ADR-001: No CSP — CCSDS-only Network Layer
Status: Accepted — 2026-04-17
Track: 1 (AX.25 Link Layer)
Context doc: ../superpowers/specs/2026-04-17-track1-ax25-design.md
Context
An external review flagged the absence of CSP (CubeSat Space Protocol) as a
project deficiency, comparing UniSat to libcsp-based flight software. CSP
provides port-based addressing, packet routing between internal nodes, and
a light authentication layer over arbitrary data links.
Decision
UniSat-1 does not implement CSP. The network-layer role is filled by
CCSDS Space Packet Protocol (CCSDS 133.0-B-2) carried inside AX.25 UI frames
on the UHF link, and directly over CCSDS ASM framing on the S-band link.
Rationale
- Topology. UniSat-1 is a point-to-point satellite ↔ ground system.
CSP is valuable when multiple internal nodes share a bus — typically
OBC ↔ EPS ↔ COMMS ↔ payload on CAN or RS-485 with independent MCUs.
UniSat packages all subsystems in a single STM32F446 OBC; there is no
internal bus for CSP to route on.
- Addressing. CCSDS APID is an 11-bit subsystem/packet-type tag that
covers every dispatch decision we need today (OBC housekeeping, EPS
telemetry, ADCS attitude, GNSS, payload data, commands). Adding CSP
ports would duplicate this with no operational benefit.
- Flash footprint.
libcsp adds ~4 KB of flash plus a routing table
that would never contain more than two entries.
- Ground-station interop. Amateur-radio ground tools (GNU Radio
Companion, SatNOGS, gpredict-based tracking software) ingest AX.25 +
CCSDS natively. CSP frames are uncommon on amateur UHF and would
require custom receiving software.
- Testability. Every byte on the wire is explainable with existing
open specs (AX.25 v2.2, CCSDS 133.0-B-2). Adding CSP adds an
independent framing layer a student must learn before touching the
link.
Consequences
- No built-in CMP (CSP Management Protocol) — we duplicate health telemetry
via CCSDS APID
OBC_HOUSEKEEPING.
- No RDP (Reliable Datagram Protocol) — reliability is managed at the
application layer via sequence numbers + ground-side retransmit. This is
acceptable because beacon frames are idempotent and commands carry their
own sequence + HMAC (planned for Track 1b).
- If UniSat-2 adds an EPS daughterboard with its own MCU over CAN, CSP
should be reconsidered at that mission boundary.
Alternatives considered
- Port
libcsp wholesale: rejected (rationale above, ~20 hours work,
zero operational benefit on current hardware).
- Thin CSP-compatible adapter on top of CCSDS APID: rejected as a
“checkbox” solution that adds maintenance burden without real value.