DETECTION OF FEATURES IN FIRST-YEAR PACK ICE BY SYNTHETIC-APERTURE-RADAR (SAR)

Observations by specialized sonars moored beneath drifting pack ice in the Beaufort Sea have been used to guide interpretation of SAR imager y under Arctic winter conditions. The sonars measured the draft, botto m-side topography and kinematic history of the ice field during the sa me time that SAR images were acquired by ERS-1 (satellite-based C-band ) and STAR-2 (aircraft-based X-band) radar systems. Inter-comparison o f the observations revealed that ridges crossing first-year pack ice c annot be detected reliably at the low incidence angles used by ERS-1. However, at the high incidence angles typical of airborne SAR, there i s sufficient contrast to enable unambiguous discrimination of ridges a nd intervening flees. A strong correlation was observed between the sp atial frequency of ridges and the average draft of the ice field. Ther efore, a count of ridges intersecting a line across a SAR image can be used as a proxy for ice draft. A linear relation was observed between radar brightness and draft of ridges. This is probably a consequence of a progressive filling of the radar pixel by the ridge sail as the d raft of the ridge increases. The relation between the frequency and ma ximum backscatter of ridges viewed by radiometrically calibrated SAR h as potential as a surrogate for the keel frequency-draft function. Est imates of ridge frequency derived via SAR can, however, be biased, sin ce both ridges and young leads appear as bright filaments in imagery. With ERS-1, there is a large variation in the microwave backscattering coefficient of sea ice during the early growth under cold conditions. This variation provides a clear indication of ice age and stage of de velopment. For older pack ice, ERS-1 can provide only an aggregate mea sure of the backscatter from unresolved ridges and level Bees in a sel ected area. The strength of this backscatter is a poor indicator of ag e or thickness because the relative contributions by ridges, frost flo wers and smooth flees cannot be determined.