Ghostline: a six-node Wi-Fi CSI motion tracker with node-to-node Links. System description and a pilot evaluation
Ghostline is a device-free motion tracking system built on six ESP32-S3 Wi-Fi boards and one consumer router. The boards' Wi-Fi driver reports channel state information (CSI) for every received frame with the sender's address, and a modified open-source firmware forwards both, so every board carries a CSI stream for the router and for each other board: seven radios give up to 36 directed Links over 21 Spans instead of a star of six. Each Link is compared with a self-forming quiet Baseline, disturbed Links vote on a floor grid, and the grid becomes anonymous Tracks. The system was built and run during a three-day hackathon in an ordinary room. We describe it in full and report a pilot evaluation with one walker: spoken cues at seven marked positions, a two-minute free walk and five minutes of empty room, synchronised to video within 0.18 s (median). Walking between marks, the Track lay a median 0.85 m from the straight path and 1.56 m (95% CI 1.29 to 2.83) from a cue-based reference trajectory (constant speed between cues, not measured positions), with 46% of walk updates within 2 m of it; the free walk was tracked in 94% of updates. The limits: a standing person is lost within seconds, a false Track appeared in 15.9% and 2.9% of updates in two empty-room periods, and informal runs that afternoon did not separate two walkers. Two offline replays through the same pipeline show where this comes from: with the node-to-node Links removed and settings unchanged, the tracked share of walk updates falls from 90% to 19% and the free walk from 96% to 60%; a plain radio tomographic image on the same per-Span evidence, at the same false rate, matches the grid and tracker at 2 m (48% against 47%) and is ahead at 1 m (36% against 21%). On this recording the Tracks depend on the Link set, and the imaging stage adds no measurable accuracy. We state what the pilot establishes and what it does not.