delay can be replaced by time-varying phase-shifting, as shown in fig. 4, thereby effectively ensuring the real-time property of DBF synthesis processing. The weighting coefficient of the k channel is
Wherein d is the channel spacing, λcIs the wavelength corresponding to the carrier frequency, theta (t) is the corresponding antenna down view angle, beta, in fig. 1cIs the antenna mounting angle. The fixed delay in fig. 4 is to compensate for Pulse Extension Loss (PEL) received by the DBF, and since it uses the fixed delay, it has little influence on the real-time performance of the process. With the delay amount of the k-th channel being
Where c is the electromagnetic wave propagation velocity, θcFor central downward viewing angle of swath, KrFrequency modulation, t, for transmitting chirp signalscFor centering of swathsThe wave time.
The weighting factor given by equation (1) uses the carrier frequency corresponding wavelength instead of the wavelengths of all signal frequencies, which is not problematic in narrow band conditions. However, if a signal with a relatively large bandwidth (the relative bandwidth is equal to the signal bandwidth/carrier frequency, and it is generally considered that the relative bandwidth is greater than 10% of the signal is a broadband signal), that is, the signal wavelength cannot be considered as a fixed value, fig. 5 shows a chirp signal under the condition of carrier frequency 9.6GHz, and a different signal bandwidth B is shown in fig. 5rThe variation of the corresponding wavelength with the pulse time (i.e. different frequency points). It can be seen that the change in signal wavelength becomes more dramatic as the bandwidth of the signal increases. The variation of the signal wavelength makes the weighting coefficient given by equation (1) no longer accurate in the frequency band range far from the center frequency, resulting in the deviation of the beam pointing direction, which will greatly affect the signal-to-noise ratio of the DBF-SAR system if the signal bandwidth is larger than the antenna array bandwidth. Although the fixed delay given by equation (2) partially compensates for the inter-channel delay differences, beam pointing deviations will still be very significant at locations away from the center of the swath, which is defined as frequency dispersion of the DBF process. Frequency dispersion causes the echoes to be amplitude weighted as shown in figure 6, which in turn affects the system signal-to-noise ratio as shown in figure 7. In fig. 7, the maximum return gain loss of the system can reach 9.8dB at 2500MHz signal bandwidth, which is not tolerable in practice and must be solved.
The invention designs a novel multi-beam real-time former, realizes full-bandwidth high-gain reception of large-bandwidth echo signals, and effectively corrects the dispersion influence of large-bandwidth DBF processing. Simulation results show that the method can basically eliminate amplitude modulation of the echo, and compared with the traditional method, the echo gain can be improved by about 9.1 dB.
Disclosure of Invention
The invention mainly aims to provide a large-bandwidth DBF-SAR processing frequency dispersion correction method based on multi-beam real-time formation, which effectively solves the problem of signal-to-noise ratio deterioration caused by dispersion on the premise of ensuring that DBF processing can be completed on the satellite in real time.
The technical scheme provided by the invention is as follows:
a large bandwidth DBF-SAR dispersion correction method, the method comprising:
transmitting a linear frequency modulation signal, and receiving an echo in a multi-channel manner in a pitching manner;
forming a plurality of sub-beams pointing to different directions by using a multi-path beam former, and realizing high-gain coverage reception of pulse ground width by using the multi-sub-beams;
the weighting vector of the multi-path beam former uses the equivalent wavelength of the high-gain narrow frequency band corresponding to each weighting vector so as to effectively reduce the influence caused by wavelength change;
each sub-beam former synthesizes signals, and a band-pass filter is used for taking out a corresponding high-gain narrow frequency band;
and recombining the high-gain narrow-band signals output by the sub-beam formers into a full-bandwidth high-gain broadband signal for subsequent imaging processing.
In the technical scheme, the SAR transmitter transmits linear frequency modulation signals at a certain pulse repetition frequency to irradiate a corresponding mapping area on the ground, the signals are received by the multi-channel antenna in a pitching mode, a directional diagram of each channel antenna covers the whole mapping band, and the signals of each channel are respectively sampled for subsequent processing.
In the above technical solution, according to the range of the lower view angle of the antenna corresponding to the pulse ground width, a plurality of narrow beams pointing to different directions are designed to perform high gain coverage on the range, and each narrow beam deviates from a certain angle, including:
the pulse ground width can be obtained by calculation according to a satellite-borne DBF-SAR signal transceiving geometric relation, and the expression is as follows:
where T is the distance-wise time, TpIs the pulse width, c is the electromagnetic wave propagation velocity, ReIs the radius of the earth, RseThe radius of the load orbit is shown, and theta (t) is the beam pointing angle at the moment t;
the expression that the wave beams are different in pointing direction and deviate from each other at an angle is
Where M is the number of narrow beams that should be large enough to cut the wideband signal into narrowband signals.
In the above technical solution, the expression of the equivalent wavelength of the narrow band is:
wherein i is a narrow beam number, satisfies
fcIs a carrier frequency, BrIs the signal bandwidth;
the weighting vector of each sub-beam former has the k channel weighting coefficient expression of the ith beam as follows:
where d is the pitch channel spacing of the antenna, βcIs the antenna normal viewing angle.
In the above technical solution, the high-gain narrow band has a narrow band range corresponding to the ith sub-beam
The passband of the bandpass filter corresponds to the narrow band range.
In the above technical solution, a plurality of paths of signals output by the sub-beam formers are summed at corresponding points in the time domain or the frequency domain to obtain a path of output signal for subsequent imaging processing.
The technical scheme adopted by the invention to solve the technical problem is shown in fig. 8, and is specifically described as follows:
multiple sub-beams pointing in different directions are formed using a multi-path beamformer to achieve high gain coverage of the pulse floor width with multiple sub-beams, as shown in fig. 9. In FIG. 9
The included angle of the lower view angle corresponding to the pulse edge can be calculated according to the geometrical relationship of the echo. The edge beam angle in fig. 9 should be equal to
Thereby ensuring high gain coverage of the entire echo by the multi-beamlet. Each sub-beam receives a narrow frequency band corresponding to the echo with high gain, and high gain reception is not guaranteed outside the frequency band.
The weighting coefficients used by the multi-path beamformer correspond to narrow bands of high-gain reception, respectively, and the wavelengths of the entire narrow band are replaced with wavelengths corresponding to the center frequency of the narrow band. This approximation is reasonable for narrow frequency bands. The weighting coefficient of the k-th signal of the ith beam former in fig. 8 can be expressed as
Wherein
M is the number of sub-beam formers, lambdaiIs the equivalent wavelength of the ith narrow band, which can be expressed as
Wherein f iscIs the carrier frequency of the signal, BrIs the total bandwidth of the signal.
Each sub-beamformer synthesizes a signal and a corresponding narrow frequency band is extracted by a band-pass filter.
The narrow bands output by the sub-beamformers are recombined into a wideband signal for subsequent imaging processing.
Drawings
FIG. 1 is a schematic diagram of a satellite-borne DBF-SAR signal receiving geometry;
FIG. 2 is a schematic diagram of a DBF receiver processing flow;
FIG. 3 is a schematic diagram showing a timing relationship between sampling points of received signals of each channel;
FIG. 4 is a schematic diagram of a conventional DBF process flow;
FIG. 5 is a diagram illustrating the relationship between pulse time and signal wavelength for different bandwidths;
FIG. 6 is a schematic diagram of the real part of the signal echo of the process flow of FIG. 4;
FIG. 7 is a diagram illustrating the signal echo pulse compression results of the processing flow of FIG. 4;
FIG. 8 is a schematic process flow diagram of the method described herein;
FIG. 9 is a schematic diagram of a signal reception geometry according to the method of the present application;
FIG. 10 is a schematic diagram of the real part of the signal echo of the process flow described in the present application;
FIG. 11 is a schematic diagram of a signal echo pulse compression result of the processing flow described in the present application;
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the following describes specific technical solutions of the present invention in further detail with reference to the accompanying drawings in the embodiments of the present invention. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
The technical scheme adopted by the invention to solve the technical problem is shown in fig. 8, and is specifically described as follows:
multiple sub-beams pointing in different directions are formed using a multi-path beamformer to achieve high gain coverage of the pulse floor width with multiple sub-beams, as shown in fig. 9. In FIG. 9
The included angle of the lower view angle corresponding to the pulse edge can be calculated according to the geometrical relationship of the echo. The edge beam angle in fig. 9 should be equal to
Thereby ensuring high gain coverage of the entire echo by the multi-beamlet. Each sub-beam receives a narrow frequency band corresponding to the echo with high gain, and high gain reception is not guaranteed outside the frequency band.
The weighting coefficients used by the multi-path beamformer correspond to narrow bands of high-gain reception, respectively, and the wavelengths of the entire narrow band are replaced with wavelengths corresponding to the center frequency of the narrow band. This approximation is reasonable for narrow frequency bands. The weighting coefficient of the k-th signal of the ith beam former in fig. 8 can be expressed as
Wherein
M is the number of sub-beam formers, lambdaiIs the equivalent wavelength of the ith narrow band, which can be expressed as
Wherein f iscIs the carrier frequency of the signal, BrIs the total bandwidth of the signal.
Each sub-beamformer synthesizes a signal and a corresponding narrow frequency band is extracted by a band-pass filter.
The narrow bands output by the sub-beamformers are recombined into a wideband signal for subsequent imaging processing.
The effect of the method is illustrated by an X-waveband DBF-SAR system, and the system design parameters are shown in the following table
TABLE 1 System design parameters
Echo generation is carried out according to the receiving geometrical relation of the DBF signals in the figure 1, and generated 16 baseband signals are respectively stored for subsequent processing.
The results of processing using the conventional method of fig. 4 are shown in fig. 6 and 7, which reveal that the conventional method processes a large bandwidth signal with a severe frequency dispersion phenomenon, with a maximum snr loss of 9.8dB at a 2.5GHz bandwidth.
As a result of processing by using the method, the real part of the echo of the synthesized signal is shown in FIG. 10, and compared with FIG. 6, the amplitude modulation phenomenon of the echo signal is basically eliminated; the result of the pulse compression of the synthesized signal is shown in fig. 11, and compared with fig. 7, the maximum gain loss is increased from-9.8 dB to about-0.7 dB, which is increased by about 9.1 dB. The dispersion effect of the large bandwidth DBF process is substantially eliminated.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.
Claims (6)
1. A large bandwidth DBF-SAR dispersion correction method, characterized in that the method comprises:
transmitting a linear frequency modulation signal, and receiving an echo in a multi-channel manner in a pitching manner;
forming a plurality of sub-beams pointing to different directions by using a multi-path beam former, and realizing high-gain coverage reception of pulse ground width by using the multi-sub-beams; the edge beam angle should be equal to theta0(t)，θ0(t) is a downward viewing angle included angle corresponding to the pulse edge, so that high-gain coverage of the multi-sub-beam on the whole echo is ensured;
the weighting vector of the multi-path beam former uses the equivalent wavelength of the high-gain narrow frequency band corresponding to each weighting vector so as to effectively reduce the influence caused by wavelength change;
each sub-beam former synthesizes signals, and a band-pass filter is used for taking out a corresponding high-gain narrow frequency band;
and recombining the high-gain narrow-band signals output by the sub-beam formers into a full-bandwidth high-gain broadband signal for subsequent imaging processing.
2. The method of claim 1, wherein the SAR transmitter transmits chirp signals at a pulse repetition frequency to illuminate a corresponding mapping area on the ground, receives the signals using multiple channel antennas in elevation, each channel antenna pattern covering the entire swath, and samples each channel signal separately for subsequent processing.
3. The method of claim 1, wherein designing a plurality of narrow beams pointing in different directions according to the range of the antenna downward viewing angle corresponding to the pulse ground width to perform high-gain coverage on the range, wherein the narrow beams are mutually deviated from a certain angle, comprises:
the pulse ground width can be obtained by calculation according to a satellite-borne DBF-SAR signal transceiving geometric relation, and the expression is as follows:
where T is the distance-wise time, TpIs the pulse width, c is the electromagnetic wave propagation velocity, ReIs the radius of the earth, RseThe radius of the load orbit is shown, and theta (t) is the beam pointing angle at the moment t;
the expression that the wave beams are different in pointing direction and deviate from each other at an angle is
Where M is the number of narrow beams that should be large enough to cut the wideband signal into narrowband signals.
4. The method of claim 1, wherein the narrow band of equivalent wavelengths is expressed as:
wherein i is a narrow beam number, satisfies
fcIs a carrier frequency, BrIs the signal bandwidth;
the weighting vector of each sub-beam former has the k channel weighting coefficient expression of the ith beam as follows:
where d is the pitch channel spacing of the antenna, βcIs the antenna normal viewing angle.
5. The method of claim 1, wherein the high-gain narrow band is defined by an ith sub-beam having a narrow band range
The passband of the bandpass filter corresponds to the narrow band range.
6. The method of claim 1, wherein a plurality of output signals from each of said sub-beamformers are summed at corresponding points in the time domain or the frequency domain to obtain an output signal for subsequent imaging processing.
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| CN113391288B (en) * | 2021-06-29 | 2022-11-08 | 内蒙古工业大学 | Satellite-borne DBF processing method and device based on multi-path group delay and storage medium |
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