In this paper we propose a method to get fine registration of high resolution multispectral images. The algorithm supposes that a coarse registration, based on ancillary information, has been already performed. It is known, in fact, that residual distortions remain, due to the combined effects of Earth rotation and curvature, view geometry, sensor operation, variations in platform velocity, atmospheric and terrain effects.
The algorithm grounds its main idea on the information-theoretic approach to register volumetric medical images of different modalities. Registration is achieved by adjustment of the relative position and orientation until the mutual information between the images is maximized. The idea is that the join information is maximized when the two images are at their best registration. This approach works directly with image data but in principle it can be applied in any transformed domain. While the original algorithm has been thought to make registration in a limited search space (i.e. translation and orientation), in the remote sensing framework the class of transformations is extended allowing scaling, shearing or a general polynomial model. The maximization of the target function is performed using both the stochastic gradient descent algorithm and the simulated annealing, since the former is known to occasionally deadlock in local maxima.
We have applied the algorithm on a SPOT-5 couple of images, achieving the registration of chips of size
256x256 pixels at time. Accuracy has been obtained comparing the results with the outcomes of a commercial software that adopts a sort of Normalized Cross-Correlation method. On 143 chips taken throughout the image, the final translation accuracy resulted well below 1 pixel and the rotation accuracy about 0.015deg.
A Ground-Based Synthetic Aperture Radar (GB-SAR) is nowadays employed in several applications. The processing of
ground-based, space and airborne SAR data relies on the same physical principles. Nevertheless specific algorithms for
the focusing of data acquired by GB-SAR system have been proposed in literature.
In this work the impact of the main focusing methods on the interferometric phase dispersion and on the coherence has
been studied by employing a real dataset obtained by carrying out an experiment. Several acquisitions of a scene with a
corner reflector mounted on a micrometric screw have been made; before some acquisitions the micrometric screw has
been displaced of few millimetres in the Line-of-Sight direction. The images have been first focused by using two
different algorithms and correspondently, two different sets of interferograms have been generated. The mean and
standard deviation of the phase values in correspondence of the corner reflector have been compared to those obtained by
knowing the real displacement of the micrometric screw. The mean phase and its dispersion and the coherence values for
each focusing algorithm have been quantified and both the precision and the accuracy of the interferometic phase
measurements obtained by using the two different focusing methods have been assessed.