KR100441590B1 - Method of generating DEM for Topography Measurement using InSAR - Google Patents

Abstract

PURPOSE: A method for forming a digital elevation model to measure geometric characteristics of an interferometric synthetic aperture radar(InSAR) is provided to obtain pass difference only due to topographic fringe on the ground of pass difference due to satellite parameters. CONSTITUTION: According to the method, a pass difference from a satellite orbit to a target on the earth is calculated using a repeat pass satellite. A pass difference due to satellite parameters becomes quantitative. A pass difference is calculated using topographic fringe after removing the pass error due to the satellite fringe on the pass difference. An interferometric phase is detected from the pass difference due to topographic fringe. And topographic elevation is obtained from the interferometric phase. Then, the elevation value is processed through multi-look processing or space filtering.

Description

Method of generating digital altitude model for measuring topographical altitude by using geometric characteristics of interferometric synthetic aperture radar {Method of generating DEM for Topography Measurement using InSAR}

The present invention relates to a method for measuring a digital elevation model (hereinafter referred to as 'DEM'), and more specifically, to calculate a path difference using only terrain parameters by quantifying a path error by a satellite parameter and removing it from the path difference. In accordance with the present invention, the present invention relates to a method for generating a digital elevation model for measuring topographical altitude by using geometric characteristics of a synthetic aperture radar for interferometry, which can increase the accuracy of DEM generation.

In general, the method of generating a DEM includes i) a method generated from a contour layer of a digital map, ii) a method using a stereo satellite image or an aerial photograph, and iii) a method using a GIS application such as a wireless network design system. .

Therefore, a DEM may be generated so that the numerical data includes the height information based on the numerical map, and a method using the ultra wideband VHF SAR (Synthetic Aperture Radar) data. The DEM may be generated by using SAR interferometry, such as using a corona satellite image.

An interferometric synthetic aperture radar (hereinafter referred to as InSAR) is a synthetic aperture radar that has two antennas spaced apart from each other and obtains a topographic altitude from the phase difference of an interference signal received by these antennas.

InSAR detects information about the earth's surface using the phase difference between two complex radar signals. Examples of applications using InSAR technology include DEM generation, Geophysical Hazard Analysis, Glacier Velocity Measurement, and Land Use Classification.

The applications using InSAR technology utilize interferometric correlations related to phase and target point characteristics by interferometry that is proportional to the path length difference between a target point and a radar position on the earth's surface.

SAR has been used to create high resolution maps using aircraft and satellites. InSAR extracts the topographical altitude by forming an interferometric beam using SAR images simultaneously generated from two antennas spaced at narrow intervals, so even if the stereoscopic optical approach cannot be applied by clouds or other obstacles Stereo maps can be produced.

However, SAR by interferometry measures altitude because the accuracy of altitude measurement varies according to the characteristics of radar and terrain, and the Interferometric Coherence Map is calculated by calculating correlation between normalized images. There is always inherent uncertainty.

In addition, the approximation method using the phase-altitude conversion in the repeated-path SAR interferometry cannot produce an accurate DEM because it cannot accurately calculate the earth surface elevation due to the path error caused by the orbital parameter.

The present invention is to solve the above problems and to provide a method for generating a high-precision DEM using the InSAR geometric characteristics by the Repeat Pass satellite. More specifically, the path errors due to satellite parameters (ie, orbital fringe errors) are removed from the path differences by quantifying the path errors due to the satellite orbits, and the path differences due to only the terrain parameters (topographic fringes). It provides a method for generating a high precision DEM using InSAR geometrical characteristics by a Repeat Pass satellite that provides highly accurate topographical information.

1 is a schematic diagram of the generation of DEM using the geometric characteristics of InSAR by a single pass aircraft of the present invention

2 is an interferometric imaging geometry using a single-path aircraft represented by an image of the present invention.

3 is an interferometric geometric characteristic using a repeating satellite of the present invention

[Figure 4] is a procedure for measuring the terrain altitude by using the geometric characteristics of the InSAR by the Repeat Pass satellite of the present invention

5 is a block diagram of a repeating path interferometry system according to the present invention

<Description of the symbols for the main parts of the drawings>

A1, A2, # 1, # 2: antenna H: altitude of sensors

B: Baseline Length D: Direct Range

δD: Direct range difference Z: Terrain altitude

θ: observation angle α: direction angle of the baseline with respect to the horizontal plane

B⒳: Horizontal component of baseline B⒵: Vertical component of baseline

P: reference surface Q: target point on the surface

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings in the detailed description. In the following description, detailed descriptions of well-known functions or configurations will be omitted when it is determined that the detailed description of the present invention may obscure the gist of the present invention. The terms to be described below are terms defined in consideration of functions in the present invention, and may be changed according to a user's intention or custom, and the definitions should be made based on the contents throughout the present specification.

According to an aspect of the present invention for achieving the above object, a digital elevation model generation method for measuring topographical altitude by using the geometric characteristics of the composite aperture radar for interferometry, using the InSAR geometric characteristics to survey the terrain altitude information A method of generating a DEM for a method comprising the steps of: a) calculating a path difference from a different satellite orbit to a target point on the earth using a repeat pass satellite; b) Path error due to satellite parameters (ie, orbital fringe error due to satellite orbit): ( Is the radar signal wavelength, Z is the surface elevation measured by both antennas A1 and A2, H is the aircraft's altitude, B is the baseline distance, θ is the look angle, D is the direct distance to the target point on the surface, α is the angle of the baseline to the horizontal plane, Is quantified according to the path difference of the reflected waves in A1 and A2; c) removing the path error by the satellite parameter from the path difference, and then calculating the path difference using a terrain parameter (topographic fringe); Detecting an interferometric phase from a path difference caused by the topographic fringe; d) a phase difference-altitude conversion step of obtaining a terrain altitude from the interferometric phase difference; And e) a filtering step of performing multiple look processing or spatial filtering of the altitude value. Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. First, the present invention, as shown in Figures 4 and 5, two antennas (A1, A2), path difference calculation means (1), path error calculation means (2), interference phase difference extraction means (3), phase-altitude It is a repeating path interferometric system comprising a conversion means (4) and filtering means of the altitude information (5).

Here, the process of generating a DEM using geometrical characteristics visualized the repetition path. Radar sensors receive one SAR complex image pair at different orbital locations at different times.

After matching the complex image pairs, the phases may be combined to generate an interference diagram having a high correlation with the topographical altitude.

The complex interferogram shows that the interferometry phase is 2π. After removing the phase of assuming that the earth is flat from the interference plot, the phase unwrapping step adds a multiple of 2π to the interferometric phase and associates it with the terrain altitude.

As described above, the Interferometric Coherence Map is generated by the correlation between normalized images and thus has inherent uncertainty in measuring altitude.

Therefore, the successful phase-to-altitude conversion requires the accuracy of the orbital parameters and the basic requirements of the iterative path interferometry system. The requirements must be: first, stable atmospheric conditions and topographic backscattering, and second, the same InSAR geometrical characteristics of the two satellite orbits in the measurement of the same target point.

Phase to altitude conversion is a very important step in the DEM generation process. Therefore, the relationship between phase and altitude must be accurately calculated. Several simplified theoretical models based on InSAR geometric characteristics have been developed to establish the relationship between topographical elevation and InSAR parameters. These models provide a one-dimensional and somewhat accurate problem solution, but they do not provide accurate results because they are simplified by assuming the model itself. Indeed, in the conventional model, the main factor affecting the path difference is caused by the displacement of the direct distance from the radar to the target point rather than by the terrain. Accordingly, the present invention provides an accurate and general SAR interferometric approximation that extracts topographical elevations based on SAR geometric characteristics. In addition, the present invention is to provide a general solution that can obtain a precise path difference, that is, accurate topographical altitude from SAR characteristics by interferometry.

Interferometry SAR technology combines two complex images to provide a complex interferogram and a coherence map using the SAR interference plot. Before generating the interference picture, two SAR images are coregistered with sub-pixel accuracy and resampled, and then the interference image is generated by multiplying the complex conjugate of the second image by the first image and generating a corresponding normalized image. The correlation gives the coherence image. The phase difference between the two images (modulo 2π) is measured from the interference plot to find the path difference δD which has a direct relationship.

(One)

Is the radar signal wavelength. The topographic altitude is obtained from the phase difference equation (1), which is influenced by orbital parameters, in particular by the interference baseline. The process of converting the phase difference to the surface altitude depends on the InSAR system.

1 is a schematic view of generation of a DEM using InSAR geometric characteristics by a single pass aircraft.

That is, it is equipped with two antennas, one of which is used as both a transmitter and a receiver, and the other is used as a bistatic receiver (a receiver in a radar system with a distance between transmitting and receiving). Pass) Schematic diagram of generating DEM using InSAR geometric characteristics by aircraft.

In the InSAR system by a single pass aircraft, when a radio wave is emitted to a point on the earth from a # 1 antenna for a transceiver, an echo signal is returned from a # 1 antenna and a # 2 antenna, respectively. The signal received from each antenna is received in units of pixels having a magnitude and a phase, and generates one two-dimensional image from each antenna. The two SAR images are arranged in a range / doppler region to detect an interferometric phase difference on a pixel-by-pixel basis. The detected interference phase difference is converted into a phase difference-terrain altitude to provide a terrain altitude in units of pixels. The accuracy of topographical elevation information is improved by performing multiple look processing on one pixel or spatial filtering by dividing an area into multiple pixels.

FIG. 2 is a geometric representation of [FIG. 1]. Interferometric Imaging Geometry using a single-path aircraft that is represented by an image of the case where # 1 and # 2 antennas are installed at A1 and A2 points, respectively. It is shown.

One application of interferometry using single path aircraft is the Shuttle Radar Topography Mission (SRTM).

Surface topography can calculate an interferogram phase by a reverse algorithem when the baseline formed by two physical antennas A1 and A2 is accurately measured.

The process of calculating the surface elevation by the orbital parameter function using the interferometer imaging geometry is as follows.

(2)

(3)

Where z is the surface elevation measured by both antennas A1 and A2, H is the aircraft altitude, B is the baseline length, θ is the look angle, D is the direct distance to the target point on the surface, and α is The angle of the baseline with respect to the horizontal plane, δD, is the path difference of the reflected waves in the radars A1 and A2.

The phase elevation at each point on one image is measured and the topographic elevation at each point is obtained by the above equations (1), (2) and (3).

Application examples using the InSAR system using a single pass satellite include ERS (Eurppean Remote Sensing) and RADARSAT. Hereinafter, based on the geometrical representation of FIG. 3, an evolution formula for a path difference in an InSAR system using a repeat pass satellite is obtained, and an approximation method considering the actual reality is obtained.

In this case, the generated error is quantified and separated into an error due to orbital fringes and an error due to topographic fringes, and the effect of each error on the phase difference is analyzed. An example is shown in Table 1.

Surface topography requires accurate measurement of the position of each trajectory in order to accurately measure the interference fringes of the reference surface and to accurately convert the phase difference and the surface elevation.

In general, uncertainty exists in the location of the interferometric trajectory, which severely distorts the image geometry.

It is necessary to understand the interferometric phase dependence of orbital accuracy in altitude measurement by InSAR.

Using two-dimensional expansion, the expression for the geometric formula path difference can be expressed as the sum of two terms. The first term δD1 relates to the flat-earth surface (hereinafter referred to as the 'plane earth surface'), which is assumed to be flat, and the second term δD2 is related to altitude.

(4)

Here, B is the baseline, X is the distance from the reference point to the target point (the point where the position of the sensor is projected on the surface), H is the altitude of the sensor, z is the topographic elevation.

δD1 is related to the flat earth surface and corresponds to the path difference induced by the baseline, and the flat earth interference figure is composed of orbital stripes, typically in the azimuth direction. Parallel to (for two parallel tracks).

δD2 is generated from the interferogram topographic fringe.

The ratio of the two path length differences is to be. (5)

For ERS satellites, H = 789 km and X is about 335 km. When the terrain elevation is about 1 km, the ratio of the path length difference is Is less than.

Therefore, the influence of the terrain on the path length difference is negligibly small compared to the influence of the range displacement.

3 shows a geometric representation of an interferometry using a repeating satellite in the case where one sensor is sequentially located in two orbital paths of point A and point B on altitude H. FIG.

Here, the heights of the sensors are H, the baseline length is B, the look angle is θ, the slant range is D, the direct distance difference is dD, the topographic elevation is z, and the horizontal component of the baseline with respect to the horizontal plane. And the vertical components are B _X and B _Z, P (X, 0) and Q (X + x, z) respectively are two target points located on or above the reference surface.

The direct distance from the sensor point A and B to the target point , to be.

The multipath geometric characteristics are divided into horizontal and vertical components and described as follows. For the two target points P and Q observed by the incident radar angle θ, the path difference between APB and AQB, i.e., (d [PA] -d [PB]) and (d [QA] -d [QB]) Calculate the path difference. d [] is the Euclidean distance.

If the target point Q is located x meters from the target point P and is located z meters above the reference point, the three-dimensional Taylor expansion equation is as follows.

In practice, (radar senser distance)

here, Is the projection of the baseline on vector n, which is perpendicular to the direction of the rectangular direction. Is the projection of the cross-target distance between the target points to the vector.

remind Due to the symmetry of, the incidence direction component of the baseline or target point has no effect on the path difference.

However, in reality, since the wave paths are different between the two tracks, an approximation equation for the path length difference associated with the different propagation paths can be obtained as follows.

Where the higher order term Is negligible, and in the case of B _z = 0, in comparison with the path difference approximation in the case of symmetry, in the orbital fringes Error occurs, and in the topographic fringes Error occurs.

TABLE 1

x = 5 km x = 50 km 0.13 m 1.66 m fringe error 2 31 0.92 · 10 ^-5 8.91 · 10 ^-5 fringe error negleglible negleglible

Table 1 shows the error calculated using the parameters corresponding to the ERS geometrical characteristics in each of 5 X 5 ms and 50 X 50 ms. It can be seen that even in a narrow region, the stripes (Orbital Fringe) error is prominent.

Is proportional to the boundary component B _X of the baseline, Is an error affecting the B _x value, which reduces the baseline accuracy.

Therefore, in order to obtain an interferogram corresponding to topographic fringes only, orbital fringes must be removed from the phase difference by interferometry.

This flattening of the phase has the effect that the interferogram phase is proportional to the topographic elevation of the horizontal plane.

In the present invention having the above-described configuration, specific embodiments have been described, but various modifications can be made without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the claims below but also by the equivalents of the claims.

According to the present invention, it is possible to obtain the path difference using only the terrain parameter by quantifying the path error by the satellite parameter and removing it from the path difference. That is, by removing the orbital fringe from the phase difference by interferometry, an interferogram corresponding to the topographic fringe is obtained, and the flattening of the phase ) Can provide accurate topographical information by generating a high-precision DEM using InSAR geometrical characteristics by Repeat Pass satellites by making the interferogram phase proportional to the topographic elevation of the horizontal plane. Will be.

Claims (2)

In the method for generating a DEM for surveying topographic elevation information using InSAR geometric characteristics, a) calculating a path difference from a different satellite orbit to a target point on the earth using a repeat pass satellite; b) Path error due to satellite parameters (ie, orbital fringe error due to satellite orbit): ( Is the radar signal wavelength, Z is the surface elevation measured by both antennas A1 and A2, H is the aircraft's altitude, B is the baseline distance, θ is the look angle, D is the direct distance to the target point on the surface, α is the angle of the baseline to the horizontal plane, Is the path difference of the reflected waves at A1 and A2) Quantifying according to; c) removing the path error by the satellite parameter from the path difference, and then calculating the path difference using a terrain parameter (topographic fringe); Detecting an interferometric phase from a path difference caused by the topographic fringe; d) a phase difference-altitude conversion step of obtaining a terrain altitude from the interferometric phase difference; And e) a method for generating a digital elevation model for measuring topographical altitude using a geometrical characteristic of a composite aperture radar for interferometry, comprising the step of performing multiple look processing or spatial filtering the altitude value. delete