Overview
The core Flight Executive (cFE) defines mission-specific secondary headers that extend the generic SPP primary header. These headers occupy the SPP secondary header field and add metadata needed by the cFE Software Bus for message routing and validation.
A Telemetry (TM) packet carries a secondary header containing a timestamp (6 bytes) used by the ground system to correlate telemetry with on-board time.
A Telecommand (TC) packet carries a secondary header containing a function code (1 byte) that identifies the specific command within the target application, and a checksum (1 byte) computed over the entire packet to detect corruption.
The cFE Software Bus routes packets using a composite Message ID derived from the SPP APID and packet type fields. This allows TM and TC packets to share the APID space without ambiguity.