Emulating Sentinel-1 Doppler Radial Ice Drift Measurements Using Envisat ASAR Data

Abstract

Using data from the Envisat ASAR instrument, this paper demonstrates how the high precision radial surface velocity product, which will become available with ESA's Sentinel-1 satellite, can complement the analysis of sea ice motion. Highresolution Doppler frequency measurements are used to estimate the sub-second line-of-sight motion of drifting sea ice in Fram Strait. We compare the method with buoy measurements as well as a recent cross-correlation algorithm for tracking ice between pairs of images. Maximum speeds measured from the time series were in the order of 20 cm/s. Using our method we measured instantaneous speeds reaching 40-60 cm/s.

Sea ice local surface topography from single-pass satellite InSAR measurements: a feasibility study

“…4b, the open water areas bounded by the light blue lines reveal alternating dark and bright strips in the SAR image. This pattern is typical for Langmuir circulation, in which streaks of ice nearly parallel to the wind direction appear on the water surface (Leibovich, 1983). The streaks are visible manifestations of the convergence zones between counter-rotating vortices that are present in the near-surface water layer, with their axes of rotation parallel to the wind.…”

“…Table 5 reveals that extreme instantaneous lineof-sight velocities, such as reported by Kraemer et al (2015), cause significant phase shifts already at very short baselines, in particular at higher radar frequencies. For line-of-sight velocities larger than 0.2 m s −1 , the change of the critical alongtrack baseline is only small (see Fig.…”

“…The former is largest in the zones of convergence and smaller in the zones of divergence. The latter changes direction between neighboring vortices (Leibovich, 1983). The large "height" variations in the open water areas of Fig.…”

“…For a given phase difference the corresponding along-track baseline depends linearly on λ and v and decreases with increasing u LOS . Typical average sea ice drift velocities range mostly from 0 to 0.35 m s −1 (1.26 km h −1 ) (Rampal et al, 2009), but for instantaneous radial velocities Kraemer et al (2015) found values up to 0.6 m s −1 from analyses of the Doppler shift of SAR signals. For TanDEM-X, the ground velocity is 7 km s −1 .…”

“…(1), surface topography and movement affect the measured interferometric phase simultaneously, it is necessary to obtain independent data of the line-of-sight velocity. This can in principle be achieved by estimating the Doppler centroid from the unfocussed SAR data as described in Kraemer et al (2015). For this method, however, a sufficient number of neighboring pixels has to be averaged, resulting in spatial resolutions on the order of hundreds of meters to a few kilometers.…”