A digital elevation model (DEM) represents the rise in the surface of the earth as a grid of elevation values. It serves as essential tools to understand the topography, hydrology and reliefs of the earth. Researchers, geologists and environmental scientists rely on DEMs for various ends, including field analysis, flood modeling and town planning.
The creation of a DEM using open-source software like Pygmtsar (Python Insar) is an excellent example of how modern technology makes complex processes accessible and fast. With one click, you can start the treatment and get the results for a DEM satellite. This example and many other examples of interactive Insar with 3D visualization are available on the Insar.dev platform.
The digital elevation model is created from radar images obtained by Sentinel-1 data in a few minutes on devices connected to the Internet. This speed and accessibility make technology adapted to everyone, without the need for high speed internet, a large disk space or a powerful computer.
Public radar satellites, such as Sentinel-1, scan our day and night planet, capturing the thoughtful signal as raster images for subsequent treatment. These images store complex phase values of the reflected signal and can be used to obtain various information on the condition and movements of the surface of the earth and atmospheric properties. The simplest form of treatment consists in calculating the amplitude of the signal to produce a gray level image, similar to optical but independent photography of the day of the day and the cloud cover.
The real potential of radar satellites is revealed when processing complex data. The Sentinel-1 radar wavelength is 56 mm, allowing the recording of precise heights and displacements of the surface of the earth to the order of fractions of one millimeter. If the surface lights up by a millimeter, the radar beam travels an additional distance of 1 millimeter to the surface and back, totaling 2 millimeters. This small delay can be recorded and treated.
In addition to surface trips, images contain what is called the atmospheric phase, which is the delay in the signal when it passes through the atmosphere. In case of strong atmospheric turbulence or cloud cover, the atmospheric phase delay can equip hundreds of millimeters of surface displacement. Consequently, a precise separation of different sources of signal change is necessary to obtain high precision surface monitoring.
For such precise calculations, it is necessary to know the orbit of the satellite with precision and to compensate for orbital errors. Solid tidal tides caused by the Moon and the Sun also influence the data, which can be offset for analytically or based on space spectrum data.
For illustrative and educational purposes, a pair of “ideal” images is chosen from a well reflection rocky area with a low cloud cover. In industrial use, various methods of processing a series of images are applied. For DEM construction, a submillimeter accuracy is not required, so for a well-chosen field, data analysis is automatically carried out without selection of parameters. When the vegetation and cloud cover are present, additional operations are necessary to separate these components from the signal and assess the accuracy of the results.
Sentinel-1, the only global satellite accessible to the public, is currently in orbit. Almost a decade of images is available, which allows us to build a detailed topography and to study surface trips, infrastructure, earthquake effects and many other processes and phenomena on our planet. Two new Sentinel-1 radar satellites and the articulation Nisar satellite NASA and the Indian Space Research Organization (ISRO) should be launched by the end of the year.
