I figured out differential GPS (DGPS) corrects incoming signal time divergencies resulting from atmosphere and malfunction of sattelites. The station knows its accurate position and accurate position of sattelites -> it calculates the distance from visible sattelites -> ideal signal travel time -> divergency from real signal travel time it is recieving. Though I haven't found the station timing. This scheme requires very accurate time and I can not imagine this technique with time correction that common GPS receiver performs. Also, does every DGPS station use atomic clock?