Physics scrutiny of WiFi-band maritime sensing scenarios. Steel skin depth
is 3.25 um at 2.4 GHz, making bulkheads utterly opaque. Saltwater
attenuation is 853 dB/m. The 'through-bulkhead WiFi radar' framing
common in conservation/maritime is wrong; the actual feasible category
is 'through-seam' sensing exploiting slot diffraction through gaskets,
hatch seals, and vent grilles.
Composite link budget for 7 maritime scenarios (ESP32-S3 121 dB budget,
10 dB SNR margin):
FEASIBLE:
- Man-overboard surface @ 200 m: +25 dB
- Cabin door, 2 mm seam: +31 dB
- Cabin door, 5 mm seam: +39 dB
- Container, 30 mm vent slot: +45 dB
IMPOSSIBLE:
- Closed 10 mm steel door: -938 dB
- Submarine pressure hull: -929 dB
- Head 30 cm underwater: -231 dB
Five feasible verticals catalogued: man-overboard surface, through-seam
crew vitals, container tamper detection, hatch-seal predictive
maintenance, engine-room thermal anomaly via condensation.
Composes with prior threads:
- R6 Fresnel envelope + slot diffraction = narrower composite envelope
- R10 link-budget primitives reused unmodified for air-side maritime
- R7 multi-link consistency essential against superstructure jammers
- R14 privacy framework transfers directly to crew-cabin monitoring
Honest scope: best-case ignores vessel vibration (5-30 Hz, in-band with
R10 gait frequencies), engine ignition noise, salt-spray, steel-surface
multipath. Maritime gait-classification is harder than land.
The romantic 'through-hull radar' is now explicitly debunked. The actual
product roadmap is gasket-leakage sensing, surface detection, and
predictive-maintenance audits.
Coordination: ticks/tick-10.md, no PROGRESS.md edit.
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