Abstract
Ice velocity is one of the products associated with the Ice Sheets Essential Climate Variable. This paper describes the intercomparison and validation of ice-velocity measurements carried out by several international research groups within the European Space Agency Greenland Ice Sheet Climate Change Initiative project, based on space-borne Synthetic Aperture Radar (SAR) data. The goal of this activity was to survey the best SAR-based measurement and error characterization approaches currently in practice. To this end, four experiments were carried out, related to different processing techniques and scenarios, namely differential SAR interferometry, multi aperture SAR interferometry and offset-tracking of incoherent as well as of partially-coherent data. For each task, participants were provided with common datasets covering areas located on the Greenland ice-sheet margin and asked to provide mean velocity maps, quality characterization and a description of processing algorithms and parameters. The results were then intercompared and validated against GPS data, revealing in several cases significant differences in terms of coverage and accuracy. The algorithmic steps and parameters influencing the coverage, accuracy and spatial resolution of the measurements are discussed in detail for each technique, as well as the consistency between quality parameters and validation results. This allows several recommendations to be formulated, in particular concerning procedures which can reduce the impact of analyst decisions, and which are often found to be the cause of sub-optimal algorithm performance.
Highlights
Results were provided by 6 groups, affiliated to the Geological Survey of Denmark and Greenland (GEUS), the Italian National Institute for Geophysics and Volcanology (INGV) and the Technical University of Denmark (DTU)
The measurements provided by group 1 (Figure 3a) show a more limited coverage compared to other groups, as well as significantly higher velocity magnitudes on ice and on bedrock outcrops, whereas the results provided by the remaining groups show local differences, in particular in the bottom-left and bottom-right corners of the image, as well as slightly varying image-wide trends (Figure 5b–e)
In the current context in which Synthetic Aperture Radar (SAR)-based ice-velocity products are being made available by an increasing number of independent providers, it would be desirable from an end-user point of view, for these products to share similar state-of-the-art properties in terms of measurement spatial resolution, accuracy, coverage, and error characterization
Summary
The currently available ice-sheet velocity products consist of continental-scale seasonal mean-velocity maps and time-series of the horizontal ice-motion components for selected areas. Generation of the latter based on SAR data, which are the focus of this study, entails post-processing and mosaicking measurements of satellite line-of-sight (LoS) and flight-path (azimuth) velocity components, derived independently for a large number of radar tracks. Participants were provided with common SAR datasets, and asked to apply their processing algorithm of choice to derive ice-velocity measurements, which were intercompared and validated against GPS data by the project consortium.
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