CN110488281A - A kind of big bandwidth DBF-SAR dispersion correction method - Google Patents

Abstract

The embodiment of the present invention discloses a large-bandwidth DBF‑SAR dispersion correction method, which includes: transmitting a chirp signal, receiving echoes from multiple channels in elevation; using a multi-channel beamformer to form multiple sub-beams pointing to different The beam realizes the high-gain coverage reception of the pulse ground width; the weighting vectors of the multi-channel beamformers use the equivalent wavelengths corresponding to the high-gain narrow-bands to effectively reduce the impact of wavelength changes; each sub-beamformer synthesizes signals to The pass filter extracts the corresponding high-gain narrow-band; the high-gain narrow-band signals output by each sub-beamformer are resynthesized into full-bandwidth high-gain broadband signals for subsequent imaging processing.

Description

A large bandwidth DBF-SAR dispersion correction method

technical field

The embodiment of the present application relates to the communication field, and more specifically, relates to a large-bandwidth DBF-SAR dispersion correction method.

Background technique

The invention relates to correcting the influence of the frequency dispersion of a large-bandwidth digital beam-forming (Digital beam-forming, DBF) spaceborne synthetic aperture radar (Synthetic Aperture Radar, SAR) on the system signal-to-noise ratio. According to application requirements, spaceborne SAR needs to achieve high-resolution and wide-range imaging. In order to ensure that the system has a high receiving gain to ensure that the system signal-to-noise ratio meets the design requirements, it should be considered to use the DBF receiving technology in pitch direction. The receiving geometry of DBF technology is shown in the figure 1.

The DBF technology uses multi-channel reception in the pitch direction. The received RF signal is subjected to low-noise amplification, down-conversion, and digitization to obtain the baseband digital signal of each channel, as shown in Figure 2. The data rate of the system is high. In order to ensure the smooth downloading of data, the DBF synthesis of multi-channel data should be completed in real time on the star, and one channel of data is synthesized before being downloaded to the ground. Therefore, DBF synthesis has high requirements for real-time performance. 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 digital interpolation method is used for time-delay processing, the number of interpolation filters will be very large, which will occupy a large number of on-board digital resources, so the DBF processing of space-borne SAR generally cannot use the real-time time-delay method.

Since the echo time delay difference of each channel is generally very small, in the case of narrowband, the time delay can be replaced by time-varying phase shifting, as shown in Figure 4, so as to effectively ensure the real-time performance of DBF synthesis processing. The weighting coefficient of the kth channel is

Where d is the channel spacing, λ _c is the wavelength corresponding to the carrier frequency, θ(t) is the downward viewing angle of the corresponding antenna in Figure 1, and β _c is the antenna installation angle. The fixed time delay in Fig. 4 is to compensate the pulse extension loss (Pulse Extension Loss, PEL) received by the DBF, since it uses a fixed time delay, it has little influence on the real-time performance of the processing. The delay of the kth channel is

Where c is the electromagnetic wave propagation velocity, θ _c is the downward angle of view at the center of the surveying swath, K _r is the modulation frequency of the transmitted chirp signal, and _tc is the echo time at the center of the swath.

The weighting coefficient given by formula (1) uses the wavelength corresponding to the carrier frequency instead of the wavelength of all signal frequencies, which is no problem under narrow-band conditions. But if use relative bandwidth (relative bandwidth=signal bandwidth/carrier frequency, it is generally believed that relative bandwidth is greater than 10% is broadband signal) larger signal, i.e. broadband signal, then signal wavelength can not be considered as a fixed value, Fig. 5 shows The linear frequency modulation signal under the condition of carrier frequency 9.6GHz, the relationship between the wavelength corresponding to different signal bandwidth B _r and the pulse time (that is, different frequency points) is obtained. It can be seen that as the signal bandwidth increases, the change of the signal wavelength becomes more severe. The change of the signal wavelength makes the weighting coefficient given by formula (1) no longer accurate in the frequency band away from the center frequency, which leads to the deviation of the beam pointing. If the signal bandwidth is larger than the antenna array bandwidth, this deviation will greatly affect the DBF - Signal-to-noise ratio of the SAR system. Although the fixed delay given by formula (2) partially compensates for the delay difference between channels, the beam pointing deviation will still be very obvious at positions far away from the center of the swath. This deviation is defined as the frequency dispersion phenomenon processed by DBF. Frequency dispersion causes the echo to be amplitude-weighted, as shown in Figure 6, which in turn affects the system SNR, as shown in Figure 7. In Figure 7, when the signal bandwidth is 2500MHz, the maximum return gain loss of the system can reach 9.8dB, which is intolerable in practice and must be resolved.

The present invention designs a brand-new multi-beam real-time former, which realizes full-bandwidth and high-gain reception of echo signals with large bandwidths, and effectively corrects the dispersion effect of DBF processing with large bandwidths. Simulation results show that the method of the invention can basically eliminate the amplitude modulation of the echo, and the echo gain can be increased by about 9.1dB compared with the traditional method.

Contents of the invention

The main purpose of the present invention is to propose a large-bandwidth DBF-SAR processing frequency dispersion correction method based on multi-beam real-time formation, which can effectively solve the signal-to-noise ratio deterioration caused by dispersion under the premise that DBF processing can be completed in real time on the star question.

The technical scheme provided by the invention is as follows:

A large bandwidth DBF-SAR dispersion correction method, the method comprising:

Transmit chirp signal, and receive echoes from multiple channels in pitch;

Use a multi-channel beamformer to form multiple sub-beams pointing in different directions, and achieve high-gain coverage reception of the pulse ground width with multiple sub-beams;

The weighting vectors of the multi-channel beamformer use the equivalent wavelengths corresponding to the high-gain narrow-band to effectively reduce the impact of wavelength changes;

Each sub-beamformer synthesizes the signal and extracts the corresponding high-gain narrow frequency band through a band-pass filter;

The high-gain narrow-band signals output by each sub-beamformer are recombined into full-bandwidth high-gain wideband signals for subsequent imaging processing.

In the above technical solution, the SAR transmitter emits a chirp signal at a certain pulse repetition frequency to irradiate the corresponding surveying area on the ground, and uses a multi-channel antenna to receive signals in the pitch direction. The antenna pattern of each channel covers the entire surveying zone, and the signals of each channel are respectively Sampling is used for subsequent processing.

In the above technical solution, according to the angle of view range of the antenna corresponding to the pulse ground width, a plurality of narrow beams pointing in different directions are designed to cover the range with high gain, and each narrow beam deviates from each other by a certain angle, including:

The pulse ground width can be calculated according to the geometric relationship of spaceborne DBF-SAR signal transmission and reception, and its expression is:

Where t is the distance time, T _p is the pulse width, c is the electromagnetic wave propagation velocity, R _e is the radius of the earth, R _se is the radius of the payload orbit, θ(t) is the beam pointing angle at time t;

The beams are directed differently, and the expression of the angle of deviation between them is

Where M is the number of narrow beams, which should be large enough to cut the wideband signal into narrowband signal.

In the above technical solution, the equivalent wavelength of the narrow frequency band is expressed as:

where i is the narrow beam number, satisfying f _c is the carrier frequency, B _r is the signal bandwidth;

The weighting vectors of each sub-beamformer, the k-th channel weighting coefficient expression of the i-th beam is:

where d is the channel spacing in the pitch direction of the antenna, and β _c is the angle of view of the normal line of the antenna.

In the above technical solution, in the high-gain narrow frequency band, the range of the narrow frequency band corresponding to the i-th sub-beam is

The passband of the bandpass filter corresponds to the narrow frequency range mentioned above.

In the above technical solution, the multiple signals output by each sub-beamformer are summed at corresponding points in the time domain or frequency domain to obtain one output signal for subsequent imaging processing.

The technical solution adopted by the present invention to solve its technical problems is as shown in Figure 8, and is specifically described as follows:

A multi-beamformer is used to form multiple sub-beams pointing in different directions to achieve high-gain coverage of the pulse ground width with multiple sub-beams, as shown in Figure 9. Figure 9 is the included angle of the downward viewing angle corresponding to the pulse edge, which can be calculated according to the geometric relationship of the echo. In Figure 9, the edge beam angle should be equal to Thus, high-gain coverage of the entire echo by multiple sub-beams is ensured. Each sub-beam receives the narrow frequency band corresponding to the echo with high gain respectively, and high-gain reception is not guaranteed outside the frequency band.

The weighting coefficients used by the multi-channel beamformer correspond to the narrow frequency bands received by the high-gain respectively, and the wavelength corresponding to the center frequency of the narrow frequency band is used to replace the wavelength of the entire narrow frequency band. For narrow frequency bands, this approximation is reasonable. Therefore, the weighting coefficient of the k-th signal of the i-th beamformer in Fig. 8 can be expressed as

in M is the number of sub-beamformers, λ _i is the equivalent wavelength of the i-th narrow frequency band, which can be expressed as

Among them, f _c is the signal carrier frequency, and B _r is the total bandwidth of the signal.

The signals synthesized by each sub-beamformer use a band-pass filter to extract a corresponding narrow frequency band.

The narrow frequency bands output by each sub-beamformer are recombined into broadband signals for subsequent imaging processing.

Description of drawings

Figure 1 is a schematic diagram of the geometric relationship of spaceborne DBF-SAR signal reception;

Fig. 2 is a schematic diagram of the processing flow of the DBF receiver;

Fig. 3 is a schematic diagram of the timing relationship of sampling points of receiving signals of each channel;

Fig. 4 is a schematic diagram of a traditional DBF processing flow;

Fig. 5 is a schematic diagram of the relationship between pulse time and signal wavelength under different bandwidths;

Fig. 6 is a schematic diagram of the real part of the signal echo in the processing flow of Fig. 4;

Fig. 7 is a schematic diagram of the signal echo pulse compression result of the processing flow in Fig. 4;

Fig. 8 is a schematic diagram of the processing flow of the method described in the present application;

Fig. 9 is a schematic diagram of the signal receiving geometric relationship of the method described in the present application;

Fig. 10 is a schematic diagram of the real part of the signal echo of the processing flow described in the present application;

Fig. 11 is a schematic diagram of the signal echo pulse compression result of the processing flow described in the present application;

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the specific technical solutions of the invention will be further described in detail below in conjunction with the drawings in the embodiments of the present invention. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

The technical solution adopted by the present invention to solve its technical problems is as shown in Figure 8, and is specifically described as follows:

A multi-beamformer is used to form multiple sub-beams pointing in different directions to achieve high-gain coverage of the pulse ground width with multiple sub-beams, as shown in Figure 9. Figure 9 is the included angle of the downward viewing angle corresponding to the pulse edge, which can be calculated according to the geometric relationship of the echo. In Figure 9, the edge beam angle should be equal to Thus, high-gain coverage of the entire echo by multiple sub-beams is ensured. Each sub-beam receives the narrow frequency band corresponding to the echo with high gain respectively, and high-gain reception is not guaranteed outside the frequency band.

The weighting coefficients used by the multi-channel beamformer correspond to the narrow frequency bands received by the high-gain respectively, and the wavelength corresponding to the center frequency of the narrow frequency band is used to replace the wavelength of the entire narrow frequency band. For narrow frequency bands, this approximation is reasonable. Therefore, the weighting coefficient of the k-th signal of the i-th beamformer in Fig. 8 can be expressed as

in M is the number of sub-beamformers, λ _i is the equivalent wavelength of the i-th narrow frequency band, which can be expressed as

Among them, f _c is the signal carrier frequency, and B _r is the total bandwidth of the signal.

The signals synthesized by each sub-beamformer use a band-pass filter to extract a corresponding narrow frequency band.

The narrow frequency bands output by each sub-beamformer are recombined into broadband signals for subsequent imaging processing.

The present invention illustrates the effect of the method with an X-band DBF-SAR system, and the system design parameters are shown in the table below

Table 1 System Design Parameters

Echo generation is performed based on the receiving geometry of the DBF signal in Figure 1, and the generated 16 baseband signals are stored for subsequent processing.

The processing results using the traditional method in Fig. 4 are shown in Fig. 6 and Fig. 7, which reveal the serious frequency dispersion phenomenon when the traditional method processes large-bandwidth signals, and the maximum signal-to-noise ratio loss reaches 9.8dB at a bandwidth of 2.5GHz.

As a result of processing using the method, the real part of the composite signal echo is as shown in Figure 10. Compared with Figure 6, the amplitude modulation phenomenon of the echo signal is basically eliminated; 7, its maximum gain loss increased from -9.8dB to about -0.7dB, an increase of about 9.1dB. The dispersion effect of large bandwidth DBF processing is basically eliminated.

The above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Anyone skilled in the art can easily think of changes or substitutions within the technical scope disclosed in the present invention. Should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (6)

1. a kind of big bandwidth DBF-SAR dispersion correction method, which is characterized in that the described method includes:

Emit linear FM signal, pitching receives echo to multichannel；

The multiple beamlets being pointed in different directions are formed using multi-beam shaper, it is wide to pulse ground with multiple wavelet Shu Shixian The high-gain of degree, which covers, to be received；

The weighing vector of multi-beam shaper is using the effective wavelength for respectively corresponding to high-gain narrow-band, wavelength is effectively reduced It is influenced caused by variation；

Each wavelet beamformer composite signal takes out corresponding high-gain narrow-band with bandpass filter；

The broadband signal that the high-gain narrow-band signal that each wavelet beamformer exports is reunited into full bandwidth high-gain is used In subsequent imaging.

2. the method according to claim 1, wherein SAR transmitter is with certain pulse recurrence frequency emission lines Property the corresponding mapping region in FM signal irradiation ground, pitching receives signal, each channel antenna direction to using multichannel antenna The entire mapping band of figure covering, samples respectively for subsequent processing each channel signal.

3. the method according to claim 1, wherein according to the corresponding antenna downwards angle of visibility of pulse ground width Range designs the multiple narrow beams being pointed in different directions and carries out high-gain covering to the range, and each narrow beam deviates from each other centainly Angle, comprising:

Pulse ground width can be calculated according to spaceborne DBF-SAR signal transmitting and receiving geometrical relationship, expression formula are as follows:

Wherein t is distance to time, T_pFor pulse width, c is propagation velocity of electromagnetic wave, R_eFor earth radius, R_seFor load track Radius, θ (t) are the beam position angle of t moment；

Each beam position is different, and the expression formula of mutual deviation angle is

Wherein M is the number of narrow beam, should be large enough to broadband signal being cut into narrow band signal.

4. the method according to claim 1, wherein the effective wavelength of the narrow-band, expression formula are as follows:

Wherein i is narrow beam number, is metf_cFor carrier frequency, B_rFor signal band It is wide；

The weighing vector of each wavelet beamformer, the kth channel weighting coefficient expressions of i-th of wave beam are as follows:

Wherein d be antenna pitching to interchannel away from β_cFor antenna normal viewing angles.

5. the method according to claim 1, wherein the high-gain narrow-band, i-th of beamlet are corresponding narrow Frequency range is

The bandpass filter passband corresponds to above-mentioned narrow-band range.

6. the method according to claim 1, wherein the multiple signals of each wavelet beamformer output are existed Time domain or frequency domain corresponding points sum to obtain output signal all the way and are used for subsequent imaging.