Assesment of the ionospheric impact on synthetic aperture radar imaging

Synthetic Aperture Radar (SAR) is the key-technology for the Earth’s Observation from space. It enables the imaging of our planet under any atmospheric condition and, being an active instrument, all day long, as it is independent on the sunlight. At least a pair of SAR images of the same portion of the Earth’s surface can be used to form a phase difference image that is called “interferogram”. This technique, known as the Interferometric Synthetic Aperture RADAR (InSAR), is among the most used techniques for geophysical applications, providing maps of, e.g., surface deformation with high accuracy (mm level). The ionosphere affects space-borne SAR measurements especially at lower frequencies: free electrons and ions modify phase and group velocities as well as the intensity and polarization of the SAR transmitted electromagnetic waves crossing the ionospheric plasma. Such modifications introduce errors in SAR products, but, vice versa, they can provide valuable information on the ionospheric conditions during the image acquisition. These errors are a function of the Total Electron Content (TEC), i.e. the columnar concentration of free electrons along the satellite-target path. TEC is a highly variable quantity influenced by several helio-geophysical parameters, such as the solar activity and the geomagnetic field conditions; moreover, it depends on the latitude, on the season and on the time of the day. This thesis has a twofold scope: evaluating the ionospheric impact on InSAR imaging and assessing the capability of InSAR to retrieve ionospheric information. To the scope, mid-latitude, night-time case studies are selected to investigate about the correlation between TEC temporal and spatial variability derived from InSAR and from a dense regional network of GNSS (Global Navigation Satellite Systems) receivers over Italy. To retrieve the TEC variability from InSAR, the correlation between the integral of the azimuth shifts and the interferometric phase in the absence of ground motions (e.g. earthquakes) and/or heavy rain events is investigated. If correlation exists, the tropospheric contribution to the interferometric phase can be assumed as negligible with respect to the ionospheric one and, consequently, the TEC variability from InSAR can be retrieved. This assumption is further supported by the correlation results between the TEC from InSAR and from GNSS, allowing also the use of TEC from GNSS to mitigate the phase advance introduced by the ionosphere. Moreover, the careful characterization of the geomagnetic and ionospheric conditions, carried out for the case studies under investigation, shows that a particular electron density formation in the ionosphere named sporadic E layer, looks like the main responsible of the “streaks” occurrence.