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TM

Used for the downlink (satellite to ground). A TM frame has a fixed length configured at link setup time. The header contains:

  • Spacecraft ID (SCID): identifies the satellite.
  • Virtual Channel ID (VCID): multiplexes multiple data streams over a single physical link. For example, real-time telemetry and stored science data can share a downlink on separate virtual channels.
  • Master Channel Frame Counter: increments for every frame on the physical link, across all virtual channels.
  • Virtual Channel Frame Counter: increments for every frame on this virtual channel specifically. COP-1 uses this counter to detect lost frames.
  • First Header Pointer: the byte offset within the data field where the first Space Packet begins. This allows the receiver to find packet boundaries even when packets span multiple frames.

The fixed frame length simplifies the coding layer (RS and ASM operate on fixed-size blocks) and enables the receiver to achieve frame synchronization without delimiters.