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Architecture

ADR 0002: no GPL code; clean-room implementation from papers

Accepted · 2026-10-31 · scout-drone/docs/adr/0002-no-gpl-clean-room-policy.md

Status: Accepted Date: 2026-10-31 Source: the stack’s no-GPL policy, set in cortex’s ADR 0002 Related: ADR 0001

Context

Section titled “Context”

cortex’s ADR 0002 rules out GPL code anywhere in cortex, for any purpose. The reason applies to scout-drone unchanged: partners redistribute the stack inside their own certified products, and any GPL obligation, even one confined to a test, is something their legal reviews would have to clear.

scout-drone’s task comes from Loquercio, Kaufmann et al., “Learning high-speed flight in the wild” (Science Robotics, 2021). That paper’s code, UZH’s agile_autonomy, is GPL-3.0. So are other UZH agile-flight projects (rpg_mpc, rpg_quadrotor_mpc, rpg_time_optimal), and several of the open flight stacks:

  • Betaflight, INAV and Cleanflight (GPL-3.0)
  • ArduPilot (GPL-3.0)
  • Paparazzi (GPL-2.0)

Permissive references exist too: PX4 (BSD-3-Clause) and UZH’s agile_flight (MIT). The task and its score come from the paper, which is not code.

Decision

Section titled “Decision”

No GPL-licensed code anywhere in scout-drone, for any purpose. Not as a dependency, not in a test target, not as a build-time tool in what scout-drone builds, and not as a reference oracle. The scope is ADR 0001’s: what scout-drone ships, the code that builds it and the tests, and not the docs site’s toolchain.

  • Everything is implemented clean-room from papers, textbooks and specifications, plus permissively licensed (MIT / BSD / Apache-2.0) references.
  • Check a reference’s license before opening its code. Reading GPL source and then writing the same thing “from scratch” is not clean-room. A GPL project’s structure may be consulted, meaning its directory layout and how it splits the problem into modules, and nothing else: never its code, its algorithms’ details, or its data, which includes tuning tables, gains, parameters and trained weights.
  • Test oracles are in-house or permissive: scout-drone’s own fields and simulator, flights it recorded itself, the results published in the paper, and permissive code such as PX4. Never compare against the output of a GPL program (agile_autonomy, Betaflight, ArduPilot, …), its simulator or its logs.
  • Conventions are not code. Following a convention that a GPL project also uses (a frame, a rotor order, a unit) is fine, when it is taken from a permissive source or a specification.
  • LGPL, MPL, AGPL and SSPL code is rejected, as a dependency and as a reference, as ADR 0001 rejects it. An exception would need an ADR of its own that supersedes this one.

Consequences

Section titled “Consequences”
  • Each component costs more to build: the design comes from the literature, and validation uses scout-drone’s own simulation and permissive references.
  • A paper’s results can be compared against; its GPL code cannot be read or run.
  • Partners can redistribute scout-drone under MIT terms with no copyleft review.
  • Current state: scout-drone’s field, scorer and baselines are written in-house. No GPL code or oracle is used, and agile_autonomy’s code has not been opened.