# FLATSIM: The ForM@Ter LArge-Scale Multi-Temporal Sentinel-1 InterferoMetry Service
## 2 The NSBAS Processing Chain Used in the FLATSIM Service  
### Main Steps  
The FLATSIM service runs the New Small Baseline (NSBAS) processing chain. The NSBAS chain is versatile, allowing for the user to choose different processing options and parameters, for example for coregistration, selection of image pairs, multi-looking, atmospheric or digital elevation model (DEM) corrections, filtering, unwrapping, or to decide the order in which steps are performed. Here, we only describe the specific steps, workflow and parameters implemented in FLATSIM. They are included in the 2.02 version of the NSBAS processing chain.  
NSBAS partly relies on a set of modified scripts and libraries originating from the legacy InSAR processing software Repeat Orbit Interferometry Package (ROI_PAC) [41], expanded to allow for the processing of a stack of SAR images, using a set of parallelized steps, as described in [19; 39; 42]. The NSBAS chain was further adapted to accommodate the specificities of Sentinel-1 images, acquired in the novel TOPS mode [40; 43]. It is inspired by the \"Small Baseline Subset\" (SBAS) approach defined in [4; 6]. As for all SBAS methods, a series of small baseline differential interferograms are computed, multi-looked, filtered, and unwrapped, before being inverted to obtain phase time series of each pixel. The main steps of the fully automatic workflow are the following:  
* Import all available S-1 data with VV polarization for a given track in a given region of interest defined by its maximum and minimum latitudes (ROI);
* Import and assemble the digital elevation model (DEM) encompassing the area covered by the downloaded S-1 data. We use the DEM from the Shuttle Radar Topography Mission (SRTM) [44];
* Select bursts for each subswath independently and retain only complete acquisitions within the chosen latitude range;
* Download precise orbits, compute perpendicular, and temporal baselines, and choose a single primary date common for the three subswatths;
* For each subswath, assemble bursts into Single Look Complex data (SLC), transform the DEM in the primary date radar geometry, and compute the geometrical transformations of all secondary SLCs on the geometry of the primary acquisition, combining orbit, DEM, and correlation offsets information;
* For each subswath, reassemble secondary SLCs using the position of the burst limits from the primary image and an improved deramping function, and resample secondary SLCs on the primary geometry;
* Define the interferometric network and computes differential interferograms for each subswath;
* For each subswath, evaluate spectral diversity parameters and correct interferograms;
* Merge the subswatths of all interferograms and of some auxiliary images using available metadata and inverted phase jumps in subswath overlaps;
* Correct interferograms from stratified atmospheric effects, multi-look, filter, unwrap, remove phase ramps in range and azimuth and reference;
* Discard possible low quality interferograms based on their phase variance or unwrapping fraction;
* Invert interferograms into time series and provide quality indicators;
* Prepare products in tiff or geogtiff format, together with metadata, previews, and text files.  
We detail below a few selected steps, necessary to understand the FLATSIM products.