Hi Alexey,
I was able to get a good result downloading and using some 4 GCPS from same web site and now, I get errors on GCPS less than 0.5 m and DEM is now correct ( from 0 m to some 1270 m)....
Still , it would be very helpful to do some blog or tutorial for RPC satellite processing as it is not very clear when to optimize or use certain parameters as f, cx, cy, b1 and b2 whose meaning is not evident in case of satellite push broom sensor....
Also, why is the altitude of 2 images 238 m ?... does not make much sense....
Edit I did a further optimization with f, cx, cy, b1 and b2 and got better results: better than 25 cm error on GCPs and better than 1 m precision on TiePts....