Analysis of polarimetric signatures of Arctic lead ice using data from AIRSAR and RADARSAT
Examensarbete för masterexamen
In December 2004 the JPL airborne synthetic aperture radar (AIRSAR) flown on the NASA DC-8 acquired fully polarimetric data in the Beaufort Sea at C, L and P-band. This work presents the backscatter coefficients (σHH, σVV, σHV), copolarized ratios (σHH/σVV) and copolarized phase differences (ϕHH-VV) at the three frequencies from various sea ice types and in particular different young sea-ice types formed in recently frozen leads. Two weeks of RADARSAT imagery from the same region as the AIRSAR data was used in order to identify when and where leads were formed and the age of the newly formed ice contained within the leads. By using a known empirical relationship based on freezing degree days, the thickness of a sea-ice layer could be estimated from its age. Several stages of new and young ice were identified and examined by using RADARSAT imagery to track back in time to find sea-ice openings: lead ice 1-2 days old, lead ice 2-3 days old, lead ice 9-14 days old, lead ice/first year ice older than 15 days and multi-year ice. Furthermore, narrow cracks with unique signatures, hypothesized to be a few hours old, found in several 1-2 days old leads have been included in the analysis. In addition to report the polarimetric signatures of these ice types, this study seeks to find which combination of polarimetric parameters that best differentiates the primary ice types as well as the stages of new and young ice within the leads. Ice that is thought to be less than one day old has the lowest backscatter coefficients for C- and L-band. This ice type also has significantly larger copolar phases for C- band than the other ice types and is the only ice type that shows negative phase differences for L-band, in correspondence with previous reported results for very young ice. Furthermore, it has the lowest observed C-band copolar ratios. Ice 1- 2 days old is characteristic for the large variability of the significantly negative P-band phases. Ice up to three days old has the lowest C-band copolar ratios. Using copolar ratios and phase to discriminate ice of age 9-14 days and older than 15 days was difficult, but L-band backscatter appears to give more contrast. The analysis of the phase information shows large variations between the thin ice types but it is hard to draw unambiguous conclusions. However, generally the phases for thin ice have much larger variability and typically have an offset relative to multi-year ice. The best separations of thin ice are found by using C-band channels and for combinations with different polarizations. C-band crosspolarized backscatter coefficient combined with C-band copolarized ratio has been determined as giving the largest separation causing less misclassification, using a discriminant classifier. Ice less than one day and multi-year ice are the ice types that separate best from the other types while the other ice types more or less overlap for most combinations. Severe mixing between ice types occurs for combination of different L-band channels. In future work the calibration quality of L- and P-band needs to be assessed. More time must be spent on trying to define different ice types and how to pick ice samples minimizing the effect of inhomogenities in the different polarimetric channels. Methods to determine normalized classification accuracies, independent of the number of samples of each class, must be considered.
Rymd- och flygteknik , Aerospace Engineering