Optical-flow sensors give the drone XY velocity by tracking surface motion as seen by a downward camera. Critical for indoor GPS-denied operation — without them, EKF3’s EK3_SRC1_VELXY has no source and the drone won’t arm.
Two options:
- PMW3901 — raw chip from PixArt. SPI interface, custom driver, ~$10.
- CXOF — Holybro’s PMW3901-based module with ArduPilot-ready UART protocol. ~$50, production-ready.
Trade-offs and integration recipe: PMW3901 limits.
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