|Paper title||Land motion monitoring service over Switzerland through interferometric multi-temporal analyses of Sentinel-1 SAR data|
|Form of presentation||Poster|
Protecting the population and their livelihood from natural hazards is one of the central tasks of Swiss state. Efficient prevention, preparation and intervention measures can be used to pre-vent or at least limit potential material damage and fatalities as a result of natural hazards. Warnings and alerts are particularly cost-effective instruments for reducing damage, as they allow emergency personnel and the population to take the prepared measures.
The Swiss Federal Office of Topography (swisstopo) therefore procures processed InSAR data to detect any changes in the terrain of the whole of Switzerland.
The object of the service is the procurement of processed InSAR data for the entire perimeter of Switzerland. The data provided by the Sentinel-1 (S1) SAR satellite constellation as part of the European Union’s Copernicus Earth observation programme are processed as the data basis for the Swiss-wide monitoring of surface motion.
The service implementation includes the analysis of all the available historical (S1), from 2014 up to November 2020, followed by annual updates, at least up to 2023. The frequency of the periodical updated could increase, up to monthly updated, if needed or considered valuable from swisstopo.
The area of interest is covering Switzerland and Liechtenstein, including a 5 km buffer, for a total surface of approximately 50’000 km2.
This area is covered by five different S1 tracks, two ascending and three descending, from October 2014 up to now. The approximate number of acquisition per track is about 300, characterized by a 6-day revisiting time, which is showing a regular sampling with no data gaps starting from November 2015.
The end-to-end workflow of the production chain includes the following steps:
- S1 Data Ingestion, transferring S1 data from external repositories into the service storage facilities;
- Core Processing
- Quality Control procedures for ensuring product quality before delivery the results to swisstopo.
Southern Switzerland is characterized by prominent topography, as it includes more than the 13% of the Alps, comprising several peaks higher than 4’000 m above sea level. In fact, the Alps cover 60% of Switzerland. Therefore, a preliminary analysis has been addressed on the creation of layover and shadow maps, for each S1 relative orbit, considering both the ascending and descending geometries. This step is helping to identify the portions of the study area where the combination of topography and the satellite acquisition geometry do not allow getting information from InSAR techniques.
Additionally, the vast mountainous areas are often affected by seasonal snow cover, which, in turn, is affecting S1 interferometric coherence over long periods, resulting in loss of data for parts of the year. To handle the periodical data decorrelation or misinterpretation of the data phase information during the snow period, a specific strategy to correctly threat these circumstances has been designed.
The Core Processing is responsible for the generation of all required products, operating on S1 and ancillary data. The deformation products will be obtained exploiting a combination of both Small Baseline subset (SBAS) and Persistent Scatterers Interferometry (PSI) methods, in order to estimate the temporal deformation at both DS and point-like PS. In the following, the terms low-pass (LP) and high-pass (HP) will be used to name the low spatial resolution and residual high spatial frequency components of signals related to both deformation and topography.
The role of the SBAS technique is twofold: on the one hand, it will provide the LP deformation time series in correspondence of DS points and the LP DEM-residual topography; on the other hand, the SBAS will estimate the residual atmospheric phase delay still affecting the interferometric data after the preliminary correction carried out by leveraging GACOS products and ionospheric propagation models.
The temporal displacement associated to PS points will be obtained applying the PSI method to interferograms previously calibrated removing the LP topography, deformation and residual atmosphere estimated by the SBAS technique. This strategy connects the PSI and SBAS methods ensuring consistency of deformation results obtained at point-like and DS targets and, therefore, provides better results with respect to the approach of executing the two methods independently from each other.
A key aspect considered in the framework of the project implementation is related to the estimation and corrections of atmospheric effects affecting the area, generally more evident over the mountainous areas.
An initial correction is applied on each interferogram through the Generic Atmospheric Correction Online Service for InSAR (GACOS), which utilizes the Iterative Tropospheric Decomposition model to separate stratified and turbulent signals from tropospheric total delays, and generate high spatial resolution zenith total delay maps to be used for correcting InSAR measurements. This atmospheric calibration procedure is intended as preliminary correction that will be later refined by the data-driven atmospheric delay estimation in order to obtain atmospheric delay maps at a much higher spatial resolution than that achievable by using external data based on numerical weather prediction such as GACOS.
GNSS data provided by swisstopo, consisting in more than 200 points over Switzerland, are used for the products calibration and later for the result validation during the quality control procedure.
The generated products consist of:
- Line-of-Sight (LOS) surface deformation time series for ascending and descending datasets in SAR geometry (Level 2a);
- Line-of-Sight (LOS) surface deformation time series for ascending and descending datasets in map geometry (Level 2b);
- Combination and projection of deformation results obtained from the overlapping ascending and descending datasets to calculate vertical and east-west deformations starting from the LOS results (Level 3).
The quality control (QC) procedures are divided into automatic QC and operator QC. The automatic QC include the analyses of point-wise indicators (coherence maps, precision maps, points density, deformation RMSE with respect to a smooth fitting model), quality indicators at sparse locations (comparison with GNSS data, consistency of stable targets) and other quality indicators (short-time interferogram variograms before and after atmospheric calibration, consistency of overlapping areas). The additional operator QC are focusing on a visual assessment of deformation maps reliability / realism leveraging also on a priori knowledge about the expected deformation behavior.
The results of this service are then going to be delivered to swisstopo that will manage the possibility of sharing the deformation maps through their national geo-portal.