How s2p find out the correct coordinates with only RPC model?

[lionlai1989]

I read through the commit history which said that the s2p is not using SRTM (DEM) anymore. I want to know how s2p can make the result (DSM) has the precise coordinates. I believe that with only RPC model, the result coordinates will be not precise. However, I found the result is really more precise than reprojecting the tiff file with DEM+RPC on panchromatic tiffs.