Here is information from Sarmap on the 'fake_gcp.xml': This is an automatic GCP generated by the software during the interferogram flattening, to correct the azimuth start time and the slant range distance of the slave image to respect the master image. It doesn't affect the master image, because it doesn't make any correction on it, only the slave. When you introduce a manual geometric GCP during the flattening, the software can manage and correct both the start time and slant range dist of master and slave, but in this case only a relative correction is feasible because the software exploits the co-registration parameter. So a better flattening will be applied, but there will be inaccuracy of geolocation due to problem in the master orbit. It is better to choose as master the most correct one. This feature is applied when: -the customer selects "automatic slave orbit Correction" in the default section, flattening -always during the SBAS process.