Conference Agenda

Copernicus Sentinel-1 with its dedicated polar acquisition scheme provides the basis for monitoring ice flow velocity of the Greenland and Antarctic ice sheets at unprecedented spatial and temporal sampling. Continuous observations of the ice sheet margins started in October 2014 and are augmented by dedicated ice sheet wide mapping campaigns which enables the operational monitoring of key climate parameters such as ice velocity and mass discharge. In 2019 additional tracks were added to the regular acquisition scheme, covering the slow-moving interior of the Greenland Ice Sheet with crossing ascending and descending orbits. This offers the opportunity for routine application of the InSAR technique to improve ice velocity products that are currently derived using the offset tracking technique. Since the failure of Sentinel-1B in December 2021 the repeat pass interval increased from 6 days to 12 days affecting the signal coherence of image pairs. InSAR can provide a better precision for velocity by one to two orders of magnitude than offset tracking, particularly in slow moving sections of ice sheets. An InSAR processing line was implemented to generate ice velocity maps from Sentinel-1 IW TOPS mode SAR (C-Band) using 6- and 12-day repeat pass data. In fast moving areas and shear zones decorrelation hampers the derivation of ice velocity from C-Band data. In these regions we use available SAOCOM StripMap Mode SAR (L-Band) with 8- to 16-day repeat observations to fill in gaps. The interferometric processing of SAOCOM data turned out to be challenging, due to the reduced performance of the orbital state vectors that are needed for image coregistration over ice sheets. Nevertheless, good coherence can be achieved, enabling accurate retrieval of ice velocity. Especially in shear zones and fast-moving regions of outlet glaciers the SAOCOM L-Band data proved to be useful to resolve high ice velocity. Remaining gaps near the terminus of very fast moving glaciers, where even L-Band decorrelates, are filled using offset tracking.

We will present a Greenland Ice Sheet ice velocity maps (50 m pixel spacing) generated by means of Sentinel-1 SAR interferometry, complemented by offset tracking on fast moving sections. For key areas we exploit the synergistic use of L- and C-band SAR from SAOCOM and Sentinel-1, respectively. We show ice velocity maps demonstrating monthly and seasonal variations of ice flow and present numbers on ice discharge for selected outlet glaciers in both Greenland and Antarctica. Acquisition requirements for Sentinel-1 as well as for upcoming L-Band SAR missions (ROSE-L) will be proposed, to enable the integration of Sentinel-1 and L-Band SAR data and support continuous and improved monitoring of ice dynamics and discharge.

Ice sheets are acknowledged by the World Meteorological Organization (WMO) and the United Nations entity tasked with supporting the global response to the threat of climate change (UNFCCC) as an Essential Climate Variable (ECV) needed to make significant progress in the generation of global climate models. Several national and international programs (NASA MEaSUREs, ESA CCI) fund efforts to generate high quality geoinformation products for Antarctica and Greenland based on satellite remote sensing data. Interferometric Synthetic Aperture RADAR (SAR) data prove particularly useful for ice sheet science. With funding from NASA, our group is producing ice velocity (IV), grounding line position (GP), Ice front position (IP), as well as basin boundaries as Earth Science Data Records (ESDR).
The ice-ocean interface of a glacier is a critical boundary and is described by the grounding line (GP), which delineates where ice detaches from the bed and becomes afloat and frictionless at its base. Here we present results from our efforts to utilize spaceborne SAR data from a variety of international missions operating in different frequency bands to generate a record of grounding line positions in Antarctica. Using double difference interferometry, the flexing of the ice shelf due to differential differences in tide levels at the acquisition times results in a dense band of fringes due to the vertical displacement. The upstream boundary of this fringe band is interpreted as the InSAR grounding line. The approach requires the availability of two interferograms (or a minimum of 3 scenes acquired), an aspect that made suitable data sparse in the past. Until 2015, only a few grounding lines were collected for any given region in Antarctica. The Sentinel-1 mission with consistent acquisitions in coastal Antarctica changed the situation dramatically, resulting in data suitable for GL measurements available more frequently. Given the mission parameters (resolution, 6/12 day repeat), measurements over fast glaciers, the targets with the highest scientific relevance, continue to pose a significant challenge due to decorrelation, particularly at the grounding line. Our strategy to address this challenge is to augment the Sentinel-1 mission with data from other missions used to their strength and availability.

X-band: Cosmo SkyMED (targeted acquisition plan for fast glaciers), ICEYE selected glaciers of high scientific interest.

C-band: Sentinel-1 (Coastal coverage, all of Antarctica), RADARSAT-2 (Best effort coverage of Ross and Ronne Ice Shelves), RCM (targeted acquisition plan for fast glaciers).

L-band: ALOS-2 PALSAR-2 (targeted acquisition plan for fast glaciers and areas with more decorrelation in C-band).

While acquisitions are not formally coordinated between missions, our recommendations for acquisitions plans were carefully developed based on each mission’s strengths as well as availability. Using this virtual constellation we are able to generate a grounding line geoinformation product that is more comprehensive w.r.t. spatial coverage for Antarctica and provides more information than any product based on a single mission. We will present an overview of data availability, detail our approach for processing and data integration and show some of the challenges faced for the various missions. This work is performed at UC Irvine and JPL under a contract with NASA MEaSUREs and Cryosphere Programs.

The boundary between ice that is grounded on the bedrock and floating ice, the grounding line, is a key attribute of marine ice sheets and ice shelves. Accurate knowledge of grounding zone configuration is essential to quantify ice sheet mass loss, understand the stability of marine ice sheets and initialise ice sheet models.

An established technique for measuring grounding line position is Differential Synthetic Aperture Radar Interferometry (DInSAR), where the vertical displacement caused by tidal motion of floating ice is precisely measured. A significant limitation of this method is that it relies on interferometric coherence between SAR image acquisitions, making measurements difficult in regions of high ice speed, ice deformation, surface accumulation and melting. Furthermore, hinge zones must be delineated from interferograms manually or using AI techniques.

Intensity feature tracking measures ice motion without the requirement for interferometric coherence and due to the off-nadir viewing geometry of SAR sensors vertical tidal motion of floating ice creates an apparent, but erroneous, horizontal motion in the range direction of the satellite viewing geometry. This is usually considered an error term when measuring ice velocity, however a limited number of studies have exploited this effect by differencing range velocity results from multiple image pairs to measure grounding line location in the differential range offset tracking method. Here we significantly build on this methodology to develop a full time-series approach to map grounding line position by measuring the correlation between modelled tidal motion and velocity tracking anomaly using the full timeseries of Sentinel-1 IW mode imagery. This method eliminates the need for manual digitization by facilitating automated delineation of grounding line by contouring the correlation field.

We validate this methodology in the Antarctic Peninsula region by comparison to existing grounding line products and Sentinel-1 DInSAR measurements concurrent with our period of observation. We demonstrate that this method is suitable for measuring the grounding line position of both large ice shelves and glaciers as narrow as 3 km. Performance is best in high tidal amplitude areas such as the Larsen-C Ice Shelf, however we demonstrate that the method also performs well in low tidal amplitude zones, such as the George VI Ice Shelf, and further show that grounding lines can be mapped at annual temporal resolution.

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In recent decades, the global atmospheric warming accelerates at a pace that is unprecedented in the past 2000 years. Glaciers and ice caps have begun to react with increased imbalance and respective volume loss. Since about 70% of the world's glaciated area outside the polar ice sheets is located at latitudes above +55°, our focus for this study is on that extended arctic region. Comprehensive observations in this vast study area cannot easily be provided other than by space-borne imagery. Given the prevalence of cloud cover in the Arctic atmosphere, microwave remotes sensing offers the great advantage of continuous and broad coverage. Thus, we use observations of the twin satellite mission TanDEM-X, which is a bistatic SAR interferometer mission optimized for terrain modelling. Previous studies have proven the resulting DEMs a reliable data source for glacier related investigations with the geodetic method.

This presentation gives insight in the surface elevation change measurements and resulting geodetic mass balance estimates of arctic glaciers and ice caps in the last decade. Surface elevation change datasets are calculated for the period of 2011/12 to 2017/2018 by creating merged datasets form selected scenes from the mission archive catalogue. We investigate glaciers identified in the Randolph Glacier Inventory (RGI) to cover more than 400,000 km², spread over the landmasses adjacent to the Arctic Ocean and the Gulf of Alaska. Therefore, the dataset comprises over 11500 single scenes forming regional change datasets, to provide the overall picture.

Each CoSSC underwent a differential InSAR processing chain. In a first step, data scenes from a consecutive acquisition in along track direction, where larger glaciated terrain was covered, are re-concatenated to continuous data takes. For each take individually differential interferograms are calculated with the TanDEM-X global DEM as elevation reference. Following phase unwrapping the differential phase converts to differential elevations. Re-added to reference elevation, we obtain absolute heights for resulting DEMs of each take. Prior to the calculation of elevation differences, a post-processing pipeline aligns the DEMs iteratively in x, y and z coordinates to compensate for systematic height errors. To derive elevation change rates, both DEMs are resampled and projected to 30x30 m ground resolution in Polar Stereographic North projection. These DEMs serve as tiles for regional rasters to eventually derive surface elevation change.

The comprehensive coverage allows for conversion into Mass Balance estimates that are provided at RGI regional level. At the date of presentation this study should provide an estimate for all RGI regions that have an Arctic territorial share.