CN105403886A - Automatic extraction method for airborne SAR scaler image position - Google Patents

Abstract

Disclosed in the invention is an automatic extraction method for an airborne SAR scaler image position. With the method, a scaler can be searched automatically in an SAR image rapidly and accurately; and the positioning precision is high. Position information of a scaler and an SAR main antenna as well as an SAR image processing Doppler central frequency is extracted; longitude and latitude information of the scaler and the SAR main antenna are projected to the same plane, and the height information is not changed; a shortest distance between the position of the scaler and the movement locus of the main antenna is calculated and a nearest slant range and a zero Doppler time are obtained; a slant range of the scaler at a wavebeam center passing-through time as well as an azimuth position of the main antenna at the wavebeam center passing-through time is determined based on strabismus geometry; the slant range and the azimuth positio are used as an initial distance position and an initial azimuth positio of the scaler in the SAR image; and then a searching window is taken by using the initial position as a center and the position of a pixel point with the highest intensity in the searching window is the position of the scaler in the SAR image.

Description

A kind of carried SAR scaler picture position extraction method

Technical field

The present invention relates to the radar exploration technique field, be specifically related to a kind of carried SAR scaler picture position extraction method.

Background technology

Airborne synthetic aperture radar (SAR) is a kind of active microwave imaging sensor, it utilizes the relative motion between radar and target that larger antenna eliminator aperture is synthesized by the method for data processing in real antenna aperture less for size, thus realizes high-resolution imaging.SAR can round-the-clock, round-the-clock detection and positioning is carried out to target, little by weather effect, be therefore widely used in the field such as military surveillance, topographic mapping.

Scaler is a kind of instrument used in SAR calibration, is divided into passive scaler and source scaler.Passive scaler is the metal device with strong retroreflection coefficient and certain geometrical shape, and source scaler is then the calibration device that can initiatively transmit, and scaler plays important role in the application of SAR.When calibrating, we need to obtain the accurate location of scaler in SAR image.At present, being all the method by manually searching mostly, determining the position of scaler in SAR image by the bright spot display of eye-observation scaler in SAR image.There is some deficiency following in the method for artificial Search and Orientation:

(1) artificial lookup method workload is large, and speed is slow, and expend time in length.Especially the situation comparatively large for scene, scaling point is more, manually searching scaling point will expend more time and efforts, have larger difficulty, lack practicality, be difficult to be applied in Larger Engineering Projects.

(2) manually may there is Wrong localization in the method for searching.Certain distortion is there is in SAR image (before not carrying out geocoding) relative to real scene, certain difficulty is brought to manually searching scaler, when especially there is the interference of other strong point scatterer around scaler, easily think non-scaler point by mistake scaler, thus the lookup result led to errors.

(3) easily there is seek error in method of manually searching.In image, the brightness of scaler center and surrounding is all brighter, is directly with the naked eye difficult to tell the brightest pixel center, therefore may cause seek error (Pixel-level error).

Summary of the invention

In view of this, the invention provides a kind of carried SAR scaler picture position extraction method, in SAR image, automatically can find scaler quickly and accurately, and setting accuracy is high.

Carried SAR scaler picture position of the present invention extraction method, comprises the steps:

Step 1, extract longitude and latitude, the elevation information of scaler, in SAR image, SAR main antenna is at longitude and latitude, the elevation information in each orientation moment, and the process doppler centroid of SAR image;

Step 2, be projected in the Oxy plane of the same coordinate system Oxyz by the latitude and longitude information of scaler and SAR main antenna, coordinate axis z is the height coordinate of scaler and main antenna;

Step 3, in coordinate system Oxyz, calculate the minimum distance of scaler position to main antenna movement locus, this minimum distance is nearest oblique distance, and the orientation moment of the main antenna position that this minimum distance is corresponding was for zero Doppler's moment;

Step 4, if SAR image is imaged on zero Doppler, then obtains the main antenna position of orientation in zero Doppler's moment and nearest oblique distance according to step 3, finds corresponding row, column number in SAR image, as the initial position of scaler in SAR image, performs step 5; If SAR image is imaged on Doppler center, the process doppler centroid of SAR image is then first utilized to calculate the angle of squint of radar beam, the nearest oblique distance obtained according to angle of squint and step 3 again calculates scaler passes through moment oblique distance at beam center, and beam center passes through the position of orientation of moment main antenna; Pass through the position of orientation of moment main antenna and the oblique square in this moment according to beam center, in SAR image, find out corresponding row, column number, as the initial position of scaler in SAR image, perform step 5;

Step 5, centered by the initial position of scaler in SAR image that step 4 obtains, get a search window, the pixel that in search window, intensity is maximum, then this position is the position of scaler in SAR image.

Further, in described step 3, scaler position to the computing method of the minimum distance of main antenna movement locus is: first calculate the distance between each orientation moment main antenna position and scaler position, then gets distance minimum value as the minimum distance of scaler position to the movement locus in main antenna each orientation moment.

Further, also comprise step 6, namely, the position coordinates of scaler step 5 obtained in SAR image is as center, get a window, a liter sampling is carried out to image in window, rises the position of pixel in SAR image that in sampling rear hatch, intensity is maximum and be the final position of scaler in SAR image.

Further, in described step 2, Gauss projection method is adopted the latitude and longitude information of scaler and SAR main antenna to be projected in the Oxy plane of coordinate system Oxyz.

Further, when scaler has time delay, correct the initial distance position of scaler that step 4 obtains, the initial distance position obtained by step 4 adds scaler time delay distance, and the distance and position of acquisition is the initial distance position after correction.

Beneficial effect:

The method can extract the position of scaler in SAR image automatically when not needing manually to check image, has the following advantages with existing artificial lookup method:

(1) achieve the robotization of searching, no longer need manually to check SAR image, save a large amount of manpowers and energy, accelerate seek rate, have a good application prospect in Practical Project.

(2) result is extracted accurately and reliably in scaler position, avoids the interpretation mistake and error that may occur when manually searching.

(3) the method can be applicable to multiple calibration mode, the method all can be adopted to search the accurate location of scaling point in radiation calibration and interference calibration.

Accompanying drawing explanation

Fig. 1 is carried SAR scaler picture position of the present invention extraction method process flow diagram.

Fig. 2 is initial coordinate and the actual coordinate schematic diagram of scaler scene graph and calculating.

Fig. 3 is that scaler is thought a little to rise the rear schematic diagram of sampling.

Fig. 4 is that scaler picture point rises the rear distance and bearing tangential section schematic diagram of sampling.

Embodiment

To develop simultaneously embodiment below in conjunction with accompanying drawing, describe the present invention.

The invention provides a kind of carried SAR scaler picture position extraction method, as shown in Figure 1, specific implementation step is as follows for its process flow diagram:

The positional information (latitude, longitude and elevation information) of step one, reading scaler, read the SAR main antenna positional information (latitude, longitude and elevation information) in each orientation moment corresponding to SAR image, and the parameter such as the process Doppler center of SAR image.

In actual applications, the positional information of scaler all provides with latitude, longitude and elevation information, simultaneously, SAR image also can provide POS data corresponding to scene image (SAR image), POS data have recorded the positional information of scene image each orientation moment main antenna, the i.e. latitude of main antenna, longitude and elevation information, and the process Doppler center that this SAR image is used in imaging process.First these data are read.

Step 2, the latitude and longitude coordinates of the latitude and longitude coordinates of scaler and SAR main antenna is projected in the plane Oxy of the same coordinate system Oxyz, coordinate axis z is the height coordinate axle of scaler and main antenna, and scaler is identical with the height value that step one obtains with the height value of main antenna.

Wherein, choose concrete coordinate system Oxyz according to projecting method, only need ensure that the latitude and longitude coordinates of scaler and the latitude and longitude coordinates of SAR main antenna project to same plane.The present invention adopts Gauss projection method, under scaler position and main antenna position are projected to Gauss plane coordinate system.Wherein, Gauss projection computing formula is as follows:

$x=X+\frac{N}{2{\mathrm{\ρ}}^{\′\′2}}\sin B\cos B\·l^{\′\′2}+\frac{N}{24{\mathrm{\ρ}}^{\′\′4}}\sin B{\cos }^{3}B(5-t^2+9{\mathrm{\η}}^2+4{\mathrm{\η}}^4)l^{\′\′4}$

$+\frac{N}{720{\mathrm{\ρ}}^{\′\′6}}\sin B{\cos }^{5}B(61-58t^2+t^4)l^{\′\′6}$

$y=\frac{N}{{\mathrm{\ρ}}^{\′\′}}\cos B\·l^{\′\′}+\frac{N}{6{\mathrm{\ρ}}^{\′\′3}}{\cos }^{3}B(1-t^2+{\mathrm{\η}}^2)l^{\′\′3}$

$+\frac{N}{120{\mathrm{\ρ}}^{\′\′5}}{\cos }^{5}B(5-18t^2+t^4+14{\mathrm{\η}}^2-58{\mathrm{\η}}^2{t}^2)l^{\′\′5}$

In above formula, B, l are respectively latitude and the longitude relative to central meridian.The Coordinate Transformation Based on Longitude-Latitude of the main antenna that the latitude and longitude coordinates of scaler and POS can be recorded by above formula is in same plane coordinate system.After projection, z-axis is height coordinate, and makes the height coordinate of scaler and main antenna be former height, remains unchanged.

Step 3, in coordinate system Oxyz, calculate scaler to the minimum distance of main antenna movement locus, this minimum distance is nearest oblique distance, and the orientation moment of the main antenna position that this minimum distance is corresponding was for zero Doppler's moment.

The main antenna position data of POS record is straight line (after motion-compensated), and calculating scaler is nearest oblique distance to the distance of this straight line.In practice, when image orientation is higher to resolution, the distance between each orientation moment main antenna position and scaling point can be calculated, and get minimum value as minimum distance, and the main antenna orientation moment that minimum distance is corresponding, be for zero Doppler's moment.

Step 4, utilization process doppler centroid calculate the angle of squint of radar beam, the nearest oblique distance obtained according to angle of squint and step 3 again calculates scaler passes through moment oblique distance at beam center, and beam center passes through the position of orientation of moment main antenna.

According to the relation processing Doppler center and angle of squint, can show that the computing formula of angle of squint β is:

$\mathrm{\β}=arcsin\frac{{\mathrm{\λf}}_{dc}}{2v}$

Wherein, λ is radar signal wavelength, f _dcfor process doppler centroid, v is Texas tower speed.

Then scaler is passed through moment oblique distance R at radar beam center and is:

R＝R ₀/cosβ

Wherein, R ₀for the nearest oblique distance that step 3 obtains;

Radar beam center is found to pass through moment oblique distance R at the range gate n of SAR image distance to correspondence _i(i.e. SAR image column number):

$n_i=round\left(\frac{R-r_0}{drange}\right)+1$

Wherein, r ₀for in SAR image the 1st row representative distance and position, drange be image distance to spacing, round () is bracket function.

Then main antenna passes through the position of orientation m in moment at beam center _ifor:

$m_i=m_0-round\left(\frac{R_{0}tan\mathrm{\β}}{dAzimuth}\right)$

Wherein, m ₀for main antenna is in the position of orientation that tiltedly square is corresponding recently, dAzimuth is that SAR image orientation is to spacing.

Step 5, the beam center finding step 4 to obtain in SAR image pass through the main antenna position of orientation m in moment _ithe oblique distance place range gate n in moment is passed through with beam center _i, by (m _i, n _i) as the initial position of scaler in SAR image, and with (m _i, n _i) centered by get the search window (search box size can adjust according to actual conditions) of 16*16 size, the pixel that in search window, intensity is maximum.The position of the pixel that intensity is maximum is scaler position.

Step 6, in order to obtain scaler position more accurately, using the scaler position that obtains in step 5 as center, the window getting 5*5 size in SAR image carries out 10 times and rises sampling, find out the pixel that the rear intensity of liter sampling is maximum, and calculate its position in SAR image, this position is the position of scaler, can obtain the sub-pixel positioning precision of scaler thus.

Since then, just complete carried SAR scaler picture position automatically to extract.

Above-mentioned scaler localization method is that supposition SAR image geometry is consistent with data acquisition geometry, and namely image becomes at Doppler center, and now the moment is passed through to being positioned at beam center in target azimuth, and distance passes through moment oblique distance to being positioned at beam center.When image is imaged on zero Doppler, directly using the column and row of the nearest oblique distance that calculates in step 3 and SAR image corresponding to zero Doppler's moment main antenna position of orientation number as the scaler initial position in step 5.Simultaneously the method for be passive scaler framing, for source scaler, if know the accurate time delay of source scaler, the method also may extend to source scaler location, now, need to correct the initial distance position of scaler in SAR image, namely need the time delay distance adding source scaler on the basis of initial distance position, the step after performing with the initial distance after correcting.

Provide an instantiation below.

The course line of carried SAR is from eastern west-bound operation, and course angle is-92.537 °, and flying height is 3410.7m.Determine that SAR scene graph orientation that scaler position adopts is to being 11296 pixels, the spacing between each pixel is 0.1787m; Distance is to being 9915 pixels, and the spacing between each pixel is 0.1874m; The distance range of SAR image is 3.5169km ~ 5.3744km.

For scaler 1, its geographic coordinate be 1 (34.83759 °, 109.53435 °, 342.897m), represent latitude, longitude and height in bracket respectively.Process doppler centroid is 348.957Hz.

Adopt Gauss projection computing formula, scaler 1 projects to the coordinate position after Oxyz plane for (3.8576e6,3.6593e5,342.897), under the main antenna position in simultaneously being recorded by POS also projects to this coordinate system.Then adopting step 3 to calculate scaler 1 to the minimum distance of main antenna track is R ₀=7429.0275m, position of orientation corresponding to zero Doppler's moment is m ₀=8477 (SAR image line numbers).The beam center of the scaler 1 then calculated according to step 4 passes through moment oblique distance distance and position n _i=6517, position of orientation m _i=6861.With (6861,6517) centered by, get the search window of 16*16 size, the position obtaining the pixel that intensity is maximum in search window is (6864,6517), this coordinate is compared with the initial coordinate calculated in step 4, and the two only differs several pixel distance, and this illustrates that the initial coordinate that the method calculates is comparatively accurately.Adopt step 6 to (6864,6517) window getting 5*5 size carry out 10 times rise sampling after, the final coordinate of scaler 1 is accurate to (6863.7,6516.9), so just obtain the exact position of this scaler in SAR image, achieve the method that scaler picture position is extracted automatically.

In sum, these are only preferred embodiment of the present invention, be not intended to limit protection scope of the present invention.Within the spirit and principles in the present invention all, any amendment done, equivalent replacement, improvement etc., all should be included within protection scope of the present invention.

Claims (5)

1. a carried SAR scaler picture position extraction method, is characterized in that, comprise the steps:

Step 1, extract longitude and latitude, the elevation information of scaler, in SAR image, SAR main antenna is at longitude and latitude, the elevation information in each orientation moment, and the process doppler centroid of SAR image;

Step 2, be projected in the Oxy plane of the same coordinate system Oxyz by the latitude and longitude information of scaler and SAR main antenna, coordinate axis z is the height coordinate of scaler and main antenna;

Step 3, in coordinate system Oxyz, calculate the minimum distance of scaler position to main antenna movement locus, this minimum distance is nearest oblique distance, and the orientation moment of the main antenna position that this minimum distance is corresponding was for zero Doppler's moment;

Step 4, if SAR image is imaged on zero Doppler, then obtains the main antenna position of orientation in zero Doppler's moment and nearest oblique distance according to step 3, finds corresponding row, column number in SAR image, as the initial position of scaler in SAR image, performs step 5; If SAR image is imaged on Doppler center, the process doppler centroid of SAR image is then first utilized to calculate the angle of squint of radar beam, the nearest oblique distance obtained according to angle of squint and step 3 again calculates scaler passes through moment oblique distance at beam center, and beam center passes through the position of orientation of moment main antenna; Pass through the position of orientation of moment main antenna and the oblique square in this moment according to beam center, in SAR image, find out corresponding row, column number, as the initial position of scaler in SAR image, perform step 5;

Step 5, centered by the initial position of scaler in SAR image that step 4 obtains, in SAR image, get a search window, the pixel position that in this search window, intensity is maximum is the position of scaler in SAR image.

2. carried SAR scaler picture position as claimed in claim 1 extraction method, it is characterized in that, in described step 3, scaler position to the computing method of the minimum distance of main antenna movement locus is: first calculate the distance between each orientation moment main antenna position and scaler position, then gets distance minimum value as the minimum distance of scaler position to the movement locus in main antenna each orientation moment.

3. carried SAR scaler picture position as claimed in claim 1 extraction method, it is characterized in that, also comprise step 6, namely, the position coordinates of scaler step 5 obtained in SAR image is as center, get a window, a liter sampling is carried out to image in window, rise the position of pixel in SAR image that in sampling rear hatch, intensity is maximum and be the final position of scaler in SAR image.

4. carried SAR scaler picture position as claimed in claim 1 extraction method, is characterized in that, in described step 2, adopts Gauss projection method the latitude and longitude information of scaler and SAR main antenna to be projected in the Oxy plane of coordinate system Oxyz.

5. the carried SAR scaler picture position extraction method as described in Claims 1 to 4 any one, it is characterized in that, when scaler has time delay, the initial distance position of the scaler that step 4 obtains is corrected, the initial distance position obtained by step 4 adds scaler time delay distance, and the distance and position of acquisition is the initial distance position after correction.