TC Sync and Channel Coding

Conformance Statement for `pkg/tcsc`, CCSDS 231.0-B-4

A2.1 GENERAL INFORMATION

A2.1.1 Identification of PICS

Field	Value
Date of Statement (DD/MM/YYYY)	25/08/2026
PICS Serial Number	ASTRO-TCSC-PICS-002
System Conformance Statement Cross-Reference	This document

A2.1.2 Identification of Implementation Under Test (IUT)

Field	Value
Implementation Name	astro/pkg/tcsc
Implementation Version	See `go.mod` / latest commit on `main`
Special Configuration	None
Other Information	Go library implementing CCSDS TC Synchronization and Channel Coding sublayer. Provides CLTU wrapping/unwrapping with BCH(63,56) forward error correction per codeblock (SEC and TED decoding modes), CCSDS pseudo-randomization, configurable start/tail sequences, and PLOP-1/PLOP-2 acquisition and idle sequence assembly.

A2.1.3 Identification of Supplier

Field	Value
Supplier	Ravi Suhag
Contact Point for Queries	GitHub, github.com/ravisuhag/astro
Implementation Name(s) and Version(s)	astro/pkg/tcsc (Go package)
System Name(s)	Astro

A2.1.4 Identification of Specification

Field	Value
Specification	CCSDS 231.0-B-4 (TC Synchronization and Channel Coding, Blue Book, Issue 4, November 2019)
Have any exceptions been required?	Yes [X] No [ ]

NOTE, Non-supported optional capabilities (LDPC and convolutional coding) are identified in section A2.2. Clause references follow the Issue 4 numbering: section 3 (BCH coding), section 4 (LDPC coding), section 5 (CLTU), section 6 (randomizer), section 7 (physical layer operations procedures), section 8 (managed parameters).

NOTE: CCSDS 231.0-B-4 publishes no PICS proforma; its annex A is the service definition. This document is therefore a coverage matrix written against the clauses above, not a filled-in proforma, and the item numbers (TCSC-n) are local to this repository.

A2.2 REQUIREMENTS LIST

Table A-1: BCH(63,56) Coding (section 3)

Item	Description	Reference	Status	Support
TCSC-1	BCH Generator Polynomial	3.3	M	Yes
g(x) = x^7 + x^6 + x^2 + 1, represented as `bchPoly = 0xC5`.
TCSC-2	BCH Codeblock Structure	3.2	M	Yes
64 bits per codeblock: 56 information bits (7 bytes) + 7 parity bits + 1 filler bit. Constants: `InfoBytes = 7`, `CodeblockBytes = 8`.
TCSC-3	BCH Systematic Encoding with Complemented Parity	3.3	M	Yes
`BCHEncode(info)`, 7-bit LFSR systematic encoding. The transmitted parity bits are the COMPLEMENT of the LFSR remainder (`parity = ^sr & 0x7F`), per the standard. Information bytes unchanged in first 7 bytes; parity in high 7 bits of the 8th byte. Pinned to the known vector: all-zeros information octets encode to last octet 0xFE.
TCSC-4	BCH Filler Bit	3.3.2	M	Yes
Filler bit (bit 0 of byte 7) is always '0' per 3.3.2. Pinned by `TestBCHEncode_FillerBitAlwaysZero`.
TCSC-5	BCH Syndrome Computation	3.5	M	Yes
`BCHDecodeWithMode()` complements the received parity bits (undoing the on-the-wire inversion) before the LFSR syndrome pass over all 63 code bits. Zero syndrome indicates no errors.
TCSC-6	SEC Decoding Mode	3.5	M	Yes
`BCHDecode()` / `ModeSEC` corrects 1 bit error per codeblock; `findErrorPosition()` searches all 63 bit positions for a syndrome match. Returns corrected information bytes and correction count. NOTE: in SEC mode a 3-bit error pattern can miscorrect; guaranteed detection of up to 3 bit errors requires TED mode.
TCSC-7	TED Decoding Mode	3.5	M	Yes
`BCHDecodeWithMode(cb, ModeTED)`, Triple Error Detection: no correction attempted; any non-zero syndrome returns `ErrUncorrectable`. Guaranteed detection of up to 3 bit errors per codeblock. Also selectable end-to-end via `UnwrapCLTUWithMode()`.
TCSC-8	BCH Uncorrectable Detection	3.5	M	Yes
SEC mode returns `ErrUncorrectable` when the syndrome is non-zero and no single-bit error position matches (2+ bit errors).
TCSC-9	BCH Error-Free Pass-Through	3.5	M	Yes
Zero syndrome: information bytes returned with 0 corrections and nil error.

Table A-2: Pseudo-Randomization (section 6)

Item	Description	Reference	Status	Support
TCSC-10	PN Sequence Generation	6.2	M	Yes
`GeneratePNSequence(length)`, 8-bit LFSR, h(x) = x^8 + x^6 + x^4 + x^3 + x^2 + x + 1, preset to all ones. Pinned to the published first 40 digits (0xFF 0x39 0x9E 0x5A 0x68). This is the TC polynomial; the TM randomizer of CCSDS 131.0-B-5 clause 10.4.2 is a different sequence and is not used here.
TCSC-11	Randomization (Send)	6.3	M	Yes
`Randomize(data)` XORs data with the PN sequence. Returns new slice; input not modified.
TCSC-12	De-Randomization (Receive)	6.3	M	Yes
Same `Randomize()` function, XOR is self-inverse. Integrated into `UnwrapCLTU()` when randomize=true.
TCSC-13	Randomization Coverage in the CLTU	6.3	M	Yes
`WrapCLTU()` pads to the codeblock boundary with 0x55 fill FIRST, then randomizes the padded buffer, so the fill octets go out randomized like the rest of the data. `UnwrapCLTU()` de-randomizes the full recovered buffer (fill included). Pinned by `TestWrapCLTU_RandomizesFillOctets`.

Table A-3: CLTU (section 5)

Item	Description	Reference	Status	Support
TCSC-14	CLTU Start Sequence	5.2.2	M	Yes
`DefaultStartSequence()` returns the standard 2-byte start sequence 0xEB90. Custom sequences supported via `WrapCLTU()` parameter.
TCSC-15	CLTU Tail Sequence	5.2.4	M	Yes
`DefaultTailSequence()` returns the standard 8-byte tail sequence 0xC5C5C5C5C5C5C579. The pattern is chosen so that it fails BCH decoding even after any single bit error, which is what terminates reception.
TCSC-16	CLTU Assembly (Send)	5.2	M	Yes
`WrapCLTU(frameData, startSeq, tailSeq, randomize)`: 0x55 padding to the 7-byte boundary, optional randomization of the padded buffer, BCH encoding of each block, start sequence prepend, tail sequence append.
TCSC-17	CLTU Data Padding	3.4, 5.2	M	Yes
Frame data padded to a multiple of `InfoBytes` (7) with the fill data of 3.4 (0x55 octets) before randomization and BCH encoding, as part of CLTU assembly.
TCSC-18	CLTU Reception and Termination	5.3	M	Yes
`UnwrapCLTU()` / `UnwrapCLTUWithMode()`: validates the start sequence, decodes codeblocks, and terminates on the tail sequence or on the FIRST codeblock that fails to decode, an exact tail match is not required, so bit errors in the tail are tolerated. Trailing octets after the CLTU are ignored.
TCSC-19	Start Sequence Validation	5.3	M	Yes
`UnwrapCLTU()` validates the start sequence; returns `ErrStartSequenceMismatch` otherwise.
TCSC-20	Minimum Length Validation	5.3	M	Yes
`UnwrapCLTU()` requires at least the start sequence plus one codeblock; returns `ErrDataTooShort` otherwise.

Table A-4: Acquisition, Idle, and PLOP (section 7)

Item	Description	Reference	Status	Support
TCSC-21	Acquisition Sequence	7.2.2	M	Yes
`AcquisitionSequence(octets)`, alternating '01' pattern (0x55 octets); defaults to the recommended minimum of 16 octets (128 bits).
TCSC-22	Idle Sequence	7.2.4	M	Yes
`IdleSequence(octets)`, alternating '01' pattern between CLTUs. The length is unconstrained by 7.2.4 and is a managed parameter; `DefaultIdleOctets = 8` is this library's practical default, not a requirement.
TCSC-23	PLOP-1	7.4	M	Yes
`UplinkSequence(PLOP1, ...)`, each CLTU preceded by its own acquisition sequence (session ends after each CLTU).
TCSC-24	PLOP-2	7.5	M	Yes
`UplinkSequence(PLOP2, ...)`, one acquisition sequence starts the session; idle sequence keeps the channel modulated between CLTUs. CCSDS-recommended procedure.
TCSC-25	Carrier Modulation Modes (CMM state machine)	7	M	No
The CMM-1..CMM-4 state machine itself is not modeled; the library assembles the symbol stream (acquisition/idle/CLTU ordering) and leaves carrier control to the ground station equipment.

Table A-5: Optional Coding Schemes

Item	Description	Reference	Status	Support	Notes
TCSC-26	LDPC Coding	4	O	No	Not implemented.
TCSC-27	Concatenated BCH + Convolutional	-	O	No	Convolutional inner code not implemented. BCH outer code is available.

A2.3 CONFORMANCE SUMMARY

Overall Statistics

Category	Total Items	Supported	Not Supported
Mandatory (M)	25	24	1
Optional (O)	2	0	2
Total	27	24	3

Non-Conformances (Mandatory Items Not Supported)

Item	Description	Reason
TCSC-25	CMM state machine
The library produces the physical-channel symbol stream (`UplinkSequence`) but does not model the carrier modulation mode transitions; carrier on/off control belongs to the transmitting equipment, not to a codec library.

Non-Supported Optional Items

Item	Description	Reason
TCSC-26	LDPC Coding	Not implemented. Specialized application for high-data-rate TC links.
TCSC-27	Concatenated BCH + Convolutional	Convolutional inner code not implemented.

Key Implementations

Area	Items	Implementation
BCH(63,56) Coding	TCSC-1-9
`BCHEncode()` (complemented parity, filler '0', all-zeros -> 0xFE vector), `BCHDecode()` / `BCHDecodeWithMode()` with SEC and TED modes.
Pseudo-Randomization	TCSC-10-13
`GeneratePNSequence()`, `Randomize()`; fill-then-randomize order in `WrapCLTU()`.
CLTU	TCSC-14-20
`WrapCLTU()`, `UnwrapCLTU()` / `UnwrapCLTUWithMode()` terminating on the first failed codeblock.
Acquisition/Idle/PLOP	TCSC-21-24
`AcquisitionSequence()`, `IdleSequence()`, `UplinkSequence()` for PLOP-1 and PLOP-2.

Wire test vectors

The octets backing this statement live in the vector corpus — 13 vectors. Each vector names the clause it comes from and carries the derivation that produced it.

File
`tcsc/bch.json`	7 vectors
`tcsc/interop.json`	6 vectors, captured from Yamcs 5.13.5

These are data files, so any implementation can check itself against the same octets. See CONTRACT.md for how, and how this is verified for what rests on a published vector versus a reading of the clause.