# A GeoNode-Based Platform for an Effective Exploitation of Advanced DInSAR Measurements
## 2 DInSAR Techniques  
Synthetic Aperture Radar (SAR) is currently one of the most relevant remote sensing systems for investigating the Earth's surface characteristics. A SAR sensor is a coherent radar system, typically mounted on board satellites and/or aircraft (and more recently on ground-based platforms and drones/UAVs) which allows to effectively map the scattering properties of the Earth's surface [24; 44]. It operates in a side-looking illumination configuration by emitting microwave pulses and measuring the amplitude and the phase of the signals backscattered from the illuminated area of the Earth's surface. The proper processing of the received backscattered radar signals (echoes), which accounts for both the physical characteristics of the imaged scene and the acquisition geometry, allows reconstructing complex radar images relevant to Earth's surface with high spatial resolution, independent from the sensor altitude [44; 45; 46]. Being an active sensor, a SAR system provides its own illumination system; therefore, it does not need any external radiation source to work, thus permitting continuous (day and night) operation. Moreover, by working at the microwave frequencies of the electromagnetic spectrum (significantly lower than those of the visible spectrum region), it can be effectively used in the presence of fog, clouds, precipitations and ash, thus allowing an all-weather imaging. SAR Interferometry (InSAR) is one of the most successful techniques based on the SAR sensor technology; it exploits the phase difference (interferogram) between pairs of SAR images, which are collected over an area of interest at different epochs (temporal baseline) and from typically different orbital positions (spatial baseline), in order to retrieve Earth's surface geophysical parameters, such as the scene topography and/or its surface deformation [24; 25]. In particular, the DInSAR technique [20; 21; 22; 23; 24; 25] allows extracting information on the radar line-of-sight (LOS) projection of the surface displacements that occurred between two temporally separated observations (tipically referred to as master and slave acquisitions), acquired with nearly the same illumination geometry over an investigated area (see Figure 1). Due to its capability to produce spatially dense maps of the surface displacements caused by a large variety of natural and anthropogenic hazards, such as earthquakes, volcanic unrests, landslides, subsidence phenomena [6; 7; 8; 9; 10; 11; 47; 48], with accuracies on the order of fractions of the used wavelength (typically ranging from few centimeters to few tens of centimeters) [20; 21; 22; 23; 24; 25], the DInSAR technique has progressively emerged as an effective, non-invasive tool in civil protection scenarios, with limited monitoring costs with respect to traditional in situ surveys. Nevertheless, standard DInSAR analyses are intrinsically limited because a single interferogram contains the information relevant to the overall deformation that occurred between the two acquisitions, whose separation in time can range from a few days to some months (or even years). Moreover, the presence of atmospheric artifacts that may affect the deformation maps can be particularly difficult to filter out from the single DInSAR interferograms, if no sophisticated atmospheric models are available [49]. To overcome these limitations, in the last 25 years several advanced DInSAR approaches have been developed, also fostered by the large availability of SAR data collected by the so-called \"first generation\" satellite systems, continuously acquiring data of the Earth's surface since the 90s (e.g., ERS-1/2, ENVISAT, RADARSAT-1) with a nearly monthly revisit time, and ground resolution ranging from a few to tens of meters.