Since early ninety’s Synthetic Aperture Radar Interferometry (InSAR) has been exploited for geophysical applications. InSAR is capable to obtain information on surface deformation and can be a useful tool for monitoring active volcanic area. Despite the capability of SAR sensors to operate in all weather and day/night conditions, SAR signal is affected by different source of noise (e.g. water vapour or instrumental noise). These latter affect the accuracy of InSAR deformation map, especially for time series analysis, where the identification of clear and electromagnetically stable point targets, the so called persistent scatterers, is very important. Passive Corner Reflectors (CRs) can be helpful for this purpose. Indeed, CRs can provide point measurements with high level of confidence thanks to their high backscattering value representing a strong persistent scatterer to be used for calibrating and validating InSAR deformation map.
In order to implement a small CRs network, for InSAR data calibration and validation, on the Fogo volcano island, three Corner Reflectors will be installed . The three CRs have been purposely designed and realised for MIA-VITA project. They are trihedral shape type, in perforated aluminium with azimuth and elevation pointing capabilities (Figure 1).
Figure 1. The first prototype of CR during the testing phase at INGV headquarter in Rome.
The size of the single triangular face is 150 cm, which guarantees a Radar Cross Section (RCS) of about 6700 m2, suitable for C-band SAR sensors, like the one on board of ENVISAT and the future SENTINEL mission, but the CRs can also be used for X-band sensor. The CRs have been realised in few weeks and they cost about € 2500 each one.
In order to maximize the effectiveness of a CR in terms of SNR (Signal to Noise Ratio), the accuracy of its orientation respect to the satellite Line Of Sight (LOS) is critical, i.e. we must take into account the orbital path with respect to north and the incidence angle of the electromagnetic wave transmitted by the radar (figure 2). For example, if we consider the case of ENVISAT satellite, to set the orientation of the CR is with the reflecting surface perpendicular to the LOS, it is necessary to account for the about 8.5° between the N direction and the orbital path (either for ascending or for descending orbits), while the incidence angle is of 40° (for IS6 acquisition mode presently working on ENVISAT SAR; envisat.esa.it).
Figure 2. Geometries of ascending and descending satellite LOS.
A preliminary evaluation for the three possible sites suitable for CR installation has been carried out by analysing either optical very high resolution and SAR images. Two sites have been identified inside the Fogo caldera, close to the village of Chã das Caldeiras, and one is located in the area close to the village of Achada Furna (figure 3). The final and precise positions of the CRs will be definitively identified after a local survey during the fieldwork campaign which is planned to be carried out on the last week of September 2011.
Figure 3.The three preliminary selected sites for Corner Reflectors set up (red triangles).