2.6 Data Storage and Transmission

The Operations and Data Management 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.

2.6.1 Real-time Contact Requests

Observers at STScI can interact in real-time with HST, usually in support of target acquisition pointing adjustments. There are two components to this interaction; data transmission downlink and vehicle/instrument command uplink. The real-time data downlink typically involves expediting the transmission of an HST image for determining a pointing correction for a subsequent observation. The real-time mode for transmitting this data is a limited resource that restricts scheduling opportunities since the data collection must occur at the same time that HST has high data rate communications with the ground. These opportunities may not coincide with other absolute timing requirements of the science observation itself or the request may simply be an unnecessary restriction in general. The real-time uplink component is less restrictive in that it simply must occur before the observation it is intended to support. However, it is more difficult to find scheduling opportunities for real-time contact requests in general. Observers who think they may require real-time support for target acquisitions or other reasons should discuss their needs and the implications for their program with STScI.

2.6.2 Onboard Data Storage

HST currently uses large capacity Solid State Recorders (SSRs) 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 instruments would generate more data than either the links or ground system could handle (see Section 6.2.2).