TerraSAR-X Capabilities in Polar Regions

DLR participates in a coordinated plan established by the space agencies for the optimum use of SAR acquisitions over the Artic and Antarctica for the remaining period of IPY and beyond. Due to the specific advantages of the X-band in respect to snow and ice properties and the high spatial resolution of the data, the contribution of TerraSAR-X focuses on topics like mapping of seasonal snow cover at high latitudes, ice sheet velocity, velocity fields of slow and fast moving glaciers, permafrost, generation of DEMs, sea ice classification. These are reflected in a coordinated proposal which is prepared by DLR and the scientific community as part of the common polar SAR acquisition plan

InSAR grounding line mapping with the TSX/TDX/PAZ constellation for fast Antarctic glaciers

The grounding line positions of Antarctic glaciers are needed as an important parameter to assess ice dynamics and mass balance in order to record the effects of climate change to the ice sheets as well as to identify the driving mechanisms for these. In order to address this need, ESA’s Climate Change Initiative (CCI) produced interferometric grounding line positions as ECV for the Antarctic Ice Sheet (AIS) in key areas. Additionally, DLR’s Polar Monitor project focuses on the generation of a near complete circum-Antarctic grounding line. Until now these datasets have been derived from interferometric acquisitions of ERS, TerrasSAR-X and Sentinel-1. Especially for some of the faster glaciers, the only available InSAR observations of the grounding line have been acquired during the ERS Tandem phases (1991/92, 1994 and 1995/96). In May 2021, a joint DLR-INTA Scientific Announcement of Opportunity was released which offers the possibility of a joint scientific evaluation of SAR acquisitions of the German TerraSAR-X/TanDEM-X and the Spanish PAZ satellite missions. These satellites are almost identical and are operated together in a constellation therefore offering the possibility of combining their acquisitions to SAR interferograms. The present study harnesses the interferometric capability of joint TSX and PAZ acquisitions in order to reduce the temporal decorrelation between acquisitions. The revisit times are reduced from 6 days (Sentinel-1 A/B) or 11 days (TSX) to 4 days (TSX-PAZ). Together, the higher spatial resolution than Sentinel-1 and the reduced temporal baseline allows imaging the grounding line at important glaciers and ice streams where the fast ice flow causes strong deformation. These are often the glaciers where substantial grounding line migration has taken place or is suspected (e.g Amundsen Sea Sector) but where current available SAR constellations cannot preserve enough interferometric coherence to image the grounding line. The potential of short temporal baselines was already shown with data from the ERS Tandem phases in the AIS_cci GLL product and more recently but only in dedicated areas with the COSMO-SkyMed constellation [Brancato, V. et al. 2020, Milillo, P. et al. 2019]. In some fast-flowing regions, InSAR grounding lines could not be updated since. For the derivation of the InSAR grounding line, 2 interferograms (PAZ-TSX) with a temporal baseline of 4-days will be formed. It is not necessary, that the acquisitions for the two interferograms fall in consecutive cycles but is advantageous to acquire the data with limited overall temporal separation to be able to assume constant ice velocity. The ice streams where potential GLLs should be generated were identified with focus on glaciers in the Amundsen Sea Sector (e.g. Thwaites Glacier, Pine Island Glacier) but also glaciers in East Antarctica (e.g. Totten, Lambert, Denman). Besides filling spatial or temporal gaps in the circum-Antarctic grounding line, the resulting interferograms will also be used for sensor cross-comparison to Sentinel-1-based grounding lines in areas where both constellations preserve sufficient coherence. Brancato, V., E. Rignot, P. Milillo, M. Morlighem, J. Mouginot, L. An, B. Scheuchl, u. a. "Grounding Line Retreat of Denman Glacier, East Antarctica, Measured With COSMO-SkyMed Radar Interferometry Data". Geophysical Research Letters 47, Nr. 7 (2020): e2019GL086291. https://doi.org/10.1029/2019GL086291. Milillo, Pietro, Eric Rignot, Paola Rizzoli, Bernd Scheuchl, Jérémie Mouginot, J. Bueso-Bello, und P. Prats-Iraola. "Heterogeneous Retreat and Ice Melt of Thwaites Glacier, West Antarctica". Science Advances 5, Nr. 1 (1. Januar 2019): eaau3433. https://doi.org/10.1126/sciadv.aau343

Velocities of Major Outlet Glaciers of the Patagonia Icefield Observed by TerraSAR-X

The capabilities of TerraSAR-X data for feature tracking by amplitude correlation over glacier surfaces are investigated. Methodical aspects of the amplitude correlation approach are described. The TerraSAR-X based velocity fields are compared with former InSAR derived velocities and field measurements on three outlet glaciers on the South Patagonia ice field

Brief Communication: Further summer speedup of Jakobshavn Isbræ

We have extended the record of flow speed on Jakobshavn Isbræ through the summer of 2013. These new data reveal large seasonal speedups, 30 to 50% greater than previous summers. At a point a few kilometres inland from the terminus, the mean annual speed for 2012 is nearly three times as great as that in the mid-1990s, while the peak summer speeds are more than a factor of four greater. These speeds were achieved as the glacier terminus appears to have retreated to the bottom of an over-deepened basin with a depth of ~1300m below sea level. The terminus is likely to reach the deepest section of the trough within a few decades, after which it could rapidly retreat to the shallower regions ~50 km farther upstream, potentially by the end of this century

The influence of scene selection on height change rates from TanDEM-X DEM differencing - results of the RAGMAC intercomparison exercise

Within the IACS working group on Regional Assessments of Glacier Mass Change (RAGMAC) an intercomparsion experiment was set up to evaluate variations of volume change results between different processing chains and sensors. For this purpose, high resolution height change maps over two alpine glaciers, Aletschgletscher and Hintereisferner, were provided as independent validation data. Here we will target the influence of TanDEM-X bistatic scene selection for reporting height change rates and their difference to the provided validation data keeping a common processing chain. Ideally, two timely relevant, bistatic end of summer acquisitions from the same orbit and a suitable height of ambiguity should be utilized to calculate a DEM difference map from TanDEM-X data. In many cases however, the ideal set of criteria cannot be met and a scene selection from the TanDEM-X mission database has to be performed. SAR acquisitions from different seasons might enhance elevation biases due to signal penetration depending on the observed region. A combination of ascending and descending orbits will potentially increase the co-registration error and leads to an uneven sampling of the terrain due to different locations of shadow and layover caused by the steep terrain around many mountain glaciers. We will attempt to formulate a set of best-practices for TanDEM-X scene selection and future data acquisition ordering when targeting height change rate calculation for mountain glaciers

Synergistic Use of Single-Pass Interferometry and Radar Altimetry to Measure Mass Loss of NEGIS Outlet Glaciers between 2011 and 2014

Mass balances of individual glaciers on ice sheets have been previously reported by forming a mass budget of discharged ice and modelled ice sheet surface mass balance or a complementary method which measures volume changes over the glaciated area that are subsequently converted to glacier mass change. On ice sheets, volume changes have been measured predominantly with radar and laser altimeters but InSAR DEM differencing has also been applied on smaller ice bodies. Here, we report for the first time on the synergistic use of volumetric measurements from the CryoSat-2 radar altimetry mission together with TanDEM-X DEM differencing and calculate the mass balance of the two major outlet glaciers of the Northeast Greenland Ice Stream: Zachariæ Isstrøm and Nioghalvfjerdsfjorden (79North). The glaciers lost 3.59±1.15 G t a−1 and 1.01±0.95 G t a−1 , respectively, between January 2011 and January 2014. Additionally, there has been substantial sub-aqueous mass loss on Zachariæ Isstrøm of more than 11 G t a−1 . We attribute the mass changes on both glaciers to dynamic downwasting. The presented methodology now permits using TanDEM-X bistatic InSAR data in the context of geodetic mass balance investigations for large ice sheet outlet glaciers. In the future, this will allow monitoring the mass changes of dynamic outlet glaciers with high spatial resolution while the superior vertical accuracy of CryoSat-2 can be used for the vast accumulation zones in the ice sheet interior