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Iterative model reconstruction for phase unwrapping
| Andrea Trucco1, Paul C. Smits1, Giancarlo Bo1,
and Gianni Vernazza2 |
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1University of Genoa, Department of Biophysical and Electronic
Engineering, Via all'Opera Pia 11A, 16145 Genova. Tel. 39-10-3532755, fax
39-10-3532134.
2University of Cagliari, Piazza Armi, Cagliari. Tel. 39-70-6755866,
fax 39-70-6755900.
fragola dibe.unige.it , smits@dibe.unige.it,
ginka70 nina.dibe.unige.it ,
vernazza diee.unica.it
http://dibe.unige.it/department/imm/group.html
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Abstract
- Phase unwrapping is one of the most important steps we have to consider
in DEM generation using SAR interferometry. The phase of the radar echoes
may only be measured modulo 2PI, but the whole phase at each pixel is needed
to obtain an elevation map. Furthermore some of the phase data may be not
reliable because of the presence of a certain amount of noise. The method
for phase unwrapping we have implemented is based on a global analysis
of the interferogram, and works in an iterative way. An approximate model
of the unwrapped phase is generated and then tested by calculating the
"residue image": this image is, by definition, the difference
between the interferogram and the model. If there are residual fringes
in the residue image, then the model must be improved. In order to construct
the model, we subdivide the interferogram into squares and then, for each
square, the local slopes in azimuth and range are estimated by means of
the power spectral density (PSD): the maximum of the PSD should be proportional
to the local slopes. Using the local slopes and a least-squares algorithm
(Gauss-Seidel relaxation) the values of the model in the vertexes of the
grid are obtained, and then by bilinear interpolation the phase at each
pixel can be evaluated. If an improvement of the model is needed, the entire
process is repeated with as the input image the residue image as a "new"
interferogram. The program stops when a certain condition is reached. The
method allows one to assign weights to each square, associating to each
square a weight proportional to the signal-to-noise ratio in that square.
- Prelimanary results using synthetic and real data interferograms
are shown.
Keywords: Phase unwrapping, Iterative model reconstruction
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,
atmospheric chemistry
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