R11 — Maritime sensing: through-bulkhead RF is impossible, through-seam works

Status: physics scrutiny + honest verdict + 10-20y vertical map · 2026-05-22

TL;DR

The romantic "through-bulkhead WiFi sensing for ships and submarines" framing is physically wrong at WiFi bands. Steel bulkheads have a skin depth of 3.25 µm at 2.4 GHz — a single millimetre of mild steel produces 2,674 dB attenuation, more than the link budget of any portable device by a factor of 10²². No amount of clever DSP recovers a signal through closed metal.

What does work is through-seam sensing — exploiting the diffraction leakage through gaskets, vent slots, hatch seals, and porthole gaskets. This thread maps which maritime scenarios are physically feasible and which aren't.

Physics

Skin depth in steel

δ = 1 / √(π·f·μ·σ)

For mild steel (σ = 1·10⁷ S/m, μ_r = 1):

A 1 mm steel sheet attenuates 2,674 dB at 2.4 GHz — utterly impassable.

Saltwater attenuation

For seawater (σ = 4.8 S/m, ε_r = 81) via the lossy-dielectric model:

Saltwater is similarly opaque. A head 30 cm underwater = 256 dB additional loss = invisible. Submarine RF comms work at VLF (10-30 kHz) for exactly this reason; WiFi-band underwater detection is hopeless.

Slot diffraction (the loophole)

For a narrow slot of width w << λ in an otherwise opaque conductor, the diffraction loss approximates:

L_slot ≈ 20·log10(λ / 2w)   when w < λ/2
       ≈ 0                   when w ≥ λ/2

At 2.4 GHz λ = 12.5 cm, so any slot wider than 6.25 cm is effectively transparent. A typical cabin-door gasket gap is 2-5 mm — significant attenuation (~22-30 dB) but well within link budget.

Composite scenarios

examples/research-sota/r11_maritime_propagation.py computes the composite (FSPL + bulk + slot + saltwater) for seven scenarios. ESP32-S3 link budget = 121 dB, 10 dB SNR margin reserved for DSP.

Verticals catalogued

✅ Feasible at WiFi bands

1. Man-overboard surface detection. ESP32 + omnidirectional antenna on a ship's mast, monitoring CSI on a beacon worn by crew. Pull-down of the beacon below the waterline → CSI signature flips from "surface scatterer with sea-state Doppler" to "no signal" within 1 second. False-positive rejection via gait-frequency-band check (R10) on the surface-state CSI.
2. Through-seam vitals in confined spaces. Submarine berth compartments, ship cabins, lifeboat interiors. Sensor in adjacent compartment monitors heart-rate / breathing via 2-5 mm gasket leakage. Use case: lone-watch monitoring without crew compromise (no camera, no microphone).
3. Container intrusion / contents change. Sea-cargo container with at least one vent slot >2 cm leaks RF. Sensor outside monitors CSI signature; sudden change indicates contents shifted or door opened. Use case: tamper detection on bonded customs cargo, long-haul container security.
4. Hatch-seal integrity audit. A known-position transmitter inside a compartment, receiver outside. Closed-and-sealed hatch → only seam leakage (specific dB attenuation per gasket condition). Drift in this attenuation over time = gasket degradation. Predictive maintenance for watertight integrity.
5. Engine room thermal-anomaly detection (via condensation). RF propagation in moist air is bandwidth-dependent. Sustained CSI-amplitude drift = condensation envelope shifting = thermal anomaly. Indirect, but adds a sensing modality to engine rooms without IR cameras.

❌ Not feasible at WiFi bands

1. Through-hull submarine comms (use VLF/ELF instead — different industry).
2. Underwater swimmer detection (use sonar / acoustic — different industry).
3. Through-watertight-bulkhead sensing into a sealed compartment with no leakage path.
4. Through-radome of any reasonable thickness (most radomes are thin enough to pass — but this isn't the use case).

Re-framed verticals (with caveats)

1. Pirate-skiff approach detection (10y). Air-link sensing from a vessel's superstructure can detect small boats approaching at radar-blind low altitudes. Range: ~100 m at 2.4 GHz (R10's foliage-less air model). The maritime version of R10's wildlife sensing.
2. Crew situational awareness in dark / smoke (15y). Through-seam vitals + breathing patterns inside compartments tell fire-control whether occupants are conscious. Real value-add when smoke obstructs cameras.
3. Whale-strike avoidance (20y). Surface-floating mammals can be detected at the surface by CSI Doppler signature; the practical issue is range (whales are slow, ship is fast — need 200+ m detection). The R6 Fresnel envelope at 200 m link length is ~3.5 m wide; large enough to catch a whale-sized target, marginal for smaller mammals.

How this composes with prior threads

• R6 (Fresnel forward model): the per-subcarrier signature of through-seam leakage is a band-passed version of the open-air signature, distorted by the slot's frequency response. Detectable, but the saliency profile differs from R5's open-room measurement.
• R10 (foliage): the through-air maritime scenarios (man-overboard, pirate-skiff) reuse R10's free-space link budget directly. ~100 m at 2.4 GHz in clear-air conditions.
• R1 (CRLB): 4-anchor multistatic on a small ship's superstructure (4 corners of a 10 m wheelhouse) achieves ~30 cm ToA position precision; >10 m operational ranges put us in the room-pose-quality regime.
• R7 (mincut adversarial): essential for maritime. Single-link spoofing is easy (jammer on the dock). Multi-link consistency over 4 superstructure sensors is the only way to harden against this.

Honest scope

• All numbers are best-case — ignore vessel vibration, electromagnetic noise from engine ignition systems, salt-spray on antennas, multipath from steel surfaces (which dominates real maritime CSI).
• Salt-spray on PCB antennas degrades them by 3-10 dB after a few hours of operation. Marine-grade conformal coating extends this, but installation is harder than land deployments.
• Vibration from engines / wave-slap modulates CSI at ~5-30 Hz. This is in-band with the gait frequencies used for R10's species classifier — making maritime gait-classification much harder than land.
• No GPS in steel compartments. Multistatic positioning would need an alternative reference (inertial + RF anchors on the vessel itself). This is solvable but adds installation complexity.
• The 200 m air-link range assumes a clear horizon. Real vessels have superstructure occluding many bearings; effective coverage is more like a 90° forward arc.

What this DOES enable

• A physically honest maritime sensing roadmap that doesn't promise through-bulkhead capability that doesn't exist.
• Clear product categories where ESP32 + RuView stack adds value: man-overboard surface detection, through-seam vitals, container tamper detection.
• A predictive-maintenance angle (hatch-seal degradation) that has no current sensor alternative.

What this DOES NOT enable

• Through-hull submarine sensing — physics says no at any practical bandwidth.
• Underwater sensing at WiFi frequencies — physics says no.
• Single-sensor multistatic localisation on a ship — vibration noise needs multi-sensor consensus.

Next ticks (R11 follow-ups)

• Through-seam frequency response measurement. Place ESP32 + known signal source on opposite sides of a cabin door with a controlled gasket gap; characterise the slot transfer function vs. the slot-diffraction model.
• Vibration-suppression filter: design a notch/comb filter that removes 5-30 Hz engine-modulation from CSI, validate on a real boat (no boat available in repo, but the filter design is reproducible).
• ADR sketch for cog-maritime-watch: man-overboard + through-seam vitals as a maritime-specific cog package. Same ADR-103 pattern as cog-person-count, different model + different feature set.

Connection back

• R5 (saliency) — through-seam slot acts as a frequency-selective filter; the saliency profile through a seam differs from open-air saliency. New experiment opportunity.
• R6 (Fresnel) — Fresnel envelope still applies through seam, but the slot acts as an additional spatial filter, restricting the effective transmit position. The composite "Fresnel-zone-AND-slot-aligned" envelope is much narrower.
• R10 (foliage) — air-side maritime scenarios reuse R10's link-budget primitives unmodified.
• R12 (eigenshift) — the structure-detection problem is even harder on ships because the natural drift floor includes vessel motion and engine vibration. PABS over Fresnel+vibration basis is the maritime version.
• R14 (empathic appliances) — through-seam vitals + the V1 stress-responsive lighting framework could plausibly become "crew wellness monitoring in confined ship cabins". Privacy framework from R14 transfers directly.