TM
cFE Telemetry packets carry data from spacecraft to ground. They extend the SPP primary header with a secondary header containing timestamp information.
Packet Structure
| Field | Size |
|---|---|
| Primary Header (SPP) | 6 bytes |
| Secondary Header | 10 bytes |
| Payload | variable |
| Minimum Total | 16 bytes |
Secondary Header
| Field | Size |
|---|---|
| Time (CDS format) | 8 bytes |
| Spare | 2 bytes |
| Total | 10 bytes |
| Field | Size | Description |
|---|---|---|
| Time | 8 bytes | CCSDS Day Segmented time (48 bits used) |
| Spare | 2 bytes | Padding for alignment |
Time Format
The time field uses CCSDS Day Segmented (CDS) format:
- 48 bits of the 64-bit field are active
- Bitmask:
0x0000_FFFF_FFFF_FFFF - Stored in network byte order
Fixed Header Values
Telemetry packets always have:
- Packet Type = 0 (Telemetry)
- Secondary Header Flag = 1 (Present)
- Sequence Flags = 0b11 (Unsegmented)
Constraints
- Maximum payload: 65534 bytes
- Time value must fit in 48 bits