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research(R11): maritime sensing — through-bulkhead impossible, through-seam works (#712)

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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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2026-05-22 01:53:51 -04:00
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examples/research-sota/r11_maritime_propagation.py
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#!/usr/bin/env python3
"""R11 — Maritime / through-bulkhead RF propagation.
See docs/research/sota-2026-05-22/R11-maritime-sensing.md.
Computes:
- Steel bulkhead RF attenuation (skin depth) at WiFi bands
- Seam-leakage diffraction loss
- Saltwater attenuation (man-overboard surface sensing)
- Composite link budget for three maritime scenarios
Pure NumPy.
"""
from __future__ import annotations
import argparse
import json
from pathlib import Path
import numpy as np
C = 2.998e8
MU_0 = 4 * np.pi * 1e-7 # H/m
EPS_0 = 8.854e-12 # F/m
# Material properties (typical values)
STEEL_SIGMA = 1.0e7 # S/m (mild steel conductivity)
SALTWATER_SIGMA = 4.8 # S/m (35 ppt at 20 deg C)
SALTWATER_EPSR = 81.0 # relative permittivity
def skin_depth_m(freq_ghz: float, sigma: float, mu_r: float = 1.0) -> float:
"""Classical skin depth: delta = 1 / sqrt(pi * f * mu * sigma)."""
f = freq_ghz * 1e9
return 1.0 / np.sqrt(np.pi * f * MU_0 * mu_r * sigma)
def bulk_attenuation_db_per_mm(freq_ghz: float, sigma: float, mu_r: float = 1.0) -> float:
"""Per-mm attenuation through bulk conductor."""
delta = skin_depth_m(freq_ghz, sigma, mu_r)
# Field decays as exp(-x/delta), power as exp(-2x/delta)
# In dB per metre: 20/(delta*ln(10)) = 8.686/delta
return 8.686 / delta / 1000 # divide by 1000 to get per-mm
def saltwater_attenuation_db_per_m(freq_ghz: float) -> float:
"""Saltwater attenuation per metre via lossy-dielectric model.
alpha = (omega/c) * Im(sqrt(eps_r - j*sigma/(omega*eps_0)))
Returns dB/m."""
omega = 2 * np.pi * freq_ghz * 1e9
eps_complex = SALTWATER_EPSR - 1j * SALTWATER_SIGMA / (omega * EPS_0)
n_complex = np.sqrt(eps_complex)
# Principal sqrt of (a - jb), b>0, has negative imag part. The wave
# attenuation coefficient is alpha = omega/c * |Im(n)| -- take abs().
alpha = omega * abs(n_complex.imag) / C # Np/m
return float(8.686 * alpha) # dB/m
def seam_diffraction_loss_db(seam_width_mm: float, freq_ghz: float) -> float:
"""Approximate diffraction loss through a narrow slot in a conductor.
For slot width w << lambda, the slot acts as a high-pass filter:
L_slot = 20 * log10(lambda / (2 * w)) when w < lambda/2
0 when w >= lambda/2
Crude but captures the 1st-order physics. Real slot antennas are more
complex; for forensic 'how much leaks through the door seal' work
this is the right scale."""
lam_mm = (C / (freq_ghz * 1e9)) * 1000
if seam_width_mm >= lam_mm / 2:
return 0.0
return max(0.0, 20 * np.log10(lam_mm / (2 * seam_width_mm)))
def maritime_scenario(name: str, freq_ghz: float, bulkhead_mm: float,
seam_mm: float, free_air_m: float,
saltwater_m: float = 0.0) -> dict:
"""Composite path loss for a maritime sensing scenario."""
# Free-space loss
fspl = 32.45 + 20 * np.log10(freq_ghz) + 20 * np.log10(max(0.1, free_air_m + 0.1))
# Bulkhead loss (if any propagation through metal)
bulk_loss = bulkhead_mm * bulk_attenuation_db_per_mm(freq_ghz, STEEL_SIGMA)
# Seam diffraction (alternative path)
seam_loss = seam_diffraction_loss_db(seam_mm, freq_ghz) if seam_mm > 0 else 999.0
# Saltwater loss
water_loss = saltwater_m * saltwater_attenuation_db_per_m(freq_ghz)
# The actual propagation path takes whichever is lower (bulk OR seam)
best_metal_path = min(bulk_loss, seam_loss)
total = fspl + best_metal_path + water_loss
return {
"scenario": name,
"freq_ghz": freq_ghz,
"fspl_db": fspl,
"bulk_loss_db": bulk_loss,
"seam_loss_db": seam_loss,
"metal_path_used": "seam" if seam_loss < bulk_loss else "bulk",
"metal_path_loss_db": best_metal_path,
"saltwater_loss_db": water_loss,
"total_loss_db": total,
"esp32_link_budget_db": 121,
"snr_margin_db": 121 - total - 10, # 10 dB SNR margin for DSP
}
def main():
parser = argparse.ArgumentParser()
parser.add_argument("--out", default="examples/research-sota/r11_maritime_results.json")
args = parser.parse_args()
# 1. Skin depth + per-mm attenuation
materials_grid = {}
for f in [2.4, 5.0]:
delta_steel_um = skin_depth_m(f, STEEL_SIGMA) * 1e6 # micrometres
att_steel = bulk_attenuation_db_per_mm(f, STEEL_SIGMA)
att_water = saltwater_attenuation_db_per_m(f)
materials_grid[f"{f}_GHz"] = {
"steel_skin_depth_um": delta_steel_um,
"steel_atten_dB_per_mm": att_steel,
"saltwater_atten_dB_per_m": att_water,
}
# 2. Three maritime scenarios
scenarios = [
maritime_scenario("man-overboard, surface-floating", 2.4,
bulkhead_mm=0, seam_mm=0, free_air_m=200, saltwater_m=0),
maritime_scenario("man-overboard, head 30 cm underwater", 2.4,
bulkhead_mm=0, seam_mm=0, free_air_m=200, saltwater_m=0.3),
maritime_scenario("crew vitals through 10 mm steel cabin door (closed)", 2.4,
bulkhead_mm=10, seam_mm=0, free_air_m=3),
maritime_scenario("crew vitals through cabin door (2 mm seam gap)", 2.4,
bulkhead_mm=10, seam_mm=2, free_air_m=3),
maritime_scenario("crew vitals through cabin door (5 mm seam gap)", 2.4,
bulkhead_mm=10, seam_mm=5, free_air_m=3),
maritime_scenario("container intrusion (steel cargo container, 2 mm walls, 30 mm vent slot)", 2.4,
bulkhead_mm=2, seam_mm=30, free_air_m=10),
maritime_scenario("through hull (submarine, 30 mm pressure hull)", 2.4,
bulkhead_mm=30, seam_mm=0, free_air_m=1),
]
out = {
"model": "skin-depth steel + lossy-dielectric saltwater + slot-diffraction seam",
"materials": materials_grid,
"scenarios": scenarios,
}
Path(args.out).parent.mkdir(parents=True, exist_ok=True)
Path(args.out).write_text(json.dumps(out, indent=2))
# Print headlines
print("=== Skin depth + bulk attenuation ===")
for fkey, m in materials_grid.items():
print(f" {fkey:>8} steel: skin={m['steel_skin_depth_um']:>6.2f} um, "
f"attenuation={m['steel_atten_dB_per_mm']:>9.1f} dB/mm "
f"saltwater={m['saltwater_atten_dB_per_m']:>6.1f} dB/m")
print()
print("=== Composite maritime scenarios @ 2.4 GHz ===")
print(f"{'Scenario':<58} {'FSPL':>6} {'Metal':>6} {'Water':>6} {'Total':>6} {'Margin':>7}")
for s in scenarios:
print(f"{s['scenario']:<58} {s['fspl_db']:>6.1f} "
f"{s['metal_path_loss_db']:>6.1f} {s['saltwater_loss_db']:>6.1f} "
f"{s['total_loss_db']:>6.1f} {s['snr_margin_db']:>+7.1f}")
print()
print(f"Wrote {args.out}")
if __name__ == "__main__":
main()
examples/research-sota/r11_maritime_results.json
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{
"model": "skin-depth steel + lossy-dielectric saltwater + slot-diffraction seam",
"materials": {
"2.4_GHz": {
"steel_skin_depth_um": 3.248736671806984,
"steel_atten_dB_per_mm": 2673.654677948628,
"saltwater_atten_dB_per_m": 852.7792439147287
},
"5.0_GHz": {
"steel_skin_depth_um": 2.2507907903927653,
"steel_atten_dB_per_mm": 3859.0881200843564,
"saltwater_atten_dB_per_m": 867.7495416795573
}
},
"scenarios": [
{
"scenario": "man-overboard, surface-floating",
"freq_ghz": 2.4,
"fspl_db": 86.07916660695635,
"bulk_loss_db": 0.0,
"seam_loss_db": 999.0,
"metal_path_used": "bulk",
"metal_path_loss_db": 0.0,
"saltwater_loss_db": 0.0,
"total_loss_db": 86.07916660695635,
"esp32_link_budget_db": 121,
"snr_margin_db": 24.92083339304365
},
{
"scenario": "man-overboard, head 30 cm underwater",
"freq_ghz": 2.4,
"fspl_db": 86.07916660695635,
"bulk_loss_db": 0.0,
"seam_loss_db": 999.0,
"metal_path_used": "bulk",
"metal_path_loss_db": 0.0,
"saltwater_loss_db": 255.83377317441858,
"total_loss_db": 341.9129397813749,
"esp32_link_budget_db": 121,
"snr_margin_db": -230.91293978137492
},
{
"scenario": "crew vitals through 10 mm steel cabin door (closed)",
"freq_ghz": 2.4,
"fspl_db": 49.88145871091758,
"bulk_loss_db": 26736.546779486278,
"seam_loss_db": 999.0,
"metal_path_used": "seam",
"metal_path_loss_db": 999.0,
"saltwater_loss_db": 0.0,
"total_loss_db": 1048.8814587109175,
"esp32_link_budget_db": 121,
"snr_margin_db": -937.8814587109175
},
{
"scenario": "crew vitals through cabin door (2 mm seam gap)",
"freq_ghz": 2.4,
"fspl_db": 49.88145871091758,
"bulk_loss_db": 26736.546779486278,
"seam_loss_db": 29.891207909453847,
"metal_path_used": "seam",
"metal_path_loss_db": 29.891207909453847,
"saltwater_loss_db": 0.0,
"total_loss_db": 79.77266662037142,
"esp32_link_budget_db": 121,
"snr_margin_db": 31.227333379628575
},
{
"scenario": "crew vitals through cabin door (5 mm seam gap)",
"freq_ghz": 2.4,
"fspl_db": 49.88145871091758,
"bulk_loss_db": 26736.546779486278,
"seam_loss_db": 21.93240773601309,
"metal_path_used": "seam",
"metal_path_loss_db": 21.93240773601309,
"saltwater_loss_db": 0.0,
"total_loss_db": 71.81386644693066,
"esp32_link_budget_db": 121,
"snr_margin_db": 39.18613355306934
},
{
"scenario": "container intrusion (steel cargo container, 2 mm walls, 30 mm vent slot)",
"freq_ghz": 2.4,
"fspl_db": 60.14065230988498,
"bulk_loss_db": 5347.309355897256,
"seam_loss_db": 6.369382728340219,
"metal_path_used": "seam",
"metal_path_loss_db": 6.369382728340219,
"saltwater_loss_db": 0.0,
"total_loss_db": 66.5100350382252,
"esp32_link_budget_db": 121,
"snr_margin_db": 44.4899649617748
},
{
"scenario": "through hull (submarine, 30 mm pressure hull)",
"freq_ghz": 2.4,
"fspl_db": 40.88207853739662,
"bulk_loss_db": 80209.64033845883,
"seam_loss_db": 999.0,
"metal_path_used": "seam",
"metal_path_loss_db": 999.0,
"saltwater_loss_db": 0.0,
"total_loss_db": 1039.8820785373966,
"esp32_link_budget_db": 121,
"snr_margin_db": -928.8820785373966
}
]
}