You've to open the .xml file in SNAP choosing a fixed resolution (each brand has different spatial resolution) and then export to a format recognized by ERDAS like GeoTIFF.
Regards
Hi,
It can be really hard to do, or at least get good results. If you could have lidar data could attempt to follow the methodology mentioned in this paper.
http://grslab.yonsei.ac.kr/paper/international/29.pdf