Ers – Envisat Tandem Data over Sea and Shelf Ice

ERS – ENVISAT Tandem (EET) data are SAR data pairs acquired by the ERS-2 and ENVISAT ASAR instruments from approximately the same orbits within 28 minutes. During two dedicated campaigns in 2007/2008 and 2008/2009 ESA specifically acquired EET pairs with baselines suited for cross-interferometry (CInSAR). At perpendicular baselines of about 2km the frequency difference and baseline effects on the reflectivity spectrum compensate and so coherent interferograms can be obtained. Sea and shelf ice has usually a relatively flat surface and so there is little topographic phase observed, even for 2km perpendicular baselines. Centimeter scale motion occurring during the 28 minute interval results in deformation phase. Motion at this rate is often observed for sea and shelf ice and is of interest to understand dynamics and stress occurring. More open sea ice moves at even much higher rates. Here EET coherence is typically lost but offset tracking and split-beam interferometry may be used to retrieve motion fields. The sensitivities of these techniques are in the order of 1/20 of a SAR image pixel. So in azimuth direction this translates to a sensitivity of about 20cm per 28 minutes interval and in cross-track direction to about 1m per 28 minutes interval. On the other hand maximum rates which can reliably be retrieved correspond to offsets of several pixels between the two acquisitions corresponding to rates up to the order of 100m per 28 minutes interval. Repeat-pass interferometry, offset tracking, and splitbeam interferometry were already applied in the past using data at different wavelengths and time intervals. So far the shortest interval available was 1 day from the ERS-1/2 Tandem mission. Now, EET data offers data at a much shorter 28 minutes interval. Consequently, faster motion rates can be investigated.