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Topographic change measurement using SAR interferometry image (Okushiri Island in Hokkaido)
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In addition to the backscatter explained so far, SAR data includes phase information on microwave dependent on distance between in-orbit SAR and a target.
Interferometry processing is a method to analyze differences of phase information by interfering two data observed from almost identical position of satellite on orbit. This analysis method is focused as a method to produce elevation data and to measure topographical change. Two SAR images used for interference are called Master Image and Slave image, respectively.
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Outline of Elevation Data Calculation Process |
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According to the triangular surveying principle, aligning two SAR data acquired from different near points on orbit targeting the same area on ground, leads to gain elevation data of the area. Geometric principle of calculating elevation data based on interferometry processing is illustrated in the following figure.
Calculation of elevation data based on SAR interferometry processing
Accordingly, work out the elevation (h) in the following formula.
h = H - r x cos Θ
Here, altitude of the satellite (H) and distance from the satellite to the target area (r) can be clarified based on the data of satellite orbit and attitude. Accordingly, as long as q (off-nadir angle) is definable, sea level of the target (h) is calculable. Thus, SAR interferometry processing is the logic of working out elevation data by calculating off-nadir angle based on the phase difference of two SAR images originated in Dr.
Accuracy of satellite altitude (H) and distance between the satellite and a target object is directly translated into the accuracy of elevation data. In this sense, the accuracy of orbit data and attitude data is critical for the calculation of elevation in SAR interferometry processing.
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Outline of Differential SAR Interferometry Processing |
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In case elevation of target object is known, topographical change can be extracted by subtracting the elevation from the value calculated by interferometry processing.
Even if elevation is unknown, extraction of topographical changes is possible based on the difference between two elevation data respectively obtained by the combination of three SAR data.
The processing is called differential SAR interferometry processing.
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