Airborne synthetic aperture radar observations and simulations for waves in ice

Abstract

The Canada Centre for Remote Sensing CV‐580 aircraft collected C‐band synthetic aperture radar (SAR) data over the marginal ice zone off the east coast of Newfoundland during the Labrador Ice Margin Experiment (LIMEX) in March 1989. One component of the LIMEX'89 program was the study of ocean waves penetrating the marginal ice zone. In this paper, we consider nearly coincidental observations of waves in ice by airborne SAR and wave‐induced ice motion measurements. We explain the wave patterns observed in the SAR imagery, and the corresponding SAR image spectra, in terms of SAR wave imaging models. These include the well‐known tilt cross‐section modulation, linear, quasi‐linear, and nonlinear velocity bunching forward mapping models (FMMs), and the assertion that the concept of coherence time limitation applies differently to the cases of waves in ice and open water. We modify the concept of the scene coherence time to include two parts: first, a decorrelation time deduced from the inherent azimuth cutoff in the nonlinear velocity bunching FMM; and second, the intrinsic scene coherence time which is a measure of the time scale over which an open water Bragg scattering patch retains its phase structure. Either of these coherence time scales could dominate the SAR image formation process, depending upon the environmental conditions (the wave spectrum and the wind speed, for example). These two coherence time scales are independently estimated based upon a quasi‐linear velocity bunching FMM applied to some of the LIMEX'89 observations. Observed SAR image spectra and forward mapped ice motion package spectra are favorably compared.