The Operations and Engineering Division at STScI constructs the HST observing schedule, and creates the command loads sent to the telescope. Communications with the spacecraft are performed via the Tracking and Data Relay Satellite System (TDRSS), a network of satellites in geosynchronous orbit. The TDRSS network supports many spacecraft in addition to
HST. Therefore, the use of the network, either to send commands or receive data from
HST, must be scheduled. Because of limited onboard command storage capacity and TDRSS availability, the command sequences for
HST observations are normally uplinked approximately once every eight hours for automatic execution by
HST. Data are downloaded ten to twenty times per day, depending on the observing schedule.
HST currently uses large capacity Solid State Recorders (SSRs) to store science data awaiting transmission to the ground. Most
HST observations are stored to the SSR and read back to the ground several hours later. Some science programs requiring very high data acquisition rates cannot be accommodated because the instruments would generate more data than either the links or ground system could handle (see
Section 6.2.2).
