CN113281747A - Satellite based on SAR imaging - Google Patents

Satellite based on SAR imaging Download PDF

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Publication number
CN113281747A
CN113281747A CN202110531395.8A CN202110531395A CN113281747A CN 113281747 A CN113281747 A CN 113281747A CN 202110531395 A CN202110531395 A CN 202110531395A CN 113281747 A CN113281747 A CN 113281747A
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satellite
baseline
sar
imaging
primary
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CN113281747B (en
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杨峰
任维佳
杜志贵
陈险峰
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Changsha Tianyi Space Technology Research Institute Co Ltd
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Changsha Tianyi Space Technology Research Institute Co Ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/9021—SAR image post-processing techniques
    • G01S13/9023—SAR image post-processing techniques combined with interferometric techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/882—Radar or analogous systems specially adapted for specific applications for altimeters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/904—SAR modes
    • G01S13/9058—Bistatic or multistatic SAR
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
    • G01S19/258—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS relating to the satellite constellation, e.g. almanac, ephemeris data, lists of satellites in view
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/29—Acquisition or tracking or demodulation of signals transmitted by the system carrier including Doppler, related
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related

Abstract

The invention relates to a satellite based on SAR imaging, which comprises an SAR system, wherein the SAR system generates a long vertical baseline with the length larger than a short vertical baseline (D) and a short horizontal baseline with the length smaller than a long horizontal baseline (C) based on a first physical baseline (A) and a second physical baseline (B), and performs optimal interference signal processing by combining the short vertical baseline (D) and the long horizontal baseline (C) so as to improve the accuracy and the measuring range of terrain height measurement while realizing high-resolution wide swath imaging. The invention can improve the accuracy and the measuring range of the terrain measurement while imaging with high resolution and wide swath.

Description

Satellite based on SAR imaging
The invention has the application number of CN201910788732.4, the application date of 08-23.2019 and the application type of invention, and the application name is divisional application of a satellite-borne SAR constellation system.
Technical Field
The invention relates to the technical field of satellite radars, in particular to a satellite based on SAR imaging.
Background
Synthetic Aperture Radar (SAR) is an active microwave imaging Radar with all-weather, all-day-time earth observation capability and has a certain penetration capability to the earth surface. The two-dimensional image with high resolution is generated by actively irradiating a ground object target to obtain a backscattering echo. The satellite-borne synthetic aperture radar satellite has important effects on the aspects of resource exploration, military reconnaissance, disasters, environmental monitoring and the like, wherein the satellite-borne synthetic aperture radar satellite is an earth observation satellite taking SAR as a payload. A transmitting device in the satellite-borne SAR system transmits radar signals to a ground interested observation area, and a receiving device in the satellite-borne SAR system receives reflected echo signals after the radar signals reach the ground. The satellite-borne SAR system generates an SAR complex image of an observation area based on the received echo signal.
The satellite-borne SAR system takes high-resolution wide swath, multi-mode imaging and the like as development targets. However, since the satellite-borne SAR has the minimum antenna area limitation, the azimuth high resolution and the observation bandwidth are a pair of inherent contradictions of a single SAR system. For the interference working mode, the single satellite-borne SAR system obtains two complex images by adopting two modes of repeated flight and single flight for interference to obtain elevation information. Although the repeated flight can realize a larger optimal baseline and obtain higher height measurement accuracy, the time interval of the two observations is longer, and the interference accuracy can be greatly reduced by the change of the ground target in the time interval; although two complex images can be obtained almost simultaneously in a single flight, two antennas with long distances cannot be loaded on the same satellite, so that a larger optimal baseline cannot be realized. Therefore, a space-borne SAR constellation system arises.
The satellite-borne SAR constellation system consists of two or more satellites with SAR as effective load. The satellites forming the constellation can keep fixed and unchangeable relative positions, and the constellation network formed in the form can effectively increase the ground coverage area and shorten the revisit period. The space-borne SAR constellation system is the best way to obtain high-resolution and wide swath SAR images, and can obtain the high-resolution wide swath SAR image by combining a plurality of low-resolution wide swath images of the space-borne SAR to improve the spatial resolution. For example, [1] Yuweidong. use distributed minisatellites to improve the azimuth resolution of spaceborne SAR [ J ]. systems engineering and electronics, 2002,24(7): 43-45. The above document [1] proposes that two satellite-borne SAR antennas with the same system parameters are arranged in a forward and backward manner along the flight direction, the front SAR antenna is in a rear-view state, and the front SAR antenna is in a forward-view state. The two satellites successively image the same ground area, but the irradiation directions of antenna beams are different, and the two satellites are equivalent to a single satellite adopting a large-beam antenna working mode. For each satellite-borne SAR, the Doppler bandwidth of the echo signal is not increased, so that the pulse repetition period of the echo signal is kept unchanged, and the mapping bandwidth is not influenced. However, the method proposed in this document can only ensure that echo distance directions are not blurred, and azimuth doppler blurring cannot be avoided. The prior art generally reconstructs the unambiguous Doppler spectrum by means of signal processing, for example [2] Dragon, Sure, Liutown, et al. distributed moonlet combined with Doppler deblurring SAR imaging [ J ]. systems engineering and electronics 2015,37(3): 544-. The literature [2] adopts non-space-variant phase, space-variant phase and residual phase caused by joint compensation of a vertical baseline to solve the problem of satellite-borne SAR Doppler ambiguity resolution under low azimuth repetition frequency sampling. The method provides space sampling for Doppler ambiguity by using a horizontal base line of a satellite-borne SAR constellation system, and solves the Doppler ambiguity by using a Capon space spectrum estimation method. This reconstruction method, while solving the problem of azimuthal doppler ambiguity, comes at the expense of the range of the ground altitude measurement. When the satellite-borne SAR constellation system has a longer vertical baseline in order to obtain higher height measurement accuracy in an interference working mode, in order to meet the accuracy requirement of a Doppler fuzzy algorithm, the measuring range of ground height measurement must be limited within a small range, so that the three-dimensional imaging performance of the satellite-borne SAR constellation system is limited.
In the satellite-borne SAR constellation system, the satellites do not need to be physically connected, so that the time interval for acquiring signals is very small while a larger optimal base line is obtained, the influence of ground target change on interference is small, and the height measurement precision can be greatly improved. For example, chinese patent publication No. CN109031297A discloses a distributed SAR configuration with a primary satellite at the center and a secondary satellite cart formation, which includes a primary satellite and a plurality of secondary satellites, wherein the primary and secondary satellites are located on the same orbital plane, the secondary satellites are uniformly distributed on a cart oval locus centered on the primary satellite, and fly around the primary satellite along the cart oval locus; under the configuration, at any time of the whole orbit operation period, an effective vertical long baseline formed by two auxiliary stars and an optimal vertical effective short baseline formed by a main star and one auxiliary star are arranged, so that optimal interference signal processing can be performed by using the long and short baselines, and a high-precision Digital Elevation Model (DEM) of a surveying and mapping area is obtained.
For example, chinese patent publication No. CN101520511B discloses a formation configuration method for distributed satellite synthetic aperture radar, which proposes a formation configuration of "concentric rings", and uses the optimal baseline combination constraint condition of multi-limit interference SAR as a design input parameter, and by designing a control rule of radar antenna view angle, the constraint condition of optimal baseline combination can be satisfied within an orbital operation period, and the elevation accuracy of DEM can be improved by a multi-baseline interference processing technique.
However, for the satellite-borne SAR constellation system disclosed in the above patent document, which is accompanied by the fact that the physical baseline between the secondary satellite and the primary satellite or between any two satellites is time-varying, the degree of coupling between the vertical baseline and the horizontal baseline is severe within one orbit period, i.e., the vertical baseline and the horizontal baseline are unclear at some point in time. Since the horizontal baseline of the baselines is used to measure the radial velocity of the moving target, the horizontal baseline contains velocity and position information of the target. However, the vertical baseline contains elevation measurement information caused by topographic fluctuation, and the coupling of the horizontal baseline and the vertical baseline causes the coupling of radial velocity information and elevation information, so that the interference phases caused by topographic change and ground target motion are difficult to distinguish, and accurate height measurement information is difficult to obtain.
In summary, the existing satellite-borne SAR constellation system mainly emphasizes that stable long and short vertical baselines are formed in the orbit period to improve the accuracy of topographic survey, and does not consider that the coupling of the vertical baselines and the horizontal baselines in the orbit period causes interference of the radial velocity information of a phase inclusion moving target, so that the accurate height measurement accuracy is difficult to obtain. Furthermore, in order to meet the accuracy requirements of solving the azimuthal doppler ambiguity algorithm, with a stable long vertical baseline, the range of the ground altitude measurement must be limited to a small range, limiting the range of the terrain altitude measurement. Accordingly, there is a need for improvements in the prior art to improve the accuracy and range of terrain height measurements while simultaneously providing high resolution wide swath imaging.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a satellite-borne SAR constellation system, which adopts a mode that a first auxiliary satellite and a second auxiliary satellite symmetrically fly around a main satellite as a center to form a passive stable orbit configuration, the first auxiliary satellite and the second auxiliary satellite can respectively form a first physical base line A and a second physical base line B which have the same length and reverse phases relative to the main satellite in any time of an orbit period, then decoupling is carried out based on the first physical baseline A and the second physical baseline B to obtain a long vertical baseline containing terrain height information and a short horizontal baseline containing speed information, and a long horizontal baseline C and a short vertical baseline D formed between the satellite and the main satellite are combined, so that the SAR system can perform optimal interference signal processing based on the long vertical baseline, the short vertical baseline D, the long horizontal baseline C and the short horizontal baseline to improve the accuracy and the range of the terrain height measurement while high-resolution wide swath imaging is performed.
According to a preferred embodiment, a space-borne SAR constellation system comprises: the SAR system comprises a main satellite taking an SAR system as a payload, a satellite flying and at least one pair of a first satellite and a second satellite flying around the main satellite. The first and second satellites form a passive stable configuration in a manner that is centrosymmetric about the primary satellite such that the first satellite forms a first physical baseline a relative to the primary satellite. The second secondary star forms a second physical baseline B with respect to the primary star. The first physical baseline A and the second physical baseline B are the same in length and opposite in phase. The companion secondary stars fly in tandem with the first primary star on tracks outside the flight tracks of the first and second secondary stars and adjacent to the primary star, thereby growing a horizontal baseline C and a short vertical baseline D relative to the primary star. The SAR system generates a vertical baseline with the length larger than the short vertical baseline D and a horizontal baseline with the length smaller than the long horizontal baseline C based on the first physical baseline A and the second physical baseline B, and performs optimal interference signal processing by combining the short vertical baseline D and the long horizontal baseline C so as to improve the accuracy and the measuring range of the terrain height measurement while imaging in a high-resolution wide swath.
According to a preferred embodiment, the orbit parameters of the companion satellite are obtained by the following steps: acquiring orbit parameters of the main satellite according to task requirements, wherein the eccentricity ratio of the orbit of the main satellite is 0; the major semi-axis, the orbit inclination angle and the ascent point right ascension of the first satellite and the second satellite are all the same as those of the main satellite, and the minor semi-axis and the major semi-axis of the flight tracks of the first satellite and the second satellite are obtained by calculation of a Hill equation, wherein the initial design value of the minor semi-axis of the flight tracks of the first satellite and the second satellite is the baseline requirement of the mission; determining an argument of the near place and an argument of the near point according to the phase difference of the flight tracks of the first satellite and the second satellite; calculating the effective vertical base length of the first physical base line A and the second physical base line B in one orbit period according to the orbit parameters of all satellites obtained by current calculation, and judging whether the effective vertical base length is smaller than the limit base length of the vertical effective base line; if the length of the effective baseline is not less than the length of the limit baseline perpendicular to the effective baseline, adjusting the minor semi-axes of the flight trajectories of the first satellite and the second satellite until the lengths of the effective perpendicular baselines of the first physical baseline A and the second physical baseline B meet the requirement; and calculating the orbit parameters of the satellite under flight under the condition that the long horizontal base line C meets the requirement of spatial sampling required by azimuth Doppler ambiguity resolution and the short vertical base line D meets the requirement of Doppler ambiguity resolution precision according to the orbit parameters of the main satellite, the first auxiliary satellite and the second auxiliary satellite obtained in the above steps.
According to a preferred embodiment, after obtaining the orbit parameters of the primary satellite, the first satellite and the second satellite, the primary satellite obtains the attitude parameters of the SAR radar antenna through a measuring device to realize precise orbit determination on the primary satellite, the first satellite and the second satellite so as to obtain the first physical baseline a, the second physical baseline B, the horizontal baseline C and the short vertical baseline D with high precision. The measuring means comprise at least a GPS receiver for attitude measurements. The GPS receiver performs the steps of: preprocessing original data, mainly including ephemeris data decoding and data synchronization; acquiring the position parameters of the SAR radar antenna through a differential positioning algorithm; obtaining an initial value of the integer ambiguity by using the position parameters of the SAR radar antenna, solving an accurate value of the integer ambiguity by using Kalman filtering and recursive search methods, and obtaining an accurate coordinate by using a carrier phase; and obtaining an inter-satellite baseline vector by obtaining the accurate coordinate value, and solving through the inter-satellite baseline vector to obtain the attitude parameter of the SAR radar antenna.
According to a preferred embodiment, after the primary satellite obtains the attitude parameters of the SAR radar antenna, the GPS receiver is connected with a GPS constellation based on the orbit parameters of the primary satellite, the first satellite and the second satellite, and at least acquires prior information of daily and monthly perturbation, sunlight pressure perturbation and atmospheric resistance perturbation to establish an orbit disturbance model of the constellation system so as to eliminate the influence of perturbation on baseline measurement; and the GPS receiver obtains the first physical baseline A, the second physical baseline B, the long horizontal baseline C and the short vertical baseline D based on attitude parameters of the SAR radar antenna and a orbit determination result obtained by an orbit disturbance model of the constellation system.
According to a preferred embodiment, before the primary satellite acquires the prior information through the GPS constellation, the primary satellite performs time synchronization to avoid the influence of clock errors. The main satellite, the accompanying auxiliary satellite, the first auxiliary satellite and the second auxiliary satellite all comprise synchronizing devices. The synchronization means comprise at least a time synchronization component. The time synchronization component is configured to: the timing pulse signals carried by all the satellites are simultaneously triggered and generated based on startup, and inter-satellite frequency difference values are obtained through inter-satellite frequency synchronization pulses to achieve time synchronization.
According to a preferred embodiment, the synchronization device further comprises a frequency synchronization component. The frequency synchronization component is configured to: the method comprises the steps of taking a frequency linear time-varying signal as a synchronous pulse, interrupting the acquisition of SAR information carried by any two satellites in a periodic mode, and processing the exchanged synchronous pulse signal to obtain the phase difference caused by frequency sources on any two satellites. Phase compensating the SAR based on the phase difference to achieve frequency synchronization.
According to a preferred embodiment, the first physical baseline a, the second physical baseline B, the horizontal baseline C and the short vertical baseline D form a horizontal baseline and a vertical baseline of a time series in the case where the primary satellite is synchronized with the companion secondary satellite and/or the first secondary satellite and the second secondary satellite by the synchronization means.
According to a preferred embodiment, after the primary satellite is connected with the GPS constellation through the synchronization device to obtain the prior information, the primary satellite obtains latitude information of a ground observation area according to the prior information of the GPS constellation and adjusts antenna angles of the satellite, the first satellite and the second satellite to maintain the same elevation ambiguity, thereby improving consistency of elevation measurement accuracy in different latitude areas.
According to a preferred embodiment, a method of on-board SAR imaging, the method comprising: imaging using a constellation of a primary satellite, a companion flying secondary satellite, and at least one pair of a first secondary satellite and a second secondary satellite flying around the primary satellite, with the SAR system as payload. The first satellite and the second satellite form passive stable configurations in a manner of being centrosymmetric with respect to the main satellite, so that the first satellite and the second satellite respectively form a first physical baseline A and a second physical baseline B which are same in length and opposite in phase with respect to the main satellite. And the accompanying auxiliary star flies in tandem with the first main star on the tracks which are outside the flight tracks of the first auxiliary star and the second auxiliary star and adjacent to the first main star. The companion satellite grows a horizontal baseline C and a short vertical baseline D relative to the first master star. The SAR system generates a longer vertical baseline and a shorter horizontal baseline based on the first physical baseline A and the second physical baseline B, and performs optimal interference signal processing in combination with the short vertical baseline D and the long horizontal baseline C to improve the accuracy and the range of the terrain measurement while performing high-resolution wide swath imaging.
According to a preferred embodiment, after the primary satellite is connected with a GPS constellation through a synchronization device to obtain prior information, the primary satellite obtains latitude information of a ground observation area based on the prior information of the GPS constellation to adjust antenna angles of SAR systems on the primary satellite, the first satellite and the second satellite to maintain the same elevation ambiguity, thereby improving consistency of elevation measurement accuracy in different latitude areas. After the antenna angles of the SAR systems on the main satellite, the accompanying flying auxiliary satellite, the first auxiliary satellite and the second auxiliary satellite keep the same first elevation ambiguity imaging, when the terrain altitude change amplitude of a ground observation area obtained based on prior information is large, the antenna angles of the SAR systems on the main satellite, the accompanying flying auxiliary satellite, the first auxiliary satellite and the second auxiliary satellite are adjusted to keep the same second elevation ambiguity different from the first elevation ambiguity, so that the SAR systems are imaged with at least two different elevation ambiguities.
The beneficial technical effects of the invention comprise one or more of the following:
1. the satellite-borne SAR constellation system adopts a mode that a first auxiliary satellite and a second auxiliary satellite symmetrically fly around a main satellite as a center to form a passive stable track configuration, the first auxiliary satellite and the second auxiliary satellite can respectively form a first physical baseline and a second physical baseline which have the same length and reverse phases relative to the main satellite within any time of a track period, so that a long vertical baseline only containing terrain height information and a short horizontal baseline only containing speed information can be obtained, the situation that the phases of the obtained ground height information are mixed with radial speed information due to the coupling of the horizontal baseline and the vertical baseline can be avoided, and the accuracy of the terrain measurement height can be improved;
2. generally, when an SAR system is used for high-resolution wide swath imaging, azimuth Doppler ambiguity can be caused by low-azimuth repeated sampling, the image obtained by the SAR needs to be subjected to Doppler ambiguity resolution, a long horizontal base line formed by the satellite flying relative to the main satellite can provide required space sampling for azimuth Doppler ambiguity resolution, and further the defect of time sampling caused by low azimuth repetition frequency is overcome, so that conditions are provided for azimuth Doppler ambiguity resolution;
3. the long horizontal base line is combined with the short horizontal base line obtained by decoupling the first physical base line and the second physical base line, optimal interference signal processing can be carried out, more interference phase information related to radial velocity can be provided, and therefore conditions are provided for moving target detection;
4. the long vertical baseline and the short vertical baseline are combined to carry out multi-baseline interference processing, so that more terrain height information can be obtained, and the accuracy of measuring the terrain height is improved when an SAR system is subjected to interference imaging;
5. because the first physical baseline, the second physical baseline and the physical baseline formed by the satellite relative to the main satellite contain the vertical baseline and can introduce the space-variant vector, phase compensation is carried out before Doppler ambiguity resolution to ensure the accurate realization of Doppler ambiguity resolution, and the prior art carries out the phase compensation based on the condition that the ground altitude information is assumed to be known, so that the length of the vertical baseline is in inverse proportion to the measuring range of the terrain altitude measurement in order to meet the requirement of the Doppler ambiguity resolution precision, thereby limiting the measuring range of the terrain altitude measurement The line and the long horizontal base line only containing speed information, the phase introduced by the vertical base line in each Doppler frequency can be obtained through multi-base line interference processing of the long vertical base line and the short vertical base line, phase compensation is carried out before Doppler ambiguity resolution so as to improve the measuring range of the terrain height under the condition of meeting the Doppler ambiguity resolution precision, and therefore the accuracy and the measuring range of the terrain height measurement are improved while the SAR system is used for high-resolution wide swath imaging.
Drawings
Fig. 1 is a simplified schematic diagram of a preferred embodiment of the present invention.
List of reference numerals
100: the main star 101: satellite for flight
200: the first satellite 201: second satellite
A: first physical baseline B: second physical baseline
C: long horizontal baseline D: short vertical base line
Detailed Description
This is explained in detail below with reference to fig. 1.
In the description of the present invention, it is to be understood that the terms "first", "second", and the like, if any, are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, the term "plurality", if any, means two or more unless specifically limited otherwise.
As used throughout this application, the word "may" is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words "include", "including", and "includes" mean including, but not limited to.
The phrases "at least one," "one or more," and/or "are open-ended expressions that encompass both association and disassociation in operation. For example, each of the expressions "at least one of A, B and C", "at least one of A, B or C", "one or more of A, B and C", "A, B or C" and "A, B and/or C" refers to a alone a, a alone B, a alone C, A and B together, a and C together, B and C together, or A, B and C together, respectively.
The terms "a" or "an" entity refer to one or more of that entity. As such, the terms "a" (or "an"), "one or more," and "at least one" are used interchangeably herein. It should also be noted that the terms "comprising," "including," and "having" may be used interchangeably.
Example 1
The embodiment also discloses an SAR system, which can be a satellite-borne SAR system, a distributed SAR system, or a distributed satellite-borne SAR system, and can be realized by the system of the invention and/or other replaceable parts. For example, the system of the present invention may be implemented using various components of the system of the present invention. The preferred embodiments of the present invention are described in whole and/or in part in the context of other embodiments, which can supplement the present embodiment, without resulting in conflict or inconsistency.
According to a preferred embodiment, a space-borne SAR constellation system comprises: a primary satellite 100, a companion flying secondary satellite 101, and at least one pair of a first secondary satellite 200 and a second secondary satellite 201 flying around the primary satellite 100, with a SAR system as payload. The first satellite 200 and the second satellite 201 form a passive stable configuration in a manner that is centrosymmetric about the primary satellite 100 such that the first satellite 200 forms a first physical baseline a with respect to the primary satellite 100. The second secondary star 201 forms a second physical baseline B with respect to the primary star 100. The first physical baseline A and the second physical baseline B are the same in length and opposite in phase. The companion satellite 101 flies in tandem with the first primary satellite 100 on a trajectory that is outside the flight trajectories of the first and second satellites 200 and 201 and adjacent to the primary satellite 100, forming a long horizontal baseline C and a short vertical baseline D with respect to the primary satellite 100. The SAR system generates a short horizontal baseline with the length larger than a short vertical baseline D and the length smaller than a long horizontal baseline C based on a first physical baseline A and a second physical baseline B, and performs optimal interference signal processing by combining the short vertical baseline D and the long horizontal baseline C to improve the accuracy and the measuring range of the terrain height measurement while imaging in a high-resolution wide swath. Preferably, the first satellite 200 and the second satellite 201 are centered on the primary satellite 100 and are designed using Hill's equations into a passively stable configuration that does not require fuel consumption. As shown in fig. 1, the motion trajectories of the first satellite 200 and the second satellite 201 are elliptical trajectories described by Hill equation. Because the main satellite 100, the first auxiliary satellite 200 and the second auxiliary satellite 201 run in the same orbital plane, the first auxiliary satellite 200 and the second auxiliary satellite 201 have the same orbital period, the same semi-major axis and the same eccentricity, and no position offset exists between the satellite bodies in the horizontal direction, so that the first auxiliary satellite 200 and the second auxiliary satellite 201 form an elliptical spatial configuration in space. Preferably, the elliptical spatial configuration formed by the first satellite 200 and the second satellite 201 is centered on the primary satellite 100. The projection of the spatial configuration on the track plane and the horizontal plane is an ellipse. The first satellite 200 and the second satellite 201 rotate around the main satellite 100 with the same period as the main satellite 100 rotates around the earth. In this way, the present invention can enable the first secondary star 200, the second secondary star 201 and the primary star 100 to maintain relative spatial positions, so as to obtain the stable first physical baseline a and the stable second physical baseline B. Furthermore, the first physical baseline a and the second physical baseline B are periodically changed with the rotation period of the elliptical configuration, so that the first physical baseline a and the second physical baseline B are mixed baselines including speed information and terrain height information. Preferably, the first physical baseline a and the second physical baseline B are identical in length and opposite in phase, and a long vertical baseline containing only terrain height information and a short horizontal baseline containing only speed information can be obtained by using, for example, a differential processing method. The invention at least has the following beneficial technical effects: the method can avoid the situation that the ground height information phase obtained by the coupling of the horizontal baseline and the vertical baseline of the first physical baseline A and the second physical baseline B is mixed with the radial speed information, thereby improving the accuracy of the topographic survey height.
Preferably, the orbit of companion satellite 101 is at least 120m from the orbit of primary satellite 100, avoiding and keeping the distance of the orbit of companion satellite 101 from the orbit of primary satellite 100 fixed, as shown in fig. 1. The physical baseline formed by the companion secondary satellite 101 relative to the primary satellite 100 may be resolved into a short vertical baseline D and a long horizontal baseline C based on the determined orbital distance. In this way, the short vertical baseline D of the present invention is always determined and kept unchanged in the stage of the orbit operation of the companion satellite 101 and the primary satellite 100, and can provide prior conditions and error analysis for the accurate orbit determination and baseline measurement of the companion satellite 101 and the primary satellite 100.
Preferably, the SAR systems on the primary satellite 100, the companion flying secondary satellite 101, the first secondary satellite 200 and the second secondary satellite 201 receive the echo signals returned by the primary satellite 100 to the ground targets. The SAR system obtains a plurality of SAR images of a ground target by adopting a low-azimuth repeated sampling mode, and forms the SAR image of a high-resolution wide swath by splicing the plurality of SAR images. Because the SAR system acquires images in a low-azimuth repeated sampling mode, azimuth Doppler blurring can be caused. Preferably, the azimuthal doppler frequency is proportional and in a one-to-one correspondence to the sine of the azimuthal instantaneous squint angle. When the azimuth repetition rate is lower than the Doppler bandwidth, the azimuth Doppler spectrum is aliased and blurred, and the azimuth Doppler frequency of the azimuth Doppler spectrum is not directly proportional to the sine of the azimuth instantaneous squint angle any more, but is formed by mixing a plurality of energies pointing to the azimuth instantaneous squint angle. The azimuth Doppler ambiguity resolution process is that aiming at each Doppler frequency, under the condition that energy of a plurality of azimuth instantaneous squint angles is aliased, energy of specific azimuth instantaneous squint angles is extracted one by one, and finally an unambiguous Doppler spectrum is reconstructed through splicing. The invention at least has the following beneficial technical effects: firstly, the long horizontal base line C and the short horizontal base line contain radial speed information and can provide conditions for moving target detection; secondly, the long horizontal base line C and the short horizontal base line can also provide spatial sampling required by azimuth Doppler ambiguity resolution, so that the defect of time sampling caused by low azimuth repetition frequency is overcome, and conditions are provided for azimuth Doppler ambiguity resolution.
Preferably, the presence of the first physical baseline a, the second physical baseline B, and the physical baseline formed by the companion secondary satellite 101 with respect to the primary satellite 100 introduces a space variant vector. The first physical baseline A, the second physical baseline B and a physical baseline formed by the companion satellite 101 relative to the primary satellite 100 are decomposed to obtain a long vertical baseline and a short vertical baseline D, and the multi-baseline interference processing of the long vertical baseline and the short vertical baseline D can obtain phase information introduced by the vertical baseline in each Doppler frequency. The invention at least has the following beneficial technical effects: firstly, phase compensation is carried out before Doppler ambiguity resolution based on the obtained phase information so as to improve the measuring range of the terrain measuring height under the condition of meeting Doppler ambiguity resolution precision; secondly, the multi-baseline interference processing of the long vertical baseline and the short vertical baseline D can also improve the accuracy of the terrain height measurement.
The invention can at least realize the following beneficial technical effects by adopting the mode:
firstly, the satellite-borne SAR constellation system of the invention adopts a mode that a first auxiliary satellite 200 and a second auxiliary satellite 201 symmetrically fly around a main satellite 100 as a center to form a passive stable track configuration, the first auxiliary satellite 200 and the second auxiliary satellite 201 can respectively form a first physical baseline A and a second physical baseline B which have the same length and reverse phases relative to the main satellite 100 within any time of a track period, so that a long vertical baseline only containing terrain height information and a short horizontal baseline only containing speed information can be obtained, the situation that the ground height information phase obtained due to the coupling of the horizontal baseline and the vertical baseline is mixed with radial speed information can be avoided, and the accuracy of the terrain measurement height can be improved;
secondly, generally, when an SAR system is used for high-resolution wide swath imaging, because low-azimuth repeated sampling causes azimuth Doppler ambiguity, the image obtained by the SAR needs to be subjected to Doppler ambiguity resolution, a long horizontal base line C formed by the satellite 100 flying along relative to the main satellite 100 can provide required space sampling for azimuth Doppler ambiguity resolution, and further make up for the defect of time sampling caused by low azimuth repetition frequency, so that conditions are provided for azimuth Doppler ambiguity resolution;
thirdly, the long horizontal base line is combined with the short horizontal base line obtained by decoupling the first physical base line and the second physical base line, optimal interference signal processing can be carried out, more interference phase information related to radial velocity can be provided, and therefore conditions are provided for moving target detection;
fourthly, the long vertical baseline and the short vertical baseline D are combined to carry out multi-baseline interference processing, so that more terrain height information can be obtained, and therefore the measurement accuracy of the terrain height is improved when an SAR system is subjected to interference imaging;
fifth, since the first physical baseline a, the second physical baseline B, and the physical baseline formed by the companion satellite 101 relative to the primary satellite 100 may introduce space-varying vectors due to the inclusion of the vertical baseline, phase compensation is performed before doppler ambiguity resolution to ensure accurate implementation of doppler ambiguity resolution in the prior art, and phase compensation is performed in the prior art based on the condition that it is assumed that the ground altitude information is known, so that the accuracy of doppler ambiguity resolution may cause the length of the vertical baseline to be inversely proportional to the range of the terrain altitude measurement, thereby limiting the range of the terrain altitude measurement, according to the present invention, the first physical baseline a and the second physical baseline B may obtain the long vertical baseline including only the terrain altitude information and the short horizontal baseline including only the velocity information through differential processing, and the physical baseline formed by the companion satellite 101 relative to the primary satellite 100 may be decomposed based on the fixed orbital distance between the companion satellite 101 and the primary satellite 100 to obtain the physical baseline including only the terrain altitude information The multi-baseline interference processing of the long vertical baseline and the short vertical baseline can obtain phase information introduced by the vertical baseline in each Doppler frequency, and phase compensation is carried out before Doppler ambiguity resolution so as to improve the measuring range of the terrain height under the condition of meeting the Doppler ambiguity resolution precision, thereby realizing that the SAR system improves the accuracy and the measuring range of the terrain height measurement while imaging in a high-resolution wide swath.
According to a preferred embodiment, the orbit parameters of the companion satellite 101 are obtained by the following steps: acquiring orbit parameters of a main satellite 100 according to task requirements, wherein the eccentricity of the orbit of the main satellite is 0; the major semi-axis, the orbit inclination angle and the ascent point right ascension of the first satellite 200 and the second satellite 201 are all the same as those of the main satellite 100, and the minor semi-axis and the major semi-axis of the flight tracks of the first satellite 200 and the second satellite 201 are obtained by calculation of a Hill equation, wherein the initial design value of the minor semi-axis of the flight tracks of the first satellite 200 and the second satellite 201 is the baseline requirement of the mission; determining an argument of a near point and a mean angle of the near point according to the phase difference of the flight trajectories of the first satellite 200 and the second satellite 201; calculating the effective vertical base length of a first physical base line A and a second physical base line B in an orbit period according to the orbit parameters of all satellites obtained by current calculation, and judging whether the effective vertical base length is smaller than the limit base length of the vertical effective base line; if the length of the effective baseline is not less than the length of the limit baseline of the vertical effective baseline, adjusting the minor semi-axis of the flight trajectories of the first satellite 200 and the second satellite 201 until the lengths of the effective vertical baselines of the first physical baseline A and the second physical baseline B meet the requirement; and calculating the orbit parameters of the satellite 101 accompanying the flight under the condition that the long horizontal base line C meets the requirement of spatial sampling required by azimuth Doppler ambiguity resolution and the short vertical base line D meets the requirement of Doppler ambiguity resolution precision according to the orbit parameters of the main satellite 100, the first satellite 200 and the second satellite 201 obtained in the steps. Preferably, assuming that the orbit of the main star 100 is circular, that is, the eccentricity is 0, 6 orbits of the main star, that is, the semi-major axis of the orbit, the orbit eccentricity, the orbit inclination, the ascent point right ascent, the argument of the perigee, and the mean-anomaly angle of the designated epoch, are obtained according to the parameters such as the orbit type, the orbit height, and the like in combination with the Hill equation set. Preferably, the Hill equation describing the motion of the satellites is:
Figure BDA0003067282240000121
wherein, the origin of the coordinate system in the formula is defined as a main satellite 100, the main satellite orbits the earth, the x axis points to the flight direction of the reference satellite, the y axis is vertical to the orbit plane of the main satellite 100, and the z axis points to the main satellite 100 back to the earth center.
Figure BDA0003067282240000122
ψkThe initial position of the kth satellite in the elliptical orbit configuration is identified, and T represents the orbit period. Preferably, the primary star 100 is the center of the first secondary star 200 and the second secondary star 201, and thus B is 0. Superior foodOptionally, the relative motion of the first satellite 200 and the second satellite 201 is elliptical in the XZ plane, and the major axis in the direction of the velocity direction X is 2 times the minor axis perpendicular to the direction of the velocity direction Z. Preferably, the first satellite 200 and the second satellite 201 move relative to each other in an independent sinusoidal motion in the y-axis. The first satellite 200 and the second satellite 201 rotate slowly in an elliptical configuration centered on the primary satellite 100. The first satellite 200, the second satellite 201 and the main satellite 100 are all in the same orbit plane, so the orbit inclination angle and the rising point right ascension of the three are all the same. Preferably, the initial value is designed by taking the baseline as the minor half axis of the elliptical orbit formed by formation flight. Since the minor semi-axis of the elliptical trajectory of the formation flying relative movement is related only to the semi-major axis and the eccentricity of the satellite, the eccentricity of the first satellite 200 and the second satellite 201 is determined by the minor semi-axis, with the semi-major axis already determined. Preferably, since the first satellite 200 and the second satellite 201 are symmetrical about the main satellite 100 as a center, that is, the first satellite 200 and the second satellite 201 are oppositely located and have a phase difference of 180 °, the perigee argument and the mean apogee angle of the first satellite 200 and the second satellite 201 can be determined. Preferably, from the orbit parameters of the primary satellite 100, the first secondary satellite 200 and the second secondary satellite 201 obtained above, the lengths of the first secondary satellite 200 and the second secondary satellite 201 in one orbit period with respect to the first physical baseline a and the second physical baseline B of the primary satellite 100, respectively, can be calculated. And projecting the earth surface based on the obtained first physical base line A and the second physical base line B to obtain an effective base line, enabling the length of the vertical effective base line to meet the requirement of Doppler ambiguity resolution precision, and enabling the length of the horizontal effective base line to meet the requirement of spatial sampling required by azimuth Doppler ambiguity resolution, so that the length of the vertical effective base line and the length of the horizontal effective base line are both smaller than the length of the corresponding limit base lines. Limit base length of vertical effective base:
Figure BDA0003067282240000131
limit base length of horizontal effective base:
Figure BDA0003067282240000132
wherein, lambda is the working wavelength of the SAR system, theta is the visual angle,
Figure BDA0003067282240000133
in the oblique view, alpha is the included angle between the connecting line of the centroids of all the satellites and the horizontal plane, namely the inclination angle of a base line, beta is the terrain slope, and R represents the average slant distance between the two satellites forming the base line and the ground target. p is a radical ofrAnd paRespectively representing the range and azimuth resolutions. R can be obtained according to formula 4, wherein a is the major axis of orbit semimajor of the star 100, ReIs the radius of the earth:
Figure BDA0003067282240000134
prand paCan be obtained according to formula 5, wherein c is the speed of light, BwIdentify radar signal bandwidth, D represents azimuth antenna size:
Figure BDA0003067282240000141
preferably, the orbit parameters of the companion satellite 101 are calculated based on the orbit design parameters of the primary satellite 100, the first secondary satellite 200 and the second secondary satellite 201 as completed under the above conditions. The orbit of the companion satellite 101 is maintained at a fixed distance from the orbit of the primary satellite 100. Preferably, the companion satellite 101 and the main satellite 100 are arranged in tandem flight. Preferably, the anteroposterior distance of the companion secondary star 101 from the primary star 100 meets the limit baseline length of the horizontal effective baseline.
The invention can at least realize the following beneficial technical effects by adopting the mode: firstly, the first auxiliary star 200, the second auxiliary star 201 and the main star 100 fly on a common rail road surface, the elliptical space configuration is passively stable, and the power system is only started when the track is corrected or the task is switched, so that the energy is saved; secondly, the orbit of the satellite-borne auxiliary satellite 101 is similar to that of the main satellite 100, and a front-back formation flying mode is adopted, so that a horizontal base line meeting the requirement of spatial sampling required by azimuth Doppler ambiguity resolution can be obtained by the satellite-borne SAR constellation system at any moment, and the imaging efficiency of the system is improved; thirdly, the spatial configuration formed by the satellite 101, the first satellite 200, the second satellite 201 and the main satellite 100 is a common rail surface configuration, which is easy to maintain and has low fuel consumption; fourthly, the constellation configuration of the invention can form stable base line and base line dip angle, and is suitable for interferometric synthetic aperture radar imaging; fifthly, the constellation space configuration can form a plurality of effective vertical baselines and a plurality of effective horizontal baselines at any time, and the plurality of effective vertical baselines and the plurality of effective horizontal baselines all meet the requirement of multi-baseline interference processing; sixth, the effective vertical baseline formed by the constellation space configuration at any time meets the requirement of Doppler ambiguity resolution precision, and the effective horizontal baseline meets the requirement of spatial sampling required by azimuth Doppler ambiguity resolution, so that the high-resolution wide swath imaging at any time of an SAR system is met, and the precision and the range of ground height measurement are improved.
According to a preferred embodiment, after obtaining the orbit parameters of the primary satellite 100, the companion flying secondary satellite 101, the first secondary satellite 200 and the second secondary satellite 201, the primary satellite 100 obtains the SAR radar antenna attitude parameters through the measuring device to realize precise orbit determination on the primary satellite 100, the companion flying secondary satellite 101, the first secondary satellite 200 and the second secondary satellite 201 to obtain the first physical baseline a, the second physical baseline B, the long horizontal baseline C and the short vertical baseline D with high precision. Preferably, the measuring means comprise at least a GPS receiver for attitude measurements. The GPS receiver performs the following steps: preprocessing original data, mainly including ephemeris data decoding and data synchronization; acquiring position parameters of the SAR radar antenna through a differential positioning algorithm; obtaining an initial value of the integer ambiguity by using a position parameter of the SAR radar antenna, solving an accurate value of the integer ambiguity by using Kalman filtering and recursive search methods, and obtaining an accurate coordinate by using a carrier phase; and obtaining an inter-satellite baseline vector by obtaining the accurate coordinate value, and solving through the inter-satellite baseline vector to obtain the attitude parameter of the SAR radar antenna.
Preferably, the GPS receiver is a high precision dual frequency GPS receiver. Preferably, a GPS masker receiver is integrated within the high-precision dual-band GPS receiver. Preferably, the GPS receiver preprocesses the received original data, then performs single-point positioning coordinate calculation, and then calculates the coordinates of the mobile coordinate station through code double-difference positioning to obtain the rough position coordinates of the SAR radar antenna. Preferably, in order to obtain accurate SAR radar antenna position coordinates, the GPS receiver needs to use a carrier phase method with centimeter-level wavelength and millimeter-level ranging error to measure the distance. Since the carrier signal is a periodic sinusoidal signal, integer ambiguity occurs when the measured distance is greater than the wavelength. Preferably, the GPS receiver obtains an initial value of the integer ambiguity based on the coarse position coordinates, and solves an accurate value of the integer ambiguity by using kalman filtering and recursive search.
According to a preferred embodiment, after the primary satellite 100 obtains the attitude parameters of the SAR radar antenna, the GPS receiver is connected with a GPS constellation based on the orbit parameters of the primary satellite 100, the satellite 101, the first satellite 200 and the second satellite 201, and at least acquires the prior information of daily and monthly perturbation, the solar pressure perturbation and the atmospheric resistance perturbation to establish an orbit disturbance model of the constellation system so as to eliminate the influence of the perturbation on baseline measurement; the GPS receiver obtains a first physical baseline A, a second physical baseline B, a long horizontal baseline C and a short vertical baseline D by a differential carrier phase measurement method based on attitude parameters of the SAR radar antenna and a orbit determination result obtained by an orbit disturbance model of a constellation system.
Preferably, the influence of the earth aspheric perturbation, the sunlight pressure perturbation, the atmospheric resistance perturbation and the like on the satellite is not negligible, and the satellite earth aspheric perturbation, the sunlight pressure perturbation, the atmospheric resistance perturbation and the like can be used as model noise to be estimated in combination with prior information. Preferably, the orbit disturbance model is a linear accumulation of accelerations of the earth non-spherical perturbation, the sunlight pressure perturbation and the atmospheric resistance perturbation which affect the satellite motion. Preferably, the differential carrier phase measurement method is to differentiate the path delay of the two stars and the orbit determination error caused by the algorithm to eliminate most common error components of the two stars, and differentiate the orbit determination results of the two stars to obtain the corresponding baseline vector. The invention can at least realize the following beneficial technical effects by adopting the mode: the GPS receiver can eliminate most common orbit determination errors of the two satellites, and corresponding compensation is carried out through algorithm optimization and establishment of an orbit disturbance model, so that the inter-satellite base line and the attitude parameters of the satellites can be accurately measured.
According to a preferred embodiment, the master satellite 100 is time synchronized to avoid clock errors before the master satellite 100 acquires a priori information via the GPS constellation. The primary satellite 100, the companion flying secondary satellite 101, the first secondary satellite 200, and the second secondary satellite 201 each include synchronization devices. The synchronization means comprise at least a time synchronization component. The time synchronization component is configured to: the timing pulse signals carried by all the satellites are simultaneously triggered and generated based on startup, and inter-satellite frequency difference values are obtained through inter-satellite frequency synchronization pulses to achieve time synchronization. Preferably, the power-on trigger timing signal may be implemented by a GPS second pulse.
According to a preferred embodiment, the synchronization means further comprise a frequency synchronization component. The frequency synchronization component is configured to: the method comprises the steps of taking a frequency linear time-varying signal as a synchronous pulse, interrupting the acquisition of SAR information carried by any two satellites in a periodic mode, and processing the exchanged synchronous pulse signal to obtain the phase difference caused by frequency sources on any two satellites. And performing phase compensation on the SAR based on the phase difference to realize frequency synchronization. Preferably, the frequency-linear time-varying signal is a chirp signal. Preferably, the primary satellite 100, the companion flying secondary satellite 101, the first secondary satellite 200, and the second secondary satellite 201 all mount 6 synchronous horn antennas to provide quasi-omni-directional beam coverage, ensuring near real-time omni-directional frequency synchronous pulse reception.
Preferably, the primary satellite 100 transmits synchronization pulses to the companion flying secondary satellite 101, the first secondary satellite 200, and the second secondary satellite 201, respectively. The acquisition of SAR data by the master satellite 100 will be periodically interrupted due to the linearly time-varying nature of the chirp signal's frequency. In one period, the synchronous pulse is transmitted from the SAR main antenna of the main satellite 100 to the horn antenna dedicated for synchronization on the satellite 101 and/or the first satellite 200 and/or the second satellite 201, the satellite 101 and/or the first satellite 200 and/or the second satellite 201 records the pulse and then transmits a short synchronous pulse back to the main satellite 100, the phase difference caused by the frequency source of the main satellite 100 relative to other satellites is obtained by processing the exchanged synchronous pulse signals, and corresponding phase compensation is performed during SAR system imaging, so that frequency synchronization is completed. Preferably, after the frequency linear time varying signal is adopted as the pulse signal, the frequency of the frequency source on the satellite can be considered as a constant, and the frequency difference can be extracted from the linear part of the phase difference of the synchronization signal, so as to complete the time synchronization.
Preferably, the SAR radar antenna on the primary satellite 100 points to the imaging area in a front-side view manner, and the first secondary satellite 200, the second secondary satellite 201 and the accompanying secondary satellite 101 observe the ground target imaging in a small-angle squint manner to point to the imaging area, so as to complete spatial synchronization. The invention can at least realize the following beneficial technical effects by adopting the above mode: firstly, time and space synchronization can ensure that main beams of two satellites simultaneously cover the same area on the ground, and can ensure that time windows of received echo signals of the two satellites are synchronized; second, frequency synchronization can reduce the interference phase error caused by the frequency drift of each of the two stars.
According to a preferred embodiment, the first physical baseline a, the second physical baseline B, the horizontal baseline C and the short vertical baseline D form a horizontal baseline and a vertical baseline of the time series in the case of the master satellite 100 being synchronized with the companion flying satellite 101 and/or the first satellite 200 and the second satellite 201 by the synchronization means. Preferably, when SAR interference imaging is performed by using the long vertical baseline and the short vertical baseline D, the phase difference obtained from the interferogram is a phase main value after winding of an unknown integer period between [ -pi, pi ], and it is necessary to restore the wound phase to a true phase difference, i.e., phase unwrapping. Preferably, the SAR system obtains synchronization information such as time, frequency, etc. based on the horizontal baseline and the vertical baseline of the time series to correct the received signal. The SAR system carries out phase compensation based on the corrected information and carries out phase unwrapping on the long vertical base line by using the short vertical base line, thereby further improving the accuracy of measuring the terrain height. The invention can at least realize the following beneficial technical effects by adopting the mode: because the short vertical baseline D has low height measurement precision, the error unwrapping can be carried out when the phase unwrapping is carried out on the long vertical baseline, and after the method and the device are adopted, the received signals are corrected by utilizing the horizontal baseline and the vertical baseline of the time sequence to obtain the synchronization information of time, frequency and the like, and the phase unwrapping is carried out by utilizing the long vertical baseline and the short vertical baseline, so that the phase unwrapping precision is improved to further improve the precision of measuring the terrain height.
According to a preferred embodiment, after the primary satellite 100 is connected with the GPS constellation through the synchronization device to obtain the prior information, the primary satellite 100 obtains the latitude information of the ground observation area according to the prior information of the GPS constellation and adjusts the antenna angles of the accompanying secondary satellite 101, the first secondary satellite 200 and the second secondary satellite 201 to maintain the same elevation ambiguity, thereby improving the consistency of the elevation measurement accuracy in different latitude areas.
Preferably, the elevation ambiguity reflects how sensitive a change in interferometric phase is to elevation changes. When the vertical baseline is longer, the elevation ambiguity is smaller, and the sensitivity of the interference phase to the elevation change is stronger. When the vertical baseline is too short, the interference phase is insensitive to elevation changes, and the accuracy of the terrain height measurement is reduced. Preferably, the elevation ambiguity is proportional to the sine of the azimuth instantaneous squint angle, and due to the rotation of the earth, the length of the effective baseline obtained by projecting the physical baseline to the ground is different at different latitudes, and the elevation ambiguity can be adjusted by adjusting the angle of the antenna. The invention can at least realize the following beneficial technical effects by adopting the mode: the primary satellite 100 can acquire latitude information of a ground observation area according to the prior information of the GPS constellation and adjust an antenna angle to maintain the same elevation ambiguity, thereby improving the consistency of elevation measurement accuracy in different latitude areas.
Example 2
The embodiment also discloses an imaging method, which may also be a radar imaging method, an imaging method of an SAR system, an imaging method of a satellite-borne SAR system, or an imaging method of a satellite-borne SAR constellation system, and the method may be implemented by the system of the present invention and/or other replaceable components. For example, the method of the present invention may be implemented using various components of the system of the present invention. This embodiment may be a further improvement and/or a supplement to embodiment 1, and repeated contents are not described again. The preferred embodiments of the present invention are described in whole and/or in part in the context of other embodiments, which can supplement the present embodiment, without resulting in conflict or inconsistency.
According to a preferred embodiment, the method comprises: imaging is performed using a constellation of a primary satellite 100, a companion flying secondary satellite 101, and at least one pair of a first secondary satellite 200 and a second secondary satellite 201 flying around the primary satellite 100, with the SAR system as payload. The first and second satellites 200 and 201 form a passive stable configuration in a centrosymmetric manner about the primary satellite 100 such that the first and second satellites 200 and 201 form first and second physical baselines a and B, respectively, of the same length and opposite phase with respect to the primary satellite 100. The companion satellite 101 flies in tandem with the first primary satellite 100 on a track that is outside the flight tracks of the first and second satellites 200 and 201 and adjacent to the first primary satellite 100. The companion satellite 101 forms a long horizontal baseline C and a short vertical baseline D relative to the first primary satellite 100. The SAR system generates a longer vertical baseline and a shorter horizontal baseline based on a first physical baseline A and a second physical baseline B, and performs optimal interference signal processing in combination with a short vertical baseline D and a long horizontal baseline C to improve the accuracy and range of the terrain measurement while imaging at a high resolution wide swath. The invention can at least realize the following beneficial technical effects by adopting the mode: the first auxiliary satellite and the second auxiliary satellite can form a passive and stable orbit configuration in a mode of symmetrically flying around the main satellite as a center, the first auxiliary satellite and the second auxiliary satellite can form a first physical baseline and a second physical baseline which are identical in length and opposite in phase in any time of an orbit period relative to the main satellite respectively, a long horizontal baseline and a short vertical baseline which are formed between the auxiliary satellite and the main satellite are combined, then decoupling is carried out based on the first physical baseline and the second physical baseline to obtain a long vertical baseline containing terrain height information and a short horizontal baseline containing speed information, and finally the SAR system carries out optimal interference signal processing based on the long vertical baseline, the short vertical baseline, the long horizontal baseline and the short horizontal baseline to improve the accuracy and the measuring range of the terrain height measurement while high-resolution wide swath imaging is carried out.
According to a preferred embodiment, after the primary satellite 100 is connected with the GPS constellation through the synchronization device to obtain the prior information, the primary satellite 100 obtains the latitude information of the ground observation area based on the prior information of the GPS constellation to adjust the antenna angles of the SAR systems on the primary satellite 100, the accompanying flying secondary satellite 101, the first secondary satellite 200, and the second secondary satellite 201 to maintain the same elevation ambiguity, thereby improving the consistency of the elevation measurement accuracy in different latitude areas. After the antenna angles of the SAR systems on the main satellite 100, the accompanying flying auxiliary satellite 101, the first auxiliary satellite 200 and the second auxiliary satellite 201 keep the same first elevation ambiguity for imaging, when the terrain altitude change amplitude of a ground observation area obtained based on prior information is large, the antenna angles of the SAR systems on the main satellite 100, the accompanying flying auxiliary satellite 101, the first auxiliary satellite 200 and the second auxiliary satellite 201 are adjusted to keep the same second elevation ambiguity different from the first elevation ambiguity, so that the SAR systems are imaged with at least two different elevation ambiguities. Preferably, the elevation ambiguity is directly proportional to the sine of the azimuthal instantaneous squint angle. The high degree of ambiguity reflects the degree of sensitivity of phase changes to high changes. When the topography of the imaging area is large, the phase principal value is discontinuous, and the phase unwrapping error is large. Preferably, the SAR system images at different elevation ambiguities based on adjusting the antenna angle to check consistency of the two imaging phase unwrapping and differences in the same imaging area, thereby reducing phase unwrapping errors. The invention can at least realize the following beneficial technical effects by adopting the mode: when the imaging is carried out on areas with large topographic relief, such as steep areas, the phase unwrapping error is reduced by imaging with different elevation fuzziness, and the relative precision is improved, so that the precision of the topographic survey height is improved.
The word "module" as used herein describes any type of hardware, software, or combination of hardware and software that is capable of performing the functions associated with the "module".
It should be noted that the above-mentioned embodiments are exemplary, and that those skilled in the art, having benefit of the present disclosure, may devise various arrangements that are within the scope of the present disclosure and that fall within the scope of the invention. It should be understood by those skilled in the art that the present specification and figures are illustrative only and are not limiting upon the claims. The scope of the invention is defined by the claims and their equivalents.

Claims (10)

1. A SAR imaging based satellite, said satellite comprising a SAR system,
the SAR system generates a long vertical baseline with a length larger than a short vertical baseline (D) and a short horizontal baseline with a length smaller than a long horizontal baseline (C) based on a first physical baseline (A) and a second physical baseline (B), and combines the short vertical baseline (D) and the long horizontal baseline (C) to perform optimal interference signal processing so as to improve the accuracy and the measuring range of the terrain height measurement while high-resolution wide swath imaging is performed.
2. The satellite of claim 1, wherein the elliptical spatial configuration of the first satellite (200) and the second satellite (201) is centered on the primary satellite (100).
3. Satellite according to one of the preceding claims, wherein the limit base length of the vertical effective base line is:
Figure FDA0003067282230000011
wherein, λ is the working wavelength of the SAR system, θ is the viewing angle, α is the included angle between the connecting line of the centroids of each satellite and the horizontal plane, i.e. the base line dip angle, β is the terrain slope, R represents the average slant distance between two satellites forming the base line and the ground target, and p is the average slant distance between two satellites forming the base line and the ground targetrIndicating the range-wise resolution.
4. Satellite according to one of the preceding claims, wherein the limit base length of the horizontal effective base line is:
Figure FDA0003067282230000012
wherein λ is the working wavelength of the SAR system，
Figure FDA0003067282230000013
For squint angles, R represents the average slant distance, p, from the ground object for the two satellites forming the baselineaIndicating the azimuth resolution.
5. Satellite according to one of the preceding claims, characterized in that the orbit of the satellite's satellite (101) is kept at a fixed distance from the orbit of the main satellite (100), the distance of the satellite (101) from the main satellite (100) being front to back, satisfying the limit base length of the horizontal effective base.
6. Satellite according to one of the preceding claims, characterized in that the master satellite (100) comprises a GPS receiver for measuring SAR radar antenna attitude parameters.
7. A satellite according to any one of the preceding claims, wherein the GPS receiver is a high accuracy dual frequency GPS receiver having a GPS masker receiver integrated therein.
8. The satellite of any of the preceding claims, wherein the primary satellite (100) adjusts the elevation ambiguities of the companion satellite (101), the first secondary satellite (200), and the second secondary satellite (201) by adjusting the antenna angles of the companion satellite (101), the first secondary satellite (200), and the second secondary satellite (201) to maintain the same elevation ambiguity between the primary satellite (100) and the secondary satellites to improve the consistency of elevation measurement accuracy in different latitudes.
9. A method of satellite imaging, the method comprising: the SAR system can perform optimal interference signal processing on the long vertical baseline, the short vertical baseline D, the long horizontal baseline C and the short horizontal baseline so as to improve the accuracy and the range of the terrain height measurement while high-resolution wide swath imaging is performed.
10. The imaging method of claim 9, characterized in that after the imaging of the first elevation ambiguity with the antenna angles of the SAR system on the primary satellite (100), the companion flying secondary satellite (101), the first secondary satellite (200) and the second secondary satellite (201) remaining the same, when the terrain elevation change amplitude of the ground observation area obtained based on the prior information is large, the antenna angles of the SAR system on the primary satellite (100), the companion flying secondary satellite (101), the first secondary satellite (200) and the second secondary satellite (201) are adjusted to remain the same and a second elevation ambiguity different from the first elevation ambiguity, so that the SAR system is imaged with at least two different elevation ambiguities.
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