CLTU

A Communications Link Transmission Unit (CLTU) wraps a TC transfer frame for uplink. The structure differs from CADU because uplink commands must be received with very high reliability — an incorrect command could endanger the mission.

The CLTU begins with a 2-byte start sequence (0xEB90) that the receiver uses to detect the beginning of a command. The TC frame is then split into 7-byte blocks, and each block is appended with a 1-byte parity computed using a BCH(63,56) code — a type of error-detecting code that can detect (but not correct) errors within the block. The CLTU ends with an 8-byte tail sequence (0xC5C5C5C5C5C5C5C5).

The receiver processes each 8-byte block independently. If the BCH check fails, the receiver knows the block is corrupted and can reject the entire CLTU. This per-block error detection complements the FEC: even if the FEC introduces a miscorrection, the BCH check provides a second line of defense.