ALOS PALSAR for offshore wind mapping
Charlotte Hasager(2), Donald Thompson(1), Frank Monaldo(1) and Merete Christiansen(2)
(1) JHU APL, 11100 Johns Hopkins Road, MD 20723-6099, United States
(2) Risoe DTU, Frederiksborgvej 399, 4000 Roskilde, Denmark
ALOS PALSAR scenes acquired over the wind farm site Horns Rev in Denmark includes 15 geo-referenced images recorded in the period between 3 October 2006 and 9 June 2007. These scenes can be processed with existing software from JHU/APL.
The first step in the data processing is calibration of digital values to the normalized radar cross section (NRCS) in dB using the ALOS PALSAR Level 1.0/1.5 Data Format (Revision I) Document from the Remote Sensing Technology Center of Japan for the data calibration. Monaldo et al. 2007 report the results of the distribution of NRCS as a function of the radar incidence angle that is different from distributions found from two airborne SAR data sets (German E-SAR and Danish EMISAR). The airborne data sets suggest a concave distribution of NRCS decreasing with increasing incidence angles. The ALOS PALSAR data, in contrast, show a peak of NRCS at intermediate incidence angles. We speculate that the reason for this deviation might be 1) a poor calibration of the ALOS PALSAR data, 2) an error related to the radar geometry, or 3) incorrect reading or application of the calibration factor. The calibration issue must be resolved before reliable wind retrievals from the ALOS PALSAR data can be performed.
Based on the two airborne data sets acquired in L-band (fully polarised), a new model function has been developed for L-band wind retrievals (LMOD). A very good agreement has been achieved between the measured and modeled NRCS for both VV and HH polarisation. The application of the LMOD function to the ALOS PALSAR scenes has resulted in maps of wind speed showing values that are much too high around the image centerline where NRCS peaks.
Future goal is to compare wind maps from ALOS PALSAR to offshore wind data from the Horns Rev site observed from a meteorological mast.
Monaldo, F.M., Thompson, D. R. and Christiansen, M.B. On ALOS PALSAR calibration and wind retrieval. Johns Hopkins University Applied Physics Laboratory, Report SRO-07-003, p. 1-7