I’ve been using ESA Sentinel-1 Toolbox (S1TBX) and SNAP (Sentinel Applications Platform) since a long time for SAR processing, since back when it was known as NEST. One issue that I’ve faced is that when any SAR intensity data is exported from SNAP into some other format, e.g. ENVI or GeoTIFF format, the coordinates are not exported. I found the solution on ESA Sentinel Toolbox Exploitation Platform (STEP) Forum, and want to share it with others too.
What’s actually going on is that S1TBX, being a focused toolset for SAR, automatically interprets the geocoding information of the SAR metadata, while the data itself is mostly not projected. However, SNAP does not carry forward this geocoding interpretation to the export function. Therefore, we need to tell SNAP to attach / imbed the geocoding information before exporting.
I’ve worked with Envisat ASAR and ALOS-1 / 2 PALSAR intensity data, and here are the solutions for both (thanks to ESA STEP forum):
- For Envisat ASAR intensity Image Mode IMG / IMP .N1 data, the simple way to make sure coordinates are exported is to apply the Radar > Ellipsoid Correction > Gelocation-Grid function. Export the output in ENVI or GeoTiff format, and opening it in any external software will now show the data with coordinates.
- For ALOS-1 / 2 PALSAR Level 1.5 CEOS data, things are a bit more complicated. For some reason, S1TBX / SNAP do not give full support for this data. The Geolocation-Grid function does not work, as it says “Source product should not be map projected.” Attempts to use the Radar > Geometric > Update Geo Reference function instead, which requires DEM to be provided, works in terms of generating the exported output, but the coordinates are not carried over. The Raster > Geometric Operations > Reproduction tool, but that doesn’t work either. The “trick” to solve this issue was discovered in an ESA STEP forum post: Use Radar > Geometric > SAR-Mosaic with only 1 image as input; the output will be geolocated correctly.
I think the geolocation issue for intensity images from other SAR sensors like TerraSAR-X and Radarsat would be quite the same. Once again, thanks to the ESA STEP Forum and its members for providing the solution.
For details and background discussion, see these ESA STEP Forum posts: