A fast time-varying scene observation inclined orbit high orbit SAR imaging system

Abstract

The application belongs to the field of satellite-borne radar synthetic aperture radar design and signal processing, and discloses a high-orbit SAR imaging system for fast time-varying scene observation, which comprises two high-orbit SAR satellites in inclined orbits and an echo signal processing module, the two high-orbit SAR satellites in inclined orbits are respectively a main satellite and an auxiliary satellite, are used for simultaneously starting to observe the same ground scene area, and receive the backscattering echo signals of the ground scene area; the echo signal processing module is used for sequentially performing distance direction pulse compression processing, cross-correlation processing and time domain back projection processing on the backscattering echo signals received by the two high-orbit SAR satellites in inclined orbits, and obtaining a high-resolution correlation function diagram. The system overcomes the defect that the synthetic aperture time of high-orbit SAR imaging is long, the azimuth imaging resolution is improved by 1-2 orders of magnitude compared with the traditional SAR imaging resolution, and the efficiency of high-orbit SAR marine application is greatly improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of spaceborne radar synthetic aperture radar (SAR) design and signal processing, and is mainly applied to long synthetic aperture time observation imaging system design and signal processing of fast time-varying scenes (such as sea surface), and particularly relates to a tilting orbit high-orbit SAR imaging system and method for time-varying scene observation. The tilting orbit high-orbit SAR refers to a SAR system running in a geosynchronous orbit with a large orbit inclination (generally greater than 5°), and the high-orbit SAR is also called geosynchronous orbit SAR. The height of the high-orbit SAR from the ground is about 36000 km. BACKGROUND

[0002] SAR has the advantages of all-weather, all-day work, high-resolution wide-range imaging, etc. SAR uses the motion of the platform to form a long synthetic aperture. For a stationary scene with stable scattering characteristics, coherent accumulation processing based on virtual long synthetic aperture data can obtain high azimuth resolution imaging. However, for fast time-varying scene imaging, the time scale of the scene scattering characteristic decorrelation may be as short as sub-second, which is much smaller than the length of the synthetic aperture time of the high-orbit SAR of hundreds of seconds. Therefore, the echo recorded within the synthetic aperture time range cannot improve the azimuth resolution through coherent accumulation.

[0003] The high-orbit SAR satellite is about 36000 kilometers away from the ground. Compared with low-orbit SAR satellites or airborne SAR systems, the high-orbit SAR satellite has the following outstanding advantages: 1. The imaging swath width reaches several hundred kilometers, and the azimuth resolution is not sacrificed. Single observation covers the entire large lake water body, cross-provincial crop planting area, ecological protection area, etc.; 2. The revisit observation time is short, and the rapid response capability is strong, which is suitable for high temporal resolution sampling dynamic change monitoring of large-scale areas; 3. The target area stays for a long time, and the stay time reaches the order of hours, so it can be used as an illumination source for low-orbit SAR systems, airborne SAR systems, etc. to provide external radiation sources, thereby realizing bistatic SAR imaging.

[0004] In 1983, the General Electric Company of the United States first proposed a S-band geosynchronous SAR system with a track inclination of 50°, which adopts a 15m-30m aperture SAR antenna and can cover the observation of the United States within 3 hours. In the past two decades, the high-orbit SAR system design and imaging processing have developed rapidly, and the research teams of the high-orbit SAR in the world are mainly concentrated in Europe, the United States and China. According to the technical route, the high-orbit SAR systems proposed by research institutions at home and abroad mainly include: a low-power near-zero-inclination high-orbit SAR system running in the geostationary orbit, a distributed small satellite cluster high-orbit SAR system ARGOS adopting a multiple-input multiple-output (MIMO) system, a near-zero-inclination orbit double-satellite correlation high-orbit SAR system (CoSAR), a large-range observation high-orbit SAR system running in the inclined geosynchronous orbit, a retrograde orbit high-orbit SAR system with less influence of satellite speed variation and earth rotation on full-orbit imaging performance, etc.

[0005] In terms of rapid time-varying scene observation, P. Dekker et al. proposed to use the relative motion of two near-zero-inclination geostationary SAR satellites to estimate the cross-correlation function of the rapid time-varying scene, but the near-zero-inclination geostationary SAR is only suitable for observation in the mid-latitude region, and there are observation blind areas in the low-latitude and high-latitude regions. In addition, the relative motion speed of the near-zero-inclination geostationary SAR satellite is small, and the synthetic aperture time is as long as several hours, which brings great challenges to imaging processing. At present, the near-zero-inclination geostationary SAR system at home and abroad is still in the stage of conceptual research. In addition, the near-zero-inclination geostationary SAR system proposed by P. Dekker et al. does not limit the length of the along-track baseline and the vertical track baseline between satellites, and the orbit segment that can be used for rapid time-varying scene observation is very limited.

[0006] The high-orbit SAR satellite of China works in the inclined geosynchronous orbit. In order to improve the application efficiency of the high-orbit SAR system of China in marine observation and fill the gap in the high-resolution imaging of the rapid time-varying scene of the inclined orbit high-orbit SAR at home and abroad, it is urgent to study the imaging method of the inclined orbit high-orbit SAR. SUMMARY

[0007] The purpose of the present application is to provide an inclined orbit high-orbit SAR imaging system for time-varying scene observation, which solves the technical problem of filling the gap in the high-resolution imaging observation of the rapid time-varying scene of the inclined orbit high-orbit SAR at home and abroad.

[0008] The purpose of the present application is achieved by the following technical solutions:

[0009] An inclined orbit high-orbit SAR imaging system for rapid time-varying scene observation includes two inclined orbit high-orbit SAR satellites and an echo signal processing module, wherein:

[0010] The two inclined orbit high-orbit SAR satellites are a primary satellite and a secondary satellite operating in the same orbital plane, and both have the same orbital semi-major axis, orbital inclination, orbital eccentricity, right ascension of the ascending node, and argument of perigee; they are used to simultaneously power on to observe the same ground scene area and receive backscattered echo signals from the ground scene area.

[0011] The echo signal processing module is used to sequentially perform range pulse compression, cross-correlation processing, and time-domain back projection processing on the backscattered echo signals received by the two inclined orbit high-orbit SAR satellites to obtain a high-resolution correlation function graph.

[0012] Furthermore, the difference in true anomaly angle Δf between the two inclined orbit high-orbit SAR satellites determines the directional distance between them and the degree of Doppler decoherence between their echoes. To ensure the echo coherence of the two inclined orbit high-orbit SAR satellites, Δf satisfies the following calculation formula:

[0013]

[0014] in:

[0015] κ—proportion factor, with a value range of 0 to 0.2;

[0016] θ az —Azimuth beamwidth, in degrees;

[0017] R s —The shortest slant range at which radar reaches the target, in meters;

[0018] ω s —The angular velocity of a satellite orbiting the Earth, measured in degrees per second, has a value of 0.00417.

[0019] V max —The maximum speed of the primary star in the Earth-fixed coordinate system (i.e., the geocentric fixed coordinate system), in meters per second;

[0020] Specifically, V max Typically, this is obtained from the orbital parameters of the primary star: based on the primary star's orbital parameters, STK software is used to simulate and generate the primary star's orbital data, and the maximum velocity V of the primary star in the Earth-solid system is calculated. max =max([V s (t1), V s (t2), ...]), V s (t1), V s(t2)…respectively represent the flight speed in the earth-fixed system at time t1, t2… in the main star orbit period.

[0021] Further, the single imaging on-time of the two oblique orbit high-orbit SAR satellites satisfies the following formula:

[0022]

[0023] Wherein:

[0024] T int —The single imaging on-time of the two oblique orbit high-orbit SAR satellites; unit: seconds;

[0025] λ—Radar wavelength, unit: meters;

[0026] Δv—The maximum flight speed difference of the two oblique orbit high-orbit SAR satellites at the same time, unit: meters / second;

[0027] ρ a —The azimuth resolution of the correlation function image required by the system application, which is determined by the satellite user according to the application requirements, for example, 5m, 10m, 20m.

[0028] Specifically, the maximum flight speed difference of the two oblique orbit high-orbit SAR satellites at the same time can be calculated by the following formula:

[0029]

[0030] Further, the specific implementation process of the echo signal processing module is as follows:

[0031] Step 1, respectively, the received backscattering echo signals of the two oblique orbit high-orbit SAR satellites are processed in the range direction to obtain the main star range direction pulse compression signal s rc1 (τ,t) and the auxiliary star range direction pulse compression signal s rc2 (τ,t), wherein τ is the distance direction sampling fast time, t is the azimuth direction sampling slow time; Specifically, the formula of the range direction pulse compression processing is as follows:

[0032]

[0033] Wherein:

[0034] s rc1 (τ,t)—The main star range direction pulse compression signal;

[0035] s rc2 (τ,t)—The auxiliary star range direction pulse compression signal;

[0036] FFT(·)—Fast Fourier transform operation;

[0037] IFFT(·)—fast inverse Fourier transform operation;

[0038] f r —distance frequency of the spectrum after fast Fourier transform;

[0039] W(f r )—windowing function for range pulse compression processing, such as the classic Kaiser window function, Hamming window function;

[0040] s1(τ,t)—backscatter echo signal received by the main satellite at time (τ,t);

[0041] s2(τ,t)—backscatter echo signal received by the auxiliary satellite at time (τ,t);

[0042] s rp1 (τ,t)—internal calibration signal collected by the internal calibration system on the main satellite at time (τ,t). The internal calibration system on the satellite is a mature technology, which is used to measure the amplitude and phase characteristics of each module of the radar system by using the internal calibrator inside the radar system;

[0043] s rp2 (τ,t)—internal calibration signal collected by the internal calibration system on the auxiliary satellite at time (τ,t).

[0044] Step 2, azimuth cross-correlation processing is performed on the main satellite range pulse compressed signal s rc1 (τ,t) and the auxiliary satellite range pulse compressed signal s rc2 (τ,t), to obtain the main satellite and auxiliary satellite cross-correlation function, which is expressed as:

[0045]

[0046] Wherein:

[0047] s cor (τ,t,Δt)—main satellite and auxiliary satellite cross-correlation function;

[0048] t m —azimuth pulse sampling time;

[0049] [·] * —complex conjugate operation;

[0050] Δt m —range of azimuth pulse sampling time corresponding to the set of average samples used in azimuth cross-correlation processing;

[0051] Δt—the time difference mark of the primary star and the secondary star cross-correlation function, that is, the difference between the azimuth pulse sampling time of the primary star range pulse compressed signal and the secondary star range pulse compressed signal in the azimuth cross-correlation processing;

[0052] s rc1 (τ, t m )—the value of the primary star range pulse compressed signal at the moment of (τ, t m );

[0053] s rc2 (τ, t+Δt)—the value of the secondary star range pulse compressed signal at the moment of (τ, t+Δt).

[0054] Step 3, performing range time domain multi-view processing on the primary star and the secondary star cross-correlation function s cor (τ, t, Δt) to obtain a multi-view cross-correlation function, which is expressed as:

[0055]

[0056] Wherein:

[0057] s mcor (τ r , t, Δt)—the multi-view cross-correlation function;

[0058] τ r —range fast time variable of the multi-view cross-correlation function;

[0059] Δτ—range fast time range corresponding to the set average sample used in the range time domain multi-view processing sample;

[0060] Specifically, Δτ is determined by the range resolution ρ r of the correlation function image, and is expressed as:

[0061]

[0062] Wherein:

[0063] ρ r —range resolution of the correlation function image, which is determined in advance by the satellite user according to the application requirement, for example, 5m, 10m, 20m;

[0064] c—speed of light.

[0065] Step 4, performing azimuth time domain backward projection processing on the multi-view cross-correlation function s mcor (τ r , t, Δt) to obtain a high-resolution correlation function image s img (τ r , t, Δt), which is expressed as:

[0066]

[0067] in:

[0068] s img (τ r (,t,Δt) — Graph of correlation function;

[0069] R(t m — at t m The distance at which the radar reaches the target at any given moment;

[0070] W a (t m — at t m Weighting functions for azimuth-time backward projection processing, such as the classic Kaiser function and Hamming window function;

[0071] s mcor (τ r ,t m ,Δt)—in (τ r ,t m The value of the cross-correlation function after multiple views at time Δt);

[0072] j—imaginary number;

[0073] Δt L —The time range of the target synthetic aperture after azimuth-direction time-domain back-projection processing. Target synthetic aperture is a well-defined and universal concept in the SAR field, determined by the time the radar beam's main lobe illuminates the target.

[0074] Compared with the prior art, the present invention has the following advantages:

[0075] (1) The system of this invention utilizes the relative motion between two inclined geosynchronous orbit high-orbit SAR satellites to improve the azimuth resolution of cross-correlation function imaging, which is 1 to 2 orders of magnitude higher than that of traditional SAR imaging. This system is suitable for high-resolution imaging applications of inclined geosynchronous orbit high-orbit SAR for rapidly time-varying scenes. Compared with near-zero inclination high-orbit SAR systems, the system of this invention has a larger latitudinal observation range, significantly increasing the proportion of orbital segments that can achieve high-resolution imaging, and filling the gap in the field of rapidly time-varying scene imaging by inclined geosynchronous orbit high-orbit SAR both domestically and internationally.

[0076] (2) This invention proposes a method for designing the true perihelion angle of two satellite orbits, processing the cross-correlation of the two satellite echoes, and calculating the azimuth resolution of the inclined orbit high-orbit SAR imaging system for time-varying scene observation. It also establishes a relationship model between the difference in the true perihelion angle of the two satellites and the radar beamwidth and satellite flight speed.

[0077] This system consists of two inclined orbit high-orbit SAR systems operating in the same orbital plane. High-resolution correlation function plots are obtained by cross-correlation processing and synthetic aperture processing of the backscattered echo signals received by the two SAR satellites. Subsequently, physical information of interest, such as backscattering coefficients, time-varying scene velocity, and elevation information, can be derived from the correlation function plots. Compared with traditional high-orbit SAR systems' long synthetic aperture time-of-flight imaging, this invention utilizes the characteristic that the decoherence time of the correlation function in rapidly time-varying scenes is much longer than the decoherence time of the scene's own scattering characteristics. This overcomes the limitation of high-orbit SAR in performing long synthetic aperture time-of-flight imaging for rapidly time-varying scenes, improving the azimuth imaging resolution by approximately one to two orders of magnitude compared to traditional SAR imaging.

[0078] The system of this invention consists of two inclined orbit high-orbit SAR systems operating in the same orbital plane. The two SAR satellites simultaneously observe the same scene. Cross-correlation processing is performed on the ground-scattered echo signals received by the two SAR satellites at equal time intervals to estimate the cross-correlation function of the rapidly time-varying scene. Then, the relative motion between the two SAR satellites is used to significantly improve the azimuth resolution of the cross-correlation function. This invention has significant potential for high-orbit SAR marine applications. Attached Figure Description

[0079] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0080] Figure 1 The azimuth distance between two satellites in an inclined orbit high-orbit SAR imaging system for observing rapidly time-varying scenes;

[0081] Figure 2 The difference in flight velocity between two satellites in an inclined orbit high-orbit SAR imaging system for observing rapidly time-varying scenes;

[0082] Figure 3 The azimuth resolution of an inclined orbit high-orbit SAR imaging system for observing rapidly time-varying scenes. Detailed Implementation

[0083] Exemplary embodiments of the present disclosure will be described in greater detail below with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it is understood that the present disclosure can be embodied in various forms without being limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be thoroughly and completely understood as a whole, and fully convey the scope of the present disclosure to those skilled in the art. It should be noted that the embodiments in the present disclosure and the features in the embodiments can be combined with each other without conflict. The present disclosure will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0084] Embodiment

[0085] The high orbit satellite is about 36000 kilometers away from the ground, and the orbit inclination of the inclined orbit high SAR is usually greater than 5°. The input satellite orbit parameters of the present embodiment are as follows: the semi-major axis of the satellite orbit is 42164 km, the orbit inclination is 16°, the orbit eccentricity is 0, the orbit right ascension of the ascending node is 90°, the orbit argument of perigee is 0°, and the true anomaly of the primary star orbit is 0°. The designed input radar wavelength is 0.24 m, the azimuth beam width is 0.7°, the observation incidence angle is 35°, and the scale factor κ = 0.2. According to the above input parameters, the true anomaly angle difference between the secondary star and the primary star is calculated to be 0.438°, so the true anomaly of the secondary star orbit is 0.438°, and then the STK software simulation is used to generate the secondary star orbit data.

[0086] Figure 1 The along-track distance of the dual-satellite of the inclined orbit high SAR imaging system for fast time-varying scene observation designed in the present embodiment varies between 12.4 km and 87.9 km in the full orbit range, and the along-track distance between the primary star and the secondary star varies between 12.4 km and 87.9 km in the full orbit range. Figure 2 The flight speed difference of the dual-satellite of the inclined orbit high SAR imaging system for fast time-varying scene observation designed in the present embodiment varies between 1.8 m / s and 6.6 m / s in the full orbit range, and the maximum flight speed difference of the two inclined orbit high SAR satellites at the same time is 6.6 m / s.

[0087] Assuming that the radar center frequency is 1.25 GHz, the corresponding radar wavelength is 0.24 m, and the along-track distance between the primary star and the secondary star is 1 km, the along-track distance between the primary star and the secondary star is 1 km, and the along-track distance between the primary star and the secondary star is 1 km. int More than 668 seconds.

[0088] Since the sea surface decorrelation time is about 3s-4s, the azimuth resolution that can be obtained by using the traditional SAR imaging method is about 166 km-616 km, which is far from meeting the needs of marine applications. Figure 3For the single imaging on-off time of two oblique orbit high orbit SAR satellites is 900 seconds, the azimuth imaging resolution of the oblique orbit high orbit SAR imaging system for fast time-varying scene observation designed in the embodiment is obtained, since the cross-correlation function of the fast time-varying scene is slowly changed in the high orbit SAR synthetic aperture time range, when the single imaging on-off time is 900 seconds, the azimuth resolution of the correlation function diagram in the full orbit range is changed between 742 m and 2744 m, at this time, 82.7% of the orbit segment resolution is better than 2 km, 48.9% of the orbit segment resolution is better than 1 km, for the marine application, the imaging resolution of 1 km to 2 km can meet the application requirement. By comparison, the azimuth imaging resolution of the system is improved by about 1 to 2 orders of magnitude than the traditional SAR imaging resolution.

[0089] Although the present application has been disclosed with the above preferred embodiments, it is not intended to limit the present application, and any person skilled in the art can make possible changes and modifications to the technical solutions of the present application by using the disclosed methods and technical contents without departing from the spirit and scope of the present application, therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present application, which does not depart from the technical solutions of the present application, all belong to the protection scope of the technical solutions of the present application.

Claims

1. A fast time-varying scene observation system of an inclined orbit high orbit SAR imaging system, characterized in that, The system comprises two inclined orbit high orbit SAR satellites and an echo signal processing module, wherein: The two inclined orbit high orbit SAR satellites are respectively a main satellite and an auxiliary satellite running on the same orbit plane, and have the same orbit semi-major axis, orbit inclination, orbit eccentricity, ascending node right ascension and perigee amplitude; they are used for simultaneously starting to observe the same ground scene area and receive the backscattering echo signals of the ground scene area; The echo signal processing module is used for sequentially performing distance direction pulse compression processing, cross-correlation processing and time domain back-projection processing on the backscattering echo signals received by the two inclined orbit high orbit SAR satellites, to obtain a high-resolution correlation function graph.

2. The fast time varying scene looking inclined orbit high altitude SAR imaging system of claim 1 wherein, The true perigee angle difference Δf of the two inclined orbit high orbit SAR satellites determines the along-track distance and the Doppler de-coherence degree between echoes of the two inclined orbit high orbit SAR satellites, in order to ensure the echo coherence of the two inclined orbit high orbit SAR satellites, Δf satisfies the following calculation formula: Wherein: κ is a proportional factor, and the value range is 0-0.2; θ az —azimuth beamwidth, unit: degree; R s — The shortest slant range to the target, in meters. ω s - the angular velocity of the satellite's revolution around the Earth, in degrees per second; V max — Maximum flight velocity of the primary star in the terrestrial system, in meters / second.

3. The fast time varying scene looking inclined orbit high altitude SAR imaging system of claim 2 wherein, The single imaging start-up working time of the two inclined orbit high orbit SAR satellites satisfies the following formula: Wherein: T int - Single imaging on time for two inclined orbit high orbit SAR satellites; unit: seconds; λ is a radar wavelength, and the unit is meter; Δv is the maximum flight speed difference of the two inclined orbit high orbit SAR satellites at the same time, and the unit is meter / second; ρ a — the azimuthal resolution of the relevant function map imaging required by the system application.

4. The fast time varying scene observing, inclined orbit, high- altitude SAR imaging system of claim 3 wherein, The maximum flight speed difference of the two inclined orbit high orbit SAR satellites at the same time is calculated by the following formula:

5. The inclined orbit high orbit SAR imaging system for fast time-varying scene observation according to claim 4, and a specific implementation process of the echo signal processing module is as follows: Step 1, respectively, to two inclined orbit high orbit SAR satellite receiving the backscattering echo signal distance to pulse compression processing, get the main star distance to pulse compression after signal s rc1 (τ,t) and auxiliary star distance to pulse compression after signal s rc2 (τ,t), wherein, τ is a distance direction sampling fast time, and t is an azimuth direction sampling slow time. Step 2, azimuth cross-correlation processing is performed on the primary star range-azimuth pulsed compressed signal s rc1 (τ, t) and the secondary star range-azimuth pulsed compressed signal s rc2 (τ, t) to obtain the primary star and secondary star cross-correlation function; Step 3, range-migration processing of the primary and secondary star cross-correlation functions s cor (τ, t, Δt) to obtain multi-aperture post cross-correlation functions; Step 4, the multi-look post cross-correlation function s mcor (τ r ,t,Δt) is processed in the azimuth direction to obtain a high-resolution correlation function graph s img (τ r ,t,Δt).

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