SAR data becomes more useful when analysts apply interferometry. This helps us monitor ground movement and create elevation models.
Displacement Measurement and Phase Differences
Interferometry is the comparison of two SAR images taken at different times. Analysts process these images to create an interferogram that maps the phase difference between the two images.
Phase differences happen when the distance between the SAR antenna and the ground changes. This can occur because of variations in the terrain. Examples include landslides, earthquakes, or subsidence.
Rather than direct displacement, the output is fractions of the phase. If all displacements are less than half a wavelength (for C-band, the wavelength is 5.6 cm), they are easy to measure. However, if the displacements are greater than half a wavelength, it can cause confusion.
Elevation Models
Accurate displacement output comes from correcting for topography. Analysts can achieve this using a digital elevation model (DEM). Researchers call the process of SAR data analysis Differential Interferometric Synthetic Aperture Radar (DInSAR).
A high-resolution digital elevation model (DEM) shows the land's shape clearly. This model helps remove topographic effects from SAR data. As a result, researchers can measure displacement more accurately.
Another approach is to use a DEM to calculate the topographic component of the interferogram. Differences in the radar signal path length because of the topography of an area cause the topographic phase component. Removing the topographic phase component from the interferogram allows us to attribute the remaining phase differences to surface displacement.
Practical Application
At Geofem, we have used DInSAR for many projects. One example is this case in Cyprus. In this case, DInSAR analysed both old and new data.
Researchers conducted this study to examine mass movement on hilly land. These movements could be signs of future landslides. Using these results along with hydrogeological and topographical data, we created a detailed susceptibility map. Analysts can update this map each season to show changes in landslide risk.
Researchers can collect SAR data regardless of weather or time of day through an ingenious process. Learn all about it in this article...
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