## 2 LiCSAR System Architecture
### LiCSAR Processing Chain
#### 2.1.2 Resampling to RSLC  
The resampling step is demanding on random-access memory (RAM) and has the longest processing runtime--over 1 h per epoch using a single processing core. The task of this step is to generate resampled RSLC files and additional supplementary files that can be used later to regenerate RSLC if needed for data reprocessing. The process of generating RSLC and the supplementary files from the epoch SLC is shown in Figure 3. The following textual description uses estimated processing time (EPT) measured for a typical frame consisting of 39 bursts (13 bursts per swath) and using a single processing core (as experienced at the facility described in Section 2.5). The resample procedure (including precise coregistration) follows the approach implemented within the GAMMA command S1_core_TOPS.  
Figure 2: General flowchart of the LiCSAR processing chain.  
After the first step of generating an epoch SLC (and MLI), a frame DEM (also multilooked by factors of \(4\times 20\)) is used to provide a DEM-assisted SLC coregistration (GAMMA command rdc_trans, EPT \(\sim\)2 min). A preliminary coregistration lookup table (LUT) is generated. In order to increase the precision of SLC coregistration, an intensity cross correlation is performed towards the frame's primary SLC image (GAMMA command offset_pwr_tracking, EPT \(\sim\)5 min) and the estimated offsets are used to update the LUT.  
In order to achieve the high precision coregistration required for interferometric processing of a specific Terrain Observation with Progressive Scan (TOPS) mode of the Sentinel-1 IWS data [51], we use the spectral diversity (SD) technique [51] that estimates a mis-registration offset in azimuth direction, based on inversion of azimuth ramps in interferograms formed from overlaps between bursts of primary and secondary SLC images. To keep a reasonable coherence in the burst overlap interferograms and thus increase reliability of the SD estimate, an existing RSLC is used instead of the primary SLC if its epoch is closer to the epoch being coregistered--this temporally nearest RSLC is recognised as RSLC3. The primary SLC can also serve as RSLC3.  
Prior to the SD estimation, a subswath offset estimation (GAMMA command S1_coreg_subswath_overlap, EPT 15 min) is applied to datasets being processed by an IPF differing from the original primary SLC; currently this step is applied only to data from the problematic IPF 2.36.  
To estimate the mis-registration using SD, we determine burst overlap regions from the primary SLC metadata (GAMMA command S1_poly_overlap). Raster coordinates of the burst overlap regions are used to generate a masked LUT. The masked LUT is applied to resample only the burst overlap regions of the secondary SLC (EPT 20 min). The SD azimuth offset is estimated in the burst overlaps between the secondary SLC and RSLC3 by applying GAMMA command S1_coreg_overlap (to determine the fine coregistration offset within the burst overlaps) on the overlap region resampled towards the RSLC3. If the coregistration offset changes by more than 0.0005 pixels, the resample and coregistration of the burst overlaps is repeated up to 5 times (EPT 60+ min). This threshold should allow for further interferometric combinations of any RSLC data with maximal residual azimuth ramp of approx. 0.05 rad [52]. Finally, the azimuth pixel offset w.r.t. the LUT is estimated and both are used to resample the original secondary epoch SLC into the RSLC.  
After the resampling process, the LiCSAR system stores the LUT and the estimated subpixel shift in azimuth direction (SD estimate) inside an offset refinement file for each RSLC. The LUT and offset refinement files can be used later if needed to quickly regenerate an RSLC from SLC data.