CCSDS is dense with acronyms and most of these docs use them without stopping to expand. This page stops.
Ordered by where it sits in the stack rather than alphabetically, because the shape is the useful part. Every expansion here is the one the standards and the package documentation use.
Packets
| APID | Application Process Identifier | |
Names the on-board application a packet came from or is going to. 11 bits, so 0 to 2047; 0x7FF is reserved for idle packets. pkg/spp | ||
| SPP | Space Packet Protocol | |
CCSDS 133.0-B-2, the packet every mission uses. pkg/spp | ||
| EPP | Encapsulation Packet Protocol | |
CCSDS 133.1-B-3, a thin wrapper for data that is not a Space Packet, such as IP. pkg/epp | ||
| PDU | Protocol Data Unit | |
One protocol's unit of transmission. Used throughout CFDP, LTP and Proximity-1. | ||
| SDU | Service Data Unit | |
What a service hands down to the layer below, or up to the layer above. | ||
| PUS | Packet Utilization Standard | |
ECSS-E-ST-70-41 pkg/pus | ||
Data link
| TM | Telemetry | |
Fixed-length downlink frames, CCSDS 132.0-B-3. pkg/tmdl | ||
| TC | Telecommand | |
Variable-length uplink frames, CCSDS 232.0-B-4. pkg/tcdl | ||
| AOS | Advanced Orbiting Systems | |
High-rate downlink frames that also carry bitstreams and opaque blocks, CCSDS 732.0-B-4. pkg/aos | ||
| USLP | Unified Space Data Link Protocol | |
One frame format for both directions, CCSDS 732.1-B-3. pkg/usdl | ||
| SCID | Spacecraft Identifier | |
Which spacecraft a frame belongs to. 10 bits in TM and TC, 8 in AOS. | ||
| VCID | Virtual Channel Identifier | |
Which stream inside the physical channel. 3 bits in TM, 6 in AOS and TC. | ||
| GVCID | Global Virtual Channel Identifier | |
Frame version, spacecraft and virtual channel together, so a channel is unique across missions. | ||
| MCID | Master Channel Identifier | |
Frame version and spacecraft, without the virtual channel. | ||
| FHP | First Header Pointer | |
Where the first packet starts inside a frame's data field. What lets a receiver find packet boundaries again after losing a frame. | ||
| OCF | Operational Control Field | |
Four octets in every frame of a channel that carries one. Where the CLCW rides home. Full duplex | ||
| FECF | Frame Error Control Field | |
The frame's CRC, two octets at the end. | ||
| FSH | Transfer Frame Secondary Header | |
Optional per-frame header, fixed length for the channel. | ||
| FHEC | Frame Header Error Control | |
Reed-Solomon over an AOS frame header, so a corrupted VCID is caught rather than mis-routed. AOS only. | ||
| MAP | Multiplexer Access Point | |
A subdivision of a TC or USLP virtual channel. | ||
| VCA | Virtual Channel Access | |
The service that carries one opaque fixed-length block per frame, with no protocol header. | ||
| M_PDU | Multiplexing PDU | |
The AOS data field shape that carries variable-length packets. | ||
| B_PDU | Bitstream PDU | |
The AOS data field shape that carries a raw octet stream. | ||
| OID | Only Idle Data | |
The reserved virtual channel, 63, that carries nothing but fill. | ||
Reliable commanding
| COP-1 | Communications Operation Procedure-1 | |
CCSDS 232.1-B-2, what makes telecommand reliable. Two state machines. pkg/cop | ||
| FOP-1 | Frame Operation Procedure | |
The ground half of COP-1: assigns sequence numbers, holds a sliding window, retransmits. | ||
| FARM-1 | Frame Acceptance and Reporting Mechanism | |
The spacecraft half: accepts, discards or locks out, and reports what it did. | ||
| CLCW | Communications Link Control Word | |
Four octets FARM-1 generates saying what it has accepted. Travels back in the OCF of a telemetry frame. | ||
| AD, BD, BC | Type-A Data, Type-B Data, Type-B Control | |
The three TC frame kinds. AD is sequence-controlled and reliable; BD bypasses the sequence check; BC carries a control directive such as Unlock. | ||
Coding and synchronization
| TMSC | TM Synchronization and Channel Coding | CCSDS 131.0-B-5, the downlink coding layer. pkg/tmsc |
| TCSC | TC Synchronization and Channel Coding | CCSDS 231.0-B-4, the uplink coding layer. pkg/tcsc |
| ASM | Attached Sync Marker | The known pattern, 0x1ACFFC1D for TM, that says a frame starts here. |
| CADU | Channel Access Data Unit | A sync marker followed by a coded, randomized frame. What actually goes on a downlink. |
| CLTU | Command Link Transmission Unit | The uplink equivalent: start sequence, BCH codeblocks, tail sequence. |
| PLTU | Proximity Link Transmission Unit | The Proximity-1 equivalent. pkg/pxsc |
| RS | Reed-Solomon | The block code CCSDS 131.0 uses on a downlink. RS(255,223) corrects 16 symbols per codeword, RS(255,239) corrects 8. |
| BCH | Bose-Chaudhuri-Hocquenghem | The code inside a CLTU codeblock. BCH(63,56) corrects one bit. |
| LDPC | Low-Density Parity-Check | A modern code CCSDS 131.0 also defines. Not implemented; pkg/tmsc stops at Reed-Solomon. |
| PLOP | Physical Layer Operations Procedures | How a CLTU sequence is delimited on the physical layer. |
| OCSC | Optical Coding and Synchronization | CCSDS 142.0-B-1, the coding layer for a laser link. Works in bits, not octets. pkg/ocsc |
Security
| SDLS | Space Data Link Security | CCSDS 355.0-B-2, encrypting or authenticating a frame's data field. pkg/sdls |
| SA | Security Association | The agreed parameters both ends configure before the link opens: algorithm, key, field widths, channels. |
| SPI | Security Parameter Index | The only part of the SA that travels on the wire. A pointer to the agreement. |
| MAC | Message Authentication Code | The tag in the security trailer that proves the frame was not forged or altered. |
| IV | Initialization Vector | The per-frame nonce for AES-GCM. Never reused, which is why an SA is not concurrency safe. |
Files and networking
| CFDP | CCSDS File Delivery Protocol | CCSDS 727.0-B-5, moving whole files. pkg/cfdp |
| DTN | Delay-Tolerant Networking | Store and forward, for links that are never all up at once. |
| BP | Bundle Protocol | The DTN network layer, RFC 5050 as profiled by CCSDS 734.2-B-1. Version 6, not BPv7. pkg/bp |
| LTP | Licklider Transmission Protocol | The DTN convergence layer for one hop, RFC 5326 as profiled by CCSDS 734.1-B-1. pkg/ltp |
| SDNV | Self-Delimiting Numeric Value | The variable-length integer BP and LTP encode nearly every field with. |
| ECOS | Extended Class of Service | The bundle block CCSDS requires on top of RFC 5050, adding a finer priority ordinal. |
Ground to ground
| SLE | Space Link Extension | Moving frames between ground systems over TCP. The only protocol here that never touches a spacecraft. pkg/sle |
| ISP1 | Internet SLE Protocol One | The transport SLE runs over, CCSDS 913.1-B-2. |
| TML | Transport Mapping Layer | ISP1's message framing: a header, then a context, heartbeat or PDU body. |
| RAF | Return All Frames | The SLE service that delivers every frame on a physical channel. |
| RCF | Return Channel Frames | Delivers frames from one virtual channel. |
| ROCF | Return Operational Control Fields | Delivers just the OCFs, so the CLCWs. |
| FCLTU | Forward CLTU | The uplink direction: the control centre sends CLTUs to the station. |
| BER | Basic Encoding Rules | How SLE's ASN.1 messages are encoded. |
Time
| CUC | CCSDS Unsegmented Time Code | A binary counter: coarse seconds and fine fractions. Smallest, and what a link should carry. pkg/tcf |
| CDS | CCSDS Day Segmented Time Code | Day count, milliseconds of day, optional sub-milliseconds. |
| CCS | CCSDS Calendar Segmented Time Code | BCD calendar fields. Widest, and readable off a hex dump. |
| TAI | International Atomic Time | Continuous, no leap seconds. A CUC Level 1 code counts TAI seconds since 1958. |
| UTC | Coordinated Universal Time | What people use. 37 seconds behind TAI since 2017. Time correlation |
Mission data
| XTCE | XML Telemetric and Command Exchange | The mission database format: what each parameter is, how it is encoded, which packet carries it. pkg/xtce |
| LDC | Lossless Data Compression | CCSDS 121.0-B-3, the Rice adaptive entropy coder, for science data. pkg/ldc |
| RHC | Robust Compression of Housekeeping Data | CCSDS 124.0-B-1, POCKET+, for telemetry that barely changes. pkg/rhc |
Assurance
| PICS | Protocol Implementation Conformance Statement | The proforma a standard ships for an implementer to fill in, clause by clause. What the conformance pages are. |
| IUT | Implementation Under Test | What a PICS is filled in about. Here, the package. |
| M, O, C | Mandatory, Optional, Conditional | The status a PICS gives each item. There are 626 mandatory and 202 optional rows across these pages. |
Reference
- The stack, how these layers fit together
- Protocol index, one page per standard
- How this is verified