Inclined orbit high-orbit SAR imaging system for rapid time-varying scene observation

Abstract

The invention belongs to the field of spaceborne radar synthetic aperture radar design and signal processing, and discloses an inclined orbit high-orbit SAR imaging system for rapid time-varying scene observation, which comprises two inclined orbit high-orbit SAR satellites and an echo signal processing module, and is characterized in that the two inclined orbit high-orbit SAR satellites are respectively a main satellite and an auxiliary satellite; the back scattering echo modules are used for simultaneously starting up to observe the same ground scene area and receiving back scattering echo signals of the ground scene area; the echo signal processing module is used for carrying out range pulse compression processing, cross-correlation processing and time domain back projection processing on back scattering echo signals received by the two inclined-orbit high-orbit SAR satellites in sequence to obtain a high-resolution correlation function diagram. According to the system, the defect of long aperture synthesis time of high-orbit SAR imaging is overcome, the azimuth imaging resolution is improved by 1-2 orders of magnitude compared with the traditional SAR imaging resolution, and the efficiency of the high-orbit SAR in the aspect of marine application is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the field of spaceborne radar synthetic aperture radar (SAR) design and signal processing, and is mainly used in the design and signal processing of long synthetic aperture time observation imaging systems for fast time-varying scenes (such as sea surface), and specifically relates to an inclined orbit high-orbit SAR imaging system and method for time-varying scene observation. Inclined orbit high-orbit SAR refers to a SAR system running in a geosynchronous orbit with a large orbital inclination (generally greater than 5°). High-orbit SAR is also called geosynchronous orbit SAR. The height of high-orbit SAR from the ground is about 36,000 km. Background Art

[0002] SAR has the advantages of working around the clock and in all weather conditions, and high-resolution large-scale imaging. SAR uses the movement of the platform to form a long synthetic aperture. For static scenes with stable scattering characteristics, high-azimuth resolution imaging can be obtained by coherent accumulation based on virtual long synthetic aperture data. However, for imaging of fast time-varying scenes, the time scale of decoherence of scene scattering characteristics may be as short as sub-seconds, which is much shorter than the length of the synthetic aperture time of high-orbit SAR of the order of hundreds of seconds. Therefore, the echoes recorded within the synthetic aperture time range cannot improve the azimuth resolution through coherent accumulation.

[0003] High-orbit SAR satellites are about 36,000 kilometers above the ground. Compared with low-orbit SAR satellites or airborne SAR systems, high-orbit SAR satellites have the following outstanding advantages: First, the imaging width reaches hundreds of kilometers without sacrificing azimuth resolution. A single observation covers the entire large lake water body, cross-provincial crop planting areas, ecological protection areas, etc.; Second, the revisit observation time is short and the rapid response capability is strong, which is suitable for high-temporal resolution sampling dynamic change monitoring in a large area; Third, the target area can be stayed for a long time, and the residence time reaches the hour level. Therefore, it can be used as an irradiation source to provide external radiation source for low-orbit SAR systems, airborne SAR systems, etc., thereby realizing dual-base SAR imaging.

[0004] In 1983, General Electric Company of the United States first proposed an S-band geosynchronous SAR system with an orbital inclination of 50°. The system uses a 15m-30m aperture SAR antenna and can cover the continental United States within 3 hours. In the past two decades, the design and imaging processing of high-orbit SAR systems have developed rapidly. The research teams of 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 domestic and foreign research institutions mainly include: low-power near-zero inclination high-orbit SAR system operating in geostationary orbit, distributed small satellite cluster high-orbit SAR system ARGOS using multiple-input multiple-output (MIMO) system, dual-satellite correlated high-orbit SAR system (Correlating SAR, CoSAR) for near-zero inclination orbit, high-orbit SAR system for large-scale observation operating in inclined geosynchronous orbit, and high-orbit SAR system in retrograde orbit whose full-orbit imaging performance is less affected by satellite velocity changes and earth rotation.

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

[0006] my country's high-orbit SAR satellite operates in an inclined geosynchronous orbit. In order to improve the application efficiency of my country's high-orbit SAR system in ocean observation and fill the gap in high-resolution imaging of fast time-varying scenes of inclined-orbit high-orbit SAR at home and abroad, it is urgent to study the imaging method of inclined-orbit high-orbit SAR. Summary of the invention

[0007] The purpose of the present invention is to provide an inclined orbit high-orbit SAR imaging system for time-varying scene observation, and the technical problem to be solved is to fill the gap in high-resolution imaging observation of fast time-varying scenes in inclined orbit high-orbit SAR at home and abroad.

[0008] The purpose of the present invention is achieved through the following technical solutions:

[0009] An inclined orbit high-orbit SAR imaging system for fast 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 respectively the primary satellite and the secondary satellite running on the same orbital plane, and both have the same orbital semi-major axis, orbital inclination, orbital eccentricity, ascending node right ascension and perigee argument; they are used to start at the same time to observe the same ground scene area and receive the backscattered echo signals of the ground scene area;

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

[0012] Furthermore, the true anomaly difference Δf of the two inclined orbit high-orbit SAR satellites determines the along-course distance of the two inclined orbit high-orbit SAR satellites and the degree of Doppler decoherence between the echoes. In order 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—proportional factor, ranging from 0 to 0.2;

[0016] θ az —Azimuth beamwidth, unit: degree;

[0017] R s —The shortest slant distance from the radar to the target, in meters;

[0018] ω s —The angular velocity of the satellite's revolution around the Earth, in degrees per second, which is 0.00417;

[0019] V max —The maximum flight speed of the primary satellite in the Earth-fixed system (i.e. the Earth-centered fixed coordinate system), unit: m / s;

[0020] Specifically, V max Usually obtained based on the orbital parameters of the primary satellite: Based on the orbital parameters of the primary satellite, the primary satellite orbital data is simulated using STK software to calculate the maximum flight speed V of the primary satellite in the ground fixed system. max =max([V s (t1), V s (t2), ...]), V s (t1), V s(t2)…represent the flight speed in the Earth fixed system at times t1, t2…in the orbital period of the primary star.

[0021] Furthermore, the single imaging start-up time of two inclined orbit high-orbit SAR satellites satisfies the following formula:

[0022]

[0023] in:

[0024] T int — Single imaging operation time of two inclined orbit high-orbit SAR satellites; unit: seconds;

[0025] λ—radar wavelength, unit: meter;

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

[0027] ρ a —The azimuth resolution of the correlation function diagram imaging required by the system application. The azimuth resolution is predetermined by the satellite user based on the application requirements, such as 5m, 10m, 20m.

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

[0029]

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

[0031] Step 1: Perform range pulse compression processing on the backscatter echo signals received by the two inclined orbit high-orbit SAR satellites to obtain the main satellite range pulse compression signal s rc1 (τ, t) and the pulse compression signal s after the auxiliary satellite distance rc2 (τ, t), where τ is the fast sampling time in the range direction and t is the slow sampling time in the azimuth direction. Specifically, the formula for pulse compression processing in the range direction is as follows:

[0032]

[0033] in:

[0034] s rc1 (τ,t)—signal after pulse compression in the distance direction of the host star;

[0035] s rc2 (τ,t)—signal after pulse compression in the distance direction of auxiliary satellite;

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

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

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

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

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

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

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

[0043] s rp2 (τ, t)—The internal calibration signal at time (τ, t) collected by the auxiliary satellite’s onboard internal calibration system.

[0044] Step 2: Compress the pulse signal s after the distance to the primary star rc1 (τ, t) and the pulse compression signal s after the auxiliary satellite distance rc2 (τ, t) is subjected to azimuth cross-correlation processing to obtain the cross-correlation function between the primary and secondary satellites, which can be expressed as follows:

[0045]

[0046] in:

[0047] s cor (τ, t, Δt) — mutual correlation function between primary and secondary stars;

[0048] t m —Azimuth pulse sampling time;

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

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

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

[0052] s rc1 (τ,t m )—in (τ,t m ) at the time when the distance from the main star is the value of the pulse compressed signal;

[0053] s rc2 (τ, t+Δt)—The value of the auxiliary satellite distance pulse compressed signal at the time (τ, t+Δt).

[0054] Step 3: The cross-correlation function s of the primary star and the secondary star cor (τ, t, Δt) performs multi-view processing in the time domain in the range direction to obtain the multi-view cross-correlation function, which is expressed as follows:

[0055]

[0056] in:

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

[0058] τ r —Range-fast time variant of multi-look cross-correlation function;

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

[0060] Specifically, Δτ is imaged by the ground resolution ρ of the correlation function graph r Decision, expressed as:

[0061]

[0062] in:

[0063] ρ r —The ground-distance resolution of the correlation function diagram imaging is predetermined by the satellite user according to application requirements, such as 5m, 10m, 20m;

[0064] c—speed of light.

[0065] Step 4: multi-view cross-correlation function s mcor (τ r ,t,Δt) is back-projected in the azimuth direction to obtain a high-resolution correlation function graph s img (τ r ,t,Δt), the formula is as follows:

[0066]

[0067] in:

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

[0069] R(t m )—at t m The distance of the radar to the target at any moment;

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

[0071] s mcor (τ r ,t m ,Δt)—in (τ r ,t m ,Δt) time value of the multi-view cross-correlation function;

[0072] j—imaginary number;

[0073] Δt L —The time range of the target synthetic aperture processed by azimuth time domain backprojection. The target synthetic aperture is a well-defined general concept in the SAR field and is determined by the time the main lobe of the radar beam illuminates the target.

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

[0075] (1) The system of the present invention utilizes the relative motion between two inclined orbit high-orbit SAR satellites to improve the azimuth resolution of cross-correlation function imaging, and the azimuth resolution is improved by 1 to 2 orders of magnitude compared with traditional SAR imaging. The system is suitable for high-resolution imaging of fast time-varying scenes by inclined geosynchronous orbit high-orbit SAR. Compared with the near-zero inclination high-orbit SAR system, the system of the present invention has a larger latitude observation range, greatly increases the proportion of orbital segments that can achieve high-resolution imaging, and fills the gap in the field of fast time-varying scene imaging by inclined orbit high-orbit SAR at home and abroad.

[0076] (2) The present invention proposes a method for designing the true anomaly angle of the dual-satellite orbit, processing the cross-correlation of the dual-satellite echoes, and calculating the system azimuth resolution of the inclined orbit high-orbit SAR imaging system for time-varying scene observation, and establishes a relationship model between the true anomaly angle difference of the dual-satellite and the radar beam width and the satellite flight speed.

[0077] The system consists of two inclined orbit high-orbit SAR systems running on the same orbital plane. A high-resolution correlation function graph is obtained by cross-correlation processing and synthetic aperture processing of the backscatter echo signals received by the two SAR satellites. Subsequently, the physical information of interest, such as the backscatter coefficient, time-varying scene velocity and elevation information, can be inverted based on the correlation function graph. Compared with the long synthetic aperture time imaging of the traditional high-orbit SAR system, the system of the present invention utilizes the characteristic that the decoherence time of the correlation function of the fast time-varying scene is much longer than the decoherence time of the scene's own scattering characteristics, overcoming the defect that the high-orbit SAR cannot perform long synthetic aperture time imaging for fast time-varying scenes, and the azimuth imaging resolution is improved by about 1 to 2 orders of magnitude compared with the traditional SAR imaging resolution.

[0078] The system of the present invention is composed of two inclined orbit high-orbit SAR systems running on the same orbital plane. The two SAR satellites observe the same scene at the same time, and the ground scattered echo signals received by the two SAR satellites at equal time intervals are cross-correlated to estimate the cross-correlation function of the fast time-varying scene, and then the relative motion between the two SAR satellites is used to greatly improve the azimuth resolution of the cross-correlation function. The present invention has important prospects in the application of high-orbit SAR in the ocean. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0080] Figure 1 The along-course distance between two satellites of the inclined-orbit high-orbit SAR imaging system for observing fast time-varying scenes;

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

[0082] Figure 3 The azimuth resolution of the inclined-orbit high-orbit SAR imaging system for observing rapidly time-varying scenes. DETAILED DESCRIPTION

[0083] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to be able to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0084] Example

[0085] The high-orbit satellite is about 36,000 kilometers above the ground, and the orbital inclination of the inclined-orbit high-orbit SAR is usually greater than 5°. The satellite orbit parameters designed and input in this embodiment are: the semi-major axis of the satellite orbit is 42164km, the orbital inclination is 16°, the orbital eccentricity is 0, the right ascension of the orbital ascending node is 90°, the orbital perigee angle is 0°, and the true anomaly angle of the primary satellite orbit is 0°. The radar wavelength designed to be input is 0.24m, the azimuth beam width is 0.7°, the observation incident angle is 35°, and the proportional factor κ=0.2. According to the above input parameters, the true anomaly angle difference between the auxiliary satellite and the primary satellite is calculated to be 0.438°, so the true anomaly angle of the auxiliary satellite orbit is 0.438°, and then the auxiliary satellite orbit data is simulated and generated using STK software.

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

[0087] Assuming that the radar center frequency is 1.25 GHz and the corresponding radar wavelength is 0.24 m, after calculation, to achieve an azimuth resolution of the correlation function graph of 1 km, the single imaging startup time T of the two inclined orbit high-orbit SAR satellites is required int Greater than 668 seconds.

[0088] Since the decoherence time of the sea surface is about 3s to 4s, the azimuth resolution that can be obtained using traditional SAR imaging methods is about 166km to 616km, which is far from meeting the needs of marine applications. Figure 3When the single imaging startup time of two inclined orbit high-orbit SAR satellites is 900 seconds, the azimuth imaging resolution of the inclined orbit high-orbit SAR imaging system for fast time-varying scene observation designed in this embodiment is that since the cross-correlation function of the fast time-varying scene changes slowly within the high-orbit SAR synthetic aperture time range, when the single imaging startup time is 900 seconds, the azimuth resolution of the correlation function graph in the entire orbit range varies between 742m and 2744m. At this time, 82.7% of the orbit segments have a resolution better than 2km, and 48.9% of the orbit segments have a resolution better than 1km. For marine applications, an imaging resolution of 1km to 2km can meet the application requirements. By comparison, the azimuth imaging resolution of the system proposed by the present invention is improved by about 1 to 2 orders of magnitude compared with the traditional SAR imaging resolution.

[0089] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. An inclined orbit high-orbit SAR imaging system for fast time-varying scene observation, characterized in that: It includes two inclined orbit high-orbit SAR satellites and echo signal processing modules, including: The two inclined orbit high-orbit SAR satellites are respectively a primary satellite and a secondary satellite operating on the same orbital plane, and both have the same orbital semi-major axis, orbital inclination, orbital eccentricity, ascending node right ascension and perigee argument; they are used to be turned on at the same time to observe the same ground scene area and receive backscattered echo signals from the ground scene area; The echo signal processing module is used to sequentially perform range-direction pulse compression processing, cross-correlation processing and time-domain backprojection processing on backscattered echo signals received by two inclined-orbit high-orbit SAR satellites to obtain a high-resolution correlation function graph.

2. The inclined orbit high-orbit SAR imaging system for fast time-varying scene observation as claimed in claim 1, characterized in that: The true anomaly difference Δf of the two inclined orbit high-orbit SAR satellites determines the along-course distance of the two inclined orbit high-orbit SAR satellites and the degree of Doppler decoherence between the echoes. In order to ensure the echo coherence of the two inclined orbit high-orbit SAR satellites, Δf satisfies the following calculation formula: in: κ—proportional factor, ranging from 0 to 0.2; θ az —Azimuth beamwidth, unit: degree; R s —The shortest slant distance from the radar to the target, in meters; ω s —The angular velocity of the satellite orbiting the Earth, in degrees per second; V max —The maximum flying speed of the main satellite in the Earth's fixed system, unit: meter / second.

3. The inclined orbit high-orbit SAR imaging system for fast time-varying scene observation as claimed in claim 2, characterized in that: The single imaging start-up working time of the two inclined orbit high-orbit SAR satellites satisfies the following formula: in: T int — Single imaging operation time of two inclined orbit high-orbit SAR satellites; unit: seconds; λ—radar wavelength, unit: meter; Δv—the maximum flight speed difference of two inclined orbit high-orbit SAR satellites at the same time, unit: meter / second; ρ a —Azimuth resolution of correlation function diagram imaging required by the system application.

4. The inclined orbit high-orbit SAR imaging system for fast time-varying scene observation as claimed in claim 3, characterized in that: The maximum flight speed difference of the two inclined orbit high-orbit SAR satellites at the same time is calculated using the following formula:

5. According to the inclined orbit high-orbit SAR imaging system for fast time-varying scene observation as claimed in claim 4, the specific implementation process of the echo signal processing module is as follows: Step 1: Perform range pulse compression processing on the backscatter echo signals received by the two inclined orbit high-orbit SAR satellites to obtain the main satellite range pulse compression signal s rc1 (τ, t) and the pulse compression signal s after the auxiliary satellite distance rc2 (τ,t), where τ is the fast sampling time in the range direction, and t is the slow sampling time in the azimuth direction; Step 2: Compress the pulse signal s after the distance to the primary star rc1 (τ, t) and the pulse compression signal s after the auxiliary satellite distance rc2 (τ, t) performs azimuth cross-correlation processing to obtain the cross-correlation function between the primary and auxiliary satellites; Step 3: The cross-correlation function s of the primary star and the secondary star cor (τ, t, Δt) performs multi-view processing in the time domain in the range direction to obtain the multi-view cross-correlation function; Step 4: multi-view cross-correlation function s mcor (τ r ,t,Δt) is back-projected in the azimuth direction to obtain a high-resolution correlation function graph s img (τ r ,t,Δt).

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