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Time Codes

CCSDS 301.0-B-4 defines standard time formats for space missions. Time stamps appear in cFE telemetry secondary headers (6 bytes), in protocol metadata, and in science data annotations. Two formats are used in LeoDOS: CUC and CDS.

Both formats use a two-part encoding. The P-field (preamble) is a 1-byte descriptor that identifies the time format, epoch, and field sizes. The T-field contains the actual time value. When sender and receiver agree on the format in advance, the P-field can be omitted and the T-field is interpreted using the implicit configuration.

Both formats reference the CCSDS epoch: 1958-01-01T00:00:00 TAI. TAI (International Atomic Time) is a monotonic time scale that does not include leap seconds, which is important for spacecraft where a discontinuity in the time reference could cause control loops to misbehave.

CUC — Unsegmented Code​

CUC encodes time as a binary count of whole seconds (the coarse field) and fractional seconds (the fine field) since the epoch. The number of bytes for each is configurable: 1--4 bytes for coarse and 0--3 bytes for fine. This flexibility allows trading off range against resolution and encoded size.

The P-field encodes:

  • Time code ID (3 bits): 001 for agency-defined epoch, 010 for the CCSDS epoch.
  • Coarse octets (2 bits): the number of coarse bytes minus one (0--3, meaning 1--4 bytes).
  • Fine octets (2 bits): the number of fine bytes (0--3).

Common configurations:

  • 4+2 (standard): 4 coarse bytes give a range of ~136 years from epoch. 2 fine bytes give ~15 us resolution (1/216≈15.3μs1 / 2^{16} \approx 15.3 \mu s). Total T-field: 6 bytes.
  • 4+0: 4 coarse bytes, no fractional part. 1-second resolution. Total T-field: 4 bytes.

CUC is used in the cFE telemetry secondary header (6-byte timestamp). It is the natural choice when the on-board clock provides a seconds-and-ticks counter.

CDS — Day Segmented​

CDS encodes time as a day count since the epoch plus milliseconds within the day. It optionally includes a sub-millisecond field for higher resolution.

The P-field encodes:

  • Time code ID (3 bits): 100 for CDS.
  • Epoch ID (1 bit): 0 = CCSDS epoch, 1 = agency-defined.
  • Day segment length (1 bit): 0 = 16-bit day count (range ~179 years), 1 = 24-bit day count (range ~45,000 years).
  • Sub-millisecond resolution (2 bits): 00 = none, 01 = 16-bit microseconds (0--999 us), 10 = 32-bit picoseconds (0--999,999,999 ps).

Common configurations:

  • 16-bit day, no sub-ms: 2 day + 4 ms = 6-byte T-field. Millisecond resolution, ~179 year range.
  • 16-bit day, us: 2 day + 4 ms + 2 us = 8-byte T-field. Microsecond resolution.
  • 24-bit day, ps: 3 day + 4 ms + 4 ps = 11-byte T-field. Picosecond resolution, extended range.

CDS is suited for ground systems and science data where wall-clock time (day + time-of-day) is more natural than a raw second count.

Choosing Between CUC and CDS​

CUC is compact and maps directly to hardware tick counters. CDS is human-readable (day + milliseconds) and convenient for correlating events with calendar dates. In practice, flight software uses CUC for telemetry timestamps and CDS for ground-originated time references and science data annotations.