Interferometric phase corrections based on ESA's Extended Timing Annotation Dataset (ETAD) for Sentinel-1

Kurzfassung

The ETAD product provides easy-to-use gridded timing corrections for Sentinel-1 level-1 data [1]. Such corrections are meant to enhance geolocation accuracy by compensating the effects of atmospheric path delay, Earth's tidal deformation and other systematic effects not captured by the SAR image processor. As a part of ETAD scientific evolution study, the capability of deriving accurate and consistent interferometric phase corrections from timing annotations is being assessed, both for conventional and multi-temporal InSAR applications (e.g. persistent scatterer interferometry). As already discussed in [1], this involves translating annotated time delays into phase offsets, and evaluating the corrections at the reference grid defined by the InSAR processing workflow. For InSAR applications, only corrections resulting in a differential phase term between acquisitions are relevant. Some ETAD corrections might cancel out on interferogram formation, or be compensated during coregistration to the reference image geometry. There is, however, a set of correction layers available in ETAD that are considered relevant for the majority of scenarios [1][2]: Tropospheric range delay correction, which accounts for changes in the signal propagation velocity due to tropospheric conditions along the traversed path. Ionospheric range delay related to ionospheric activity, modelled as a function of the slant total electron content. Timing corrections related to solid Earth tidal deformations caused by the gravitational force of the Sun and Moon. Instrument timing calibration constant in range, which can lead to a differential phase term in case of changes in the ETAD configuration between the generation of two products, or in the instrument configuration between SLC acquisitions (also if S1-A and S1-B acquisitions are combined). Ocean tidal loading is another well-known source of solid Earth deformation signal with a significant impact on InSAR time series in many coastal regions [3]. It is not a part of ETAD yet but we investigate the effect in our evolution study as a possible future ETAD product extension. While most relevant corrections layers generally vary smoothly in space (e.g. solid Earth tides or ionospheric range delay), tropospheric corrections have a strong dependence on the topography. When applying ETAD corrections to high resolution InSAR data, with minimal or no multi-looking, accurate interpolation of the tropospheric corrections to the output grid is required, which involves accounting for the dependence on topography at the interpolation stage. Findings from the ETAD pilot study groups [1], and in particular from the IREA-CNR team, confirmed that neglecting this step results in artefacts in the (differential) ETAD tropospheric phase screens when applying the product to full-resolution Sentinel-1 interferograms. Our first experiments using a local estimate of the tropospheric range-delay-to-height-derivative to compensate for height effects during spatial interpolation have succeeded in providing meaningful differential tropospheric corrections for a common reference grid. Although the range delay to height derivative can be estimated (under certain conditions) directly from the available ETAD layers in its current version (planned to become an operational product by Spring 2023) it is foreseen that a more robust estimate is generated and delivered as an additional layer in a future release of the ETAD product. In the final publication we plan to showcase the additional ETAD correction layers, including the tropospheric delay to height gradient as well as ocean tidal loading corrections. Study cases in the European Alps (strong topography) and French Brittany region (ocean loading) will be shown to assess the use of ETAD for InSAR corrections. Acknowledgement The authors thank all the research groups that participated in the ETAD pilot study for their valuable feedback on the product when applying it in SAR applications such as offset tracking, InSAR processing, data geolocation and geocoding, and stack co-registration. List of participating institutions in alphabetical order: Caltech, DIAN srl, DLR, ENVEO, IREA-CNR, JPL, Joanneum Research, NORCE, PPO.labs, TRE ALTAMIRA, University of Jena, University of Leeds, University of Strasbourg. The S1-ETAD scientific evolution study, contract No. 4000126567/19/I-BG, is financed by the Copernicus Programme of the European Union implemented by ESA. Views and opinion expressed are however those of the author(s) only and the European Commission and/or ESA cannot be held responsible for any use which may be made of the information contained therein. [1] Gisinger, C., Libert, L., Marinkovic, P., Krieger, L., Larsen, Y., Valentino, A., Breit, H., Balss, U., Suchandt, S., Nagler, T., Eineder, M., Miranda, N.: The Extended Timing Annotation Dataset for Sentinel-1 - Product Description and First Evaluation Results. IEEE Transactions on Geoscience and Remote Sensing, vol. 60, pp. 1-22, 2022. doi: 10.1109/TGRS.2022.3194216 [2] A. Parizzi, R. Brcic and F. De Zan: InSAR Performance for Large-Scale Deformation Measurement. IEEE Transactions on Geoscience and Remote Sensing, vol. 59, no. 10, pp. 8510-8520, Oct. 2021, doi:10.1109/TGRS.2020.3039006 [3] Yu, C., Penna, N. T., Li, Z., "Ocean tide loading effects on InSAR observations over wide regions,2 in Geophysical Research Letters, 47, 2020. Doi: 10.1029/2020GL088184 [1] The S1 ETAD pilot study set up by ESA between January and September 2022 aimed to provide early access to ETAD products to expert users, promoting independent validation and supporting the definition of eventual improvements of the product. The SETAP Processor was hosted in the Geohazard Exploitation Platform to allow for processing by the pilot participants and the hosting was supported by ESA Network of Resources Initiative.