Polarimetric PALSAR System Model Assessment and Calibration
Ridha Touzi(1) and N. Shimada(2)
(1) Canada Centre for Remote Sensing, 588 Booth St., Pttawa, Ont. K1A 0Y7, Canada
(2) Japan Aerospace Exploration Agency, Tsukuba, Japan, 104-6023, Japan
Like L-band JERS-1 SAR, ALOS PALSAR is affected by Faraday rotation. The eventual presence of Faraday rotation in addition to the uncertainty regarding the actual isolation of the H-V PALSAR antenna, lead to mixed conclusions (from the ALOS Cal-VAL team) regarding the actual isolation of the H-V PALSAR antenna. In the last Pol-In’07 workshop, it was concluded that the antenna isolation is not better than -25 dB, and this should make the system calibration more complicate in the presence of Faraday rotation, and mainly for areas with significant topographic relief.
In this paper, ALOS-PALSAR data sets collected over various calibration sites are used for the assessment and validation of the polarimetric PALSAR system model. In each site, one or several corner reflectors has been deployed during the acquisition.
PALSAR data collected over the Amazonian forest and the CCRS calibration site in Ottawa will be used. DLR and the Chalmers university of technology are thanked for having kindly provided us with the data collected over their calibration sites in Germany and Sweeden, repectively. First, the PALSAR system parameters are assessed under low Faraday rotation conditions, using the Amazonian forest data sets The Freeman-Van Zyl calibration technique , which symmetrizes the system prior to the estimation of the distortion matrix elements, is adopted for this study. Van-Zyl’s technique for determination of system distortion matrix elements is reconsidered, and additional equations  are used to optimize the estimation of the calibration system unknowns even when the azimuthally symmetric reference target is of very low HV return in comparison with HH, VV, and the HH-VV cross-correlation. The new method is validated, and the effect of the azimuthally symmetric reference target’s multi-polarization return on the accuracy of the system parameters, is assessed. The method is then applied to two data sets collected in July 2006 over the Amazonian forest calibration site, with a corner reflected deployed for calibration purposes. It is shown that the system is highly isolated with an antenna H-V isolation better than -35 dB. The temporal stability of the system is also validated using the four data sets collected over Ottawa. The analysis of the response of a corner reflector deployed in the Ottawa calibration site confirms the low H-V antenna isolation, and indicates insignificant Faraday rotation errors during the four measurements. The deployment of a corner reflector is still required to correct for Faraday rotation errors  that might increase in the future. For practical reasons, we have introduced a new calibration method [4, 5], which uses human-made and natural target polarization characteristics for calibration of the polarimetric data. The new calibration method is based on the Touzi-Decomposition introduced in , which permits a unified and roll-invariant decomposition of coherent and partially coherent target scattering. Point and distributed target orientation and scattering parameters are used to measure and remove Faraday and channel imbalance errors from the PALSAR data, which are firstly corrected for the antenna gain and channel cross-talk incidence angle variations derived from the Amazonian forest measurements.
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