Muti-Aperture Focusing and Atmospheric Pahse Screen Estimation in Geosynchnrous SAR
Recchia, Andrea1; Monti Guarnieri, Andrea1; Broquetas, Antoni2; Ruiz Rodon, Josep2; Giudici, Davide3; Tebaldini, Stefano1
1Politecnico di Milano, ITALY; 2Universitat Politècnica de Catalunya, SPAIN; 3Aresys s.r.l., ITALY

A Synthetic Aperture Radar in geosynchronous orbit would be the natural complement to Low-Earth Orbit (LEO) SAR systems. A similar system has been first proposed at the end of seventies [1] and then re-considered later [2], but it has never been implemented, due to technological and processing constraints. The system proposed by us [3], suitable for regional coverage (e.g. Europe), assumes zero or negligible orbit inclination and compensates the spread losses by long integration time. We define such system as "TeLecom COMPatible" (TLCOMP) as the whole RADAR sensor would be implemented as a payload in a commercial telecommunication satellite. The main advantages of the TLCOMP system are the fast revisit time, being the same region imaged twice a day, and the South-North Line Of Sight (orthogonal to conventional LEO SAR system LOS). The proposed GEOSAT system would be a valid and powerful aid to Earth environment study, allowing the near real time monitoring of volcanoes, glaciers, landslides and atmosphere.
The main limitation to the proposed TLCOMP concept is the atmosphere turbulence. Indeed the fluctuation of the delay during the long integration time (up to 8 hours), due to the Atmospheric Phase Screen (APS), would generate azimuth resolution loss and image blurring [4] also very stable targets. The APS appears as an additional delay in the radar acquisitions and then as an additional phase term in the impulse response function. It is related to the variation of atmospheric refractivity along the GEOSAR line of sight. If not correctly compensated the APS results in an increase of the azimuth resolution w.r.t. the achievable minimum optimal value. The APS is a stochastic process and shall be analyzed through a statistical approach [5]. The final image degradation is proportional to the APS power, i.e. to the variance of the process.
The present paper proposes an efficient method for the APS estimation and compensation, performed at the same time of the azimuth compression of the GEOSAR data. The goal is achieved through a multi-aperture processing, where the acquired data are divided into azimuth sub-apertures [6], processed in hierarchical sets of iterations. The sub-apertures, separately processed at coarse resolution, are exploited for the differential APS compensation, and then recombined into larger apertures to increase resolution of both data and APS. At the last step the full resolution azimuth compressed data are obtained along with the estimated APS map. The proposed method, allowing to exploit parallelization techniques, would reach a very high computational efficiency.
The paper will show the results and the performances of the proposed method over GEOSAR simulated data.

[2] W. N. Edelstein, S. N. Madsen, A. Moussessian, and C. Chen, "Concepts and technologies for synthetic aperture radar from MEO and geosynchronous orbits" Proc. SPIE 5659, Enabling Sensor and Platform Technologies for Spaceborne Remote Sensing, January 2005
[3] C. Prati, F. Rocca, D. Giancola, and A. Monti Guarnieri, "Passive Geosynchronous SAR system reusing backscattered digital audio broadcasting signals," IEEE Transactions on Geoscience and Remote Sensing, vol. 36, no. 6, pp. 1973-1976, November 1998. [4] D. Bruno, and S. E. Hobbs, "Radar Imaging From Geosynchronous Orbit: Temporal Decorrelation Aspects," IEEE Transactions on Geoscience and Remote Sensing, vol.48, no.7, pp.2924-2929, July 2010
[5] L. Iannini, and A. M. Guarnieri, "Atmospheric phase screen in ground-based radar: Staistics and compensation," IEEE Transactions on Geoscience and Remote Sensing Letters, vol.8, no.3, pp.537-541, May 2011
[6] K. H. Wu, and M. R. Vant, "Coherent sub-aperture processing techniques for synthetic aperture radar," Canadian Communications Research Center, Rep. 1388, January 1984