The detectors in the focal planes of the X-ray and XUV mirrors record each photon separately. The PSPC instrument, for example, delivers a 2-dimensional position of the photon event in detector coordinates, the pulse height of the event as a measure of its energy and the arrival time. The data of each event are immediately written to the on-board tape recorder together with auxiliary information from different subsystems (housekeeping and attitude data). During the five to six contiguous contacts per day between the satellite and its ground station -- the German Space Operation Center (GSOC) at Oberpfaffenhofen/Weilheim -- the on-board recorded tape data are transmitted to the ground. About 20% of the data are immediately passed on via a dedicated line to the ROSAT Scientific Data Center (RSDC) for instrument health and data quality checks. The full set of telemetry data plus additional auxiliary information on orbit, attitude, commands, and the mission program is delivered to the RSDC and, in the case of WFC data, to the WFC data center in the U.K. a few days later.
Besides telemetry and command handling GSOC also implements the mission time line and provides the primary attitude solution.