2.5 Data Storage and Transmission

2.5.1 Onboard Data Storage

The Operations and Engineering Division at STScI constructs the HST observing schedule and the actual command loads to be sent to the telescope. Communications with the spacecraft are performed via the Tracking and Data Relay Satellite System (TDRSS) which consists of a set 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 return data, 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 8 hours. HST then executes the observations automatically. Data are downloaded ten to twenty times per day depending on the observing schedule.

2.5.1

Onboard Data Storage

HST currently uses large capacity Solid State Recorders (SSRs) to store science data before 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).