It’s true that it does not easily spring to mind that the images captured by Earth observation satellites, nowadays with submetric spatial resolution at best, can produce displacement measurements with a precision measured in centimetres or millimetres, as the experts claim. But what exactly does this involve?
Let’s first look at optical sensors such as Pléiades (spatial resolution of 0.5 m) and the precision that can be obtained with this type of images. For example, in the case of an open-pit mining site, two digital elevation models (DEM) generated with photogrammetric techniques using stereoscopic pairs acquired on different dates can be compared with determine the location of changes in elevation corresponding to the extraction or piling of rocky material. It is generally accepted that the DEMs produced using Pléiades images, to use this example, have a precision in the order of decimetres. But this is far from being to the centimetre, not to mention the millimetre. For this reason, we must look at radar technology and a very specific way of processing these data.
This involves technology known as “differential radar interferometry”, which uses radar images (at least two) taken on a given study site. This technology makes use of synthetic aperture radar (SAR), which can operate at different wavelengths, i.e., at 6 cm for the C-band of the Canadian satellite RADARSAT-2, or at 3 cm for the X-band of the TerraSAR-X and COSMO-SkyMed satellites, to name but a few. In addition to their use for studying the distribution of sea ice, the extent of flooding or even mapping wetlands, these SAR images may indeed be used to measure vertical displacements of the Earth’s surface to the centimetre or millimetre.
And how is this possible? It is important to first know that SAR works by transmitting pulses toward the Earth’s surface and recording the portion which is reflected back to the sensor. To be more specific, SAR records the time difference between the transmitted pulse (short sinusoidal signal of centimetric wavelength) and the reception of the reflected energy, its intensity and its phase. In the case of mapping sea ice, floods and wetlands, intensity is of greatest interest. This intensity is based on several factors, including geometry, roughness and humidity of the target on the ground. In interferometry, it is mostly the phase that interests us. Indeed, the phase difference between the two acquisitions taken from different, but close orbits, known as an “interferogram”, provides for each pixel information on topography or ground deformation or displacement. In the latter case, we speak of differential interferometry (DInSAR), since we are interested in the movement that took place between the two consecutive images taken during a given time interval.