CN102288962B - Real-time motion compensation method for ultra-wide band synthetic aperture radar (UWB SAR) - Google Patents

Abstract

The invention provides a real-time motion compensation method for an ultra-wide band synthetic aperture radar (UWB SAR), which is characterized in that motion parameters output by a single-point global positioning system (GPS) are used for carrying out real-time motion compensation of the UWB SAR, the flight time is segmented according to Ta, when the current moment is a starting point of an imaging synthetic aperture, an imaging coordinate system is built, and if not, the next steps are carried out: step 1, eliminating the single-point GPS position jumping errors; step 2, calculating the transverse motion errors according to the single-point GPS positioning information; step 3, calculating the heading speed errors; and step 4, calculating the sight line direction motion errors and compensating the motion errors in the airborne UWB SAR sight line direction. In the method of the invention, the information processing is only carried out on the output data of a navigation system, the motion parameter evaluation does not need to be carried out, the storage space and the calculation quantity are saved, and the large-area imaging compensation precision is high. In addition, the cost is low, and the realization is easy.

Description

A kind of Research on UWB SAR real-time motion compensation method

Technical field

The invention belongs to UWB (Ultra Wide Band, ultra broadband) SAR (Synthetic ApertureRadar, synthetic-aperture radar) signal processing technology field, relate to a kind of UWB SAR real-time motion compensation method, particularly use GPS (Globe Positioning System, GPS) output movement parameter to carry out the method for UWB SAR real-time motion compensation.

Background technology

UWB SAR refers to that radar system signal relative bandwidth is greater than 25% SAR, equal the ratio of transmitted signal bandwidth and radar system centre frequency due to relative bandwidth, UWB SAR is usually located at the UHF/VHF wave band, belong to low-frequency range SAR, UWB SAR possesses the leafage of penetrating and Subsurface imaging ability simultaneously, is one of the study hotspot in current SAR field.UWB SAR obtains the orientation high resolving power needs the large angle that accumulates, be installed on the UWB SAR on airborne platform, length of synthetic aperture corresponding to large accumulation angle can reach several kilometers, within an aperture time, airborne platform departs from the desirable at the uniform velocity relative motion error of straight line image quality is had a great impact, and must carry out motion compensation.

The kinematic error of airborne platform is divided into attitude error and translation error.The compensation of attitude error can realize by the sensing (being referred to as direction of visual lines in the present invention) that the attitude information that receives navigational system output is adjusted the antenna beam center in real time, no longer discusses in the present invention.Translation error can cause UWB SAR echo data in the orientation to distance to envelope migration and phase error occur, cause defocusing and geometric distortion of image.Only can't eliminate translation error by adjusting direction of visual lines.Because Airborne UWB SAR requires to possess the real time imagery ability, therefore must carry out the real-time motion compensation of translation error in imaging process.

Traditionally, can be divided on approach from realizing for the real-time motion compensation technology of translation error: based on the real-time motion compensation of radar return data, the real-time motion compensation of based on motion measured value of parameters.

Can estimate from original echo or coarse resolution image data based on the real-time motion compensation method of radar return data that by autofocus algorithm obtaining kinematic parameter carries out error compensation.Because UWB SAR single aperture data volume is very large, adopt this method to estimate that kinematic parameter required memory space and operand are all very large, and most of autofocus algorithm can only be estimated the kinematic error of low frequency; In addition, carry out based on the local strong point target in scene because autofocus algorithm is many, estimate that the kinematic parameter that obtains is difficult to fit like a glove with the actual motion of SAR airborne platform, during the large tracts of land imaging, the motion compensation effect is not good.

The real-time motion compensation method of based on motion measured value of parameters is by carrying out error compensation with the navigational system output movement parameter of installing on airborne platform.The long-term bearing accuracy that conventional I NS (Inertial Navigation System, inertial navigation system) provides is drifted about in time, and the bearing accuracy in short-term of GPS is not high, can not export attitude information.Integrated navigation technology combines this two kinds of independent sources of information with complementary characteristic, utilizes INS to improve the precision in short-term of data, utilizes GPS to guarantee the long-time stability of data, thereby is applied in recent years SAR motion compensation field.It is reported, at present, the UWB SAR system of external development adopts the real-time motion compensation method of based on motion measured value of parameters more, mainly depend on the precision of kinematic parameter due to compensation effect, need to additionally install expensive high precision navigational system additional on airborne platform, as high precision INS/ differential GPS, bearing accuracy reaches 0.1 meter, rate accuracy is higher than 0.03 meter per second, and this method successful Application in U.S. Lynx SAR system, has obtained high-resolution UWB SAR image.

High precision INS is expensive, and same the existence located drift for a long time; Differential GPS operating distance is limited, easily is disturbed.In China, the INS/GPS integrated navigation system bearing accuracy that is equipped with on airborne platform is generally on the low side, and the drift of INS output valve in 100 seconds reaches tens of rice, and the GPS output data rate mostly is 1-2Hz and location saltus step error.Concerning UWB SAR, owing to realizing that high-resolution imaging required aperture reaches several kilometers, the impact of above-mentioned measuring error is large, carries out the poor effect of UWB SAR real-time motion compensation based on airborne INS/GPS integrated navigation system.

Just because of there are the problems referred to above, the method of UWB SAR real-time motion compensation is carried out in utilization based on single-point GPS output movement parameter, after being processed, single-point GPS kinematic parameter uses it in the real-time motion compensation of UWB SAR, can make up the deficiency of original airborne INS/GPS integrated navigation system, improve the quality of UWBSAR imaging.This method has no document at present concrete report.

Summary of the invention

Goal of the invention: in order to make up the deficiency of existing UWB SAR real-time motion compensation method, the invention provides a kind of UWB SAR real-time motion compensation method based on single-point GPS.This method has made up the low defective of the original integrated navigation system bearing accuracy of airborne platform, and has advantages of that operand is little, is suitable for the real-time motion compensation of Airborne UWB SAR.Compare with the real-time motion compensation method based on the radar return data, only need the output data of navigational system are carried out information processing, need not to carry out kinematic parameter and estimate, greatly saved storage space and operand, large tracts of land imaging compensating precision is higher.With compare based on the real-time motion compensation method of high precision INS/ differential GPS, this method only need install a single-point GPS additional, effectively improved the positioning precision of navigational system by data processing method, and cost is lower, is easy to realize.

Technical scheme of the present invention is: a kind of Research on UWB SAR real-time motion compensation method, it is characterized in that, and the real-time motion compensation that the kinematic parameter that uses single-point GPS to export carries out UWB SAR imaging comprises:

If the Data Update cycle T of single-point GPS _GPS, comprise at the kinematic parameter of moment t output: the longitude λ (t) of airborne platform, latitude With height h (t); And the speed of airborne platform: the east orientation speed V in day coordinate system northeastward _e(t), north orientation speed V _n(t) with day to speed V _z(t).If airborne platform is the acceleration threshold value γ in day coordinate system=[γ northeastward _E, γ _N, γ _Z].If Polaroid aperture time of the T of UWB SAR _a

In UWB SAR imaging process, will the flight time press T _aSegmentation is equivalent to flight path according to Polaroid aperture length segmentation.

Within any one period flight time, to any time t ∈ [n ₀T _a, (n ₀+ 1) T _a), n ₀＞1 and n ₀Be integer, carry out following treatment step:

During the first situation, if t=n ₀T _a, set up imaging coordinate system, comprise the steps:

Utilize following formula to calculate Desired Track angle β corresponding to current time t _refWith desirable speed of a ship or plane V _ref:

$\left\{\begin{array}{c}{\mathrm{\β}}_{\mathrm{ref}}=\frac{1}{T}{\∫}_{t-T}^t\mathrm{\β}\left(t^{\′}\right){\mathrm{dt}}^{\′}\\ V_{\mathrm{ref}}=\frac{1}{T}{\∫}_{t-T}^{t}V\left(t^{\′}\right){\mathrm{dt}}^{\′}\end{array}\right.$

Wherein, β (t ') and V (t ') are respectively the instantaneous flight-path angles of constantly t ' airborne platform and along the instantaneous speed of a ship or plane of flight-path angle direction, and β (t ')=arctg[V _n(t ')/V _e(t ')],

T ∈ [T _a/ 3,2T _a], choose according to actual conditions, and airborne platform stability being better, the value of T is less.

With the initial time of this imaging of UWB SAR, i.e. n constantly ₀T _aThe time, the position at airborne platform place is initial point, with β _refBe oriented to X-axis, on surface level perpendicular to β _refDirection be Y-axis, day to set up the imaging coordinate system of Airborne UWB SAR for Z axis.

During the second situation, if t ≠ n ₀T _a, establish t=n ₀T _a+ t ₀, t ₀∈ (0, T _a), based on above-mentioned imaging coordinate system, Airborne UWB SAR is carried out motion compensation, specifically comprise the steps:

The first step is eliminated single-point GPS position hopping error.

What most GPS in the market adopted is the localization method of smoothing the phase of carrier wave pseudorange, since in May, 2000 U.S. government stop at add interference in satellite clock after, the bearing accuracy of single-point GPS has had large increase.But, being subjected to the impact of the factors such as GPS relative positioning, radio wave propagation error and calculation method, the locating information in adjacent two moment phenomenon of can undergoing mutation, be called the position hopping error sometimes.

Utilize following formula to calculate the current time t velocity equivalent of airborne platform in day coordinate system northeastward:

Wherein,

For at moment n ₀T _aThe latitude of airborne platform, a are average equatorial radius, and e is eccentricity of the earth.

Utilize following formula to calculate the velocity equivalent difference:

ΔV(t)＝[V _E(t)-V _E(t-T _GPS)，V _N(t)-V _N(t-T _GPS)，V _Z(t)-V _z(t-T _GPS)]

Utilize following formula to judge and whether occur the position hopping error in locating information:

Δ V (t)＞γ T _GPS(formula one)

If formula one is set up, the position hopping error appears, revise according to the following formula the locating information of current time t single-point GPS:

With the high value of longitude and latitude of revising

Replace t measured value constantly Namely obtain removing the locating information after the position hopping error.

If formula one is false, the position hopping error does not appear, need not to revise.

Second step calculates the translation motion error according to single-point GPS locating information.

Utilize following formula, will go the single-point GPS locating information of current time t after saltus step, be transformed into the solid rectangular coordinate system of ground heart from the WGS-84 coordinate system:

Then, utilize following formula that locating information is transformed into sky, northeast coordinate system from the solid rectangular coordinate system of ground heart:

Wherein, (x _e0, y _e0, z _e0) be moment n ₀T _aThe time UWB SAR the imaging coordinate system initial point coordinate in day coordinate system northeastward.

Utilize following formula that locating information is transformed into imaging coordinate system from sky, northeast coordinate system again:

$\left[\begin{array}{c}{x}_i\left(t\right)\\ y_i\left(t\right)\\ z_i\left(t\right)\end{array}\right]=\left[\begin{array}{ccc}{\cos \mathrm{\β}}_{\mathrm{ref}}& -\sin {\mathrm{\β}}_{\mathrm{ref}}& 0\\ \sin {\mathrm{\β}}_{\mathrm{ref}}& \cos {\mathrm{\β}}_{\mathrm{ref}}& 0\\ 0& 0& 1\end{array}\right]\left[\begin{array}{c}{x}_g\left(t\right)\\ y_g\left(t\right)\\ z_g\left(t\right)\end{array}\right]$

The translation motion error of utilizing following formula to try to achieve three change in coordinate axis direction in imaging coordinate system is respectively:

$\left\{\begin{array}{c}\mathrm{\Δx}\left(t\right)=x_i\left(t\right)-V_{\mathrm{ref}}\·\left({t-n}_0{T}_a\right)\\ \mathrm{\Δy}\left(t\right)=y_i\left(t\right)\\ \mathrm{\Δz}\left(t\right)=z_i\left(t\right)\end{array}\right.$

In the 3rd step, calculate the course velocity error.

The purpose of this step is that to eliminate airborne platform inhomogeneous along the sampling interval that the velocity perturbation of Desired Track direction causes.

Utilize following formula to calculate current time t airborne platform along the speed of a ship or plane V of Desired Track direction _a(t):

V _a(t)＝V(t)·cos[β(t)-β _ref]

Then, calculate V _a(t) and V _refBetween relative error V _err(t):

V _err(t)＝V _a(t)-V _ref

Adjust the pulse recurrence interval (Pulse RepeatInterval is called for short PRI) of UWB SAR according to relative error Verr (t), guarantee that UWB SAR can realize along the uniform sampling of Desired Track direction.

In the 4th step, calculate the direction of visual lines kinematic error.

The direction of visual lines kinematic error can be decomposed into the empty constant error on line of sight of distance and and apart from the space-variant error on line of sight, be located at that in the imaging region of current time t, the nearest oblique distance of certain point and Desired Track is R _B, the direction of visual lines kinematic error of calculating according to the following formula this some correspondence is:

Δr(R _B，t)＝Vx(t)cos[Ψ(R _B)]-Vz(t)sin[Ψ(R _B)]

In following formula, Ψ (R _B) be R _BCorresponding depression angle.

Calculate according to the following formula the empty constant error on line of sight Δ r (R of distance of UWB SAR _S, t):

Δr(R _S，t)＝Vx(t)cos[Ψ(R _S)]-Vz(t)sin[Ψ(R _S)]

In following formula, R _SFor the reference oblique distance of UWB SAR is arrived in current time t mapping with center line.

What calculate according to the following formula UWB SAR is Δ r apart from the space-variant error on line of sight _II(R _B, t):

Δr _II(R _B，t)＝Δr(R _B，t)-Δr(R _s，t)

Utilize the empty constant error on line of sight Δ r (R of distance _S, t) with apart from space-variant error on line of sight Δ r _II(R _B, t), the penalty function of generated error realizes that current time t is to the direction of visual lines kinematic error compensation of Airborne UWB SAR.

The invention has the beneficial effects as follows:

1, the technical scheme that adopts the present invention to propose need not high precision INS and differential GPS, only needs single-point GPS just can realize the real-time motion compensation of Airborne UWB SAR, and its operating distance is not subjected to the restriction of fixed receiver position, ground, forms simply, and cost is lower.

2, adopt the method for the first step in the present invention can eliminate position hopping error in the output locating information of single-point GPS, and the method calculated amount is very little, is fit to very much process in real time.

3, adopting the method that proposes in the present invention to set up imaging coordinate system and calculate the translation motion error, can reduce to resolve the fluctuation range of rear single-point GPS translation motion error, is mainly to reduce side-play amount, reduces the difficulty of UWB SAR motion compensation.

Description of drawings

Fig. 1 is based on the process flow diagram of the UWB SAR real-time motion compensation method of single-point GPS;

Fig. 2 is the imaging coordinate system schematic diagram of setting up in the present invention;

Fig. 3 is the experiment effect figure that eliminates single-point GPS position hopping error;

Fig. 4 is the experiment effect comparison diagram that calculates the translation motion error;

Fig. 5 is the UWB SAR imaging results of not carrying out motion compensation;

Fig. 6 is the UWB SAR imaging results that the method for employing the present invention proposition is carried out real-time motion compensation.

Embodiment

Below in conjunction with accompanying drawing, the specific embodiment of the invention is done further detailed explanation.

Fig. 1 is based on the process flow diagram of the UWB SAR real-time motion compensation method of single-point GPS.In the UWBSAR imaging process, will the flight time press T _aSegmentation if current time is the starting point of imaging synthetic aperture, is set up imaging coordinate system, if not, carry out the described step of Fig. 1: the first step, eliminate single-point GPS position hopping error; Second step calculates the translation motion error according to single-point GPS locating information.The 3rd step, calculate the course velocity error, the concrete grammar that the real-time PRF that adopts in the present invention adjusts is seen document: Li Jianyang etc., " the real-time PRF of UWB SAR adjusts ", modern radar, 2009, the 31 the 4th phases of volume, p34-p37; In the 4th step, calculate the direction of visual lines kinematic error.but by the empty constant error on line of sight of distance with generate the concrete grammar list of references A.Moreira andY.H.Huang of penalty function apart from the space-variant error on line of sight, " Airborne SAR Processing of Highly Squinted Data Using a ChirpScaling Approach with Integrated Motion Compensation ", IEEE Trans.onG.R.S.Vol.32 No.5, 1994, p1029-p1040 and document G.Fornaro, " Trajectory Deviationsin Airborne SAR:Analysis and Compensation ", IEEE Trans.on A.E.S.Vol.35No.3 July 1999, p997-p1007.

Fig. 2 is the imaging coordinate system schematic diagram of setting up in the present invention.The initial point of sky, northeast coordinate system and imaging coordinate system is all the imaging aperture starting point of UWB SAR, is designated as O, and the north orientation axle clamp angle of the X-axis of imaging coordinate system and sky, northeast coordinate system is β _ref, i.e. Desired Track angle, the Z axis of imaging coordinate system overlaps with the celestial axis of sky, northeast coordinate system, and the Y-axis of imaging coordinate system and XZ plane consist of right hand rectangular coordinate system.

Fig. 3 is the experiment effect figure that eliminates single-point GPS position hopping error.Adopt the position hopping error concealment step that proposes in the first step of the present invention to the result of single-point GPS test figure, in the measurement data figure of Fig. 3 top, thin solid line is the original displacement data that single-point GPS records, there is the saltus step error, the precise displacement data of thick dotted lines for adopting the differential GPS testing equipment to record, thin dotted lines eliminates for the 3rd step in employing the present invention the displacement data that obtains after the position hopping error; The error comparison diagram of front and back is processed in Fig. 3 below for the single-point gps measurement data, thin solid line is the displacement error of raw data relative difference GPS measured value, have obvious saltus step, thin dotted lines is the displacement error of relative difference GPS measured value after elimination saltus step error, and saltus step is eliminated substantially.Illustrate and utilize the first step of the present invention can eliminate saltus step error in single-point GPS displacement data.

Fig. 4 is the experiment effect comparison diagram that calculates the translation motion error.In flight experiment, after the method that adopts desired track angle to set up imaging coordinate system and employing the present invention proposition is set up imaging coordinate system, single-point GPS locating information is resolved respectively the fluctuation range of observing the translation motion error in two kinds of imaging coordinate systems.In figure, solid line is depicted as and is 162.1938 ° according to desired track angle and sets up imaging coordinate system, resolves the flight path curvilinear path fluctuating range that obtains large, and the flight path average reaches-163.9m; In figure, dotted line is depicted as and adopts 30 seconds average flight-path angles to resolve as 160.3983 °, the Desired Track angle of imaging the flight path curve that obtains, and the track fluctuating range reduces, and the flight path average is reduced to-29.7m.The imaging coordinate system that adopts the present invention to set up is described, the fluctuation range of resolving rear translation motion error reduces, and has reduced off-set value, can avoid introducing excessive space-variant error in the compensation of UWB SAR imaging moving.

Fig. 5 is the UWB SAR imaging results of not carrying out motion compensation, and due to the impact of kinematic error, image is very fuzzy, resolution that can't computed image; Fig. 6 is the UWB SAR imaging results after the method for employing the present invention proposition is carried out motion compensation, and the image focusing effect after compensation obviously improves, and image resolution ratio is 1.5m, proves that motion compensation process proposed by the invention is effective.

The above is only the preferred embodiment of the present invention; should be pointed out that for those skilled in the art, under the prerequisite that does not break away from the principle of the invention; can also make some improvements and modifications, these improvements and modifications also should be considered as protection scope of the present invention.

Claims (1)

1. a Research on UWB SAR real-time motion compensation method, is characterized in that, the real-time motion compensation that the kinematic parameter that uses the single-point global position system GPS to export carries out Research on UWB SAR UWB SAR imaging comprises:

If the Data Update cycle T of single-point GPS _GPS, comprise at the kinematic parameter of moment t output: the longitude λ (t) of airborne platform, latitude

With height h (t); And the speed of airborne platform: the east orientation speed V in day coordinate system northeastward _e(t), north orientation speed V _n(t) with day to speed V _z(t); If airborne platform is the acceleration threshold value γ in day coordinate system=[γ northeastward _E, γ _N, γ _Z]; If Polaroid aperture time of the T of UWB SAR _α

In UWB SAR imaging process, will the flight time press T _αSegmentation;

Within any one period flight time, to any time t ∈ [n ₀T _α, (n ₀+ 1) T _α], n ₀＞1 and n ₀Be integer, carry out following treatment step:

If t=n ₀T _α, set up imaging coordinate system, comprise the steps:

Utilize following formula to calculate Desired Track angle β corresponding to current time t _refWith desirable speed of a ship or plane V _ref:

Wherein, β (t ') and V (t ') are respectively the instantaneous flight-path angles of constantly t ' airborne platform and along the instantaneous speed of a ship or plane of flight-path angle direction, and β (t ')=arctg[V _n(t ')/V _e(t ')],

T ∈ [T _α/ 3,2T _α];

With moment n ₀T _αThe time, the position at airborne platform place is initial point, with β _refBe oriented to X-axis, on surface level perpendicular to β _refDirection be Y-axis, day to set up the imaging coordinate system of Airborne UWB SAR for Z axis;

If t ≠ n ₀T _α, establish t=n ₀T _α+ t ₀, t ₀∈ (0, T _α), based on above-mentioned imaging coordinate system, Airborne UWB SAR is carried out motion compensation, specifically comprise the steps:

The first step is eliminated single-point GPS position hopping error;

Utilize following formula to calculate the current time t velocity equivalent of airborne platform in day coordinate system northeastward:

Wherein,

For at moment n ₀T _αThe latitude of airborne platform, a are average equatorial radius, and e is eccentricity of the earth;

Utilize following formula to calculate the velocity equivalent difference:

ΔV(t)＝[V _E(t)-V _E(t-T _GPS),V _N(t)-V _N(t-T _GPS),V _Z(t)-V _Z(t-T _GPS)]

Utilize following formula to judge and whether occur the position hopping error in locating information:

Δ V (t)＞γ T _GPS(formula one)

If formula one is set up, the position hopping error appears, revise according to the following formula the locating information of current time t single-point GPS:

With the high value of longitude and latitude of revising

Replace t measured value constantly

Namely obtain removing the locating information after the position hopping error;

If formula one is false, the position hopping error does not appear, need not to revise;

Second step calculates the translation motion error according to single-point GPS locating information

Utilize following formula, will go the single-point GPS locating information of current time t after saltus step, be transformed into the solid rectangular coordinate system of ground heart from the WGS-84 coordinate system:

Then, utilize following formula that locating information is transformed into sky, northeast coordinate system from the solid rectangular coordinate system of ground heart:

Wherein, (x _e0, y _e0, z _e0) be moment n ₀T _αThe time UWB SAR the imaging coordinate system initial point coordinate in day coordinate system northeastward;

Utilize following formula that locating information is transformed into imaging coordinate system from sky, northeast coordinate system again:

The translation motion error of utilizing following formula to try to achieve three change in coordinate axis direction in imaging coordinate system is respectively:

In the 3rd step, calculate the course velocity error;

Utilize following formula to calculate current time t airborne platform along the speed of a ship or plane V of Desired Track direction _a(t):

V _a(t)＝V(t)·cos[β(t)-β _ref]

Then, calculate V _α(t) and V _refBetween relative error V _err(t):

V _err(t)＝V _a(t)-V _ref

According to relative error V _err(t) adjust the pulse recurrence interval PRI of UWB SAR, guarantee that UWB SAR can realize along the uniform sampling of Desired Track direction;

In the 4th step, calculate the direction of visual lines kinematic error;

Be located at that in the imaging region of current time t, the nearest oblique distance of certain point and Desired Track is R _B, the direction of visual lines kinematic error of calculating according to the following formula this some correspondence is:

Δ r (R _B, t)=Δ x (t) cos[Ψ (R _B)]-Δ z (t) sin[Ψ (R _B)] in following formula, Ψ (R _B) be R _BCorresponding depression angle;

Calculate according to the following formula the empty constant error on line of sight Δ r (R of distance of UWB SAR _s, t):

Δ r (R _S, t)=Δ x (t) cos[Ψ (R _S)]-Δ z (t) sin[Ψ (R _S)] in following formula, R _sFor the reference oblique distance of UWB SAR is arrived in current time t mapping with center line;

What calculate according to the following formula UWB SAR is Δ r apart from the space-variant error on line of sight _II.(R _B, t):

Δr _II.(R _B,t)＝Δr(R _B,t)-Δr(R _s,t)

Utilize the empty constant error on line of sight Δ r (R of distance _s, t) with apart from space-variant error on line of sight Δ r _II(R _B, t), the penalty function of generated error realizes that current time t is to the direction of visual lines kinematic error compensation of Airborne UWB SAR.

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