CN105301576A - Down-looking synthetic aperture laser imaging radar nonlinear correction method - Google Patents

Down-looking synthetic aperture laser imaging radar nonlinear correction method Download PDF

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CN105301576A
CN105301576A CN201510270195.6A CN201510270195A CN105301576A CN 105301576 A CN105301576 A CN 105301576A CN 201510270195 A CN201510270195 A CN 201510270195A CN 105301576 A CN105301576 A CN 105301576A
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CN105301576B (en
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栾竹
孙建锋
周煜
刘立人
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Shanghai Institute of Optics and Fine Mechanics 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
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/497—Means for monitoring or calibrating
    • 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
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/88—Lidar systems specially adapted for specific applications
    • G01S17/89—Lidar systems specially adapted for specific applications for mapping or imaging
    • G01S17/90—Lidar systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Electromagnetism (AREA)
  • Optical Radar Systems And Details Thereof (AREA)
  • Radar Systems Or Details Thereof (AREA)

Abstract

The invention discloses a down-looking synthetic aperture laser imaging radar nonlinear correction method. Through nonlinear coefficients, a phase error function is directly calculated for correction. No reference channel needs to be added, the system is simple, a nonlinear term is adopted to calculate an error function, and total correction is realized. The method is applied to the down-looking synthetic aperture laser imaging radar.

Description

Orthoptic synthetic aperture laser imaging radar non-linear correction method
Technical field
The present invention relates to Orthoptic synthetic aperture laser imaging radar, particularly a kind of direct-view synthesizing bore diameter laser radar non-linearity bearing calibration, to improve the range resolution of synthetic aperture imaging radar.
Background technology
Synthetic aperture laser imaging radar has two kinds of theory structures, one is side-looking synthetic aperture laser imaging radar, another is that Orthoptic synthetic aperture laser imaging radar is [see document 1:LirenLiu, Coherentandincoherentsynthetic-apertureimagingladarsandl aboratory-spaceexperimentaldemonstrations, Appl.Opt., 52 (4): 579 ~ 599 (2013). with document 2:Z.Luan, J.Sun, Y.Zhou, L.Wang, M.YangandL.Liu, " Down-lookingsyntheticapertureimagingladardemonstratorand itsexperimentsover1.2kmoutdoor, " ChineseOpticsLetters, 12 (11), 111101-1 ~ 4 (2014) .].Under Scan pattern, the data processing of two kinds of synthetic aperture laser imaging radars generally all adopts cross rail to Fourier transform and straight rail to phase place quadratic term matched filtering imaging algorithm.Because data phase will cause cross rail to resolution broadening in cross rail to the nonlinearities change in the Fourier transform time, need to correct.In first technology [see document 3: Xu Nan, Lu Wei, Liu Li people. the simulation and analysis [J] of non-linear chirp scan-filtering correcting algorithm one by one in synthetic aperture laser imaging radar. Acta Optica, 2009,29 (1): 47 ~ 54] a kind of bearing calibration is given for side-looking synthetic aperture laser imaging radar, increase reference channel in systems in which, according to the phase nonlinear result of variations of reference channel, the phase nonlinear error of estimating signal passage, corrects to received signal according to a certain percentage.First reference channel adds the complicacy of system hardware, especially not easily realizes in Orthoptic synthetic aperture laser imaging radar.Next is that the linear term comprised in reference channel signal is also present in correction signal, weakens calibration result, can not correction of Nonlinear completely.Last the method is only for side-looking synthetic aperture laser imaging radar system.
Summary of the invention
The object of the invention is to overcome above-mentioned the deficiencies in the prior art, propose the bearing calibration of a kind of direct-view synthesizing bore diameter laser radar non-linearity, by nonlinear factor, directly calculate the short-cut method that phase error function carries out correcting.Do not need to increase reference channel, system is simple.Adopt nonlinear terms error of calculation function, for correcting completely.Be applicable to Orthoptic synthetic aperture laser imaging radar.
Technical solution of the present invention is as follows:
A kind of Orthoptic synthetic aperture laser imaging radar non-linear correction method, the method is by known nonlinear factor, and the nonlinear phase directly in estimation target echo compensates, then carries out Fourier transform, reduces distance to imaging pulse width.
Its feature is that the method comprises the following steps:
1) looking at Synthetic Aperture Laser Radar system straight is that s (t), s (t) can be expressed as follows through the target current complex signal of autodyne reception, signal is divided in time span M pulse signal s mt (), each pulse interval is T, and pulse width is T f,
2) measure or calculate the nonlinear factor a of system 1, a 2..., a k;
3) the pulse signal s of m=1 is got 1t () is carried out distance and is obtained frequency spectrum S to Fourier transform 1(f); Similarly, by m=2 ... the pulse signal s of M mt () is carried out distance respectively and is obtained frequency spectrum S to Fourier transform m(f), f is frequency,
4) addition of the quadratic sum of M spectral amplitude is obtained be chosen for S 2f 1/2 of () maximal value is reference threshold G;
5) generate scanning filter, sweep limit is whole frequency range, is spaced apart 1/T f.Centre frequency is f n, scanning strip bandpass filter is expressed as:
P(f)＝rect[T f(f-f n)]
6) by scanning filter and S 2f () is multiplied, acquired results, if be greater than threshold value G, records the centre frequency f of now wave filter n, there is point target;
7) if be less than threshold value G, whole spectrum scan has been judged whether, as unfinished, i.e. f nbe less than frequency spectrum maximal value, f n=f n+ 1/T f, return step 5);
8) if complete whole spectrum scan, pulse sequence number m is initialized as 1;
9) m frequency spectrum S is got m(f), target sequence number n is initialized as 1;
10) get the n-th target, frequency is f n, calculate f nrespective coordinates x n;
11) the n-th target respective filter P (f)=rect [T is calculated f(f-f n)];
12) step coefficient a is 2. utilized 1, a 2..., a k, step coordinate x 10. n, generate the error function e of the n-th target n(t):
Wherein, λ is optical maser wavelength, and β is rate of change, is system constants;
13) by step 9) S m(f) and step 11) wave filter P (f) be multiplied, product carries out inverse Fourier transform, obtains the time signal s of the n-th target m(t, n):
s m(t,n)＝FFT -1[S m(f)P(f)]；
14) by step 12) with step 13) result be multiplied, obtain the signal s of the n-th target after gamma correction mc(t, n)=s m(t, n) e n(t).
15) correct if do not complete all N number of point targets, get next point target, n=n+1, return step 10);
16) correct if complete all N number of point targets, by the point target results added after correction, obtain the pulse signal after correcting
17) if do not complete the signal correction of all M pulse, m=m+1, step 9 is returned);
18) if complete the signal correction of all M pulse, the signal after correcting is obtained according to the traditional algorithm of document [1], to correct after signal through distance to Fourier transform and azimuth match filtering, obtain two dimensional compaction image.
Technique effect of the present invention
In Orthoptic synthetic aperture laser imaging radar, echoed signal obtains electric current complex signal s (t) after autodyne detection.
The nonlinearities change of system bits phase is the reason producing Range Imaging pulse strenching, and to scan the system of the phase change of generation, sweep velocity is expressed as Taylor series:
Above formula each rank coefficient a konly relevant with system inherent characteristic, can measure or calculate.A 0for ideal velocity, a 1, a 2..., a kfor nonlinear factor, introduce the nonlinearities change of speed.
Putative signal is divided into M distance to pulse, gamma correction is to the signal s in the burst length for the distance of some m mt () carries out, therefore the electric current complex signal expression formula of N number of point target is abbreviated as:
Wherein, N is number of targets, x nbe the n-th target range to coordinate, s m(t, n) is the electric current complex signal of the n-th point target, A n(x n, t) be the current signal amplitude of the n-th point target, Φ n(x n, t) be the n-th point target phase place, Φ n(x n, 0) and be the n-th target initial phase, β is system constants.First exponential term is linear function ideally, and second exponential term is nonlinearity erron, coefficient a k(k>=1) is nonlinear factor.3rd exponential term is used for azimuth match filtering.
The electric current complex signal of target is carried out Fourier transform (FFT) for frequency spectrum S m(f):
Frequency location corresponding to the n-th target is with distance x nbe directly proportional. represent convolution, the Section 2 of convolution represents shape and the width of frequency spectrum.
The frequency choosing the n-th point target is bandpass filter centre frequency, and wave filter is:
P(f)＝rect[T f(f-f n)]
By bandpass filter and distance frequency spectrum S mf () is multiplied, then carry out inverse Fourier transform, can obtain the time-domain signal s of the n-th target m(t, n):
s m(t,n)＝FFT -1[S m(f)P(f)]
According to the distance of nonlinear factor and the n-th impact point to Coordinate generation error correction function, choose k level and be similar to:
Time-domain signal s m(t, n) and error correction signal e m(t, n) is multiplied, and obtains signal s after the correction of the n-th impact point nrt () is as follows:
The gamma correction signal of all N number of point targets is calculated one by one, is then added, obtain the electric current complex signal of m range pulse after gamma correction
Above-mentioned correction is completed to all M range pulse, obtains total correction signal:
Signal after correction is pressed to the imaging algorithm in document [1], namely distance is to Fourier transform, azimuth match filtering, obtains two dimensional compaction image.
Accompanying drawing explanation
Fig. 1 is method block diagram of the present invention.
Embodiment
For the Orthoptic synthetic aperture laser imaging radar system of document [2], this system acceptance to marking current be s (t).Time span is divided into M pulse, recurrent interval 1s, pulse width is T f, M=100, T f=0.3s.The beam orthogonal plane of system of distance 1.2km is objective plane, and far field toes are 2m, and spot center is that distance is to true origin, radar motion direction be orientation to, vertical radar direction of motion be distance to, reflection spot number of targets N=2, distance is respectively-1m and 0.5m to coordinate.
The nonlinear bearing calibration of Orthoptic synthetic aperture laser imaging radar of the present invention, see Fig. 1, the method data handling procedure is as follows:
1. s (t) can be expressed as follows, and signal is divided in time span 100 pulse signal s mt (), each pulse interval is 1s, and pulse width is 0.3s.
2. the source of nonlinearity erron is sweep velocity nonlinearities change in time, records speed change curves (being similar to the 3rd), obtains nonlinear factor a 1=-0.05, a 2=0.02, therefore speed is expressed as:
v(t)≈a 0+a 1t+a 2t 2＝0.01-0.05t+0.02t 2
3. the pulse signal s of m=1 is got 1t () is carried out distance and is obtained frequency spectrum S to Fourier transform 1f (), by the pulse signal s of m=2 ~ 100 mt () is carried out distance respectively and is obtained S to Fourier transform mf (), f is frequency.
4. the quadratic sum of 100 spectral amplitudes is added and obtains be chosen for S 2f 1/2 of () maximal value is reference threshold G.
5. generate scanning filter, sweep limit is whole frequency spectrum, is spaced apart 3.3Hz.Centre frequency is f n, scanning strip bandpass filter is expressed as:
P(f)＝rect[T f(f-f n)]
6. by scanning filter and S 2f () is multiplied, acquired results, if be greater than threshold value G, records the centre frequency f of now wave filter n, there is point target.
If be 7. less than threshold value G, judge whether whole spectrum scan, as unfinished, i.e. f nbe less than frequency spectrum maximal value, f n=f n+ 3.3, return step 5..
If 8. complete whole spectrum scan, the centre frequency higher than threshold value is point target, has two, f 1=-626Hz, f 2=313Hz.
Pulse sequence number m is initialized as 1.
9. m frequency spectrum S is got mf (), target sequence number n is initialized as 1.
10. get the n-th target, frequency is f n, calculate f nrespective coordinates x n.
Distance corresponding is respectively x to coordinate 1, x 2, can by centre frequency f 1, f 2be calculated as follows:
Wherein systematic parameter D, λ, f xcalculated by document [2].Laser wavelength lambda=532nm, magnification D=1000,
Cylindrical mirror focal distance f x=60mm.
calculate the n-th target respective filter P (f)=rect [3.3 (f-f n)].
utilize step coefficient a 2. 1, a 2, step coordinate x 10. n, generate the error function e of the n-th target n(t):
e n(t)＝exp[-6.3×10 4jx n(-0.025t 2+0.007t 3)]
Wherein λ=532nm is optical maser wavelength, and system constants β is calculated by document [2] systematic parameter, magnification
D=1000, cylindrical mirror focal distance f x=60mm,
step S 9. m(f) and step wave filter P (f) be multiplied, product carries out inverse Fourier transform, obtains the time signal s of the n-th target m(t, n).
s m(t,n)＝FFT -1[S m(f)P(f)]
by step with step result be multiplied, obtain the signal s of the n-th target after gamma correction mc(t, n)=s m(t, n) e n(t).
if do not complete all N number of point targets to correct, get next point target, n=n+1, return step 10..
correct if complete all 2 point targets, by the point target results added after correction, obtain the pulse signal after correcting
if do not complete the signal correction of all 100 pulses, m=m+1, return step 9..
if complete the signal correction of all 100 pulses, obtain the signal after correcting according to the traditional algorithm of document [1], obtain two dimensional compaction image.

Claims (1)

1. an Orthoptic synthetic aperture laser imaging radar non-linear correction method, is characterized in that the method comprises the following steps:
1) looking at Synthetic Aperture Laser Radar system straight is that s (t), s (t) can be expressed as follows through the target current complex signal of autodyne reception, signal is divided in time M pulse signal s mt (), each pulse interval is T, and pulse width is T f,
s ( t ) = Σ m = 1 M s m ( t ) = Σ m = 1 M rect ( t - mT T f ) s ( t ) ;
2) measure or calculate the nonlinear factor a of system 1, a 2..., a k;
3) the pulse signal s of m=1 is got 1t () is carried out distance and is obtained frequency spectrum S to Fourier transform 1(f); Similarly, by m=2,3 ..., the pulse signal s of M mt (), carries out distance respectively and obtains frequency spectrum S to Fourier transform m(f):
S m ( f ) = ∫ t = 0 T f s m ( t ) exp ( - 2 πft ) dt ,
Wherein, f is frequency;
4) addition of the quadratic sum of M spectral amplitude is obtained be chosen for S 2f 1/2 of () maximal value is reference threshold G;
5) generate scanning filter, sweep limit is whole frequency range, is spaced apart 1/T f, centre frequency is f n, scanning strip bandpass filter is expressed as:
P(f)＝rect[T f(f-f n)]
6) by scanning filter and S 2f () is multiplied, acquired results, if be greater than threshold value G, records the centre frequency f of now wave filter n, there is point target;
7) if be less than threshold value G, whole spectrum scan has been judged whether, as unfinished, i.e. f nbe less than frequency spectrum maximal value, make f n=f n+ 1/T f, return step 5);
8) if complete whole spectrum scan, pulse sequence number m is initialized as 1;
9) m frequency spectrum S is got m(f), target sequence number n is initialized as 1;
10) get the n-th target, frequency is f n, calculate f nrespective coordinates x n;
11) the n-th target respective filter P (f)=rect [T is calculated f(f-f n)];
12) utilize step 2) coefficient a 1, a 2..., a k, step 10) coordinate x n, generate the error function e of the n-th target n(t):
e n ( t ) = exp [ - j 2 π λ βx n ( a 1 2 t 2 + a 2 3 t 3 + . . . a k k + 1 t k ) ]
Wherein, λ is optical maser wavelength, and β is rate of change, is system constants;
13) by step S 9. m(f) and step wave filter P (f) be multiplied, inverse Fourier transform is carried out to product, obtains the time signal s of the n-th target m(t, n):
s m(t,n)＝FFT -1[S m(f)P(f)]
14) by step 12) with step 13) result be multiplied, obtain the signal of the n-th target after gamma correction: s mc(t, n)=s m(t, n) e n(t);
15) correct if do not complete all N number of point targets, get next point target, make n=n+1, return step 10);
16) correct if complete all N number of point targets, by the point target results added after correction, obtain the pulse signal after correcting s m c ( t ) = Σ n = 1 N s mc ( t , n ) ;
17) if do not complete the signal correction of all M pulse, make m=m+1, return step 9);
18) if complete the signal correction of all M pulse, the signal after correcting is obtained to correct after signal through distance to Fourier transform and azimuth match filtering, obtain two dimensional compaction image.
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