2.6 Data Storage and Transmission
The Observations and Data Management Division constructs the HST observing schedule and sends the actual command loads to the Science Institute Mission Operations Room (SIMOR) at STScI. 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, command sequences for HST observations are normally uplinked approximately once every 8 hours. HST then executes the observations automatically.
2.6.1 Real-time Contact Requests
Observers at STScI can interact in real-time with HST for specific purposes (e.g., certain target acquisitions). However, real-time interactions are difficult to schedule and are generally used in exceptional circumstances only. In the past three cycles of HST usage there have been no real-time contacts.
2.6.2 Onboard Data Storage
HST currently uses a large capacity Solid State Recorder (SSR) to store science data before transmission to the ground. Except when real-time access is required, 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 SIs would generate more data than either the links or ground system could handle (see Section 6.2.3).
