On-board data handling

The on-board data handling (OBDH) subsystem of a spacecraft is the subsystem which carries and stores data between the various electronics units and the ground segment, via the telemetry, tracking and command (TT&C) subsystem.

In the earlier decades of the space industry, the OBDH function was usually considered a part of the TT&C, particularly before computers became common on board. In recent years, the OBDH function has expanded, so much that it is generally considered a separate subsystem to the TT&C, which is these days concerned solely with the RF link between the ground and the spacecraft.

Functions commonly performed by the OBDH are:

  • Reception, error correction and decoding of telecommands (TCs) from the TT&C
  • Forwarding of telecommands for execution by the target Avionics
  • Storage of telecommands until a defined time ('time tagged' TCs)
  • Storage of telecommands until a defined position ('position tagged' TCs)
  • Measurement of discrete values such as voltages, temperatures, binary statuses etc.
  • Collection of measurements made by other units and subsystems via one or more data busses, such as MIL-STD-1553
  • Real-time buffering of the measurements in a data pool
  • Provision of a processing capability to achieve the aims of the mission, often using the data collected
  • Collation and encoding of pre-defined telemetry frames
  • Storage of telemetry frames in a mass memory
  • Downlinking of telemetry to the ground, via the TT&C
  • Management and distribution of time signals
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