A meaningful step has been completed in the field of radar-interferometry at ISSIA seat of Bari. The interferometric phase, which is normally measured in the principal component alone (i.e. within a cycle of 6,28 rad), is finally measurable in its entirety, directly correlated to the distance between the interferometer and the observed object such as to other quantities that influence its value, the possibility of subtle displacements of targets on the ground or the atmospheric delay on the propagation of radar beam. New possibilities of monitoring and early warning seem, therefore, achievable in the management of natural hazards like: tectonics and seismology; volcanology; landslides; ice-stream dynamics and global scale climatology.
Applied to an airborne interferometer, the technique allowed to measure the height of soil with respect to a reference plane, avoiding to count the interferometric fringes in the geometric domain. The advantage is important if it is considered that the commonly used technique demands a good readability of the fringe patterns, which mean an elevated coherence or similarity of signals from the two points of view used by the interferometer, and the absence of ambiguities, which are generated from noise and local undersampling of the phase, and make its interpretation not univocal. Both conditions are difficult to find in nature, where the norm is not that one in Fig. 1 and 2 but that one shown in Fig. 3 of the coherence limited to few scattered points.
The measurement technique we use (Fig. 4 and 5) it is known as: "differential analysis of interferometric phases in the frequency domain". It is based on the computation of multiple interferograms at frequencies not far from the carrier frequency of radar. A linear regression is applied to the phase samples related to single cells of soil, and concurs to obtain, from the value of intercepts with the axis of ordinates, the direct reading of the total number of cycles (Fig. 6) lost from standard interferometry. All that to expense of a greater computational load of about one order of magnitude, in the generation of the set of multiple interferograms; while the implementation of a new class of wide-band radars is demanded, like those foreseen on the initiative of Alenia Spazio and ASI (the Italian Space Agency) for the COSMO/SkyMed constellation.
In the new technique, the classic definition of coherence is replaced by a different quality parameter represented by the standard deviation (std) of sampled phase values with respect to the straight line obtained from the linear regression. In our current experiment (X-band SAR, with 400 MHz of transmitted bandwidth), values of std < 0,02 rad have been found on 5% of pixels in the interferogram, providing an accuracy in the definition of intercept within ± 3,14 rad (Fig. 7). In particular, these pixels are numerous also in the natural environment and are partially distributed in areas with low coherence, indicating so that sound measures could be achieved in contexts of long temporal baseline, like those normally used in geodynamics studies.
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