Denman Glacier, East Antarctica, holds an ice volume equivalent to a 1.5 m rise in global sea level. Using satellite radar interferometry from the COSMO-SkyMed constellation, we detect a 5.4 ± 0.3 km grounding line retreat between 1996 and 2017-2018. A novel reconstruction of the glacier bed topography indicates that the retreat proceeds on the western flank along a previously unknown 5 km wide, 1,800 m deep trough, deepening to 3,400 m below sea level. On the eastern flank, the grounding line is stabilized by a 10 km wide ridge. At tidal frequencies, the grounding line extends over a several kilometer-wide grounding zone, enabling warm ocean water to melt ice at critical locations for glacier stability. If warm, modified Circumpolar Deep Water reaches the sub-ice-shelf cavity and continues to melt ice at a rate exceeding balance conditions, the potential exists for Denman Glacier to retreat irreversibly into the deepest, marine-based basin in Antarctica. Plain Language SummaryUsing satellite radar data from the Italian COSMO-SkyMed constellation, we document the grounding line retreat of Denman Glacier, a major glacier in East Antarctica that holds an ice volume equivalent to a 1.5 m global sea level rise. The grounding line is retreating asymmetrically. On the eastern flank, the glacier is protected by a subglacial ridge. On the western flank, we find a deep and steep trough with a bed slope that makes the glacier conducive to rapid retreat. If warm water continues to induce high rates of ice melt near the glacier grounding zone, the potential exists for Denman Glacier to undergo a rapid and irreversible retreat, with major consequences for sea level rise.

The bistatic interferometric coherence is affected by different sources of error, among which volume decorrelation, which quantifies the amount of noise caused by volume scattering mechanisms. This represents a key quantity not only for the performance assessment of interferometric (InSAR) products, but also for a large variety of scientific applications, ranging from land cover classification to physical parameters estimation, such as ice structure, forest height and biomass retrieval. The magnitude of volume decorrelation can be derived from the total interferometric coherence by properly compensating for all other decorrelation sources. Considering that temporal decorrelation can be neglected for a bistatic system such as TanDEM-X, the remaining decorrelation components can be estimated from the SAR scene characteristics and the system parameters. In the scientific community, it is common practice to approximate the volume decorrelation with the coherence or to compute it by compensating for the signal-to-noise ratio (SNR) decorrelation only, which typically represents the predominant decorrelation component. The aim of this work is to assess the impact of different decorrelation sources in detail and to provide the readers with a practical procedure for a precise computation of the volume decorrelation from TanDEM-X bistatic data. In particular, we concentrate on the two most relevant decorrelation components: the SNR and the quantization components. Regarding the former, we estimate the noise equivalent sigma nought directly from real SAR data and we provide the users with a set of polynomial coefficients for the retrieval of the system noise floor for each operational TanDEM-X StripMap beam used for the generation of the global Digital Elevation Model (DEM). These values are then combined with the backscatter for the retrieval of the scenebased SNR and of the corresponding decorrelation. Concerning the latter, we analyze its dependency on the backscatter local statistics and quantization rate and we provide the reader with a set of empirical look-up-tables (LUT) for quantifying its impact on the coherence. Finally, we provide reasonable assumptions for all other remaining decorrelation sources, discussing two application scenarios in the fields of forest mapping and forest height estimation, which demonstrate the added value of the proposed methodology.

Abstract

TanDEM-X is a spaceborne mission consisting in two satellites that are operated simultaneously for bistatic SAR acquisitions. The main objective of the mission is the systematic acquisition of a global and homogeneous digital elevation model (DEM) in bistatic stripmap mode. The close formation of the satellites makes the system very flexible and allows the commanding of a diversity of challenging experimental modes as bistatic spotlight or alternating bistatic stripmap modes. This paper gives an overview of the TanDEM-X experimental modes, focused on the analysis of already executed scientific orders, giving an overview of the different possibilities in commanding and image acquisition geometries. Also a first image characterization of the modes is included, in terms of image quality assessment and performance compliance.

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