P-band SAR satellite ionosphere joint measurement and compensation method and system

Abstract

The present invention provides a P-band SAR satellite ionosphere joint measurement and compensation method and system, including: using a dual-frequency measurement method to obtain a background ionosphere TEC value; near the non-equator, according to the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value, obtaining an ionosphere TEC inversion measurement value; near the equator, obtaining an equatorial region ionosphere TEC value; using the background ionosphere TEC value, the ionosphere TEC inversion measurement value and the equatorial region ionosphere TEC value to compensate for the phase error caused by the range dispersion effect; using the phase gradient self-focusing method to compensate for the phase error caused by the ionosphere scintillation effect, and obtaining a P-band SAR image. Compared with the prior art, the present invention can effectively suppress the influence of the dispersion effect, the scintillation effect and the Faraday rotation effect, and ensure the acquisition of high-quality quantitative P-band SAR remote sensing images.

Description

Technical Field

[0001] The present invention relates to the field of synthetic aperture radar satellite overall design and data application processing technology, and in particular to a P-band SAR satellite ionosphere joint measurement and compensation method and system. Background Art

[0002] Electromagnetic wave signals are affected by the ionosphere during propagation, which will introduce errors such as signal delay, phase shift, and polarization rotation. This effect is even more serious for P-band spaceborne synthetic aperture radar (SAR). The ionospheric effect errors caused by the ionosphere, such as dispersion effect, scintillation effect, and Faraday rotation effect, will cause serious distortion of the echo signal, thereby affecting the image quality of the P-band spaceborne SAR system. Therefore, it is necessary to measure the total electron content (TEC) value of the ionosphere and compensate for the error caused by it to meet the quantitative remote sensing application requirements of P-band SAR images.

[0003] Although there are many ways to detect the ionosphere, ground-based radars are difficult to achieve global coverage and cannot achieve same-field detection; occultation detection cannot achieve real-time measurement of the ionosphere at the sub-satellite point and the ground observation area, and the spatial resolution is low. In addition to conventional ground-based radars and space-based occultation detection, in order to solve the same-field observation problem of SAR satellites, satellites usually use dual-frequency measurement methods or far-ultraviolet single-photon imager measurement methods or Faraday rotation angle estimation methods. However, the dual-frequency measurement method can directly use the SAR broadband signal characteristics to divide it into upper and lower sub-bands to achieve background ionosphere TEC measurement, with an accuracy of about 1TECU, and cannot achieve high-precision measurement and effective dispersion effect compensation; the far-ultraviolet single-photon imager measurement method has a relative measurement accuracy of up to 3%, but the absolute measurement accuracy is not high; the TEC measurement method based on Faraday rotation angle estimation mainly relies on the Faraday rotation effect of the ionosphere, and performs inversion estimation through the data domain, with an accuracy of up to 0.5TECU, but the Faraday rotation effect of the ionosphere near the equator is very weak, and inversion estimation cannot be achieved.

[0004] Patent document CNCN113740821A discloses a method for estimating and compensating two-dimensional space-variable scintillation phase error of satellite-borne P-band SAR, including: S1, dividing the entire SAR image into blocks to obtain mask sub-images; S2, using LML-PGA for each mask sub-image to obtain sub-block SPE estimation values; S3, splicing the sub-block SPE estimation values ​​of the same range direction and different azimuth directions in azimuth to obtain the full-aperture SPE estimation of each block range gate; S4, using range interpolation to perform full-aperture SPE estimation and range upsampling to obtain SPE global estimation; S5, performing global SPE compensation for satellite-borne P-band SAR image data affected by ionospheric scintillation effects. However, this system cannot effectively suppress the influence of dispersion effect, scintillation effect and Faraday rotation effect on P-band SAR remote sensing images. Summary of the invention

[0005] In view of the defects in the prior art, the object of the present invention is to provide a P-band SAR satellite ionosphere joint measurement and compensation method and system.

[0006] A P-band SAR satellite ionosphere joint measurement and compensation method provided by the present invention comprises:

[0007] Step 1: In the P-band spaceborne SAR system, a broadband linear frequency modulation signal is used. When the P-band SAR is turned on for observation, a full broadband signal is transmitted, and a first narrowband filter is set at the receiving end to receive the upper subband echo signal, and a second narrowband filter is set to receive the lower subband echo signal;

[0008] Step 2: Using the dual-frequency measurement method, pulse compression processing is performed on the upper sub-band echo signal and the lower sub-band echo signal respectively to obtain the time delay difference between the upper sub-band echo signal and the lower sub-band echo signal, and the background ionosphere TEC value is obtained according to the time delay difference;

[0009] Step 3: Near the non-equator, the Faraday rotation angle introduced by the ionosphere is obtained based on the P-band SAR echo data and the background ionosphere TEC value, and then the ionosphere TEC inversion measurement value is obtained based on the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value;

[0010] Step 4: Near the equator, the far ultraviolet single photon imager measurement method is used, and the reference calibration method is used to realize the TEC measurement of the equatorial region ionosphere to obtain the TEC value of the equatorial region ionosphere;

[0011] Step 5: Use the background ionospheric TEC value, the non-equatorial ionospheric TEC inversion measurement value and the equatorial ionospheric TEC value to compensate for the phase error caused by the range dispersion effect;

[0012] Step 6: Use the phase gradient self-focusing method to compensate for the phase error caused by the ionospheric scintillation effect and obtain the P-band SAR image.

[0013] Preferably, step 1 comprises:

[0014] Step 101: using the linear frequency modulation width signal characteristics of the P-band spaceborne SAR system, outputting a broadband signal during transmission;

[0015] Step 102: Setting a first narrowband filter and a second narrowband filter in the P-band spaceborne SAR system to receive the upper subband echo signal and the lower subband echo signal respectively.

[0016] Step 2 includes:

[0017] Step 201: performing pulse compression processing on the upper sub-band echo signal and the lower sub-band echo signal and extracting the peak position to obtain the delay difference;

[0018] Step 202: Obtain the background ionospheric TEC value based on the time delay difference, the carrier frequency inversion calculation of the upper sub-band echo signal and the lower sub-band echo signal.

[0019] Preferably, step 3 comprises:

[0020] Step 301: In a non-equatorial region, obtain the Faraday rotation angle introduced by the ionosphere according to the satellite-borne polarimetric SAR data;

[0021] Step 302: Obtain an ionospheric TEC inversion measurement value according to a mapping relationship between the Faraday rotation angle and the first ionospheric TEC value.

[0022] Preferably, step 5 comprises:

[0023] Step 501: obtaining the pulse width and modulation rate after the influence of ionospheric dispersion effect according to the background ionospheric TEC value, the non-equatorial region ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value;

[0024] Step 502: Generate a new matched filter function according to the pulse width and the modulation frequency to compensate for the phase error caused by the range dispersion effect.

[0025] Preferably, step 6 comprises:

[0026] Step 601: Based on the SAR image scene characteristics and the scintillation phase error model, the phase error caused by the ionospheric scintillation effect is compensated using the phase gradient autofocusing method.

[0027] A P-band SAR satellite ionosphere joint measurement and compensation system provided by the present invention comprises:

[0028] Module M1: In the P-band spaceborne SAR system, a broadband linear frequency modulation signal is used. When the P-band SAR is turned on for observation, a full broadband signal is transmitted. A first narrowband filter is set at the receiving end to receive the upper subband echo signal, and a second narrowband filter is set at the receiving end to receive the lower subband echo signal.

[0029] Module M2: Using the dual-frequency measurement method, pulse compression processing is performed on the upper sub-band echo signal and the lower sub-band echo signal respectively to obtain the time delay difference between the upper sub-band echo signal and the lower sub-band echo signal, and the background ionosphere TEC value is obtained according to the time delay difference;

[0030] Module M3: Near the non-equator, the Faraday rotation angle introduced by the ionosphere is obtained based on the P-band SAR echo data and the background ionosphere TEC value, and then the ionosphere TEC inversion measurement value is obtained based on the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value;

[0031] Module M4: Near the equator, the far ultraviolet single photon imager measurement method is used, and the reference calibration method is used to achieve the TEC measurement of the equatorial region ionosphere to obtain the TEC value of the equatorial region ionosphere;

[0032] Module M5: Compensate the phase error caused by the range dispersion effect by using the background ionospheric TEC value, the non-equatorial ionospheric TEC inversion measurement value and the equatorial ionospheric TEC value;

[0033] Module M6: The phase gradient self-focusing method is used to compensate for the phase error caused by the ionospheric scintillation effect to obtain a P-band SAR image.

[0034] Preferably, module M1 comprises:

[0035] Submodule M101: With the help of the linear frequency modulation width signal characteristics of the P-band spaceborne SAR system, it outputs a broadband signal during transmission;

[0036] Submodule M102: a first narrowband filter and a second narrowband filter are set in the P-band spaceborne SAR system to receive the upper subband echo signal and the lower subband echo signal respectively.

[0037] Module M2, including:

[0038] Submodule M201: performing pulse compression processing on the upper subband echo signal and the lower subband echo signal and extracting the peak position to obtain the delay difference;

[0039] Submodule M202: Obtain the background ionospheric TEC value based on the time delay difference, the carrier frequency inversion calculation of the upper subband echo signal and the lower subband echo signal.

[0040] Preferably, module M3 comprises:

[0041] Submodule M301: In non-equatorial regions, the Faraday rotation angle introduced by the ionosphere is obtained based on the spaceborne polarimetric SAR data;

[0042] Submodule M302: According to the mapping relationship between the Faraday rotation angle and the first ionospheric TEC value, the ionospheric TEC inversion measurement value is obtained.

[0043] Preferably, module M5 comprises:

[0044] Submodule M501: According to the background ionospheric TEC value, the ionospheric TEC inversion measurement value in the non-equatorial region and the ionospheric TEC value in the equatorial region, the pulse width and modulation rate after the influence of the ionospheric dispersion effect are obtained;

[0045] Submodule M502: Generates a new matched filter function according to the pulse width and modulation frequency to compensate for the phase error caused by the range dispersion effect.

[0046] Preferably, module M6 comprises:

[0047] Submodule M601: Based on the SAR image scene characteristics and the scintillation phase error model, the phase gradient autofocusing method is used to compensate for the phase error caused by the ionospheric scintillation effect.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] 1. Starting from the imaging quality of P-band spaceborne SAR, the present invention designs a method of combining the dual-frequency measurement method under the same field of view, the TEC measurement method based on Faraday rotation angle estimation, and the far-ultraviolet single-photon imager measurement method to meet the application requirements of continuous high-precision TEC measurement in the entire observation domain.

[0050] 2. The present invention realizes dual-frequency measurement by using a broadband linear frequency modulation signal when the P-band SAR is turned on. In non-equatorial areas, a high-precision TEC value is obtained by inversion using the Faraday rotation angle estimation based on the dual-frequency measurement results and echo data, thereby solving the winding problem and the efficiency problem.

[0051] 3. Aiming at the phenomenon that the Faraday effect in the ionosphere in the equatorial region is not obvious, the present invention uses a far-ultraviolet single-photon imager to perform high-precision TEC relative measurement, and obtains high-precision absolute TEC measurement through reference calibration.

[0052] 4. The present invention provides high-quality data for quantitative remote sensing applications, performs distance dispersion effect error compensation based on high-precision measurement of the ionosphere, and simultaneously compensates for the azimuth imaging error caused by the space-varying scintillation effect, and adopts a phase gradient self-focusing ionospheric scintillation phase error compensation method to achieve high-precision two-dimensional focusing. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0054] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0055] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0056] Figure 1 It is a schematic diagram of the process of the present invention, such as Figure 1 As shown, the present invention provides a P-band SAR satellite ionosphere joint measurement and compensation method, comprising:

[0057] Step 1: In the P-band spaceborne SAR system, a broadband linear frequency modulation signal is used. When the P-band SAR is turned on for observation, a full broadband signal is transmitted. A first narrowband filter is set at the receiving end to receive the upper subband echo signal, and a second narrowband filter is set to receive the lower subband echo signal.

[0058] Preferably, step 1 comprises: step 101: outputting a broadband signal during transmission by means of a linear frequency modulation width signal characteristic of a P-band spaceborne SAR system; step 102: setting a first narrowband filter and a second narrowband filter in the P-band spaceborne SAR system to receive an upper subband echo signal and a lower subband echo signal, respectively.

[0059] Among them, radar frequency band refers to the frequency range of radar waves. In the radar industry, radar operating frequencies are divided into several bands, from low to high: high frequency (HF), very high frequency (VHF), ultra high frequency (UHF), L band, S band, C band, X band, Ku band, K band and Ka band. Since the earliest radars used meter waves, this band is called P (Previous) band.

[0060] Specifically, in the P-band spaceborne SAR system, the broadband linear frequency modulation signal form is fully utilized. When the P-band SAR is turned on for observation, the full broadband signal is first transmitted, and then two narrowband filters are set at the receiving end to receive two sub-band echo signals respectively. The two narrowband filters may be a first narrowband filter and a second narrowband filter, and the two sub-band echo signals may be an upper sub-band echo signal and a lower sub-band echo signal.

[0061] The upper sub-band echo signal and the lower sub-band echo signal have different frequency bands.

[0062] Specifically, based on the characteristics of the P-band SAR satellite broadband linear frequency modulation signal, a width signal is emitted when the SAR is turned on for observation. The first narrowband filter and the second narrowband filter are set at the receiving end to separate the upper and lower sub-bands of the broadband signal, forming two echo signals of different frequency bands, providing effective data for the dual-frequency measurement method.

[0063] In an optional implementation, the bandwidth range is recommended to be above 50 MHz, which can achieve a background ionosphere measurement accuracy better than 1 TECU. TECU is a total electron content unit, which is used to represent the number of electrons, and 1 TECU represents 10 to the 16th power of electrons.

[0064] Step 2: Use the dual-frequency measurement method to perform pulse compression processing on the upper sub-band echo signal and the lower sub-band echo signal respectively to obtain the time delay difference between the upper sub-band echo signal and the lower sub-band echo signal, and obtain the background ionospheric TEC value based on the time delay difference.

[0065] Preferably, step 2 comprises: step 201: performing pulse compression processing on the upper sub-band echo signal and the lower sub-band echo signal and extracting the peak position to obtain the time delay difference; step 202: obtaining the background ionospheric TEC value based on the time delay difference, the carrier frequency inversion calculation of the upper sub-band echo signal and the lower sub-band echo signal.

[0066] Specifically, the dual-frequency measurement method is used to perform pulse compression and correlation processing on the upper sub-band echo signal and the lower sub-band echo signal respectively to measure the time delay difference, and the background ionospheric TEC value is obtained by inversion estimation based on the obtained time delay difference.

[0067] Exemplarily, the delay difference measurement is realized by demodulating, pulse compressing and correlating two echo signals with different carrier frequencies.

[0068] Specifically, pulse compression is performed on the upper sub-band echo signal and the lower sub-band echo signal respectively, and the positions of the peaks are taken respectively to calculate the pseudo-range values ​​of the upper sub-band echo signal and the lower sub-band echo signal.

[0069] Preferably, the background ionospheric TEC value can be inverted and estimated according to formula (1) and formula (2).

[0070] Specifically, formula (1) is expressed as:

[0071]

[0072] Where ρ′0 represents the pseudorange measurement value of the upper subband; ρ′1 represents the pseudorange measurement value of the lower subband, T g0 and T g1 They represent the group delay values ​​under different carrier frequencies respectively.

[0073] Formula (2) is expressed as:

[0074]

[0075] Among them, T g Represents the group delay value; R z represents the slant distance from the antenna to the target; c represents the speed of light; e represents the charge per unit electron; m represents the mass of the electron; ε0 represents the dielectric constant of vacuum; ω c represents the carrier angular frequency; H represents the nominal orbit altitude; TEC is the total electron content of the background ionosphere to be inverted.

[0076] Step 3: Near the non-equator, the Faraday rotation angle introduced by the ionosphere is obtained based on the P-band SAR echo data and the background ionosphere TEC value, and then the ionosphere TEC inversion measurement value is obtained based on the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value.

[0077] Preferably, step 3 comprises: step 301: in a non-equatorial region, obtaining a Faraday rotation angle introduced by the ionosphere based on satellite-borne polarimetric SAR data; step 302: obtaining an ionospheric TEC inversion measurement value based on a mapping relationship between the Faraday rotation angle and the first ionospheric TEC value.

[0078] Specifically, in order to further obtain higher-precision ionospheric TEC values, near the non-equator, the P-band spaceborne SAR signal will be affected by the ionospheric Faraday rotation effect. With the help of the spaceborne P-band SAR echo data and the background ionospheric TEC value obtained by the dual-frequency measurement method, the Faraday rotation angle introduced by the ionosphere is accurately estimated. Then, based on the mapping relationship between the Faraday rotation angle and the first ionospheric TEC value, small-scale and high-precision TEC inversion estimation measurement is achieved to obtain the ionospheric TEC inversion measurement value.

[0079] In the present invention, based on the dual-frequency measurement method, in order to further obtain a higher-precision small-scale ionospheric TEC value, in the non-equatorial region, the P-band spaceborne SAR signal is affected by the ionospheric Faraday rotation effect, resulting in polarization domain errors. With the help of spaceborne polarization SAR data, the Faraday rotation angle introduced by the ionosphere is accurately estimated, and then the ionospheric TEC value inversion estimation is realized according to the mapping relationship between the Faraday rotation angle and the first ionospheric TEC value, and the ionospheric TEC inversion measurement value is obtained. In the entire estimation process, in order to improve the inversion accuracy, the use of spectrum segmentation processing is considered.

[0080] For example, in non-equatorial regions, the Faraday rotation effect is used to invert and estimate high-precision ionospheric TEC values, and the background ionospheric TEC values ​​obtained by the dual-frequency measurement method are used as the initial values ​​of the inversion estimation, eliminating the entanglement problem of Faraday rotation angle estimation and improving the efficiency of the inversion estimation.

[0081] Preferably, the P-band spaceborne SAR signal will be affected by the Faraday rotation effect of the ionosphere, resulting in polarization domain errors. Faraday rotation refers to the rotation of the polarization plane relative to the incident wave under the influence of the electromagnetic field when a linearly polarized radio wave passes through an electromagnetic field. The rotation angle is called the Faraday rotation angle. With the help of spaceborne polarization SAR data, the Faraday rotation angle introduced by the ionosphere is first accurately estimated, and then the TEC inversion measurement is realized based on the mapping relationship between the Faraday rotation angle and the TEC value of the first ionosphere, and the ionospheric TEC inversion measurement value is obtained. In the entire estimation process, in order to improve the inversion accuracy, spectrum segmentation processing is adopted. Through theoretical and simulation analysis, combined with the spatial distribution law of TEC, reasonable parameters are selected to achieve the optimal TEC inversion, which can improve the spatial resolution to better than 100 meters and the TEC inversion accuracy to better than 0.5TECU.

[0082] Preferably, the Faraday rotation angle is expressed as:

[0083]

[0084] Where, f represents the frequency of electromagnetic wave; K Ω is a constant; B e represents the strength of the earth's magnetic field; TEC represents the total electron content in the ionosphere in the direction perpendicular to the ground; θ represents the viewing angle of the satellite-borne SAR antenna; Ψ represents the angle between the earth's magnetic field and the propagation direction of the radar electromagnetic wave, where the propagation direction of the radar electromagnetic wave is the pointing direction of the antenna beam.

[0085] Step 4: Near the equator, the far-ultraviolet single-photon imager measurement method is used, and with the help of reference calibration means, the ionospheric TEC measurement in the equatorial region is achieved to obtain the TEC value of the equatorial region.

[0086] Specifically, in order to further obtain higher-precision ionospheric TEC values, near the equator, due to the weak Faraday rotation effect of the ionosphere, the far-ultraviolet single-photon imager measurement method is adopted, and with the help of reference calibration means, small-scale high-precision TEC measurement in the same field of view is achieved to obtain the ionospheric TEC value in the equatorial region.

[0087] Preferably, in the equatorial region, since there is no obvious ionospheric Faraday selection effect, in order to further obtain high-precision ionospheric measurement values, in the area near the equator, a far-ultraviolet single-photon imager is used to conduct ground observations in the same field of view, and the ionospheric altitude oxygen atomic airglow and nitrogen molecular airglow are monitored in real time in the P-band spaceborne SAR detection area. The ionospheric electron density TEC and electron density changes in the area can be obtained by inversion, and the real-time monitored TEC and TEC change data provide a basis for ionospheric compensation for P-band spaceborne SAR on orbit.

[0088] Preferably, the detection field of the far-ultraviolet single-photon imager covers the P-band spaceborne SAR detection area, and the working modes include night mode and day mode. The night mode measures the oxygen atomic 135.6nm band airglow, and the day mode measures the oxygen atomic 135.6nm airglow and nitrogen molecular LBH band airglow at the same time. The relative measurement accuracy can be better than 0.3TECU, and the high-precision TEC absolute value can be obtained by reference calibration.

[0089] Step 5: Use the background ionospheric TEC value, the ionospheric TEC inversion measurement value and the equatorial ionospheric TEC value to compensate for the phase error caused by the range dispersion effect.

[0090] Preferably, step 5 comprises: step 501: obtaining the pulse width and frequency modulation rate after the influence of ionospheric dispersion effect according to the background ionospheric TEC value, the ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value; step 502: generating a new matched filter function according to the pulse width and the frequency modulation rate to compensate for the phase error caused by the range dispersion effect.

[0091] In the present invention, step 5 can be used to improve the imaging performance in the distance direction.

[0092] Specifically, based on the measured background ionospheric TEC values, the inverted TEC values ​​of the non-equatorial region ionospheric TEC and the equatorial region ionospheric TEC values, the pulse width and modulation rate after the influence of the ionospheric dispersion effect are obtained according to the calculation formula, which are then brought into a new matched filter function. After re-matched filtering, the range error correction of the P-band spaceborne SAR can be achieved, thereby improving the range imaging performance.

[0093] Exemplarily, range dispersion error compensation is performed based on the measured background ionospheric TEC values, the non-equatorial region ionospheric TEC inversion measurement values, and the equatorial region ionospheric TEC values, mainly compensating the pulse width and modulation rate of the correction signal to achieve range precise matching filtering and improve the range imaging performance.

[0094] Preferably, based on the measured background ionospheric TEC value, the non-equatorial region ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value, a new pulse width and modulation rate in the two-way case can be obtained. The expression of the pulse width under the influence of the ionosphere is formula (4):

[0095]

[0096] Among them, τ′ represents the new pulse width; τ represents the original pulse width; R z represents the slant distance from the antenna to the target; c represents the speed of light; e represents the charge per unit electron; m represents the mass of the electron; ε0 represents the dielectric constant of vacuum; ω c represents the carrier angular frequency; B represents the signal bandwidth; H represents the nominal orbit altitude; TEC represents the total electron content of the background ionosphere obtained by the dual-frequency measurement method.

[0097] The frequency modulation rate is expressed as formula (5):

[0098]

[0099] Among them, α′ represents the new frequency modulation slope; Δτ represents the difference between the new pulse width and the original pulse width. After adjusting the matched filter function and the expression of the received echo signal, the influence of the ionospheric dispersion effect on the SAR range signal is corrected to a certain extent.

[0100] Step 6: Use the phase gradient self-focusing method to compensate for the phase error caused by the ionospheric scintillation effect and obtain the P-band SAR image.

[0101] Preferably, step 6 comprises: step 601: based on the SAR image scene characteristics and the scintillation phase error model, using the phase gradient self-focusing method to compensate for the phase error caused by the ionospheric scintillation effect.

[0102] Specifically, in order to obtain high-quality P-band SAR images, the phase gradient self-focusing method is used to compensate for the scintillation error caused by the ionospheric scintillation effect and achieve two-dimensional precise focusing.

[0103] Specifically, in order to realize the compensation of space-variant scintillation effect and achieve high-quality P-band spaceborne SAR imaging, on the basis of SAR image scene characteristics and scintillation phase error model, the phase gradient self-focusing method is used to realize ionospheric scintillation phase error compensation, which can effectively suppress the influence of ionospheric scintillation effect on imaging quality and improve the azimuth imaging performance of SAR images.

[0104] In the present invention, the scintillation effect is caused by the irregular body of ionospheric electron density, and the P-band spaceborne SAR signal passes through the ionosphere to cause additional phase. As the satellite moves, the signal passing through the ionosphere sweeps across the ionospheric phase screen to produce different additional phases. Moreover, the nonlinear disturbance of the ionospheric phase caused by the time-varying property of the ionosphere is unfavorable to the radar signal propagating through the ionosphere, which seriously affects the imaging quality of the P-band spaceborne SAR in azimuth. The scintillation error compensation processing is realized based on the phase gradient autofocusing algorithm to obtain high-quality P-band spaceborne SAR imaging results, and provide high-performance data for quantitative remote sensing applications.

[0105] Preferably, phase gradient autofocusing is a non-parametric model algorithm that does not use a finite-order phase error model, but directly estimates the phase error. The algorithm performs optimal estimation based on the maximum likelihood estimation theory and makes full use of the phase error information on multiple distance units. For scenes with strong targets, the estimation result has high robustness.

[0106] In order to meet the requirements of high-precision ionospheric TEC value measurement and error compensation, the present invention jointly adopts a dual-frequency measurement method, a TEC measurement method based on Faraday rotation angle estimation, and a far-ultraviolet single-photon imager measurement method to achieve high-precision TEC measurement in the same field of view as the P-band SAR satellite observation area, and effectively solves the high-precision TEC measurement in the equatorial area; based on the dual-frequency measurement method, the winding problem of TEC estimation based on Faraday rotation angle is solved, and the estimation inversion efficiency is improved; based on the high-precision ionospheric TEC value, high-precision compensation for range dispersion effect is performed, and at the same time, for the space-varying scintillation effect, a phase gradient self-focusing ionospheric scintillation phase error compensation method is adopted to achieve high-precision two-dimensional focusing, thereby ensuring the acquisition of high-quality quantitative P-band SAR remote sensing images.

[0107] The present invention relates to the overall system design and data application processing of a P-band synthetic aperture radar satellite. With the continuous development of synthetic aperture radar satellite technology, it is required not only to obtain higher resolution and larger observation width, but also to realize penetrating detection and quantitative remote sensing application. Compared with the SAR payload frequency band carried by existing satellites, the P-band SAR has stronger penetration ability. The P-band multi-polarization SAR satellite has strong penetration and polarization interference ability. It has the characteristics of interferometric SAR being sensitive to the vertical structure of vegetation space and polarization being sensitive to the shape and direction of vegetation. It provides effective space-based remote sensing means for forest resource monitoring and management and ecological environment protection, DEM acquisition of vegetation coverage area, crop drought monitoring, infrastructure planning and construction in frozen soil coverage area, glacier change monitoring, geological disaster emergency monitoring, etc. However, the P-band SAR satellite is seriously affected by the space ionosphere, and the influence in different regions and different time periods is also different. In order to meet the high-precision quantitative remote sensing application of the P-band SAR satellite, it is necessary to measure the ionosphere and compensate for the errors caused by the ionosphere effect.

[0108] The present invention realizes high-precision measurement of background ionosphere in different regions and high-precision compensation of ionosphere effect. Aiming at the characteristics of ionosphere effect, the present invention adopts dual-frequency measurement method, Faraday rotation angle estimation method and far-ultraviolet single-photon imager measurement method to obtain high-precision background ionosphere TEC values ​​of all observation regions. The dual-frequency measurement method provides an initial value for the Faraday rotation angle estimation method, solves the entanglement problem and improves the estimation efficiency. Meanwhile, aiming at the phenomenon that the Faraday rotation effect is not obvious near the equator, the far-ultraviolet single-photon imager measurement method is used as a supplementary method. With the help of the high relative measurement accuracy and reference calibration, the high-precision background ionosphere TEC value near the equator is obtained. Secondly, the high-precision detection result of the background ionosphere is organically combined with the ionosphere effect compensation. The high-precision ionosphere TEC value is used to reconstruct the range-direction matched filter function to realize the range-direction ionosphere dispersion effect error compensation, and the azimuth ionosphere scintillation effect error compensation is realized by the phase gradient self-focusing method.

[0109] Specifically, due to the influence of the ionospheric refraction effect, electromagnetic waves will produce a "bending effect" when propagating, especially the P-band ionospheric refraction effect is more obvious, which makes the actual transmission distance of the P-band electromagnetic wave longer, which is called "pseudorange". Using the characteristics of the P-band satellite-borne SAR wide linear frequency modulation signal, full bandwidth transmission, two narrowband filters with different frequencies are set at the receiving end to obtain two unrelated carrier frequency echo signals, and pulse compression processing is performed separately. The peak position is taken as the pseudorange measured under the two carrier frequencies, which are expressed as ρ0' and ρ1' respectively. The relationship between them and the group delay satisfies the relationship in formula (1) and formula (2).

[0110] Substituting formula (2) into formula (1) can further obtain the background ionospheric TEC value. This method is called dual-frequency measurement method, and its measurement accuracy is better than 1TECU.

[0111] In order to further improve the measurement accuracy of the background ionospheric TEC value, the P-band satellite-borne SAR signal in the non-equatorial region will be affected by the Faraday rotation effect of the ionosphere. The P-band electromagnetic wave will produce a rotation of the polarization plane relative to the incident wave under the influence of the electromagnetic field. The rotation angle is called the Faraday rotation angle. The Faraday rotation angle introduced by the ionosphere can be accurately estimated with the help of satellite-borne polarimetric SAR data, and then the high-precision ionospheric TEC value, that is, the ionospheric TEC inversion measurement value, can be obtained. In order to solve the entanglement problem of Faraday rotation angle estimation and improve the measurement efficiency, the present invention first adopts a dual-frequency measurement method to measure the background ionospheric TEC value of the observation area. The measurement result is used as the initial value of the Faraday rotation angle estimation, which can effectively solve the entanglement problem and improve the estimation efficiency. The measurement accuracy can reach 0.5TECU.

[0112] Further, based on the Faraday rotation angle estimation method, a high-precision background ionospheric TEC value, i.e., an ionospheric TEC inversion measurement value, can be obtained. However, for the equatorial region, the Faraday rotation effect is not obvious, so this method cannot be used. For this reason, the present invention uses a far-ultraviolet single-photon imager on a P-band SAR satellite to monitor the ionospheric height oxygen atom airglow and nitrogen molecule airglow in real time with the SAR in the same field of view in the equatorial observation area, and the ionospheric electron density TEC and electron density changes in the region can be obtained by inversion. The working modes of the far-ultraviolet single-photon imager include night mode and day mode. The night mode measures the oxygen atom 135.6nm band airglow, and the daytime simultaneously measures the oxygen atom 135.6nm airglow and the nitrogen molecule LBH band airglow. The advantage of the far-ultraviolet single-photon imager is that the relative measurement accuracy is high, which can reach 0.3TECU. For this reason, a reference calibration method is used to obtain a high-precision absolute TEC value, which provides a basis for the P-band satellite-borne SAR to perform ionospheric compensation on orbit.

[0113] Furthermore, the influence of ionospheric effect on P-band spaceborne SAR mainly includes dispersion effect and scintillation effect, which affect the imaging performance in range and azimuth respectively. Therefore, according to the characteristics of dispersion effect and scintillation effect, the present invention, firstly, aims at the dispersion effect error, uses the ionospheric TEC value inversion result obtained by the dual-frequency measurement method, the Faraday rotation angle estimation method and the far ultraviolet single photon imager measurement method to compensate the image data for the error, so as to realize the dispersion effect range error compensation processing; secondly, aims at the scintillation effect error, adopts the compensation method based on the phase gradient self-focusing algorithm, makes full use of the redundancy of the ionospheric TEC value inversion result and the phase error, estimates the derivative of the phase error introduced by the scintillation effect to the azimuth signal, thereby completes the estimation of the phase error, and performs error compensation on the image data, so as to realize the scintillation effect azimuth error compensation processing.

[0114] In summary, the P-band SAR satellite ionosphere joint measurement and compensation method of the present invention is aimed at the problem that P-band SAR satellites are greatly affected by the ionosphere. In order to meet the needs of quantitative remote sensing applications, an innovative new method based on dual-frequency measurement, Faraday rotation angle estimation and far-ultraviolet single-photon imager measurement is proposed to obtain high-precision background ionosphere TEC of the P-band satellite-borne SAR observation area, and the dispersion effect compensation is performed based on the TEC value obtained by inversion estimation and the scintillation effect compensation is performed based on the phase gradient self-focusing method, and finally a two-dimensional high-quality imaging effect can be obtained. First, the problem of high-precision ionosphere TEC value measurement of the entire observation area in the same field of view is solved; second, the error compensation problem caused by the ionosphere effect is solved, which lays the foundation for the quantitative remote sensing application of P-band SAR satellites.

[0115] The present invention provides a P-band SAR satellite ionosphere joint measurement and compensation system, comprising:

[0116] Module M1: In the P-band spaceborne SAR system, a broadband linear frequency modulation signal is used. When the P-band SAR is turned on for observation, a full broadband signal is transmitted. A first narrowband filter is set at the receiving end to receive the upper subband echo signal, and a second narrowband filter is set to receive the lower subband echo signal.

[0117] Preferably, module M1 comprises: submodule M101: with the help of the linear frequency modulation width signal characteristics of the P-band spaceborne SAR system, it outputs a broadband signal during transmission; submodule M102: setting a first narrowband filter and a second narrowband filter in the P-band spaceborne SAR system to receive the upper subband echo signal and the lower subband echo signal respectively.

[0118] Module M2: Using the dual-frequency measurement method, pulse compression processing is performed on the upper sub-band echo signal and the lower sub-band echo signal respectively to obtain the time delay difference between the upper sub-band echo signal and the lower sub-band echo signal, and the background ionosphere TEC value is obtained according to the time delay difference.

[0119] Preferably, module M2 comprises: submodule M201: performing pulse compression processing on the upper subband echo signal and the lower subband echo signal and extracting the peak position to obtain the time delay difference; submodule M202: obtaining the background ionospheric TEC value based on the time delay difference, the carrier frequency inversion calculation of the upper subband echo signal and the lower subband echo signal.

[0120] Module M3: Near the non-equator, the Faraday rotation angle introduced by the ionosphere is obtained based on the P-band SAR echo data and the background ionosphere TEC value, and then the ionosphere TEC inversion measurement value is obtained based on the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value.

[0121] Preferably, module M3 comprises: submodule M301: in the non-equatorial region, obtaining the Faraday rotation angle introduced by the ionosphere according to the satellite-borne polarization SAR data; submodule M302: obtaining the ionospheric TEC inversion measurement value according to the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value.

[0122] Module M4: Near the equator, the far-ultraviolet single-photon imager measurement method is used, and with the help of reference calibration means, the TEC value of the ionosphere in the equatorial region is obtained.

[0123] Module M5: The phase error caused by the range dispersion effect is compensated by using the background ionospheric TEC value, the ionospheric TEC inversion measurement value and the equatorial ionospheric TEC value.

[0124] Preferably, module M5 comprises: submodule M501: obtaining pulse width and frequency modulation rate after the influence of ionospheric dispersion effect according to background ionospheric TEC value, non-equatorial region ionospheric TEC inversion measurement value and equatorial region ionospheric TEC value; submodule M502: generating a new matched filter function according to pulse width and frequency modulation rate to compensate for the phase error caused by range dispersion effect.

[0125] Module M6: The phase gradient self-focusing method is used to compensate for the phase error caused by the ionospheric scintillation effect to obtain a P-band SAR image.

[0126] Preferably, module M6 comprises:

[0127] Submodule M601: Based on the SAR image scene characteristics and the scintillation phase error model, the phase gradient autofocusing method is used to compensate for the phase error caused by the ionospheric scintillation effect.

[0128] Compared with the prior art, the present invention has the following beneficial effects:

[0129] 1. The P-band SAR satellite ionosphere joint measurement and compensation method of the present invention is to obtain high-precision background ionosphere TEC value to compensate for the range error caused by the ionosphere dispersion effect. First, the TEC value of the large-scale background ionosphere is obtained by using the dual-frequency measurement method, and then it is used as the initial value of the Faraday rotation angle estimation method to solve the winding problem and greatly improve the estimation efficiency.

[0130] 2. The P-band SAR satellite ionosphere joint measurement and compensation method of the present invention is to obtain high-precision background ionosphere TEC value in the P-band spaceborne SAR observation area. Near the equator, due to the unobvious Faraday rotation effect, the Faraday rotation angle estimation method cannot be used. The far ultraviolet single photon imager measurement method is used to achieve high-precision ionosphere relative measurement, and then the high-precision TEC absolute value is obtained through reference calibration, which effectively solves and compensates for the low accuracy of ionospheric dispersion effect compensation near the equator in the past, and provides basic data for high-precision dispersion compensation in the full observation domain of P-band spaceborne SAR.

[0131] 3. The P-band SAR satellite ionosphere joint measurement and compensation method of the present invention is, firstly, based on the high-precision background ionosphere TEC value obtained by inversion measurement, a new matched filter function is constructed to achieve precise focusing in the distance direction; secondly, in order to compensate for the influence of the scintillation effect on the azimuth imaging of the P-band satellite-borne SAR, a scintillation error compensation process is implemented based on a phase gradient autofocusing algorithm to obtain two-dimensional precise focusing.

[0132] 4. The P-band SAR satellite ionosphere joint measurement and compensation method of the present invention innovatively proposes and designs the comprehensive use of multiple means for the first time, effectively solving the problem of poor imaging performance of P-band spaceborne SAR due to the influence of the ionosphere, and greatly improving the accuracy of quantitative remote sensing inversion applications of P-band SAR satellites.

[0133] Those skilled in the art know that, in addition to implementing the system, device and its various modules provided by the present invention in a purely computer-readable program code, it is entirely possible to implement the same program in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers and embedded microcontrollers by logically programming the method steps. Therefore, the system, device and its various modules provided by the present invention can be considered as a hardware component, and the modules implementing various programs included therein can also be considered as structures within the hardware component; the modules implementing various functions can also be considered as both software programs implementing the method and structures within the hardware component.

[0134] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A P-band SAR satellite ionosphere joint measurement and compensation method, characterized in that: include: Step 1: In the P-band spaceborne SAR system, a broadband linear frequency modulation signal is used. When the P-band SAR is turned on for observation, a full broadband signal is transmitted, and a first narrowband filter is set at the receiving end to receive the upper subband echo signal, and a second narrowband filter is set to receive the lower subband echo signal; Step 2: using a dual-frequency measurement method, respectively performing pulse compression processing on the upper sub-band echo signal and the lower sub-band echo signal to obtain a time delay difference between the upper sub-band echo signal and the lower sub-band echo signal, and obtaining a background ionosphere TEC value according to the time delay difference; Step 3: In the non-equatorial region, the Faraday rotation angle introduced by the ionosphere is obtained according to the P-band SAR echo data and the background ionosphere TEC value, and then the ionosphere TEC inversion measurement value is obtained according to the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value; Step 4: In the equatorial region, the far-ultraviolet single-photon imager measurement method is used, and the reference calibration method is used to measure the ionospheric TEC in the equatorial region to obtain the ionospheric TEC value in the equatorial region; Step 5: using the background ionospheric TEC value, the non-equatorial region ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value to compensate for the phase error caused by the range dispersion effect; Step 6: Use the phase gradient self-focusing method to compensate for the phase error caused by the ionospheric scintillation effect and obtain the P-band SAR image.

2. The P-band SAR satellite ionosphere joint measurement and compensation method according to claim 1, characterized in that: The step 1 comprises: Step 101: using the linear frequency modulation width signal characteristics of the P-band spaceborne SAR system to output a broadband signal during transmission; Step 102: setting the first narrowband filter and the second narrowband filter in the P-band spaceborne SAR system to receive the upper subband echo signal and the lower subband echo signal respectively; The step 2 comprises: Step 201: performing pulse compression processing on the upper sub-band echo signal and the lower sub-band echo signal and extracting the peak positions to obtain the delay difference; Step 202: Obtain the background ionospheric TEC value according to the time delay difference, the upper subband echo signal and the carrier frequency inversion calculation of the lower subband echo signal.

3. The P-band SAR satellite ionosphere joint measurement and compensation method according to claim 1, characterized in that: The step 3 comprises: Step 301: In a non-equatorial region, obtaining the Faraday rotation angle introduced by the ionosphere according to satellite-borne polarimetric SAR data; Step 302: Obtain the ionospheric TEC inversion measurement value according to the mapping relationship between the Faraday rotation angle and the first ionospheric TEC value.

4. The P-band SAR satellite ionosphere joint measurement and compensation method according to claim 3, characterized in that: The step 5 comprises: Step 501: obtaining a pulse width and a modulation rate after being affected by an ionospheric dispersion effect according to the background ionospheric TEC value, the non-equatorial region ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value; Step 502: Generate a new matched filter function according to the pulse width and the modulation frequency to compensate for the phase error caused by the range dispersion effect.

5. The P-band SAR satellite ionosphere joint measurement and compensation method according to claim 3, characterized in that: The step 6 comprises: Step 601: Based on the SAR image scene characteristics and the scintillation phase error model, the phase error caused by the ionospheric scintillation effect is compensated using the phase gradient autofocusing method.

6. A P-band SAR satellite ionosphere joint measurement and compensation system, characterized in that: include: Module M1: In the P-band spaceborne SAR system, a broadband linear frequency modulation signal is used. When the P-band SAR is turned on for observation, a full broadband signal is transmitted. A first narrowband filter is set at the receiving end to receive the upper subband echo signal, and a second narrowband filter is set at the receiving end to receive the lower subband echo signal. Module M2: using a dual-frequency measurement method, performing pulse compression processing on the upper sub-band echo signal and the lower sub-band echo signal respectively, obtaining a time delay difference between the upper sub-band echo signal and the lower sub-band echo signal, and obtaining a background ionosphere TEC value according to the time delay difference; Module M3: in the non-equatorial region, the Faraday rotation angle introduced by the ionosphere is obtained according to the P-band SAR echo data and the background ionosphere TEC value, and then the ionosphere TEC inversion measurement value is obtained according to the mapping relationship between the Faraday rotation angle and the first ionosphere TEC value; Module M4: In the equatorial region, the far-ultraviolet single-photon imager measurement method is used, and the reference calibration method is used to realize the TEC measurement of the equatorial region ionosphere to obtain the TEC value of the equatorial region ionosphere; Module M5: compensating for the phase error caused by the range dispersion effect by using the background ionospheric TEC value, the non-equatorial region ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value; Module M6: The phase gradient self-focusing method is used to compensate for the phase error caused by the ionospheric scintillation effect to obtain a P-band SAR image.

7. The P-band SAR satellite ionosphere joint measurement and compensation system according to claim 6, characterized in that: The module M1 comprises: Submodule M101: With the help of the linear frequency modulation width signal characteristics of the P-band spaceborne SAR system, it outputs a broadband signal during transmission; Submodule M102: setting the first narrowband filter and the second narrowband filter in the P-band spaceborne SAR system to receive the upper subband echo signal and the lower subband echo signal respectively; The module M2 comprises: Submodule M201: performing pulse compression processing on the upper subband echo signal and the lower subband echo signal and extracting the peak position to obtain the delay difference; Submodule M202: Obtain the background ionospheric TEC value according to the time delay difference, the carrier frequency inversion calculation of the upper subband echo signal and the lower subband echo signal.

8. The P-band SAR satellite ionosphere joint measurement and compensation system according to claim 6, characterized in that: The module M3 comprises: Submodule M301: in the non-equatorial region, obtaining the Faraday rotation angle introduced by the ionosphere according to the satellite-borne polarimetric SAR data; Submodule M302: obtaining the ionospheric TEC inversion measurement value according to the mapping relationship between the Faraday rotation angle and the first ionospheric TEC value.

9. The P-band SAR satellite ionosphere joint measurement and compensation system according to claim 8, characterized in that: The module M5 comprises: Submodule M501: obtaining the pulse width and modulation rate after the influence of ionospheric dispersion effect according to the background ionospheric TEC value, the non-equatorial region ionospheric TEC inversion measurement value and the equatorial region ionospheric TEC value; Submodule M502: Generate a new matched filter function according to the pulse width and the modulation frequency to compensate for the phase error caused by the range dispersion effect.

10. The P-band SAR satellite ionosphere joint measurement and compensation system according to claim 8, characterized in that: The module M6 comprises: Submodule M601: Based on the SAR image scene characteristics and the scintillation phase error model, the phase gradient autofocusing method is used to compensate for the phase error caused by the ionospheric scintillation effect.

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