CN111474545B - AIS-based ship position calculation and refocusing method and device in SAR image - Google Patents

AIS-based ship position calculation and refocusing method and device in SAR image Download PDF

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CN111474545B
CN111474545B CN202010324490.6A CN202010324490A CN111474545B CN 111474545 B CN111474545 B CN 111474545B CN 202010324490 A CN202010324490 A CN 202010324490A CN 111474545 B CN111474545 B CN 111474545B
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speed
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acceleration
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CN111474545A (en
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仲利华
吕旖旎
仇晓兰
刘佳音
韩冰
丁赤飚
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Aerospace Information Research Institute of CAS
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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
    • 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
    • G01S13/9027—Pattern recognition for feature extraction
    • 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
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/06—Systems determining position data of a target
    • G01S13/42—Simultaneous measurement of distance and other co-ordinates
    • 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
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/50—Systems of measurement based on relative movement of target
    • G01S13/58—Velocity or trajectory determination systems; Sense-of-movement determination systems
    • 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
    • G01S13/9029—SAR image post-processing techniques specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time
    • 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
    • G01S19/38—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
    • G01S19/39—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/42—Determining position

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Abstract

A ship position calculating and refocusing method and device in SAR images based on AIS comprises the following steps: s1, acquiring first GPS data and second GPS data; s2, acquiring the space coordinates of the ship and the space coordinates of the SAR platform; s3, obtaining a first distance process, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform; s4, obtaining imaging time according to the first distance process, and obtaining the static space coordinate of the ship at the imaging time according to the first GPS data; s5, obtaining a second distance course, a second speed and a second acceleration of the ship according to the static space coordinate and the space coordinate of the SAR platform; s6, obtaining the position offset of the ship according to the first speed and the second speed; s7, compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship; and S8, moving the focused image of the ship by the position offset to obtain the real position of the ship.

Description

AIS-based ship position calculation and refocusing method and device in SAR image
Technical Field
The disclosure relates to the technical field of SAR (synthetic aperture radar), in particular to a ship position calculating and refocusing method and device in an SAR image based on AIS (automatic identification system).
Background
Synthetic Aperture Radar (SAR) is an important means for marine monitoring and ship target monitoring due to its all-weather and all-weather working capability. The SAR realizes azimuth high-resolution imaging by using Doppler effect generated by relative motion between a radar and a target. However, due to the motion of the sea vessel, additional unknown doppler frequencies are introduced, resulting in positional shifts and azimuthal defocusing of the vessel in the image. In addition, the SAR acquires a target microwave band image, so that the judgment on the information such as the type and the length of the ship is not accurate enough, and the image interpretation personnel are not used for understanding and explaining. Meanwhile, as an effective means for traditional marine vessel monitoring, an Automatic Identification System (AIS) for ships can provide rich target static and dynamic information. Therefore, the AIS information and the SAR image are fused, and more accurate and rich target information can be obtained. The key to the AIS and SAR fusion process is the location matching of the vessel in both sensors. In the prior art, there are two types of matching methods for the two. One is a matching method from the SAR image position to the AIS position, by detecting the ship in the SAR image and estimating the radial velocity of the ship, using the radial velocity to calculate the distance of the ship from the true position in the SAR image, moving the ship to the true position, and then matching with the nearby AIS information. However, most SAR satellites are in a single-channel system at present, and even if the satellites such as RadarSAT-2, GF-3 and the like have azimuth dual channels, only a few imaging modes adopt the dual-channel system. According to research and measurement in the field of marine remote sensing, at least 30% of ships are not tailed when underway. Therefore, estimating the radial velocity from the SAR image has a great limitation, and does not have the capability of estimating the azimuth velocity in principle. The other method is a matching method from the AIS position to the SAR image position, the navigation speed is projected to the radar beam sight line direction through the course and the navigation speed information reported by the AIS, the radial speed component is estimated and obtained, the deviation position of the ship in the SAR image at the navigation speed is calculated by using the radial speed, and the matched ship is searched on the SAR image according to the deviation position. The method needs complex coordinate conversion and does not have the capability of estimating the azimuth speed.
Disclosure of Invention
Technical problem to be solved
A ship position calculating and refocusing method and device in SAR images based on AIS at least solve the estimation of the relative speed and acceleration of a ship so as to accurately determine the real position of the ship and obtain a focused image.
(II) technical scheme
The utility model provides a ship position calculation and refocusing method in SAR image based on AIS, including: s1, detecting the ship by respectively adopting an AIS system and an SAR platform, and respectively obtaining first GPS data and second GPS data; s2, respectively obtaining the space coordinate of the ship and the space coordinate of the SAR platform according to the first GPS data and the second GPS data; s3, obtaining a first distance process, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform; s4, taking the minimum time of the first distance course as the imaging time, and obtaining the static space coordinates of the ship at the imaging time according to the first GPS data; s5, obtaining a second distance course, a second speed and a second acceleration of the ship according to the static space coordinate and the space coordinate of the SAR platform; s6, obtaining the position offset of the ship according to the first speed and the second speed; s7, compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship; and S8, moving the focused image of the ship by the position offset to obtain the real position of the ship.
Optionally, step S1 further includes: the first GPS data is converted to a cartesian coordinate system.
Optionally, step S3 includes: s31, obtaining a first distance process of the ship relative to the SAR platform according to the space coordinates of the ship and the space coordinates of the SAR platform, and obtaining a first distance process RshipThe formula for calculation of (t) is:
Figure GDA0003547101230000021
wherein, the first and the second end of the pipe are connected with each other,
Figure GDA0003547101230000022
is the spatial coordinate of the vessel, [ x ]sar(t)ysar(t)zsar(t)]The space coordinate of the SAR platform is obtained;
s32, obtaining a first speed by calculating a first derivative of the first distance process; and S33, obtaining a first acceleration by calculating a second derivative of the first distance history.
Optionally, step S5 includes: s51, obtaining a second distance process of the ship according to the static space coordinate and the space coordinate of the SAR platform, and obtaining a second distance process RstaticThe formula for calculation of (t) is:
Figure GDA0003547101230000031
wherein, [ x ]static(t)ystatic(t)zstatic(t)]Is the stationary space coordinates of the vessel;
s52, obtaining the second speed by calculating the first derivative of the second distance process; and S53, obtaining the second acceleration by calculating a second derivative of the second distance history.
Optionally, step S6 includes: s61, obtaining a speed difference between the first speed and the second speed; s62, obtaining the position offset of the ship according to the speed difference, and calculating the formula as follows:
Figure GDA0003547101230000032
where Δ x is the amount of positional deviation, Δ V rAnd R is the speed difference, R is the distance between the SAR platform and the ship, and V is the speed of the SAR platform.
Optionally, step S7 includes: s71, slicing the ship and carrying out an azimuth IFFT operation to transform the ship into a range-Doppler domain; s72, compensating the ship azimuth motion in the range-Doppler domain to compensate the function H'aThe following:
Figure GDA0003547101230000033
wherein, faIs the azimuth frequency, kaFor azimuthal frequency modulation, katIn order to compensate for the frequency modulation,
Figure GDA0003547101230000034
wherein λ is a radar wavelength of the SAR platform, and Δ A is a difference between the first acceleration and the second acceleration;
and S73, performing azimuth FFT operation on the compensated slices to obtain a focused image of the ship.
Optionally, step S1 further includes removing outliers in the first GPS data and the second GPS data.
Optionally, step S2 includes: and performing interpolation fitting on the first GPS data and the second GPS data to obtain the space coordinate of the ship and the space coordinate of the SAR platform.
Another aspect of the present disclosure provides an apparatus for calculating and refocusing a ship position in an AIS-based SAR image, including: the system comprises a first acquisition module, a second acquisition module and a third acquisition module, wherein the first acquisition module is used for respectively adopting an AIS (automatic identification system) and an SAR (synthetic aperture radar) platform to detect a ship and respectively acquiring first GPS data and second GPS data; the second acquisition module is used for respectively acquiring the space coordinate of the ship and the space coordinate of the SAR platform according to the first GPS data and the second GPS data; the third acquisition module is used for acquiring a first distance process, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform; the fourth acquisition module is used for taking the minimum moment of the first distance process as the imaging moment and acquiring the static space coordinate of the ship at the imaging moment according to the first GPS data; the fifth acquisition module is used for acquiring a second distance process, a second speed and a second acceleration of the ship according to the static space coordinate and the space coordinate of the SAR platform; the sixth acquisition module is used for acquiring the position offset of the ship according to the first speed and the second speed; the compensation module is used for compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship; and the translation module moves the focused image of the ship by the position offset to obtain the position of the ship.
Optionally, the first acquisition module further comprises converting the first GPS data to a cartesian coordinate system.
(III) advantageous effects
The method disclosed by the invention has the advantages that GPS data directly acquired by an AIS system and an SAR platform are utilized, approximate equivalence does not exist in the processing process, a distance process is obtained by carrying out position fitting and interpolation on the AIS system and the SAR platform, the relative radial speed and acceleration of a ship relative to a static target are obtained by derivation of the distance process, and position deviation correction and azimuth refocusing are carried out; the calculated speed and acceleration can be used for correcting position offset and eliminating azimuth false targets caused by motion under a multi-channel SAR system.
Drawings
Fig. 1 schematically illustrates a step diagram of a method of ship position calculation and refocusing in AIS based SAR images according to an embodiment of the present disclosure;
FIG. 2 schematically illustrates a flow chart of a method of ship position calculation and refocusing in AIS based SAR images according to an embodiment of the present disclosure;
FIG. 3A schematically illustrates an image of a vessel before refocusing according to an embodiment of the disclosure;
FIG. 3B schematically shows an image of a vessel after refocusing according to an embodiment of the disclosure;
fig. 4 schematically illustrates a block diagram of a ship position calculation and refocusing apparatus in AIS based SAR images according to an embodiment of the present disclosure.
Detailed Description
A method for calculating and refocusing a ship's position in an AIS-based SAR image, as shown in fig. 1, comprising: s1, detecting the ship by respectively adopting an AIS system and an SAR platform, and respectively obtaining first GPS data and second GPS data; s2, respectively obtaining the space coordinate of the ship and the space coordinate of the SAR platform according to the first GPS data and the second GPS data; s3, obtaining a first distance process, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform; s4, taking the minimum time of the first distance process as the imaging time, and obtaining the static space coordinate of the ship at the SAR platform imaging time according to the first GPS data; s5, obtaining a second distance course, a second speed and a second acceleration of the ship according to the static space coordinate and the space coordinate of the SAR platform; s6, obtaining the position offset of the ship according to the first speed and the second speed; s7, compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship; and S8, moving the focused image of the ship by the position offset to obtain the real position of the ship.
Fig. 2 schematically illustrates a flow chart of a method of ship position calculation and refocusing in AIS based SAR images according to an embodiment of the present disclosure. The first GPS data may be obtained by the AIS system and the second GPS data may be obtained by the SAR platform. The preprocessing of the first GPS data and the second GPS data comprises the processing of removing abnormal values in the first GPS data and the second GPS data and/or converting the abnormal values into a unified coordinate system. And respectively obtaining a first distance process and a second distance process according to the first GPS data and the second GPS data, and obtaining a first speed and a first acceleration and a second speed and a second acceleration by differentiating the first distance process and the second distance process. And obtaining a position offset according to the first speed, the first acceleration, the second speed and the second acceleration, simultaneously carrying out IFFT operation, motion compensation operation and FFT operation on the azimuth direction of the ship to obtain a focused image of the refocused ship, and then calculating the position offset of the translation position of the focused image of the ship to obtain the repositioning position and the image of the ship.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings. In the embodiment of the disclosure, in the process of moving a ship on the water surface, defocusing and position deviation can occur in the imaging process of the SAR platform, and the ship can be well refocused and positioned by adopting the method in the disclosure. The methods of the present disclosure are described in detail below.
And S1, respectively detecting the ship by adopting the AIS system and the SAR platform, and respectively obtaining the first GPS data and the second GPS data.
And an AIS receiving system is adopted to receive position information SAR platforms sent by an AIS system on the ship, and the ship is monitored and imaged through a radar system. The AIS system obtains first GPS data about the vessel, second GPS data for the SAR platform.
And S2, respectively obtaining the space coordinates of the ship and the space coordinates of the SAR platform according to the first GPS data and the second GPS data.
And removing abnormal values in the first GPS data and the second GPS data. And converting the longitude and latitude coordinates in the first GPS data into a Cartesian coordinate system. And extracting effective position values in the first GPS data and the second GPS data, and carrying out interpolation fitting on the effective position values, wherein for example, a Chebyshev interpolation fitting method can be adopted, the time interval after interpolation is 0.001 second, and the time range is not less than 10 seconds before the imaging time of the SAR platform image and 10 seconds after the imaging time. Obtaining the space coordinates of the ship after interpolation
Figure GDA0003547101230000061
And spatial coordinates [ x ] of the SAR platformsar(t)ysar(t)zsar(t)]。
And S3, obtaining a first distance process, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform.
Wherein the first speed and the first acceleration are respectively the speed and the acceleration of the ship relative to the SAR platform.
S31, obtaining a first distance process of the ship relative to the SAR platform according to the space coordinates of the ship and the space coordinates of the SAR platform, and obtaining a first distance process RshipThe formula for calculation of (t) is:
Figure GDA0003547101230000062
wherein the content of the first and second substances,
Figure GDA0003547101230000063
is the spatial coordinate of the vessel, [ x ]sar(t)ysar(t)zsar(t)]The space coordinate of the SAR platform is obtained;
s32, obtaining a first speed by calculating a first derivative of the first distance process;
first speed V is obtained by taking the first derivative of the first distance course of the shipship(t), the calculation formula is as follows:
Figure GDA0003547101230000064
and S33, obtaining a first acceleration by calculating a second derivative of the first distance history.
The second derivative is obtained for the first distance history of the vessel, i.e. the first derivative is obtained for the first speed to obtain the first acceleration Aship(t), the calculation formula is as follows:
Figure GDA0003547101230000065
and S4, taking the minimum time of the first distance course as the imaging time, and obtaining the static space coordinates of the ship at the imaging time according to the first GPS data.
Acquiring a static space coordinate [ x ] of the ship at the SAR platform imaging time in the first GPS data static(t)ystatic(t)zstatic(t)]。
S5, obtaining a second distance process, a second speed and a second acceleration of the ship according to the static space coordinate and the space coordinate of the SAR platform; wherein the second velocity and the second acceleration are a velocity and an acceleration of the stationary point relative to the SAR platform, respectively.
S51, obtaining a second distance process of the ship according to the static space coordinate and the space coordinate of the SAR platform, and obtaining a second distance process RstaticThe formula for (t) is:
Figure GDA0003547101230000071
wherein, [ x ]static(t)ystatic(t)zstatic(t)]Is the stationary space coordinates of the vessel;
s52, a first derivative is obtained for the second distance process to obtain a second speed;
obtaining a second speed V by taking a first derivative of a second distance history of the vesselstatic(t), the calculation formula is as follows:
Figure GDA0003547101230000072
and S53, obtaining a second acceleration by calculating a second derivative of the second distance history.
Second acceleration A is obtained by taking the second derivative of the second distance history of the vessel, i.e. the first derivative of the second speedstatic(t), the calculation formula is as follows:
Figure GDA0003547101230000073
s6, obtaining the position offset of the ship according to the first speed and the second speed;
s61, obtaining the speed difference DeltaV between the first speed and the second speedrThe calculation formula is as follows:
ΔVr＝Vship(timage)-Vstatic(timage)
s62, obtaining the position offset of the ship according to the speed difference, and calculating the formula as follows:
Figure GDA0003547101230000074
where Δ x is the amount of positional deviation, Δ V rAnd R is the speed difference, R is the distance between the SAR platform and the ship, and V is the speed of the SAR platform.
S7, compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship;
s71, slicing the ship and carrying out an azimuth IFFT operation to transform the ship into a range-Doppler domain;
the vessel slices are extracted and subjected to an azimuthal IFFT operation, transformed to the range-doppler domain, where the vessel azimuthal motion induced defocus is compensated for.
S72, compensating the ship azimuth motion in the range-Doppler domain to compensate the function H'aThe following:
Figure GDA0003547101230000081
wherein f isaIs the azimuth frequency, kaFor azimuthal frequency modulation, katIn order to compensate for the frequency modulation,
Figure GDA0003547101230000082
wherein λ is a radar wavelength of the SAR platform, and Δ A is a difference between the first acceleration and the second acceleration;
and S73, performing azimuth FFT operation on the compensated slices to obtain ship refocused images.
Compensation for over H 'in range-Doppler domain'aAnd then, FFT operation is carried out along the azimuth direction, and a moving ship slice with good focusing is obtained. As shown in fig. 3A, the image of the ship before refocusing and in fig. 3B, the image of the ship after refocusing, from which it can be seen that the ship after refocusing has a higher picture definition with almost no defocus.
And S8, moving the focused image of the ship by the position offset to obtain the real position of the ship.
And (4) shifting the ship slice after the focusing is finished by the offset delta x along the azimuth direction, and filling the original slice position with the surrounding sea clutter to finish the relocation of the ship.
In summary, the method in the disclosure utilizes the AIS system and the GPS data directly acquired by the SAR platform, there is no approximate equivalence in the processing process, the doppler 0 time (i.e. the ship imaging 0 time) can be accurately acquired, the azimuth speed can be obtained by fitting, the speed does not need to be estimated in the SAR image or the original echo, and the method is more robust. Obtaining a distance process by performing position fitting and interpolation on the AIS system and the SAR platform, obtaining the relative radial speed and acceleration of the ship relative to a stationary target by derivation of the distance process, and performing position offset correction and azimuth refocusing; the calculated speed and acceleration can be used for correcting position offset and eliminating azimuth false targets caused by motion under a multi-channel SAR system.
In another aspect, the present disclosure further provides an apparatus 300 for calculating and refocusing a ship position in an AIS-based SAR image, as shown in fig. 4, including a first obtaining module 310, a second obtaining module 320, a third obtaining module 330, a fourth obtaining module 340, a fifth obtaining module 350, a sixth obtaining module 360, a compensation module 370, and a translation module 380, wherein: the first acquisition module 310 is configured to respectively probe a ship by using an AIS system and an SAR platform, respectively obtain first GPS data and second GPS data, and convert the first GPS data into a cartesian coordinate system; the second obtaining module 320 is configured to obtain a space coordinate of the ship and a space coordinate of the SAR platform according to the first GPS data and the second GPS data; the third obtaining module 330 is configured to obtain a first distance history, a first speed, and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform; the fourth obtaining module 340 is configured to obtain a static space coordinate of the ship at the imaging time of the SAR platform according to the first GPS data, by using the first distance history minimum time as the imaging time; a fifth obtaining module 350, configured to obtain a second distance history, a second speed, and a second acceleration of the ship according to the stationary spatial coordinate and the spatial coordinate of the SAR platform; a sixth obtaining module 360, configured to obtain a position offset of the ship according to the first speed and the second speed; a compensation module 370, for compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship; and the translation module 380 moves the focused image of the ship by the position offset to obtain the real position of the ship.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. A ship position calculating and refocusing method in SAR images based on AIS comprises the following steps:
s1, detecting the ship by respectively adopting an AIS system and an SAR platform, and respectively obtaining first GPS data and second GPS data;
s2, respectively obtaining the space coordinate of the ship and the space coordinate of the SAR platform according to the first GPS data and the second GPS data;
s3, obtaining a first distance course, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform;
s4, taking the minimum time of the first distance course as an imaging time, and obtaining the static space coordinates of the ship at the imaging time according to the first GPS data;
s5, obtaining a second distance course, a second speed and a second acceleration of the ship according to the static space coordinate and the space coordinate of the SAR platform;
S6, obtaining the position offset of the ship according to the first speed and the second speed;
wherein the step S6 includes:
s61, obtaining the speed difference between the first speed and the second speed;
s62, obtaining the position offset of the ship according to the speed difference, wherein the calculation formula is as follows:
Figure FDA0003547101220000011
wherein Δ x is the position offset, Δ VrThe speed difference is obtained, R is the distance between the SAR platform and the ship, and V is the speed of the SAR platform;
s7, compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship;
wherein the step S7 includes:
s71, slicing the ship and performing an azimuth IFFT operation to transform the ship into a range-Doppler domain;
s72, compensating the ship direction movement in the range-Doppler domainPaid function H'aThe following were used:
Figure FDA0003547101220000021
wherein f isaIs the azimuth frequency, kaFor azimuthal frequency modulation, katIn order to compensate for the frequency modulation,
Figure FDA0003547101220000022
wherein λ is a radar wavelength of the SAR platform, and Δ A is a difference between the first acceleration and the second acceleration;
s73, performing azimuth FFT operation on the compensated slices to obtain a focused image of the ship;
And S8, moving the focused image of the ship by the position offset to obtain the real position of the ship.
2. The method of claim 1, said step S1 further comprising:
converting the first GPS data to a Cartesian coordinate system.
3. The method according to claim 1, the step S3 comprising:
s31, obtaining a first distance process of the ship relative to the SAR platform according to the space coordinate of the ship and the space coordinate of the SAR platform, wherein the first distance process RshipThe formula for calculation of (t) is:
Figure FDA0003547101220000023
wherein the content of the first and second substances,
Figure FDA0003547101220000024
is the spatial coordinate of the vessel, [ x ]sar(t) ysar(t) zsar(t)]Is the spatial coordinates of the SAR platform;
s32, obtaining the first speed by calculating the first derivative of the first distance history;
s33, obtaining the first acceleration by calculating a second derivative of the first distance history.
4. The method according to claim 1, the step S5 comprising:
s51, obtaining a second distance process of the ship relative to the SAR platform at the imaging moment according to the static space coordinate and the space coordinate of the SAR platform, wherein the second distance process RstaticThe formula for calculation of (t) is:
Figure FDA0003547101220000025
wherein, [ x ]static(t) ystatic(t) zstatic(t)]Is the stationary space coordinates of the vessel;
s52, obtaining the second speed by calculating the first derivative of the second distance history;
And S53, obtaining the second acceleration by calculating the second derivative of the second distance history.
5. The method of claim 1, wherein the step S1 further comprises eliminating outliers in the first and second GPS data.
6. The method according to claim 1, the step S2 comprising:
and carrying out interpolation fitting on the first GPS data and the second GPS data to obtain the space coordinate of the ship and the space coordinate of the SAR platform.
7. An apparatus for vessel position calculation and refocusing in AIS based SAR images, comprising:
the system comprises a first acquisition module, a second acquisition module and a third acquisition module, wherein the first acquisition module is used for respectively adopting an AIS (automatic identification system) and an SAR (synthetic aperture radar) platform to detect a ship and respectively acquiring first GPS data and second GPS data;
the second acquisition module is used for respectively acquiring the space coordinate of the ship and the space coordinate of the SAR platform according to the first GPS data and the second GPS data;
the third acquisition module is used for acquiring a first distance process, a first speed and a first acceleration of the ship according to the space coordinates of the ship and the space coordinates of the SAR platform;
the fourth acquisition module is used for taking the minimum moment of the first distance process as the imaging moment and acquiring the static space coordinate of the imaging moment of the ship according to the first GPS data;
A fifth obtaining module, configured to obtain a second distance history, a second speed, and a second acceleration of the ship according to the stationary space coordinate and the space coordinate of the SAR platform;
a sixth obtaining module, configured to obtain a position offset of the ship according to the first speed and the second speed; the method comprises the following steps:
obtaining a speed difference between the first speed and the second speed;
obtaining the position offset of the ship according to the speed difference, wherein the calculation formula is as follows:
Figure FDA0003547101220000031
wherein Δ x is the position offset, Δ VrThe speed difference is obtained, R is the distance between the SAR platform and the ship, and V is the speed of the SAR platform;
the compensation module is used for compensating the ship azimuth motion according to the first acceleration and the second acceleration to obtain a focused image of the ship; the method comprises the following steps:
slicing the vessel and performing an azimuthal IFFT operation to transform to a range-doppler domain;
compensating the ship azimuth motion in the range-Doppler domain by a compensation function H'aThe following were used:
Figure FDA0003547101220000041
wherein f isaIs the azimuth frequency, kaFor azimuthal frequency modulation, katIn order to compensate for the frequency modulation,
Figure FDA0003547101220000042
wherein λ is a radar wavelength of the SAR platform, and Δ A is a difference between the first acceleration and the second acceleration;
Performing azimuth FFT operation on the compensated slices to obtain a focused image of the ship;
and the translation module moves the focused image of the ship by the position offset to obtain the real position of the ship.
8. The apparatus of claim 7, the first acquisition module further comprising converting the first GPS data to a Cartesian coordinate system.
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