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The ROSAT command and data handling system (CDHS) collects on-board data, sends them to ground, receives commands from ground, and decodes the command addresses and distributes then to the appropriate subsystems. The system consists of a redundant Intel 10-bit processor system with 40Mbyte ROM and 74Kbyte RAM, two tape recorders which are able to store 700Mbits of data from ROSAT, a redundant transponder system, and an S-band bi-directional antenna system. Since there are only five or six contacts per day of roughly eight-minute duration, the data are initially stored on tape and then dumped to ground with a data rate of 1Mbits . In addition to this link, housekeeping data are transmitted to ground during contacts via a 8 kbit secondary S-band link. Commands, generated on ground, are sent to the spacecraft via an S-band command link at a capacity of 1kbits or 10 commands per second. Command verification further slows this link. The software of all subsystems on board can be updated via the command link in order to react to changing requirements. This feature was used extensively for the reconfiguration of the attitude control system after loss of several hardware components.