Performance evaluation of Sentinel-1 derived DEMs using Copernicus DEM and ICESat-2

Kurzfassung

Digital elevation models (DEMs) hold a key role as the main input data for a large variety of applications as they provide an accurate representation of the Earth’s surface and its corresponding topographic parameters. Scientific applications require current, reliable and precise elevation information to be implemented in research to generate correct and valuable results. The performance and accuracy of DEMs must therefore be subject to quality assessment. Synthetic Aperture Radar (SAR) principles and methodology allow the generation of DEMs independent from day light and cloud coverage which act as a massively limiting interference factor in optical imagery. In 2014, a C-band radar mission Sentinel-1 launched within the Copernicus Programme of the European Space Agency (ESA), providing open access to radar data of regular global coverage (Braun 2021). The capabilities of DEM derivation from Sentinel-1 imagery are limited by parameters like slope and vegetation coverage as well as by the mission design itself as the large temporal baseline of two or more Sentinel-1 acquisitions hinders the stability of the interferometric phase and degrades the coherence. One crucial step in the analysis of SAR data and therefore optimizing its potential for DEM generation is the preprocessing of suitable interferometric data pairs, a complex workflow which includes coregistration, interferogram formation, phase unwrapping and terrain correction (Braun 2021). Within the scope of the TanDEM-X mission by the German Aerospace Center (DLR), consisting of two satellites (Terra-SAR-X and Tandem-X), a global DEM was generated from bistatic X-Band interferometric SAR products acquired between December 2010 and January 2015 (Rizzoli 2017). The TanDEM-X mission is the original source of the radar data from which the Copernicus DEM (COP-DEM) by ESA was derived. The COP-DEM is available in varying resolutions with the 30m resolution COP-DEM being openly accessible. The Ice, Cloud and land Elevation Satellite-2 (ICESat-2) mission by National Aeronautics and Space Administration (NASA) provides openly accessible data since October 2018. ICESat-2 data is a continuous, equally distributed, high resolution reference dataset and allows to independently evaluate the performance of SAR-derived DEMs. An Advanced Topographic Laser Altimeter System (ATLAS) on board of ICESat-2 fires shots of photons in 532nm wavelength separated into six beams arranged in three pairs. Each pair consists of a strong and a weak beam with an energy ratio of 4:1 (Neuenschwander und Pitts 2019). Terrain and canopy heights retrievals are stored in the ATL08 data product of ICESat-2 and computed for fixed 12x100m land segments in which a valid height value is given if a threshold of 50 signal photons is met in order to reliably represent the land surface within the segment. ICEsat-2 provides a large variety of parameters and flags to filter the data adapted to the needs of the research purpose. In this paper, we will generate several DEMs derived from Sentinel-1 data and perform a quality and accuracy evaluation. As reference data, we will use highly accurate ICESat-2 data points and two reference DEMs, TanDEM-X in 10m resolution and Copernicus DEM in 30m resolution. The main focus of this study is the correct processing and application of various parameter filter techniques of ICESat-2 data to provide accurate height reference data for the accuracy assessment of SAR-derived DEMs and the evaluation on DEM quality in consideration of different land cover types and topographic conditions regarding study sites from different continents.