Comms Track: Spacecraft Links and Flight Software Basics · Module 4 of 4 · about 15 min
Packets and protocols: how the data is packaged
Spacecraft and ground systems from different companies and countries need to understand each other, so the industry uses shared standards. Most come from the Consultative Committee for Space Data Systems (CCSDS), a group of space agencies including NASA and ESA. Their standards are called Recommended Standards and are published as "Blue Books", free to download.
The workhorse is the Space Packet, defined in CCSDS 133.0-B. Every packet starts with a primary header of exactly 6 bytes (48 bits). Inside it, a 3-bit version number, a 1-bit packet type (0 for telemetry, 1 for telecommand), a 1-bit flag saying whether a secondary header follows, and an 11-bit Application Process Identifier, or APID. Then come sequence flags, a 14-bit sequence count and a 16-bit length field. After the header comes the data.
The APID tells the ground which application or data stream a packet belongs to: say, power telemetry versus camera data. With 11 bits there are 2,048 values, and the all-ones value, 2047, is reserved for idle packets that just fill time on the link. The sequence count lets the ground spot dropped or out-of-order packets. If you have ever looked at a network packet in Wireshark, this will feel familiar, and that is a useful bridge to mention if you are coming from IT or networking.
Packets ride inside larger frames defined by other CCSDS standards for the link layer, much like IP packets ride inside Ethernet frames. Ground software then unpacks frames into packets, packets into named telemetry points, and shows them on operators' screens. Engineers who can trace a value from a sensor, through a packet, to a plot on the ground are valuable in testing and operations.
Security has become part of the job. Commands sent to a spacecraft need to be authenticated so nobody else can send them, and many newer missions encrypt links as well. CCSDS publishes security standards for space links alongside its packet and frame standards. Showing you think about who could send a command, not just whether the packet parses, sets you apart.
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Module quiz
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