A DOPPLER SPECTRAL METHOD OF IDENTIFYING WATER AND ICE IN MARGINAL ICE-ZONE IMAGERY

Complex synthetic aperture radar imageries at C band and X band acquir ed during March 1989 in the Greenland Sea are specially processed. Thi s processing yields Doppler spectra from two backscatter regions near the marginal ice zone: (1) newly forming ice inside a tongue-shaped re gion called the Odden and (2) open water adjacent to the Odden. We obs erve a consistent broadening of the Doppler bandwidth in the water spe ctra compared to the ice spectra. Short ocean waves (wavelengths < app roximately 10 m) are attenuated as they travel into the ice, whereas l ong ocean gravity waves (wavelengths much greater than 10 m) exhibit l ittle attenuation. A model is derived describing the Doppler bandwidth in terms of the platform velocity, the short waves, and the long wave s. The short subresolution waves and the long gravity waves contribute to the Doppler broadening in the ocean. However, for the gentle ocean swell observed here we show that the long-wave contribution can be ne glected. Therefore the differential broadening measured between the wa ter and the ice is primarily due to the short random waves. The Dopple r bandwidth broadening is DELTAf(a) almost-equal-to 30 Hz at C band an d DELTAf(a) almost-equal-to 40 Hz at X band, whereas the measured band width due to the platform velocity (155 m/s) is approximately 75 Hz at both C band and X band. The broadening can also be given in terms of the scene correlation time; 2tau(c) almost-equal-to 22 ms and 16 ms at C band and X band, respectively, under certain assumptions.