From 1987, the research activities on SAR data processing at POLIMI have been mainly devoted to problem solving for specific SAR missions. Many problems stem from the fact that the available satellite missions (e.g., SEASAT, ERS-1, ERS-2, Radarsat) have not been designed for interferometrical applications.

Here is a summary of the POLIMI activities on SAR data processing with present activities in italics and most recent research in bold.

* 1 - Phase preserving focusing. A focusing technique (w-k) that preserves the phase of the scatterers has been introduced in 1987 and tested on SEASAT data [1, 2, 3]. It is now the basic processing used at POLIMI and many other sites to focus satellite and airborne SAR images to be exploited for interferometric applications [4]. A phase preserving test suggested by ESA shows that the phase dispersion and the phase constant term introduced by the - focusing are less than 0.1 and 10^-4 degrees respectively.

* 2 - Doppler Centroid ambiguity estimation. A blind deconvolution technique for DC ambiguity estimation in the case of the {\bf SIR-C/X-SAR} data [5] has been developed as well as a technique for efficient focusing of SAR data with time and space varying DC [6].

* 3 - Interferogram generation. A software code made in POLIMI for interferogram generation [11,10] from ERS-1 and ERS-2 images has been extensively tested at ESA-ESRIN and then distributed via Internet. Its features include:

- image registration within the limit imposed by coherence (e.g., 1/50th of a pixel for coherence better than 0.4),

- coherence map generation;

- azimuth and range local frequencies (linked to terrain slopes) estimation;

- local spectral shift filtering [7,8];

- fringe generation and filtering.

Recent and ongoing research activities:

(3 a) - Quick-look interferometric processing for ERS data (i.e., generation of interferograms and coherence maps of a 30 x 100 km area with a 100 x 100 m resolution in about 8 minutes on a typical low-cost workstation).

(3 b) - Optimization of the tradeoff between local spatial resolution and phase noise of the interferogram.

(3 c) - Study of efficient and phase preserving interpolators for image registration in case of single pass interferometry (e.g., the envisaged 3rd SIR-C/X-SAR mission) where the theoretical coherence is close to 1.

* 4 - Phase unwrapping. Both local (i.e., ghost lines) and global (i.e. Least Mean Squares) techniques have been developed, compared, and combined to get the unwrapped phase with the minimum interaction with the operator [11].

Recent and ongoing research activities:

(4 a) - Phase unwrapping using multibaseline interferometric SAR images.

(4 b) - 3-D target reconstruction (e.g., getting resolution in foreshortening and layover areas) using multibaseline images [13, 14, 16, 18].

* 5 - Phase preserving ScanSAR focusing. A technique has been developed for efficient phase preserving ScanSAR focusing [15, 17]. The technique has been tested on Radarsat data simulated using ERS-1 data. Repeat-pass ScanSAR interferometry has also been analyzed and simulated using ERS-1 data.

Recent and ongoing research activities:

(5 a) - Use of Radarsat data.

(5 b) - Phase consistent block mosaicking both in azimuth and in range (subswaths).

* 6 - Combination of ascending and descending satellite passes. Ascending and descending interferometric ERS-1 passes have been exploited to overcome the problem of foreshortening and layover due to the steep off-nadir angle [12]. Foreshortening areas in one pass are well imaged in the other if not in shadow.

* 7 - Differential interferometry. Differential interferometry for centimetric terrain motion estimation has been tested in different situations: a landslide in St. Etienne de Tinee, subsidence along the Adriatic coast in Italy and in the area of Pozzuoli (Naples) [19]. From a methodological point of view a criterion for image selection and methods for differential phase generation that do not need phase unwrapping have been identified.

Recent and ongoing research activities:

(7 a) Use of the interferometric quick look for image browsing.

* 8 - Interference cancellation. A technique for e.m. interference cancellation has been successfully tested on SAR P-band airborne images.

* 9 - Future activity: X-SAR boom motion compensation. One of the main problems envisaged for the next SIR-C/X-SAR mission is the effect of the 60m boom oscillations on the single pass interferogram generation (especially in X-band). We plan to develop techniques for estimating and compensating these effects from the data themselves.

REFERENCES

[1] Cafforio, C. Prati, and F. Rocca, 1991, "SAR data focusing using seismic migration techniques," IEEE Transactions on AES, vol. 27-2, pp. 194-207.

[2] Cafforio, C. Prati, F. Rocca, 1989, "Synthetic Aperture Radar: a new application for wave-equation techniques," Geophysical Prospecting, vol. 37, pp. 809-830.

[3] Cafforio, C. Prati, F. Rocca, 1991, "Full resolution focusing of SEASAT SAR images in the frequency-wave number domain," International Journal of Remote Sensing, vol. 12, no. 3, pp. 491-510.

[4] Prati, F. Rocca, A. Monti Guarnieri, E. Damonti, 1990, "Seismic migration for SAR focusing: Interferometrical applications," IEEE Transactions on Geoscience and Remote Sensing, vol. 28, no. 4, pp. 627-640.

[5] Prati, C., F. Rocca, Y. Kost, and E. Damonti, 1991, "Blind deconvolution for Doppler centroid estimation in high frequencies SAR, " IEEE Transactions on Geoscience and Remote Sensing, vol. 29, no. 6, pp. 934-941.

[6] Prati, F. Rocca, 1992, "Focusing SAR data with time-varying Doppler centroid," IEEE Transactions on Geoscience and Remote Sensing, vol.30, no. 3, pp. 550-559.

[7] Prati, C. and F. Rocca, 1993, "Improving slant range resolution of stationary objects with multiple SAR surveys," IEEE Transactions on AES, vol. 29, no. 1, pp. 135-144.

[8] Gatelli, A. Monti Guarnieri, F. Parizzi, P. Pasquali, C. Prati, F. Rocca, 1994, "The wavenumber Shift in SAR Interferometry," IEEE Transactions on Geoscience and Remote Sensing, vol. 32, no. 4, July 1994.

[9] Prati, C., F. Rocca, and A. Monti Guarnieri, 1994, "Topographic Capabilities of SAR exemplified with ERS-1, " Geo-Information-Systems, vol. 7, no. 1, February 1994, pp. 17-22.

[10] Prati, C., and F. Rocca, "Process for generating Synthetic Aperture Radar Interferograms," U.S. Patent N.5, pp. 332, 999, July 26, 1994.

[11] Prati, C., F. Rocca, A. Monti Guarnieri, and P. Pasquali, "ERS-1 SAR interferometric techniques and applications," ESRIN, Frascati, June 1994.

[12] Pasquali, P., R. Pellegrini, C. Prati, and F. Rocca, "Combination of interferograms from ascending and descending orbits," IGARSS '94, Pasadena, CA, pp. 733-735.

[13] Fortuny, E. Holmer, A. J. Sieber, Pasquali, C. Prati, and F. Rocca, "Validating SAR interferomertry applications by using EMSL," IGARSS '94, Pasadena, CA, pp. 736-738.

[14] Prati, C., F. Rocca, and Monti Guarnieri, "Measuring volumetric scattering effects with SAR interferometry," PIERS '94, Noordwjik.

[15] Guarnieri, Monti, C. Prati, and F. Rocca, "Interferometry with SCANSAR, " PIERS '95, Seattle, WA, (USA), p. 158.

[16] Pasquali, P., C. Prati, C., F. Rocca, M. Seymour, J. Fortuny, E. Ohlmer, and A. Sieber, "A 3-D SAR experiment with EMSL data," PIERS '95, Seattle, WA, (USA), p.362.

[17] Guarnieri, Monti, C. Prati, and F. Rocca, "Interferometry with SCANSAR, " IGARSS '95, Firenze, pp. 550-552.

[18] Pasquali, P., C. Prati, F. Rocca, M. Seymour, J. Fortuny, E. Ohlmer, and A. Sieber, "A 3-D SAR experiment with EMSL data," IGARSS '95, Firenze, pp. 784-786.

[19] Prati, C., F. Rocca, and Monti Guarnieri, "Monitoring surface deformations with SAR interferometry," Int. Symp. on Retieval of bio and geophysical parameters from SAR data for land applications, Toulouse, October 1995.

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