# ADR-0003: Sync derived state only; raw telemetry stays local

## Status

Accepted (2026-07-08)

## Context

Each drone produces high-rate sensor telemetry and on-board video detections —
far more than the mesh can carry. But peers only need enough to coordinate
flight: where everyone is, where they are heading, what was detected. Pushing
raw feeds across the air would saturate the radio and drain batteries for data
nobody reads in flight.

## Decision

Only **derived state** crosses the air — position, attitude, detections — as
small pub/sub broadcasts (~5 Hz). **Raw telemetry stays on local NVMe** until
the drone lands, then travels to the warehouse during offload
([ADR-0002](ADR-0002-one-storage-format.md)). The design is **event-driven**:
new derived data triggers downstream action through hooks; nothing polls asking
"anything new yet?".

## Consequences

- Air bandwidth scales with the number of peers and the broadcast rate, not
  with sensor resolution.
- The warehouse is the only place with the full picture; in-flight decisions use
  the local window plus peer broadcasts.
- Broadcast staleness per peer becomes a first-class health signal
  ([`../07-observability.md`](../07-observability.md)).
- What syncs and what does not is specified in
  [`../04-swarm-sync.md`](../04-swarm-sync.md).