Envisat ASAR polarization experiments in the Peter the Great Bay, Japan Sea: Preliminary results.

L. Mitnik, V. Dubina, O. Konstantinov
V.I. Il'ichev Pacific Oceanological Institute FEB RAS
43 Baltiyskaya St., 690041 Vladivostok, Russia
Phone: 7-4232-312-854 Fax: 7-4232-312-573
mitnik@poi.dvo.ru

ABSTRACT

Among the new specificities of the Envisat ASAR, polarization diversity makes the instrument very promising. The Alternating Polarization (AP) mode provides two simultaneous images from the same area with three possible polarization combinations: HH and VV, HH and HV, VV and VH with high spatial resolution as in single polarization mode but with degraded radiometric resolution. Series of the ASAR images with AP was acquired over the Peter the Great Bay, Japan Sea in September 2003 and in May-September 2004. Only HH and VV polarization combination was used with two different mean incidence angles: 18.3° (4 pairs of images) and 22.5° (3 pairs of images). Additionally, the same scenes were sensed by ERS-2 SAR with a time delay about 30 min after ASAR acquisition. The main aim was to study the polarimetric signatures of the sea surface caused by the oceanic dynamic phenomena as well as by natural and anthropogenic slicks for later use for their detection and classification. During ASAR and SAR data acquisition, ground-truth measurements of sea surface temperature, wind speed and direction as well as polarization images of the sea surface recorded by a polarization spectrophotometer were arranged at POI Marine stations and at several coastal stations. Wind speed during satellite observations did not exceed 7 m/s and as a result the radar signatures of eddies, currents, internal waves and slicks were revealed on the SAR images. The HH and VV radar cross-section σ° obtained from Envisat ASAR images decreases with angle of incidence θ and the polarization ratio R increases with θ. Variations of R in the transition areas from the clean sea surface to slick reached 2-4 dB that calls for further investigations. Detailed comparison of σ° and sea surface roughness parameters (wave spectra and wave height variations, slope histograms, etc.) retrieved from coastal optical polarization observations was carried out and the first results of the joint analysis are discussed. Comparison of Envisat ASAR and ERS SAR scenes allowed estimating the sea surface current velocities using slicks as a tracer.

Last Update: 2005-03-16