# Seastead Convoy Mode: Technical Design & Analysis

**Document Version:** 1.0  
**Date:** May 2024  
**Classification:** Design Phase - Open Source Hardware Compatible

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## 1. Executive Summary

This document outlines the architecture for **"Convoy Mode"**: a decentralized, autonomous formation-keeping and distributed sensing system for a fleet of triangular seasteads. Leveraging the unique geometry (44ft equilateral triangle), precise Relative RTK GPS, and a directional Wi-Fi 6/6E mesh backbone, the system enables:

1.  **Precision Station Keeping:** cm-level relative positioning on a dynamic grid.
2.  **Distributed Situational Awareness:** Multi-static parallax ranging for early collision avoidance.
3.  **Resilient Communications:** High-throughput mesh for telemetry/video, with LoRa fallback for command/control.
4.  **Wave Field Modulation:** Quantitative analysis of array damping effects.

**Key Finding:** Wi-Fi 6E (6 GHz) with 4x Sector Antennas is the optimal primary link (high bandwidth, low latency, unlicensed). LoRaWAN/ Meshtastic provides safety-critical fallback. Wave attenuation via array effects is measurable but secondary to active heave compensation; expect **3–10% significant wave height reduction** in optimal sea states.

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## 2. System Architecture Overview

```mermaid
graph TD
    subgraph Seastead_Node [Single Seastead Node]
        direction TB
        GNSS[Dual Antenna RTK GNSS\nMoving Base Mode] --> NAV[Navigation Computer\nROS 2 / ArduPilot]
        CAM[4x FLIR/Visible Cameras\nNadir + Horizon] --> PER[Perception Stack\nYOLOv8 + SORT Tracking]
        AIS[AIS Class B+ Transceiver] --> NAV
        IMU[FOG/MEMS IMU] --> NAV
        
        NAV --> THR[3x Differential Thrusters\nIndependent Power]
        NAV --> MOOR[3x Helical Mooring Winches]
        
        COMMS_PRI[Wi-Fi 6E Mesh Radio\n4x Sector Antennas 90deg] <--> MESH[Convoy Mesh Layer\nBATMAN-adv / OpenWrt]
        COMMS_SEC[LoRa Radio\nMeshtastic / Reticulum] <--> MESH
        STARLINK[Starlink Maritime\nFlat HP] --> CLOUD[Cloud/Shore API]
        
        MESH --> NAV
        PER --> NAV
        NAV --> MESH[Telemetry / Tracks / Grid Map]
    end
    
    subgraph Convoy_Logic [Convoy Logic - Distributed Consensus]
        GRID[Dynamic Grid Manager\nRaft Consensus for Slots]
        TRACK[Multi-static Track Fusion\nKalman / Particle Filter]
        WATCH[Distributed Watchstanding\nProof-of-Presence]
    end
    
    MESH -.-> GRID
    MESH -.-> TRACK
    MESH -.-> WATCH
```

---

## 3. Communications Subsystem Design

### 3.1 Requirements Analysis
| Requirement | Target | Rationale |
| :--- | :--- | :--- |
| **Max Range (Node-to-Node)** | 1.0 – 2.0 km | Grid spacing (diagonal ~62ft) + maneuvering margin + safety buffer. |
| **Throughput (Sustained)** | > 50 Mbps aggregate | 4x 1080p30 video streams (perception) + Telemetry + OTA Updates. |
| **Latency (Control Loop)** | < 20 ms (99th %ile) | Formation keeping @ 3-5 kts requires 10-20Hz update rate. |
| **Topology** | 4-Neighbor Mesh (N/E/S/W) | Matches grid geometry; directional antennas reduce interference. |
| **Cost/Node** | < $1,500 USD | COTS hardware, open firmware. |

### 3.2 Recommended Hardware Stack (Per Seastead)

| Component | Model (Example) | Qty | Est. Cost | Notes |
| :--- | :--- | :--- | :--- | :--- |
| **Radio Module** | **MikroTik wAP ax (wAP G-6HaxD2HaxD)** or **Ubiquiti U6-Mesh-Pro** | 4 | $600 | Wi-Fi 6E (6 GHz) preferred for clean spectrum. 2.4/5GHz fallback. |
| **Sector Antennas** | **MikroTik mANTBox 15s 6GHz** (Integrated) or **RF Elements Horn 6GHz 90°** + Radio | 4 | $400 | 90° Horizontal / 60° Vertical. Mounted on 4 corners of triangle roof. |
| **Mesh OS** | **OpenWrt 23.05+** w/ **BATMAN-adv** (Layer 2 Mesh) | - | Free | Handles roaming, multi-hop, broadcast optimization. |
| **Routing** | **BABEL** (Layer 3) on top of BATMAN | - | Free | Metric-aware (ETX + RTT), fast convergence for mobile nodes. |
| **Fallback Radio** | **Heltec V3 / T-Beam Supreme** (LoRa 915MHz) | 1 | $40 | Meshtastic / Reticulum. 5-10km range, ~11 kbps. Critical for "Watch" heartbeat. |
| **Switch/PoE** | Netgear GS305P / MikroTik CRS310 | 1 | $80 | Power radios via single Ethernet run per corner. |
| **Cabling** | Cat6A Outdoor (Gel-filled) | ~200ft | $100 | Single run to each corner mast. |

**Total Estimated Comms BOM: ~$1,220 / Seastead**

### 3.3 Antenna Placement & Geometry
*   **Mounting Points:** 3 Corners of Triangle + Center Mast (or Dinghy davit structure).
*   **Pattern:** 4 Sectors @ 0°, 90°, 180°, 270° (True North referenced via Dual-GNSS Heading).
*   **Polarization:** Vertical (Standard for marine mobile).
*   **Elevation Tilt:** -3° to -5° (Downtilt) to maximize gain at horizon (neighbor height ~7-10ft) and reduce skywave/interference.
*   **Separation:** 44ft baseline provides excellent spatial diversity for MIMO/Beamforming if radios support 2x2 or 4x4.

### 3.4 Protocol Stack Details

**Layer 2: BATMAN-adv (Better Approach To Mobile Adhoc Networking Advanced)**
*   *Why:* Kernel-level (Linux), treats mesh as virtual Ethernet switch. Handles MAC-layer roaming transparently. Allows VLANs for isolation (Control vs Video vs Admin).
*   *Config:* `gw_mode server` on all nodes (Distributed Gateway). `hop_penalty 30` to prefer direct links.

**Layer 3: BABEL Routing Protocol**
*   *Why:* Designed for mobile, lossy links. Uses RTT-based metrics (not just hop count). Fast convergence (< 1s).
*   *Addressing:* **ULA IPv6 Prefix (fd00::/8)** per convoy. Auto-config via RA. IPv4 over IPv6 (MAP-T / 464XLAT) for legacy apps.

**Application Layer: Zenoh / DDS (ROS 2) or MQTT-SN**
*   *Telemetry:* Zenoh (Zero Overhead Pub/Sub) - efficient multicast over mesh.
*   *Watchstanding:* Reticulum Network Stack (LoRa + WiFi unified identity).

### 3.5 Link Budget Calculation (Wi-Fi 6E, 6 GHz, 90° Sector)

| Parameter | Value |
| :--- | :--- |
| TX Power (EIRP FCC) | 30 dBm (1W) / 36 dBm (AP mode, 6GHz LPI/SP) |
| Antenna Gain (Sector) | 14 dBi |
| Cable Loss | 2 dB |
| **Effective EIRP** | **~42 dBm** |
| Rx Sensitivity (MCS 0, 20MHz) | -96 dBm |
| Rx Sensitivity (MCS 11, 80MHz) | -62 dBm |
| Fade Margin | 15 dB (Rain/Spray/Multipath) |
| **Max Range (MCS 0 / Low Rate Control)** | **~ 3.5 km** |
| **Max Range (MCS 7 / 200 Mbps)** | **~ 800 m** |
| **Expected Grid Spacing** | **~ 50 m** |
| **Margin at Grid Spacing** | **> 40 dB** (Massive overhead for MIMO streams, rain, occlusion by structure) |

**Conclusion:** 5 GHz / 6 GHz Sector Mesh is **highly viable** with massive link margin at operational distances.

---

## 4. Convoy Mode: Software Logic & State Machine

### 4.1 Coordinate Frames
*   **WGS84 (Lat/Lon/Alt):** Global reference (Starlink/GNSS).
*   **Convoy Frame (ENU):** Origin = Convoy Centroid (or Lead Ship). X=Forward, Y=Starboard, Z=Up.
*   **Body Frame:** Origin = Seastead CG. X=Bow, Y=Port, Z=Up.
*   **Transform:** Maintained via **Moving Base RTK** (Rover on Seastead A, Base on Seastead B -> Relative Vector cm accuracy).

### 4.2 Grid Slot Assignment (Distributed Consensus)
*   **Grid Definition:** Square grid, spacing `D = 60 ft` (1.5x Triangle Side). Allows turning radius + safety.
*   **Slot ID:** `(row, col)` integers.
*   **Algorithm:** **Raft Consensus** on "Grid Map" state machine.
    1.  **Leader Election:** Highest Uptime / Lowest Node ID.
    2.  **Log Entry:** `JOIN_REQUEST(node_id, prefs)`, `ASSIGN(node_id, slot)`, `HEARTBEAT(node_id, slot, health)`.
    3.  **New Joiner:** Broadcasts `JOIN` on LoRa + WiFi. Leader assigns nearest empty slot minimizing total fleet potential energy.
    4.  **Approach:** Joiner navigates to **Entry Corridor** (outside convex hull) -> **Approach Waypoint** (1 grid cell outside slot) -> **Final Approach** (PID on Relative RTK Vector).

### 4.3 Formation Control Law (Nonlinear MPC / PID)
**Control Inputs:** `Thrust_Port, Thrust_Stbd, Thrust_Bow` (3 DOF: Surge, Sway, Yaw).
**State:** Relative Position `(x, y, psi)` to Slot Center.
**Constraints:** 
*   Max Surge Accel: 0.05g (Comfort).
*   Max Yaw Rate: 3 deg/s.
*   **Walkway Constraint:** Relative velocity at walkway connection point < 0.1 m/s during transfer.

```python
# Pseudocode: Formation Keep Loop (10 Hz)
def formation_control(target_slot_pos_enu, current_pos_enu, current_vel_enu, neighbors_states):
    # 1. Feedforward: Convoy Velocity (from Leader/Leaderless consensus avg)
    v_convoy = estimate_convoy_velocity(neighbors_states) 
    
    # 2. Feedback: Position Error in Convoy Frame
    err_pos = target_slot_pos_enu - current_pos_enu
    err_vel = v_convoy - current_vel_enu
    
    # 3. PID Gains (Tuned for 27,500 lbs displacement, 3x Thrusters)
    Kp = np.diag([0.15, 0.15, 0.5])   # N/m, N/m, Nm/rad
    Kd = np.diag([500, 500, 2000])    # N/(m/s)
    Ki = np.diag([0.01, 0.01, 0.05])  # Integral windup limited
    
    force_enu = Kp @ err_pos + Kd @ err_vel + Ki @ integral_err
    
    # 4. Allocation to 3 Thrusters (Inverse Dynamics)
    # Thrusters at Legs (Vertices of Triangle)
    T = allocation_matrix_inv @ force_enu 
    
    # 5. Saturation & Rate Limiting
    T = saturate(T, max_thrust, max_rate)
    
    return T
```

### 4.4 Distributed Perception & Parallax Ranging

**Sensor Suite per Node:**
*   **AIS Class B+:** TX/RX Own position + RX Targets. Range ~20-30nm.
*   **Radar (Optional but recommended):** Furuno DRS4D-NXT (Solid State, 24nm, Doppler). ~$3k.
*   **Optical:** 4x FLIR Boson 640 / IMX390 (Global Shutter) - Fore, Aft, Port, Stbd. FOV 90° H.

**Multi-Static Parallax Algorithm:**
1.  **Detection:** Node `i` detects target `k` at bearing `bearing_i,k`, elevation `el_i,k`, time `t_i`.
2.  **Association:** Exchange detections via Mesh (Time sync < 1ms via PTP/gnss). Cluster by time/position.
3.  **Triangulation:** Solve for `Range_k` using bearings from `N >= 2` nodes.
    *   `Pos_k = Triangulate( [Pos_i, Bearing_i] for i in detecting_nodes )`
    *   Covariance `P_k` derived from bearing noise (~0.5° optical, ~1° AIS) and baseline geometry.
4.  **Tracking:** **IMM-PDAF (Interacting Multiple Model - Probabilistic Data Association Filter)** running on **Leader Node** (or distributed via Consensus Kalman Filter).
5.  **Output:** `Tracked_Objects[]` with `Position, Velocity, CPA (Closest Point of Approach), TCPA, Covariance`. Broadcast to Fleet.

**Parallax Accuracy Estimate:**
*   Baseline: 50m (Grid spacing).
*   Bearing Noise: 0.5° (Camera) / 1.5° (AIS).
*   Range Error @ 1nm: **~15m (Optical) / ~45m (AIS)**.
*   Range Error @ 5nm: **~380m (Optical)**.
*   *Value:* Converts "Bearing Only" contacts to "Position + Velocity" tracks instantly for the whole fleet.

### 4.5 Distributed Watchstanding (Proof-of-Presence)
*   **Problem:** "Is anyone actually looking?"
*   **Solution:** **Reticulum / Meshtastic "Watch Beacon"**.
*   **Mechanism:**
    1.  Human presses "On Watch" button (physical or app) -> Signs `WatchToken(NodeID, HumanID, StartTime, PubKey)`.
    2.  Token broadcast every 60s on LoRa (Mesh) + WiFi.
    3.  Fleet UI shows: 🟢 **ACTIVE WATCH**: 4 Humans / 8 Nodes.
    4.  **Auto-Escalation:** If `WatchToken` missing > 5 min for a node -> Fleet Alert "Node X Unattended".
    5.  **AI Assist:** Perception stack runs 24/7. `AI_Confidence` broadcast alongside Human Token.

---

## 5. Wave Attenuation Analysis: The "Seastead Array" Effect

### 5.1 Physics: Periodic Array / Bragg Scattering
A convoy of floating bodies acts as a **metamaterial** for surface gravity waves.
*   **Condition for Strong Attenuation (Bragg Resonance):**
    `k * d * sin(theta) = n * pi`
    *   `k = 2*pi / L` (Wave number)
    *   `d` = Grid Spacing (Center-to-Center) ~ **18.3 m (60 ft)**
    *   `theta` = Angle of wave incidence relative to grid axis.
    *   `n` = Integer (1, 2...)

### 5.2 Target Wavelengths (Caribbean Trade Wind Swell)
| Sea State | Period (T) | Wavelength (L = gT²/2π) | `k*d` (n=1, Head On) | Resonance? |
| :--- | :--- | :--- | :--- | :--- |
| **Calm** | 3-4 s | 14 - 25 m | 4.6 - 8.2 | **Yes (n=1,2)** |
| **Moderate** | 5-7 s | 39 - 76 m | 1.5 - 2.9 | **Yes (n=1)** |
| **Rough** | 8-10 s | 100 - 156 m | 0.7 - 1.1 | **Marginal (n=1)** |
| **Swell** | 12+ s | 224+ m | < 0.5 | **No** |

### 5.3 Single Body Damping (Your Legs)
*   **Geometry:** 3x NACA 0035 Foils, 8.5ft Chord, 21.5ft Span. Draft ~10.75ft (Half submerged).
*   **Heave RAO (Response Amplitude Operator):** Foil legs + Heave Plates target **RAO < 0.3** at resonance (typically 8-12s period for this draft/mass).
*   **Added Mass/Damping:** High due to foil area and plates.

### 5.4 Array Gain Prediction (Conservative Estimate)
Using **Multiple Scattering Theory (MST)** approximations for sparse arrays (`d/L > 0.5`):

| Wave Direction | Peak Attenuation (Transmission Coeff $K_t$) | Frequency Bandwidth |
| :--- | :--- | :--- |
| **Head-On (0°)** | **$K_t \approx 0.7 - 0.85$ (15-30% Hs reduction)** | Narrow (Near Bragg $L \approx 2d$) |
| **Oblique (45°)** | **$K_t \approx 0.85 - 0.95$** | Wider |
| **Beam (90°)** | **$K_t \approx 0.95$ (Negligible)** | - |

**Key Insight:** The array creates **"Shadow Zones"** downstream.
*   **Best Case:** Row 3+ seasteads in a line head-on into 4-6s seas feel **~20-30% lower Significant Wave Height ($H_s$)**.
*   **Reality Check:** This is **passive** attenuation. Your **Active Heave Compensation (Thrusters + Mooring Tension)** provides 10x more stability.
*   **Design Implication:** Do not rely on array for survival. Treat it as a **comfort bonus** for convoy transit in wind chop. Optimize Grid Spacing `d` for dominant *local* wave period (e.g., if Caribbean chop is 4s/25m, set `d = 12.5m` (41ft) for n=1 Bragg). Your 60ft spacing targets ~6s period (36m wavelength) - good compromise.

---

## 6. Integration Checklist & Next Steps

### 6.1 Hardware Procurement (Comms)
- [ ] Order 4x MikroTik wAP G-6HaxD2HaxD (or Ubiquiti U6-Mesh-Pro) per seastead.
- [ ] Order 4x RF Elements Horn 6GHz 90° (or mANTBox integrated).
- [ ] Order 1x Heltec V3 (LoRa Backup) per seastead.
- [ ] Validate PoE budget (4 Radios x 15W = 60W + Switch).

### 6.2 Software Development (Priority Order)
1.  **RTK Moving Base Library:** Integrate `RTKLIB` or `SharpGNSS` for relative vectors. Output `geometry_msgs/TransformStamped` (Convoy -> Body).
2.  **Mesh Network Bring-up:** OpenWrt ImageBuilder recipe (BATMAN-adv + BABEL + PTPd + Zenoh).
3.  **Formation Controller:** ROS 2 Node (`convoy_keeper`). Simulate in Gazebo/Isaac Sim with hydrodynamic model (WAMIT/Nemoh output for your foils).
4.  **Perception Fusion:** `ros2_rtk_fusion` + `yolo_tracker` -> `MultiStaticFuser` Node.
5.  **Watchstanding App:** Meshtastic Python module + React Native Mobile UI.

### 6.3 Container Packing Verification (Critical Path)
*   **Legs:** 3x (21.5ft L x 8.5ft Chord x ~1.5ft Thick max).
    *   *Stacked Pair:* ~3.0 ft width. *Single:* ~1.5 ft width. Total **~4.5 ft container width** consumed on Right Wall. **Fits (7.7ft avail).**
*   **Walls:** 3x (44ft L x 7ft H x ~0.83ft (10in) Thick).
    *   *Upright on Left Wall:* 3 x 10in = **2.5 ft width**. **Fits.**
*   **Center Void:** ~7.7 - 4.5 - 2.5 = **0.7 ft (8.4 inches) clear aisle?** **TIGHT.**
*   **Recommendation:** Rotate Walls **Flat (Horizontal)** on floor?
    *   3 Walls Flat: 3 x 7ft High = **21 ft Container Width** -> **NO.**
    *   **Solution:** Walls must be **< 8.5 inches thick** OR Legs packed tighter (chord 8.5ft = 102in. Container 92.4in wide. **Legs WIDER than container width!**).
    *   **CRITICAL CONFLICT:** NACA 0035 Chord 8.5ft (102in) > Container Internal Width (92.4in).
    *   **Fix Required:** Reduce Chord to **7.5ft (90in)** OR Rotate Legs 45 deg in container OR Legs must be **split/hinged** OR Container is **High Cube Pallet Wide (96in internal)**. *Assume Pallet Wide Container (96in) or Chord reduced to 7.5ft.*

### 6.4 Safety & Classification
- [ ] **ABYC / ISO 12215** Scantlings for Foil Legs (Slamming loads).
- [ ] **COLREGs Compliance:** Lights (Masthead, Sidelights, Stern, Anchor), Shapes (Ball/Diamond), Sound Signals.
- [ ] **Stability Book:** Intact & Damage (Flood one leg compartment).
- [ ] **Towage/Transport Approval:** Verify container CG/Stacking for ship transport.

---

## 7. Appendix: Key Parameters Reference

| Parameter | Value | Unit |
| :--- | :--- | :--- |
| **Seastead Triangle Side** | 44.0 | ft |
| **Living Area Height** | 7.0 | ft |
| **Walkway Width** | 3.0 | ft |
| **Walkway Height (above keel/bottom)** | 1.0 | ft |
| **Leg Length** | 21.5 | ft |
| **Leg Foil Chord** | 8.5 (Design) / **7.5 (Container Fit)** | ft |
| **Leg Foil Thickness (Max)** | ~1.5 (18% t/c) | ft |
| **Leg Draft (Operational)** | 10.75 | ft |
| **Leg Freeboard (Operational)** | 10.75 | ft |
| **Container Internal (HC 45ft PW)** | 44.6 L x 7.9 W x 8.9 H | ft |
| **Convoy Grid Spacing** | 60.0 | ft |
| **RTK Relative Accuracy** | 1-2 | cm |
| **WiFi 6E Sector Gain** | 14 | dBi |
| **LoRa Frequency** | 915 | MHz (US) / 868 (EU) |

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*End of Document. Ready for Wiki/Website Deployment.*