CN105549008B - A kind of parameter optimization method of the big spaceborne Spotlight SAR Imaging system of strabismus of variable element - Google Patents
A kind of parameter optimization method of the big spaceborne Spotlight SAR Imaging system of strabismus of variable element Download PDFInfo
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- 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
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- 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
- G01S13/9004—SAR image acquisition techniques
- G01S13/9011—SAR image acquisition techniques with frequency domain processing of the SAR signals in azimuth
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- 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
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- 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
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- G—PHYSICS
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- 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
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Abstract
The invention discloses a kind of method for optimally designing parameters of the big spaceborne Spotlight SAR Imaging system of strabismus of variable element, including：First, earth observation task basic parameter is obtained；2nd, calculate as derived from basic parameter systematic parameter, three, fitting satellite orbit coordinate multinomial, estimate satellite starting, terminate sampling time scale；4th, azimuth sample points, earth's surface drift angle sequence corresponding to calculating sampling time interval, scene center are set；5th, sampled satellite position sequence, the oblique distance sequence and time sampling interval sequence of scene center are determined；6th, checking non-emissive pulse is blocked and the constraints without substar echo interference；7th, the time-varying adjustment law of radar platform parameter is calculated.The present invention provides theoretical foundation and implementation method for the system design considerations of the big spaceborne Spotlight SAR Imaging of strabismus of variable element.
Description
Technical Field
The invention relates to the technical field of Synthetic Aperture Radars (SAR), in particular to a parameter optimization method of a variable-parameter large squint space-borne bunching SAR system.
Background
The satellite-borne bunching SAR is a high-resolution earth observation microwave imaging tool, the parameter design of the traditional satellite-borne SAR system needs to be carried out according to a radar imaging mode, and then a proper imaging processing method is selected according to the system parameters and the space geometric configuration of a radar platform. However, for the satellite-borne beamforming SAR system under the large strabismus data acquisition configuration, most of the imaging processing methods are disabled due to the strong two-dimensional coupling introduced in the echo signal by the large strabismus configuration. The Polar Format Algorithm (PFA) has the potential of processing large squint bunching SAR data, corrects two-dimensional coupling of echoes in a wave number domain according to the equivalent characteristics of a signal subjected to linear frequency modulation in time and frequency domains, and finally realizes image focusing by Fast Fourier Transform (FFT). However, a large squint data acquisition configuration will result in an effective range-wavenumber bandwidth much smaller than the range-wavenumber bandwidth at the azimuthal sampling instant alone, which will result in a deterioration of the range-wise resolution, and in severe cases the effective range-wavenumber bandwidth may be zero, at which point the imaging process cannot be completed. In order to overcome the defects, the patent applicant provides a satellite-borne large squint bunching SAR system based on time-varying parameters, the distribution of echoes in a wave number domain is adjusted from a system design level, and then a PFA algorithm can be adopted to complete high-resolution imaging processing.
Disclosure of Invention
The invention aims to solve the design problem of a variable-parameter large-squint spaceborne bunching SAR system, and provides a set of complete parameter optimization method of the variable-parameter large-squint spaceborne bunching SAR system. In the process of designing system parameters, the method simultaneously considers the constraint of two-dimensional resolution requirement, two-dimensional aliasing-free sampling, emission pulse shielding-free and off-satellite point echo interference-free on the system parameter design, and provides the minimum two-dimensional sampling point number meeting the earth observation task, so that the calculation amount can be controlled to the minimum degree, and the imaging processing efficiency of the system is maximized.
The invention relates to a parameter optimization method of a variable parameter large squint satellite-borne bunching SAR system, which comprises the following steps of:
the method comprises the steps of firstly, obtaining basic parameters of a ground observation task, wherein the basic parameters comprise target scene parameters, basic reference parameters of radar emission signals, basic parameters of radar space sampling and signal oversampling factors.
And step two, calculating SAR system parameters derived from various basic parameters in the step one, wherein the SAR system parameters comprise radar emission signal derived parameters, two-dimensional signal sampling derived parameters and space sampling derived parameters.
Step three, according to the inputThe orbit sampling point sequence fits a polynomial of a satellite orbit coordinate by using a least square method, and the time scale t of the initial sampling point of the satellite orbit meeting the requirement of the azimuth resolution is estimated start And time scale t of ending sampling point end 。
Step four, setting the number of azimuth sampling points, calculating the sampling time interval under the uniform azimuth sampling condition, and counting the number of sampling points between t start And t end After up-sampling the satellite coordinate sequence by M times, calculating the earth surface deflection angle sequence alpha of each point relative to the scene center dense 。
Step five, calculating a surface deflection angle sequence alpha corresponding to the azimuth sampling points and meeting the requirement that echo data are uniformly distributed in the wave number domain along the azimuth direction tgt According to α tgt Determining a sequence of space-variant satellite sample positions P satF Calculating P satF Skew distance sequence R between each sampling position and scene center cF Calculate P satF Time sample interval sequence I satF 。
Step six, verifying P satF Whether the constraint conditions of no transmit pulse shielding and no satellite point echo interference are met. If the constraint is not satisfied, use Δ N a Increasing the number of azimuth sampling points for the step length, performing iterative operation from the fourth step to the sixth step, and when P is reached satF And simultaneously, constraint conditions of no transmitted pulse shielding and no echo interference of the satellite points are met, or the iterative operation is stopped when the number of azimuth sampling points reaches the iteration upper limit.
Step seven, according to P satF And calculating the time-varying adjustment rule of the radar platform parameters according to the relative position relation with the scene central point.
The invention has the advantages that:
(1) The parameter optimization design method of the variable parameter large squint satellite-borne bunching SAR system is provided, and a theoretical basis and an implementation method are provided for the system design of the variable parameter large squint satellite-borne bunching SAR;
(2) The method considers the two-dimensional resolution characteristic, has no aliasing sampling, no shielding of a transmitted pulse and no constraint of the interference of the echo of the satellite points on the parameters of a system platform and the parameters of space sampling, and can completely receive the echo of a target scene;
(3) According to the method, based on the time-frequency equivalent characteristic of a signal after two-dimensional line-solving frequency modulation, time-varying radar platform parameters are designed for correcting the distance wave number with strong space-variant characteristics under a large squint condition, so that a focused image has good distance resolution capability.
Drawings
FIG. 1 is a flow chart of a method of the present invention;
FIG. 2 is a schematic diagram of the present invention for finding a unit vector perpendicular to the earth's surface passing through the center of the scene;
FIG. 3 is a flow chart of a method of determining a time scale of start and end samples of a satellite orbit;
FIG. 4 is a schematic diagram of the distribution of the echo in the wavenumber domain after the adjustment of the radar platform parameters is completed in the present invention.
FIG. 5 is a schematic representation of spatial sample sequence coordinates of a satellite in an embodiment of the invention;
FIG. 6 is a schematic error diagram of an actual surface deviation angle sequence and a target surface deviation angle sequence in an embodiment of the present invention;
FIG. 7 is a schematic representation of a sequence of spatial sampling time intervals for a satellite in an embodiment of the present invention;
FIG. 8 is a schematic diagram of a time-varying adjustment rule of radar platform parameters according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The invention discloses a parameter optimization method of a variable parameter large squint spaceborne spotlight SAR system, which is shown in a flow chart of figure 1 and comprises the following steps:
the method comprises the steps of firstly, obtaining basic parameters of a ground observation task, wherein the basic parameters comprise target scene parameters, basic reference parameters of radar emission signals, basic parameters of radar space sampling and signal oversampling factors. The method comprises the following specific steps:
(1) Acquiring target scene parameters of the earth observation task, including the distance width S of the scene wr Azimuthal width S wa Distance direction observation resolution ρ r And azimuthal observation resolution ρ a ；
(2) Obtaining basic reference parameters of SAR Linear Frequency Modulated (LFM) emission signals, including reference carrier Frequency f c0 And a reference pulse width T p0 ；
(3) Obtaining space sampling basic parameters of SAR (synthetic aperture radar), including satellite orbit height H and satellite in-orbit running speed V s Scene center P sce In the earth rotating coordinate system E g Coordinate of (x) sce ，y sce ，z sce ) SAR at synthetic aperture center time t c Sample point position P sat0 At E g Coordinate of (x) sat0 ，y sat0 ，z sat0 ) And satellite orbit by P sat0 Coordinate sequence A of centered track sampling points sat0 Wherein the earth rotates a coordinate system E g The definition is as follows:
origin of coordinates: earth core marked as O g ；
X-axis is denoted X g : in the equatorial plane, points to the zero degree longitude direction;
y axis is denoted as Y g : in the equatorial plane, points to the east longitude 90 degrees direction;
z axis is denoted as Z g : along the axis of rotation of the earth, pointing to the positive north pole (90 degrees north latitude).
(4) A signal oversampling factor gamma is obtained.
And step two, calculating SAR system parameters derived from various basic parameters in the step one, wherein the SAR system parameters comprise radar emission signal derived parameters, two-dimensional signal sampling derived parameters and space sampling derived parameters. The method specifically comprises the following steps:
(1) Calculating derived reference parameters of radar transmitted signals, including reference bandwidth B of LFM signals 0 Reference frequency K r0 ：
Wherein: and c represents the speed of light.
(2) Calculating t under an ellipsoid earth model c Time SAR relative P sce Angle of incidence beta 0 ：
In the earth model of ellipsoid, let P sce Unit vector perpendicular to earth surface and deviating from earth centerAnd Z g The intersection point of the axes is O d As shown in fig. 2. Let O d At Z g The coordinate of the axis being z od Then z is od Can be obtained by calculation of space geometric relationship
Wherein: b is the length of the semiminor axis in the earth ellipsoid model. From the formula (3) can be found
Wherein
Is provided with P sce Point of direction P sat0 Can be represented asThen beta is 0 Can be calculated from the following formula
Where | represents the modulo operation.
(3) Calculating range-wise sampling requirements of the signal, including a reference sampling rate F s0 Number of sampling points N of aliasing-free minimum distance r Distance wave number support domain width K yB ：
(4) Calculating the azimuthal sampling requirement of the signal, including the range alpha of the earth's surface accumulation opening angle over the synthetic aperture time B Minimum width K of azimuth wavenumber support field xB Number of sampling points in the minimum azimuth direction without aliasing amin ：
Where atan (-) represents the arctan function.
Step three, fitting the satellite orbit seat by using a least square method according to the input orbit sampling point sequenceA target polynomial to estimate the time scale t of the initial sample point of the satellite orbit that meets the azimuth resolution requirement start And time scale t of ending sampling point end . The method comprises the following specific steps:
(1) Respectively according to A sat0 Fitting a polynomial of the coordinates with respect to the azimuth sampling time t:
the description is given by taking the fitting of polynomial in X coordinate as an example. Let X sat0 Is composed of A sat0 The dimension of the matrix formed by the extracted X coordinates is Qx 1, wherein Q is A sat0 The number of sampling points contained in (a). Let W be the highest order of the polynomial fit, t i Is A sat0 At each sampling point in t c Time coordinate of zero point, where i =1,2, \ 8230;, Q, by t i And W an observation matrix G that can generate a polynomial fit of
Polynomial fitting coefficient matrix CoE of X coordinate to be solved x Is composed of
CoE x ＝[e 0 ,e 1 ,e 2 ,…,e W ] T (14)
Wherein the superscript T denotes the matrix transposition, i.e. CoE x Is a matrix with dimension (W + 1) × 1. To this least squares problem transformation to find CoE under least squares constraint x The solution of (c).
GCoE x ＝X sat0 (15)
As can be seen from the optimization solution method, the least squares solution of equation (15) can be expressed as
CoE x ＝(G T G) -1 G T X sat0 (16)
Where the superscript-1 represents the inverse of the matrix.
Repeating the calculation processes shown in the formulas (14) to (16) on the Y coordinate and the Z coordinate to be solved to obtain a polynomial fitting coefficient matrix CoE of the Y coordinate and the Z coordinate y And CoE z 。
(2) Estimating the time scale t of the starting point of the satellite orbit meeting the requirement of the azimuth resolution start And the time scale t of the ending point end ：
Calculation of A sat0 Mean azimuth time interval Δ t of middle sampling points 0 As follows
Wherein P is sat Is represented by A sat0 Coordinates of the sample points in the sequence.
Computing vectorsProjection vector at image planeThenCan be expressed as
With t c At an initial time, at 0 For step-wise incremental azimuth time, coE is used x 、CoE y And CoE z Estimating the satellite coordinates P of the current time satE0 . Memory vectorThe projection vector at the imaging plane isThen theCan be expressed as
Is provided withAnd withIncluded angle is alpha T Then α is T Can be expressed as
If α is T <α B /2, then with Δ t 0 And (5) continuously increasing the azimuth time t for the step length, and repeating the calculation of the expressions (19) and (20) until alpha is reached T >α B And/2, the azimuth moment at the moment is t end 。
Similarly, with t c The time is the initial time and is delta t 0 Repeating the calculation of the formula (19) and the formula (20) until alpha is reached for gradually reducing the azimuth time by step size T >α B And/2, the azimuth moment at the moment is t start . FIG. 3 illustrates solving for t start 、t end An iterative computation method of time.
Step four, setting the number of azimuth sampling points, calculating the sampling time interval under the uniform azimuth sampling condition, and counting the number of sampling points between t start And t end The satellite coordinate sequence between the earth surface and the ground surface is up-sampled by M times to calculate the earth surface declination sequence alpha of each point relative to the scene center dense . The method specifically comprises the following steps:
setting the number of azimuth sampling points to N a . When the fourth step is executed for the first time, let N a ＝N amin 。N a Uniform azimuth sampling time interval delta t corresponding to each azimuth sampling point d Can approximately represent
Time interval delta t of uniform azimuth sampling sequence after M times of up-sampling dU Can be expressed as
With t start As a starting time, t end For the end time,. DELTA.t dU Generating a sampling time sequence after M times of up-sampling for the step length and according to the CoE x 、CoE y And CoE z Estimating the satellite coordinate value at each sampling point to generate an up-sampled satellite coordinate sequence P satd . At the moment, the earth surface declination sequence alpha dense Can be expressed as
Wherein
Step five, calculating a surface deflection angle sequence alpha corresponding to the azimuth sampling points and meeting the requirement that echo data are uniformly distributed in the wave number domain along the azimuth direction tgt According to α tgt Determining a sequence of space-variant satellite sample positions P satF Calculate P satF Slope distance sequence R of each sampling position and scene center cF Calculate P satF Time sample interval sequence I satF . The method specifically comprises the following steps:
number of sampling points N for azimuth direction a The surface deflection angle sequence alpha of the echo data which is uniformly distributed in the wavenumber domain along the azimuth direction is satisfied tgt Can be expressed as
Take alpha in sequence tgt Has a value ofα dense Find the closest value in P satd Middle make alpha dense Obtaining the closest alpha tgt The coordinates of the satellite sampling points of each point value are the final satellite coordinates in data acquisition, so that the final non-uniform satellite sampling sequence P can be obtained satF 。P satF At each sampling position pair P sce Is a sequence of pitches R cF Can be expressed as
P satF Time series I of sampling intervals between sampling points satF Can be expressed as
Step six, verifying P satF Whether the constraint conditions of no transmit pulse shielding and no satellite point echo interference are met. If the constraint is not satisfied, use Δ N a Increasing the number of azimuth sampling points for the step length, performing iterative operation from the fourth step to the sixth step, and when P is reached satF And simultaneously, constraint conditions of no transmitted pulse shielding and no echo interference of the satellite points are met, or the iterative operation is stopped when the number of azimuth sampling points reaches the iteration upper limit. The method specifically comprises the following steps:
if P satF If the constraint condition of no transmitted pulse occlusion is satisfied, it is required to ensure that the delay of the echo leading edge of the point at the nearest scene target is larger than that of the current transmitted pulse trailing edge, and the delay of the echo trailing edge of the point at the farthest scene target is smaller than that of the next transmitted pulse leading edge. At this time, the space geometric relationship can be calculated
Wherein M is 0 The number of the transmission pulse intervals between the transmission signal and the corresponding receiving signal in the data acquisition process is shown, and δ p is the protection time when the receiving and transmitting beams are switched.
If P satF If the echo interference without the satellite point is satisfied, the method can satisfy one of the following two conditions: firstly, the leading edge time delay of a subsatellite point echo appearing in a current echo window is larger than the trailing edge time delay of a scene echo; secondly, the time delay of the back edge of the echo of the subsatellite point appearing in the current echo window is less than the time delay of the front edge of the echo of the scene, the echo width of the subsatellite point is set to be two pulse widths, the echo of the scene of the kth transmitting pulse and the echo of the subsatellite point of the pth transmitting pulse are received at the same pulse interval, and the first condition corresponds to the following relation
The second case corresponds to the following relationship
The values of k in the equations (30) and (31) are the same as those in the equation (28). If P satF If the constraint conditions formed by the equations (28), (29) and (30) are satisfied, or the constraint conditions formed by the equations (28), (29) and (31) are satisfied, the spatial sampling parameter design is considered to meet the system parameter design requirement for completely receiving the echo of the target scene area. At this time P satF I.e. the final sequence of sample points for the satellite. If P satF That is, if the constraint conditions of equations (28), (29), and (30) are not satisfied, and the constraint conditions of equations (28), (29), and (31) are not satisfied, the number of azimuth sampling points is considered to be N a Fails in the design of the spatial sampling mode. In this case, Δ N is required a Increasing the step size by N a Repeating the fourth step to the sixth step to carry out iterative operation until P satF Can satisfy the condition of no emissionPulse occlusion and no satellite point echo interference.
In addition, N is increased under the condition of satisfying non-aliasing sampling a The resolution of the earth observation cannot be improved, but the processing load of the imaging system is increased, and the imaging processing efficiency of the system is reduced. Therefore, when the constraint condition meeting the requirements of no transmit pulse shielding and no echo interference of the satellite points is obtained by using an iterative method, an allowable N is set a The maximum value N is obtained aIM If is when N a Less than N aIM At no time is any P satF The sequence can simultaneously meet the constraint conditions of no transmitted pulse shielding and no satellite-borne point echo interference, and then the variable-parameter large squint satellite-borne bunching SAR system meeting the given ground observation requirement condition does not exist, and the iterative operation is terminated.
Step seven, according to P satF And calculating the time-varying adjustment rule of the radar platform parameters according to the relative position relation with the scene central point. The method specifically comprises the following steps:
(1) Calculating P by adopting the methods described in the formulas (3) to (6) in the second step satF Of each sample position relative to the scene center P sce Angle of incidence sequence beta F 。
(2) P is calculated by adopting the method described in the step five Chinese formula (23) satF Of each sample position relative to the scene center P sce Surface declination sequence alpha F 。
(3) According to beta F 、α F And radar platform reference parameter f c0 、K r0 、F s0 、T p0 Calculating the regulation law of the time-varying radar platform parameters, including the carrier frequency f c Frequency of modulation K r Pulse width T p Sum signal sampling rate F s The time-varying adjustment rule.
In order to avoid the spatial configuration of large squint, the signal after the linear frequency modulation is released has the distance wave number with serious space variation, and the radar platform parameters of the variable-parameter large squint space-borne bunching SAR system are timely adjusted along with the relative position relation between the radar and a target scene so as to improve the distribution mode of the signal in the wave number domain. Specifically, the adjustment rule of the radar parameter can be divided into the following two ways:
the first method is as follows: sampling rate F of radar transmission pulses s Pulse width T p Constant, regulated carrier frequency f c And a frequency K r As follows
The second method comprises the following steps: frequency modulation K of radar transmitted pulse r Adjusting the carrier frequency f of the transmitted signal without change c Sampling rate F s And a pulse width T p As follows
Fig. 4 illustrates the distribution of the echo in the wave number domain after the parameter adjustment is completed by any parameter adjustment method. Although the two radar parameter adjustment methods are different in the adjustment objects, the effects are the same. In practical application, which method should be adopted to adjust the parameters of the radar platform needs to be selected according to the actual hardware platform composition.
And completing system parameter design of the variable-parameter large squint satellite-borne spotlight SAR.
Examples
The invention is applied to the concrete examples, and the concrete steps are as follows:
the method comprises the steps of firstly, obtaining target scene parameters of a ground observation task, basic reference parameters of radar emission signals and basic parameters of radar space sampling, and obtaining signal oversampling factors. The basic parameters obtained are shown in table 1.
TABLE 1 basic parameters of variable-parameter large squint spaceborne spotlight SAR earth observation task
And step two, calculating SAR system parameters derived from various basic parameters in the step one, wherein the SAR system parameters comprise radar emission signal derived parameters, two-dimensional signal sampling derived parameters and space sampling derived parameters. The calculated radar system derived parameters are shown in table 2
Table 2 variable parameter large squint satellite-borne spotlight SAR earth observation task derived parameters
Thirdly, fitting a polynomial of a satellite orbit coordinate by utilizing a least square method according to the input orbit sampling point sequence, and estimating the time scale t of the initial sampling point of the satellite orbit meeting the requirement of azimuth resolution start And time scale t of ending sampling point end 。
And setting the polynomial fitting order W =5 of the three-dimensional coordinate of the satellite orbit, and calculating the coefficient matrix CoE of the fitting polynomial of the three-dimensional coordinate of the satellite orbit by adopting a least square method for the radar parameters given in the first step and the second step x 、CoE y And CoE z The parameters of (a) are shown in table 3.
TABLE 3 polynomial fitting coefficients for three-dimensional coordinates of satellite orbits within synthetic aperture
From A sat0 Calculating to obtain the average azimuth time interval delta t of the sampling points 0 ＝0.0013s。
At Δ t 0 Obtaining t for step size iterative calculation start ＝-3.4742s，t end ＝3.3297s。
Step four, setting the number of azimuth sampling points, calculating the sampling time interval under the condition of uniform azimuth sampling, and aligning the sampling time interval between t start And t end Up-sampling of satellite coordinate sequences in betweenCalculating the earth surface deflection angle sequence alpha of each point relative to the scene center after M times dense 。
When the step four is executed for the first time, the number N of the azimuth sampling points is made a =12005. An up-sampling multiple M =8 is set.
Calculating to obtain N a Time interval delta t of uniform azimuth sampling corresponding to each azimuth sampling point d ＝5.38×10 -4 s,8 times the time interval Δ t of uniform azimuth sampling after upsampling dU ＝6.72×10 -5 s
When N is present a =12005, satellite coordinate relative to P sce Surface declination sequence alpha dense The statistical information of (a) is shown in table 4.
TABLE 4 alpha dense Statistical information (N) a ＝12005)
| Parameter name | Numerical value |
| Number of sampling points | 101169 |
| α dense Minimum declination angle (rad) | -0.0369 |
| α dense Maximum deflection angle (rad) | 0.0369 |
Step five, calculating a surface deflection angle sequence alpha corresponding to the azimuth sampling points and meeting the requirement that echo data are uniformly distributed in the wave number domain along the azimuth direction tgt According to α tgt Determining a sequence of space-variant satellite sample positions P satF Calculate P satF Slope distance sequence R of each sampling position and scene center cF Calculate P satF Time sample interval sequence I satF 。
Calculated alpha tgt 、P satF 、R cF And I satF The statistical information of (a) is shown in table 5.
TABLE 5 variable parameter large squint space-borne spotlight SAR spatial sampling parameter (N) a ＝12005)
Step six, verifying P satF Whether the constraint conditions of no transmit pulse shielding and no satellite point echo interference are met. If the constraint is not satisfied, the value is determined by Delta N a Increasing the number of azimuth sampling points for the step length, performing iterative operation from the fourth step to the sixth step, and when P is the number of azimuth sampling points satF And simultaneously, constraint conditions of no transmitted pulse shielding and no echo interference of the satellite points are met, or the iterative operation is stopped when the number of azimuth sampling points reaches the iteration upper limit.
Setting the iteration upper limit of the azimuth sampling point number as N aIM In this case let N aIM ＝14407。
Verified as N a =12005, P satF The constraints of no transmit pulse occlusion and no intersatellite point echo interference can not be satisfied simultaneously. By Delta N a =10 step size increase sampling point N a Is verified to obtain the value of N a 12646, P satF The constraint conditions of no transmit pulse shielding and no echo interference of the satellite points can be simultaneously met. At this time N a To be by Delta N a And calculating the minimum azimuth sampling point number which can meet the requirement of the earth observation task for the step length.
Step seven, according to P satF And calculating the time-varying adjustment rule of the radar platform parameters according to the relative position relation with the scene central point.
In the task of performing earth observation by using the variable-parameter large-squint satellite-borne beamforming SAR system in this embodiment, the parameter adjustment statistical information of the time-varying radar platform is shown in table 6.
TABLE 6 parameter-variable large squint spaceborne spotlight SAR radar platform parameter adjustment statistical information
The system parameter optimization design of the variable parameter large squint satellite-borne spotlight SAR is finished, the coordinates (X, Y and Z coordinates) of the satellite sampling points obtained by the design are shown in figure 5, and the actual surface declination sequence alpha F Deviation angle sequence alpha with target earth surface tgt The error values of (a) are shown in fig. 6, the azimuth sampling time intervals are shown in fig. 7, fig. 8 shows time-varying radar platform parameters, wherein fig. 8 (a) shows the variation law of the carrier frequency and the modulation frequency in the first parameter adjustment mode, and fig. 8 (b) shows the variation law of the carrier frequency, the sampling rate and the pulse width in the second parameter adjustment mode.
The invention provides a complete parameter-variable large-squint satellite-borne bunching SAR system parameter optimization design method, which provides a theoretical basis and an implementation method for the design of a radar platform and a working mode of a variable-parameter large-squint satellite-borne bunching SAR system.
Claims (1)
1. A parameter optimization method of a variable parameter large squint satellite-borne bunching SAR system comprises the following steps:
the method comprises the following steps of acquiring basic parameters of a ground observation task, wherein the basic parameters comprise target scene parameters, basic reference parameters of radar emission signals, basic parameters of radar space sampling and signal oversampling factors, and specifically comprises the following steps:
(1) Acquiring target scene parameters of the earth observation task, including the distance width S of the scene wr Azimuthal width S wa Distance-direction observation resolution ρ r And azimuthal observation resolution ρ a ；
(2) Obtaining basic reference parameters of radar emission signals, including reference carrier frequency f c0 And referencePulse width T p0 ；
(3) Acquiring radar space sampling basic parameters including satellite orbit height H and satellite in-orbit running speed V s Scene center P sce In the earth rotating coordinate system E g Coordinate of (x) sce ，y sce ，z sce ) SAR at synthetic aperture center time t c Sample point position P sat0 At E g Coordinate of (x) sat0 ，y sat0 ，z sat0 ) And satellite orbit by P sat0 Coordinate sequence A of centered orbital sampling points sat0 Wherein the earth rotates a coordinate system E g The definition is as follows:
origin of coordinates: earth core marked as O g ；
X-axis is denoted X g : in the equatorial plane, points to the zero degree longitude direction;
y axis is denoted as Y g : in the equatorial plane, points to the east longitude 90 degrees direction;
z axis is denoted as Z g : the direction of the north pole, namely the direction of 90 degrees of north latitude, is pointed along the rotation axis of the earth;
(4) Acquiring a signal oversampling factor gamma;
step two, calculating SAR system parameters derived from various basic parameters in the step one, wherein the SAR system parameters comprise radar emission signal derived parameters, two-dimensional signal sampling derived parameters and space sampling derived parameters; the method specifically comprises the following steps:
(1) Calculating derived reference parameters of radar transmitted signals, including reference bandwidth B of LFM signals 0 Reference frequency K r0 ：
Wherein: c represents the speed of light;
wherein: t is p Represents the pulse width;
(2) Calculating t under an ellipsoid earth model c Time SAR relative P sce Angle of incidence beta 0 ：
In the earth model of ellipsoid, let P sce Unit vector perpendicular to earth surface and deviating from earth centerAnd Z g The intersection point of the axes is O d Is provided with O d At Z g The coordinate of the axis being z od Then z is od Calculated from the space geometric relationship
Wherein: b is the length of the semiminor axis in the earth ellipsoid model; from the formula (3)
Wherein
Is provided with P sce Point of direction P sat0 Is represented asThen beta is 0 Calculated from the following formula
Wherein | represents a modulo operation;
(3) Calculating range-wise sampling requirements of the signal, including a reference sampling rate F s0 Number of sampling points N of aliasing-free minimum distance r Distance wave number support domain width K yB ：
(4) Calculating the azimuthal sampling requirement of the signal, including the range alpha of the earth's surface accumulation opening angle over the synthetic aperture time B Minimum width K of azimuthal wavenumber support domain xB Number of sampling points in the minimum azimuth direction without aliasing amin ：
Wherein atan (-) represents the arctan function;
thirdly, fitting a polynomial of the satellite orbit coordinate by utilizing a least square method according to the input orbit sampling point sequence, and estimating the time scale t of the initial sampling point of the satellite orbit meeting the requirement of the azimuth resolution start And time scale t of ending sampling point end (ii) a The method comprises the following specific steps:
(1) Respectively according to A sat0 The coordinates in the X, Y and Z directions of (1) fit a polynomial of the coordinates with respect to the azimuth sampling time t:
the X coordinate polynomial fitting specifically comprises: let X sat0 Is composed ofA sat0 The dimension of the matrix formed by the extracted X coordinates is Qx 1, wherein Q is A sat0 The number of sampling points contained in (a); let W be the highest order of the polynomial fit, t i Is A sat0 At each sampling point of c Is the time coordinate of zero point, where i =1,2, \8230;, Q, is represented by t i And W generates an observation matrix G of polynomial fit to
Polynomial fitting coefficient matrix CoE of X coordinate to be solved x Is composed of
CoE x ＝[e 0 ,e 1 ,e 2 ,...,e W ] T (14)
Wherein the superscript T denotes the matrix transposition, i.e. CoE x Is a matrix with dimension (W + 1) × 1; the least squares problem is converted to CoE under the least squares constraint to solve the following equation x The solution of (2);
GCoE x ＝X sat0 (15)
the least squares solution of equation (15) is represented as
CoE x ＝(G T G) -1 G T X sat0 (16)
Where the superscript-1 represents the inverse of the matrix;
similarly, a polynomial fitting coefficient matrix CoE of Y and Z coordinates is obtained y And CoE z ；
(2) Estimating the time scale t of the starting point of the satellite orbit meeting the requirement of the azimuth resolution start Time scale t with end point end ：
Calculation of A sat0 Mean azimuth time interval Δ t of middle sampling points 0 As follows
Wherein P is sat Is shown as A sat0 Coordinates of sampling points in the sequence;
calculating a vectorProjection vector at image planeThen theIs shown as
With t c As an initial time, at 0 For step-wise incremental azimuth time, coE is used x 、CoE y And CoE z Estimating satellite coordinates P of the current time satE0 (ii) a Memory vectorThe projection vector at the imaging plane isThenIs shown as
Is provided withAndincluded angle is alpha T Then α is T Is shown as
If α is T <α B 2, then by Δ t 0 And (4) repeating the calculation of the expressions (19) and (20) until alpha is increased for the step length to continue increasing the azimuth time t T >α B And/2, the azimuth moment at the moment is t end ；
Similarly, with t c The time is the initial time and is delta t 0 Repeating the calculation of the formula (19) and the formula (20) until alpha is reached for gradually reducing the azimuth time by step size T >α B The azimuth moment at the moment is t start ；
Step four, setting the number of azimuth sampling points, calculating the sampling time interval under the uniform azimuth sampling condition, and counting the number of sampling points between t start And t end The satellite coordinate sequence between the earth surface and the ground surface is up-sampled by M times to calculate the earth surface declination sequence alpha of each point relative to the scene center dense ；
The method comprises the following specific steps:
setting the number of azimuth sampling points to N a When step four is executed for the first time, let N a ＝N amin ，N a Uniform azimuth sampling time interval delta t corresponding to each azimuth sampling point d Is shown as
Time interval delta t of uniform azimuth sampling sequence after M times of up-sampling dU Is shown as
With t start As starting time, t end For the end time,. DELTA.t dU Generating a sampling time sequence after M times of up-sampling for the step length and according to the CoE x 、CoE y And CoE z Estimating the satellite coordinate value at each sampling point to generate an up-sampled satellite coordinate sequence P satd At the moment, the surface declination angle sequence alpha dense Is shown as
Wherein
Step five, calculating a surface deflection angle sequence alpha corresponding to the azimuth sampling points and meeting the requirement that echo data are uniformly distributed in the wave number domain along the azimuth direction tgt According to α tgt Determining a sequence of space-variant satellite sample positions P satF Calculate P satF Slope distance sequence R of each sampling position and scene center cF Calculating P satF Time sample interval sequence I satF ；
The method specifically comprises the following steps:
number of sampling points N for azimuth a The surface deflection angle sequence alpha of the echo data which is uniformly distributed in the wavenumber domain along the azimuth direction is satisfied tgt Is shown as
Take alpha in sequence tgt Value of (1) is in alpha dense Find the closest value in, let P satd Middle make alpha dense Obtaining the closest alpha tgt The coordinates of the satellite sampling points of each point value are the satellite coordinates of the final data acquisition, and the final non-uniform satellite sampling sequence P is obtained satF ，P satF At each sampling position pair P sce Is a sequence of pitches R cF Is shown as
P satF Time series I of sampling intervals between sampling points satF Is shown as
Step six, verifying P satF Whether the constraint conditions of no transmit pulse shielding and no satellite point echo interference are met; if the constraint is not satisfied, the value is determined by Delta N a Increasing the number of azimuth sampling points for the step length, performing iterative operation from the fourth step to the sixth step, and when P is reached satF Simultaneously, constraint conditions of no transmitted pulse shielding and no echo interference of an off-star point are met, or the number of azimuth sampling points reaches the upper iteration limit, and the iterative operation is stopped;
the method specifically comprises the following steps:
if P satF Satisfies the constraint condition of no-emission pulse shielding, and is obtained by calculating the space geometric relationship
Wherein M is 0 The number of the transmission pulse intervals between the transmission signal and the corresponding receiving signal in the data acquisition process is represented, and δ p is the protection time during switching of the receiving beam and the transmitting beam;
if P satF If the echo interference without the satellite point is satisfied, the method can satisfy one of the following two conditions: firstly, the leading edge time delay of a subsatellite point echo appearing in a current echo window is larger than the trailing edge time delay of a scene echo; secondly, the time delay of the back edge of the echo of the subsatellite point appearing in the current echo window is less than the time delay of the front edge of the echo of the scene, the width of the echo of the subsatellite point is set to be two pulse widths, and the echo of the scene of the kth transmitting pulse and the echo of the subsatellite point of the pth transmitting pulseReceived at the same pulse interval, the first case corresponds to the following relationship
The second case corresponds to the following relationship
The values of k in the formulas (30) and (31) are the same as the values of k in the formula (28); if P satF If the constraint conditions formed by the formulas (28), (29) and (30) are satisfied, or the constraint conditions formed by the formulas (28), (29) and (31) are satisfied, the spatial sampling parameter design is considered to meet the system parameter design requirement for completely receiving the echo of the target scene area; at this time P satF The final sampling point sequence of the satellite is obtained; if P satF That is, if the constraint conditions of equations (28), (29) and (30) are not satisfied, and the constraint conditions of equations (28), (29) and (31) are not satisfied, the number of azimuth sampling points is considered to be N a Fails in the spatial sampling mode of (1), in terms of Δ N a Increasing the step size by N a Repeating the fourth step to the sixth step to carry out iterative operation until P satF The constraint conditions of no transmit pulse shielding and no echo interference of the satellite points are met;
set N a The maximum value N is obtained aIM If when N is present a Less than N aIM At no time is any P satF If the sequence simultaneously meets the constraint conditions of no transmitted pulse shielding and no satellite-borne point echo interference, the variable-parameter large squint satellite-borne bunching SAR system meeting the given ground observation requirement condition is considered to be absent, and the iterative operation is terminated;
step seven, according to P satF Calculating a time-varying adjustment rule of radar platform parameters according to the relative position relation with the scene central point; the method specifically comprises the following steps:
(1) Calculating P by adopting the method described in the formulas (3) to (6) in the second step satF Each sample inPosition relative to the scene center P sce Angle of incidence sequence beta F ；
(2) P is calculated by adopting the method described in the step five Chinese formula (23) satF With respect to the scene center P for each sample position sce Surface declination sequence alpha F ；
(3) According to beta F 、α F And radar platform reference parameter f c0 、K r0 、F s0 、T p0 Calculating the regulation law of time-varying radar platform parameters, including carrier frequency f c Frequency modulation rate K r Pulse width T p Sum signal sampling rate F s Time-varying regulation rules;
selecting an adjustment rule of radar parameters according to actual conditions, wherein the adjustment rule of the radar parameters is divided into the following two modes:
the first method is as follows: sampling rate F of radar transmission pulses s Pulse width T p Constant, regulated carrier frequency f c And a frequency K r As follows
The second method comprises the following steps: frequency modulation K of radar emission pulses r Adjusting the carrier frequency f of the transmitted signal without change c Sampling rate F s And a pulse width T p As follows
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