Compact versus full POLARIMETRIC SAR
(1) Canada Centre for Remote Sensing (CCRS), 588 Booth St., Ottawa,, Ontario K1A 0Y7, Canada
Radar polarimetry has finally reached its golden age with the recent launch of fully polarimetric satellite SARs; TerraSAR and ALOS PASAR with experimental modes, and more recently RADARAT2 with operational polarimetric modes at various incidence angles (between 20 and 40) and fine and standard 9 and 24 m resolution. This stage has been reached thanks to a big effort in research and development being invested since the release of the first polarimetric radar concepts in the 1950th. Recently, the compact mode concept firstly suggested in the 1970th has been resuscitated [1, 2]. Basically, the Compact SAR transmits at a polarization that is a combination of equally weighted horizontal and vertical polarizations (at 45 degree or shifted in phase by 90 degree for a circular polarization). Souyris et al  have shown that the compact SAR provides information similar to the one provided by fully polarimetric SAR for azimuthally symmetric targets. They showed that it is possible to reconstruct from the Compact SAR measurements equivalent to the Cloude-Pottier parameters and H . The reconstructed , H, combined with the Wishart distribution lead to a classification of agriculture fields and forested areas similar to the one obtained with a fully polarimetric SAR. Similar conclusions were reached by K. Raney , who promotes in particularly the hybrid SAR (transmitting at circular-polarization and receiving at horizontal and vertical polarization) whose architecture is much simpler than conventional circularly polarized SAR. Since Compact SAR provides the double swath that fully polarimetric SAR can provide, Compact polarimetry has been gained a significant support (versus polarimetry), and it is urgent to promote again the significant advantage of using full polarimetric SAR versus Compact SAR for key applications.
Recently, we have shown that the target scattering cannot be unambiguously characterized by a real entity, as it is currently done with the Cloude . We have introduced a new decomposition, the Touzi decomposition , which characterizes uniquely target scattering with a complex entity, the symmetric scattering type of magnitude s and phase s and the target helicity. We have shown that target scattering phase s is much more powerful than scattering type of magnitude s (equivalent to Cloude for target of symmetric scattering) for wetland classification  . In contrast to s (and ), s separate well conifers from deciduous trees during leafy conditions. s separates also well poor fens from bogs, two classes that cannot be discriminated using the radiometric information provided by s, , and the multi-polarization HH-HV and VV . The equivalent parameters reconstructed from Compact polarimetry are generated. It is shown that the Compact parameters lead to a poor classification in comparison with the one obtained with fully polarimetric information when the polarimetric scattering phase information is fully exploited. Fully polarimetric SAR perform also better than Compact SAR at the presence of targets of low cross-polarization signal. Finally, compact SAR is compared with conventional dual polarization HH-HV and implications for the design of new missions are considered.
 J.-C. Souyris, P. Imbo, R. Fjortoft, S. Mingot, and J.-S. Lee, “Compact
polarimetry based on symmetry properties of geophysical media: The π/4 mode,” IEEE Trans. Geosci. Remote Sens., vol. 43, no. 3, pp. 634–646, Mar. 2005.
 R.K. Raney, “Hybrid-Polarimty SAR architecture”,
IEEE Trans. Geosci. Remote Sens., vol. 45, no11, pp3397-3404, Nov. 2007.
 R. Cloude and E. Pottier, “A review of target decomposition theorems in radar polarimetry”, IEEE Trans. Geoscience Rem. Sens., Vol. 34, No. 2, pp. 498-518, March 1996.
 Touzi, R., “Target Scattering Decomposition in Terms of Roll-Invariant Target Parameters ”, IEEE Trans. Geoscience Rem. Sens., Vol. 45, No. 1, Jan. 2007.
 R. Touzi, A. Deschamps and G. Rother, “Wetalnd characterization using polarimetric Radarsat-2 capability”, Can. J. Rem. Sens., Vol. 33, No. 1:S56–S67, 2007