Sar motion error extraction and compensation method and device based on metamaterial tag

Abstract

The application provides a SAR motion error extraction and compensation method and device based on a metamaterial tag, comprising: constructing a transmitting waveform containing a point frequency signal and a linear frequency modulation signal, and the point frequency signal and the linear frequency modulation signal are located in the same period; constructing a motion error model, taking the slant range difference between an ideal track and an actual track as input, and establishing a motion error model for decomposing the motion error into a range space invariant component and a range space variant component; using a metamaterial tag with a known phase modulation frequency, receiving the transmitting waveform and generating a tag echo; using the tag echo of the point frequency signal to obtain the modulation frequency and the azimuth Doppler shift frequency of the metamaterial tag; based on the tag echo of the linear frequency modulation signal and the modulation frequency and the azimuth Doppler shift frequency, calculating a motion error signal; using the motion error term to compensate for the motion error, and realizing compensation and focused imaging of the platform motion error.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of synthetic aperture radar, and particularly relates to a SAR motion error extraction and compensation method and device based on a metamaterial tag. BACKGROUND

[0002] A hundred-kilogram miniaturized synthetic aperture radar (Mini-SAR) carried by a small unmanned platform can be used for high-resolution images. However, the errors of the motion of the small unmanned platform (such as speed changes, non-linear motion, and attitude fluctuations) will cause phase errors of echo signals, affecting the imaging quality. Therefore, motion compensation is a key step in SAR imaging.

[0003] At present, commonly used motion compensation methods include a combination navigation system (an inertial measurement and a positioning unit are combined to realize error measurement), Doppler parameter estimation, and a self-focusing algorithm. For motion compensation methods relying on a combination navigation, in the imaging of a hundred-kilogram high-resolution Mini-SAR system, in order to achieve a good compensation effect, an optical fiber gyroscope is generally used. Such a combination navigation device has a weight of thousands of kilograms, which is much higher than the weight of the hundred-kilogram Mini-SAR itself, and brings a burden to the load of the unmanned platform. The Doppler parameter estimation and the self-focusing algorithm cannot well solve the problem of motion compensation of the hundred-kilogram Mini-SAR because of problems such as that the compensation accuracy is greatly affected by scene characteristics, the calculation amount is large, and the time consumption is long.

[0004] The prior art has the following defects:

[0005] 1. The combination navigation system relies on a high-precision inertial measurement unit (such as an optical fiber gyroscope), resulting in a large increase in the weight of the system (thousands of kilograms), which is much higher than the weight of the hundred-kilogram Mini-SAR itself, and limits the carrying capacity of the small unmanned platform.

[0006] 2. The compensation accuracy of the Doppler parameter estimation and the self-focusing algorithm is greatly affected by scene characteristics, and the calculation complexity is high and the time consumption is long, which makes it difficult to meet the needs of real-time or efficient imaging of the Mini-SAR. And relying on the characteristics of echo data, the performance decreases under complex scenes or low signal-to-noise ratio conditions, affecting the imaging quality. SUMMARY

[0007] To solve the above technical problems, the application provides a SAR motion error extraction and compensation method based on a metamaterial tag. By placing a metamaterial tag in the imaging scene, the metamaterial tag receives the signals transmitted by the SAR system and returns modulated signals. By designing a corresponding signal processing method, the platform motion error is extracted and compensated, and a focused SAR image is realized. The specific technical scheme is as follows:

[0008] The SAR motion error extraction and compensation method based on the metamaterial tag comprises the following steps:

[0009] constructing a transmitted waveform containing a point frequency signal and a linear frequency modulation signal, and the point frequency signal and the linear frequency modulation signal are located in the same period;

[0010] constructing a motion error model, taking the slant range difference between an ideal track and an actual track as input, and establishing a motion error model for decomposing the motion error into a range space invariant component and a range space variant component;

[0011] using a metamaterial tag with a known phase modulation frequency to receive the transmitted waveform and generate a tag echo; using the tag echo of the point frequency signal to obtain the modulation frequency and the azimuth Doppler shift frequency of the metamaterial tag; and based on the tag echo of the linear frequency modulation signal and the modulation frequency and the azimuth Doppler shift frequency, calculating a motion error signal;

[0012] compensating for the motion error using the motion error term, so as to realize compensation for the platform motion error and focused imaging.

[0013] The SAR motion error extraction and compensation device based on the metamaterial tag comprises:

[0014] a waveform construction module, which constructs a transmitted waveform containing a point frequency signal and a linear frequency modulation signal, and the point frequency signal and the linear frequency modulation signal are located in the same period;

[0015] a model construction module, which constructs a motion error model, takes the slant range difference between an ideal track and an actual track as input, and establishes a motion error model for decomposing the motion error into a range space invariant component and a range space variant component;

[0016] a motion error signal calculation module, which uses a metamaterial tag with a known phase modulation frequency to receive the transmitted waveform and generate a tag echo; uses the tag echo of the point frequency signal to obtain the modulation frequency and the azimuth Doppler shift frequency of the metamaterial tag; and based on the tag echo of the linear frequency modulation signal and the modulation frequency and the azimuth Doppler shift frequency, calculates a motion error signal;

[0017] a compensation module, which compensates for the motion error using the motion error term, so as to realize compensation for the platform motion error and focused imaging.

[0018] An electronic device, comprising: one or more processors; a memory for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors implement the method.

[0019] A computer-readable storage medium having stored executable instructions, which, when executed by a processor, cause the processor to implement the method.

[0020] The present application has the following beneficial effects:

[0021] The present application arranges a metamaterial label in an imaging scene, uses modulation characteristics of the SAR signal, realizes high-precision extraction and compensation of motion error, receives the SAR transmitting signal through the metamaterial label and returns a modulated signal, extracts platform motion error through the signal processing method and process given by the present application, and completes motion difference compensation.

[0022] The present method can avoid using a combined navigation device, greatly reduces the total weight of the SAR system, and can be carried by a kilogram-level load-carrying unmanned aerial vehicle to realize high-resolution imaging, and expands the application platform of the Mini-SAR system.

[0023] The motion error extraction and compensation method has the characteristics of simple calculation, high compensation precision, can effectively reduce the processing hardware complexity and power consumption of the Mini-SAR system, and is suitable for high-resolution Mini-SAR systems (resolution better than 0.1 meters). BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Fig. (a) is a waveform time domain graph of the transmitting signal of the present application;

[0025] Figure 1 Fig. (b) is a waveform frequency graph of the transmitting signal of the present application;

[0026] Figure 2 Fig. is an imaging geometric relationship graph of the FMCW strip SAR of the present application;

[0027] Figure 3 Fig. is a processing flow chart of the SAR motion error extraction and compensation method based on the metamaterial label of the present application. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other. In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme.

[0029] The application discloses a SAR motion error extraction and compensation method based on a metamaterial label.

[0030] The processing flow of the SAR motion error extraction and compensation method based on the metamaterial label is as shown in Figure 3 The steps of the scheme are as follows:

[0031] Step 1, transmit waveform design, used for determining the transmit waveform of the system, and on the basis of the transmit waveform, deducing a motion error model and a compensation method.

[0032] The application intends to realize motion error extraction and compensation by using a modulation label, and a joint transmit waveform of a point frequency signal and a linear frequency modulation signal is designed for extracting distance modulation frequency and azimuth modulation frequency information, as shown in (a) and (b) of Figure 1 Figure 1 Wherein, (a) in Figure 1 is a waveform time domain diagram of a transmit signal, Figure 1 (b) in is a waveform frequency diagram of the transmit signal.

[0033] Figure 1 The transmit signal is as shown in (a) and (b).Neglecting the initial phase of a signal constant term, the transmit signal expression is as follows: Figure 1

[0034] (1)

[0035] Wherein, is an exponential function with a natural constant e as a base, is a transmit waveform repetition period, is a point frequency signal time length in a period, is a linear frequency modulation signal time length, is a transmit signal, , are amplitudes of the point frequency signal and the linear frequency modulation signal respectively, is a starting frequency, is a transmit signal azimuth direction propagation time, is a transmit signal distance direction propagation time, is a linear frequency modulation, is a natural number, The distance to the target is the sum of the distance to the target and the distance to the target.

[0036] Step 2, motion error model construction, for analyzing the causes of motion error formation, and establishing a mathematical model for motion error compensation to provide theoretical basis.

[0037] First, through careful analysis of the root cause of distance error, the corresponding motion compensation algorithm can be better proposed to solve this problem. For this purpose, the distance error is further derived, and the imaging geometry of FMCW (Frequency Modulated Continuous Wave, FMCW for short) radar strip SAR is as shown in Figure 2

[0038] Assume that the transmission signal propagates in the azimuth direction , the ideal position of the platform is point , and the coordinates are , but the actual motion trajectory is offset, and the position point becomes , and the corresponding coordinates are , is the error introduced in the x direction along the flight line, is the error introduced in the y direction across the flight line, is the error introduced in the z direction across the flight line. At time, the distance from the APC (Antenna Phase Center, APC for short) to the target point can be expressed as:

[0039] (2)

[0040] Wherein, the coordinates of the target point are defined as , , , all change with the azimuth time. In the forward-looking mode, the radar has a small squint angle relative to the target. The Taylor series expansion is performed on the above formula, and the first order is retained, and the , high order terms are ignored, and the following is obtained:

[0041] (3)

[0042] (4)

[0043] Wherein, is the ideal slant range of the radar to the target under ideal flight conditions, is the radar squint angle, is the slant range of the radar center sight line,​ This refers to the distance of the target from the center line of sight. According to FMCW SAR theory, scene echoes without motion errors can be determined based on the ideal slant range. Established, also based on slant range with error Motion error models for FMCW SAR can be constructed.

[0044] In formula (3), The slant range error introduced by non-uniform motion in the flight path direction represents the azimuth spatial variability of motion error. The slant distance error introduced into the YZ plane, , From the radar's downward-facing perspective, this error term changes with the range direction, thus exhibiting range-varying characteristics. Therefore, motion error is typically considered... It can be decomposed into two parts: distance invariant in space and distance variable in space, as shown in the following formula:

[0045] (5)

[0046] in, This represents the slant range variable. Based on the above equation, the motion error model of the FMCW SAR scene echo signal under motion error conditions can be obtained, as shown in the following equation:

[0047] (6)

[0048] in, This indicates the frequency modulation frequency of a linear frequency modulation signal. Indicates the distance-to-amplitude window. Represents the azimuth amplitude window. Represents signal amplitude. Let λ be the wavelength of the transmitted signal at its center frequency, and c be the speed of light. This motion error model describes the echo information under ideal slant range and motion error slant range conditions.

[0049] Step 3: Motion error extraction. Based on the motion error model, a method for extracting motion errors is proposed to provide a foundation for the next step of motion compensation.

[0050] In the motion error extraction method based on modulated tags, the tag is equivalent to a point target with the function of modulating SAR signals, and the modulation frequency is set to 1. The initial phase of the nth modulation signal is Where n is a positive integer, and the modulation method is phase modulation. Under the condition that the transmitted signal is linear frequency modulated, according to the FMCW SAR motion error model and modulation target theory, the tag echo is represented as:

[0051] (7)

[0052] where, is the range to the target tag, the amplitude information is ignored here.

[0053] Let , , represents the phase component generated by the modulator, is the echo signal component of the target, and in contains the motion error component. From the formula, the motion error term is contained in , which is independent of , and a certain method is needed to remove .

[0054] When the system transmits a point frequency waveform, according to the dechirp receiver theory, ignoring the signal amplitude, the point frequency signal obtained is as follows:

[0055] (8)

[0056] In order to obtain the two-dimensional spectrum of the above formula, further analysis is done on . From the law of frequency-modulated continuous wave radar transmitting pulses, the initial phase of the modulated waveform in the azimuth direction can be obtained, is the initial phase of the modulated signal, and the modulated signal phase can be expressed as:

[0057] (9)

[0058] Let , , then:

[0059] (10)

[0060] where, , is the initial phase change angular frequency between pulses. In this way, the azimuth frequency drift caused by non-coherent modulation is obtained, and the expression is as follows:

[0061] ;

[0062] The two-dimensional Fourier transform of formula (8) is obtained, and the two-dimensional spectrum can be expressed as:

[0063] (11)

[0064] where, is the range frequency, is the phase in the two-dimensional spectrum, The representative azimuth range window. It is worth mentioning that in the distance direction, the signal-to-noise ratio of the tag signal can be greatly improved by designing a reasonable modulation frequency, so that the tag signal is far away from the frequency area where the scene echoes are located, which is very beneficial to improve the acquisition accuracy of the motion error. Further, define the motion speed of the platform, then the azimuth frequency and the Doppler center frequency The expression is as follows:

[0065] (12)

[0066] (13)

[0067] wherein, is the center frequency of the transmitted signal, is the center slant range of the imaging scene, and the Doppler parameter estimation method is applied to the point frequency echo signal. The slant angle of the radar beam irradiation and the speed of the platform .

[0068] In this way, by analyzing the received point frequency signal, the modulation frequency of the modulated tag and the azimuth Doppler offset frequency are obtained. At the same time, the azimuth Doppler center frequency and the platform motion speed are also accurately estimated, and the ideal distance of the tag with the azimuth time change These parameters provide support for subsequent motion compensation. The above results are brought into formula (7), and the following can be obtained:

[0069] (14)

[0070] wherein, represents the error caused by the platform motion, and further removes , known items, and ignores the constant phase The following expression can be obtained:

[0071] (15)

[0072] Let:

[0073] (16)

[0074] wherein, represents the signal error caused by the platform motion error, so it can be known that the platform motion error signal of the position where the tag is located is obtained by the tag.

[0075] Step 4, motion error compensation and imaging, on the basis of acquiring motion error, through the error compensation and imaging method proposed in this step, the motion error compensation and focusing imaging of the platform can be realized.

[0076] Conjugate multiplication of formula (6), (16) is obtained:

[0077] (17)

[0078] The above formula shows that, like the traditional motion compensation method, the motion error compensation with the label as the reference point is completed. It needs to be specially pointed out that the motion error of the line of sight and the heading is compensated at one time, the narrow beam assumption is not used, and the practicability of the method is enhanced.

[0079] After completing the motion error compensation, and the imaging processing method of the traditional ideal point target is the same, the signal is further compressed and stolt interpolated, and the focusing imaging can be completed, which will not be described here.

[0080] The application also provides a SAR motion error extraction and compensation device based on a metamaterial label, comprising:

[0081] The waveform construction module constructs a transmission waveform containing a point frequency signal and a linear frequency modulation signal, and the point frequency signal and the linear frequency modulation signal are located in the same period.

[0082] The model construction module constructs a motion error model, takes the slant range difference between the ideal track and the actual track as the input, and establishes a motion error model for decomposing the motion error into a range-space invariant component and a range-space variant component.

[0083] The motion error signal calculation module uses the metamaterial label with a known phase modulation frequency to receive the transmission waveform and generate a label echo; the label echo of the point frequency signal is used to obtain the modulation frequency and the azimuth Doppler shift frequency of the metamaterial label; and based on the label echo of the linear frequency modulation signal and the modulation frequency and the azimuth Doppler shift frequency, the motion error signal is calculated.

[0084] The compensation module compensates the motion error by using the motion error term, so as to realize the compensation and focusing imaging of the platform motion error.

[0085] The application also provides an electronic device, comprising: one or more processors; a memory for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors implement the method.

[0086] The application also provides a computer readable storage medium, which stores executable instructions, and the instructions are executed by a processor to make the processor implement the method.

Claims

1. A method for SAR motion error extraction and compensation based on metamaterial tag, characterized in that, The method comprises the following steps: constructing a transmitting waveform containing a point frequency signal and a linear frequency modulation signal, and the point frequency signal and the linear frequency modulation signal being located in the same period; constructing a motion error model, taking the difference in slant range between an ideal track and an actual track as input, and establishing a motion error model for decomposing the motion error into a range-space invariant component and a range-space variant component; using a metamaterial tag with a known phase modulation frequency to receive the transmitting waveform and generate a tag echo; and using the tag echo of the point frequency signal to obtain the modulation frequency and the azimuth Doppler shift frequency of the metamaterial tag; calculating a motion error signal based on the tag echo of the linear frequency modulation signal and the modulation frequency and the azimuth Doppler shift frequency; compensating for the motion error by using the motion error term, so as to realize compensation for the platform motion error and focused imaging. The motion error model is constructed specifically as follows: The propagation time of the transmitted signal in the azimuth direction The ideal position coordinates of the carrier platform are The actual motion trajectory has an offset, and the position point corresponds to coordinates , is the error introduced in the x direction along the course, is the error introduced in the y direction across the course, is the error introduced in the z direction across the course, and at the moment , the distance from the phase center of the radar antenna to the target point is expressed as: （3） （4） wherein, is the ideal slant range of the radar to the target under ideal track conditions, , is the coordinate of the target, is the radar squint angle, is the slant range of the radar center line of sight, is the distance of the target from the center line of sight, is the slant range error introduced by the non-constant velocity of the track direction, representing the azimuthal variability of the motion error, is the slant range error introduced by the Y-Z plane, , is the radar down-look angle; The motion error is decomposed into a range space-invariant and a range space-variant part as follows: （5） wherein, representing the slant range variable, from the above equation, a motion error model of the scene echo signal in the presence of motion error is obtained, as shown in the following equation: （6） where, is the transmitted signal range propagation time, is the chirp rate of the transmitted signal, denotes the range dimension amplitude window, represents the azimuth dimension amplitude window, represents the signal amplitude, is the motion error model of the scene echo signal, λ is the wavelength of the center frequency of the transmitted signal, and c is the speed of light.

2. The method of claim 1, wherein, In one repeating period, the sum of the duration of the point frequency signal and the duration of the linear frequency modulation signal is equal to the length of the period, and the point frequency signal is used to isolate the tag echo in the azimuth frequency domain, and the linear frequency modulation signal is used to maintain the range resolution, and the two share the same radio frequency channel through time multiplexing.

3. The method of claim 2, wherein, The expression of the transmitting signal is as follows: （1） wherein is an exponential function with the natural constant e as base, is the repetition period of the transmitted waveform, is the time length of the point frequency signal within one period, is the time length of the chirp signal, is the transmitted signal, , are the amplitudes of the point frequency signal and the chirp signal, respectively, is the start frequency, is the propagation time of the transmitted signal in azimuth direction, is the chirp rate, is a natural number, is the rectangular window envelope function of the point frequency signal and the chirp signal in range direction.

4. The method of claim 3, wherein, The metamaterial tag is equivalent to a point target, has a modulation function on a SAR signal, and the modulation frequency is , the initial phase of an nth modulation signal is , n is a positive integer, and the modulation mode is phase modulation.

5. The method of claim 4, wherein, Under the condition that the transmitting signal is a linear frequency modulation, according to the motion error model, the tag echo is expressed as: （7） wherein, is the distance to the tag; Let , , represent the phase component generated by the modulator, is the echo signal component of the target; When the point frequency waveform is transmitted, the obtained point frequency signal is as follows: （8） The initial phase of the modulated waveform in azimuth time is given by The modulated signal phase is given by （9） Let , , then: （10） wherein , is the pulse-to-pulse initial phase change angular frequency, azimuth frequency drift amount , the expression is as follows: ； The two-dimensional Fourier transform of formula (8) is performed to obtain a two-dimensional frequency spectrum expressed as: （11） where, is the range rate, is the phase in the two-dimensional spectrum, represents the azimuth amplitude window, defined is the motion velocity of the platform, then the azimuth frequency and the Doppler center frequency The expression is as follows: （12） （13） wherein, is the center frequency of the transmitted signal, is the slant range of the center of the imaging scene, the Doppler parameter estimation method is applied to the point frequency echo signal to obtain the slant angle of the radar beam irradiation , and the velocity of the platform , by analyzing the received point frequency signal, the modulation frequency of the modulation tag is obtained , and the azimuth Doppler shift frequency , the ideal distance of the tag with the change of the azimuth time is obtained , the above results are brought into formula (7), and the tag echo result : （16） wherein, represent errors due to platform motion.

6. The method of claim 5, wherein, The conjugate multiplication of formula (6) and (16) is performed to obtain: （17） The motion error compensation with the tag as the reference point is completed.

7. A device for SAR motion error extraction and compensation based on metamaterial tag, which implements the method of claim 1, characterized in that, The method comprises the following steps: a waveform construction module for constructing a transmitting waveform containing a point frequency signal and a linear frequency modulation signal, and the point frequency signal and the linear frequency modulation signal being located in the same period; a model construction module for constructing a motion error model, taking the difference in slant range between an ideal track and an actual track as input, and establishing a motion error model for decomposing the motion error into a range-space invariant component and a range-space variant component; a motion error signal calculation module for using a metamaterial tag with a known phase modulation frequency to receive the transmitting waveform and generate a tag echo; and using the tag echo of the point frequency signal to obtain the modulation frequency and the azimuth Doppler shift frequency of the metamaterial tag; calculating a motion error signal based on the tag echo of the linear frequency modulation signal and the modulation frequency and the azimuth Doppler shift frequency; a compensation module for compensating for the motion error by using the motion error term, so as to realize compensation for the platform motion error and focused imaging.

8. An electronic device, comprising: The method comprises the following steps: one or more processors; a memory for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors implement the method in any one of claims 1 to 6.

9. A computer-readable storage medium, characterized in that, An executable instruction is stored thereon, and the executable instruction is executed by a processor to enable the processor to implement the method in any one of claims 1 to 6.

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