How do i select a pair of Sentinel 1 SLC images for DEM generation. Can i just pick two images of different dates and run them through the SNAP processes for DEM generation?
Firstly, they need to be of the same track (relative orbit), that means they must cover the same area, at least in range direction. Images which only partly cover the same are (perpendicular to the flight direction) can not serve as pairs for interferometry.
Secondly, for DEM generation small temporal baselines should be preferred. A pair of Sentinel-1A and B can be 6 days apart at minimum. A pair of 2 Sentinel-1A scenes is 12 days apart at minimum. The longer the time between the images the higher is temporal decorrelation.
Please also see here:
https://sentinels.copernicus.eu/web/sentinel/user-guides/sentinel-1-sar/revisit-and-coverage
But don’t expect too much from Sentinel-1 InSAR DEM generation. Especially if you have vegetation cover coherence is quite low because C-band sensors are more sensitive to smaller changes.