Christian BIGNAMI, MIAVITA team member,
On September 2011, the installation of a small network of Corner Reflectors (CRs) in Fogo, Cape Verde, has been successfully completed. The scope of this network is to provide some reference points useful for the calibration and validation of Interferometric results achieved by means of Synthetic Aperture Radar Interferometry (InSAR). In detail, the network is composed by three CRs specifically designed and realised for MIA-VITA project. They are trihedral shape type, in perforated aluminium and can be used for C-band (e.g. ENVISAT or future SENTINEL mission) and X-band SAR sensors (like COSMO-SkyMed system).
The shape and size (1.5m base edge) allows to obtain a Radar Cross Section (RCS) suitable to have a clear, high reflective point in the SAR image. The nominal values of RCS can be calculated by using the following expression:
where a is the length of trihedron side and l is the wavelength of the SAR sensor we are considering. Assuming a C-band SAR, like ENVISAT-ASAR (5.3 GHz), we obtain a RCS = 6762 m2. This value can be considered enough to have the desired reference point. Indeed, the corresponding value of the SAR backscattering coefficient, i.e. the normalized RCS, is equal to:
where A is the image pixel area. Considering the pixel size of the ENVISAT SAR image of 4 x 12 m, in azimuth and ground-range directions for single look data, we have a nominal backscattering coefficient of about 21 dB, which is definitely higher than other natural surfaces, and therefore guarantees a high Signal-to-Noise Ratio (SNR).
The three sites where the CRs have been installed are shown in Figure 1. Two CRs have been set up inside the Fogo caldera, close to the village of Chã das Caldeiras, and one on the volcano south flank in the area close to the village of Fonte Aleixo.
Figure 1. Google Earth image showing the 3 CRs installation sites (red triangles).
The two CRs in the caldera have been oriented to be used for descending orbit data, while the CR in Fonte Aleixo is for ascending SAR acquisitions. The installation campaign started on September 26, 2011 and ended on September 28. Figure 2 shows the three CRs in their operating positions.
In order to test the effectiveness of the installation campaign a pair of SAR data images from ENVISAT satellite have been obtained from European Space Agenciy (ESA). One images was acquired on 30 October2011, on ascending orbit, and one scene is dated 4 November 2011, on descending orbit. The data were radiometrically corrected to extract the backscattering coefficient from the images, and were geocoded (by using the SRTM DEM) to locate the position of the three bright points corresponding to the CRs. Figures 3 to 5 show the SAR backscattering images related to the small portions where the CRs are installed.
Figure 2. The three CRs successfully in operation. Upper, middle and bottom images refer to the two CRs close to Chã das Caldeiras and the one close to Fonte Aleixo, respectively.
Figure 3. SAR image detail of the area where the CR1 was installed. The clear bright point inside the green circle is the SAR response of the CR.
Figure 4. SAR image detail of the area where the CR2 was installed. The clear bright point inside the green circle is the SAR response of the CR.
Figure 5. SAR image detail of the area where the CR3 was installed. The clear bright point inside the green circle is the SAR response of the CR.
The maximum value of the pixels corresponding to each CR in the SAR calibrated images are 9.2 dB for CR1 and 9.7 dB for CR2, for the descending orbit image, and 10.7 dB for CR3 for the ascending orbit image. The discrepancies with respect to the nominal values (21dB) indicate that the pointing of the CRs is not very precise, i.e. the axes of the trihedrons are not precisely lined up to the LOS (Line Of Sight) of the SAR. Despite that, the CRs show a higher SAR response than the surrounding natural scatterers (on average between -11 and -6.6 dB for the three sites, within an area of 200m radius). It means that they are clearly visible and can be still used as references for SAR product.