Conference Agenda

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Session Overview
Session 3B - Ice Sheet retrieval and techniques
Wednesday, 22/Mar/2017:
10:40am - 12:20pm

Session Chair: Andrew Shepherd
Session Chair: H Jay Zwally

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10:40am - 11:00am

Cryosat-2 and Sentinel-3 SAR-Mode Altimetry Performance over the Antarctic Ice Sheet

François Boy1, Jérémie Aublanc2, Thomas Moreau2, Frederique Remy3, Denis Blumstein1, Nicolas Picot1, Pierre Thibaut2


During the past 30 years, Earth’s polar region has been continuously monitored by satellite altimetry. Thanks to their wide spatial coverage and relatively high temporal sampling, radar altimeters have greatly improved our knowledge of the ice-sheets topography and our understanding of the ice sheets dynamics. Over that time period, all radar altimeter missions flying over ice-sheet surfaces (GeoSat to Altika) have been operating in conventional Low Resolution Mode (LRM). Despite constant improvements in resolution and precision, LRM instruments still suffer from several limitations and uncertainties, notably due to their large radar footprint: 10km to 20km (depending on mission).

Unlike its predecessors, the Cryosat-2 and Sentinel-3A satellites carry on-board a new generation of radar altimeter instrument operating in a Synthetic Aperture Radar (SAR) mode. This mode allows reducing the alongtrack resolution to 300 meters, that would make it possible to capture finer-scale topographic variations of icesheet surfaces. The SAR-mode performances have been thoroughly analyzed in open ocean with Cryosat-2 data. However their abilities in monitoring of ice-sheet surfaces have still to be assessed.

SAR-mode radar altimeter data have been recently analyzed over the Antarctica continent, with sporadic Cryosat-2 acquisitions performed in winter 2014 and Sentinel-3A SRAL data acquired in spring 2016. Those data have been processed through the CNES prototype chains (the CPP for Cryosat-2 and the S3PP for Sentinel-3A) and the use of innovative and dedicated algorithms to this particular surface. This paper presents a comprehensive study of the SAR-mode performance over ice-sheet surfaces in comparison with LRM one, focusing on analyses of the waveform shapes, the accuracy of the retrieved surface elevation , its sensitivity to surface slope and penetration effects into the snow/ice layers. This work clearly demonstrates the improved icesheet surface measuring capability offered by SAR-mode altimetry with respect to conventional radar altimetry.

To finish, we will present a R&D study driven by CNES which aims at building a Digital Elevation Model over Antarctica by exploiting data from Cryosat-2, Sentinel-3 and Altika missions. In this part, we will focus on the different challenges adressed through this initiative.

11:00am - 11:20am

Comparison of Interferometric and Non-Interferometric SAR Altimetry over Ice Sheets

Malcolm McMillan1, Andrew Shepherd1, Alan Muir2, Julia Gaudelli2, Robert Cullen3

1University of Leeds, United Kingdom; 2University College London, United Kingdom; 3ESA, ESTEC, The Netherlands

The launch of CryoSat-2 in April 2010 provided the first spaceborne interferometric SAR altimeter measurements over the Polar Ice Sheets. The mission’s novel interferometric mode of operation was specifically designed to enable monitoring of rapidly-changing coastal regions of Greenland and Antarctica, where complex topography has limited the reliable retrieval of geophysical signals by past altimeter missions. In the 6.5 years since launch, CryoSat-2 has demonstrated the utility of interferometric altimeter systems for polar monitoring, and their value for establishing the contribution of ice sheets to global sea level rise. Given that CryoSat-2 had a design lifetime of 3.5 years, and has been in extended operations since October 2013, there is a need to investigate the expected performance of other sensors. In particular, as the requirement for continuity of long term measurements grows, it is important to assess the capacity of a non-interferometric mission to retrieve estimates of ice sheet elevation and elevation change.
Here we present the results of the CryoSat Follow-on SAR trade-off study, a 6 month project funded by ESA to investigate the relative performance of CryoSat-2 interferometric and non-interferometric altimetry. By processing CryoSat-2 SARIn data without interferometric information, and relocating the echoing point using an external slope model, we emulate ice sheet margin SAR retrievals from the CryoSat-2 system. In total, we generate 4 years of CryoSat-2 pseudo-SAR data covering the entire Antarctic Ice Sheet margin. We compare these measurements to interferometric retrievals spanning the same period, to assess the relative accuracy and precision of estimates of ice sheet elevation and elevation change derived from the two modes of operation.

11:20am - 11:40am

Influence of Retracker on Ice-Volume and Mass Change Estimates of Greenland and Antarctica

Veit Helm1, Angelika Humbert1,2, Stefan Ligtenberg3, Peter Kuipers Munneke3

1Alfred Wegener Institut, Germany; 2University of Bremen, Germany; 3Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, The Netherlands

Recent contribution of ice sheets to sea level change is topic with relevance for the society and a challenging topic for glaciologists. For the assessment of the contribution of ice sheets to sea level change robust, consistent processing, as well as the estimation of uncertainties is important. For this purpose we analyse altimeter data of the two ESA satellites CryoSat-2 and ENVISAT covering a time period from 2002 to 2017 and will present a time series of volume change and as a final product mass change estimates using a firn densification model provided by IMAU. This presentation focuses on large differences arising from using different re-tracker strategies applied to data acquired over the Greenland and Antarctic ice sheets. We will present a set of estimates derived from three different re-tracking approaches. To verify our results we will compare our radar-altimetry elevation change rates with rates obtained from ICESat data covering the same time period (2003 to 2009), as well as ENVISAT using the same interpolation approach. Additionally, we will compare our altimetric derived mass change estimates with the time series observed by GRACE.

11:40am - 12:00pm

On a Path Towards the Reassessment of Antarctic Volume Change: Synthesis of ESA CryoSat-2 Radar and NASA Airborne and Satellite Laser Altimetry Observations

Johan Nilsson, Alex Gardner

Jet Propulsion Laboratory, California Institute of Technology, United States of America

The West Antarctic Ice Sheet has experienced rapid changes in its surface elevation in response to glacier thinning that results from accelerated flow of glaciers feeding into the Amundsen and Bellingshausen Seas. Changes in the surface elevations over the East Antarctic Ice Sheet and over “stable” ice shelves are much more subtle(~ ± 1 cm a-1) and approach the limit of detectability from space. Here we present a reassessment of changes in grounded ice volume for the Antarctic Ice Sheet using a new technique to merge radar and laser altimetry, from both airborne and satellite data, to produce long-term and robust assessments of elevation change rates for the Antarctic Ice Sheet. The applied merging procedure allows the local solution to estimate the inherent elevation bias between laser and radar-derived elevations, due to signal penetration of the radar wave into low-density firn in the upper strata of the ice sheets surface.

To determine the volume change of the Antarctic Ice Sheet a total of five altimetry missions were used, consisting of both airborne and satellite observations. Airborne surface elevations were gathered form NASA’s Operation IceBridge (Land, Vegetation, and Ice Sensor (LVIS), Airborne Topographic Mapper (ATM) and the Riegl Laser Altimeter from University of Texas) and satellite derived elevations in the form of the Ice, Cloud, and land Elevation Satellite (ICESat) and ESA’s CryoSat-2 mission. These multi-temporal surface elevations, spanning the time period of 2003-2016, were then merged locally at 1-km resolution using an adaptive least-squares minimization, accounting for both the spatial and temporal data availability in the solution. This point wise solution provides ice sheet wide estimates of elevation change with accompanying time series that were then gridded to 5 km resolution using optimal interpolation.

The quality of the merged solution was further judge by local comparison of elevation change rates estimated over the same time period from multi-mission satellite crossover analysis. Finally, the derived volume change rates are placed in the context of other published estimates of Antarctic Ice Sheet volume change spanning the time period of 2003-2016.

12:00pm - 12:20pm

Combining Data Sets to Improve the Vertical and Spatial Resolution of Cryosat-2 Elevation-Change Mapping.

Benjamin E. Smith1, Alex Huth2

1University of Washington APL, United States of America; 2University of Washington Dept. of Earth and Space Sciences, United States of America

Cryosat-2 is currently unique for providing dense, well-located measurements of elevation and elevation change throughout coastal Greenland and Antarctica. The point-of-closest-approach (POCA) measurements allow seasonal recovery of elevation and elevation change at horizontal resolutions of a few km, with sub-meter precision, but recovering smaller-scale features, at higher spatial or temporal resolution, can be difficult using these data alone. In this presentation, we discuss techniques to include additional data in global solutions for elevation and elevation change. Additional sources of information include swath-processed Cryosat-2 returns, laser-altimetry data, stereo-photogrammetry DEMs (digital-elevation models), and surface-slope information from sunlit optical imagery. As an example, Cryoat-2 data can be used to estimate biases in DEMs from Worldview stereo pairs, allowing their use in estimating elevation and elevation change in areas where other control data are not available. We provide examples for how each type of data can be used in glaciological problems, including outlet glacier elevation change, ice-shelf change mapping, and subglacial lake discharge mapping.

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