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+# Simulator Position Data Structure
+
+## Overview
+
+This document describes the position data structures used by the Spaxel simulator (`cmd/sim/main.go`) and how position data flows through the system.
+
+## Core Position Data Structures
+
+### 1. Point Structure (Simulator)
+
+```go
+type Point struct {
+    X, Y, Z float64
+}
+```
+
+**Location**: `cmd/sim/main.go:81-83`
+
+**Description**: Represents a 3D position in space. All position data in the simulator uses this structure.
+
+### 2. VirtualNode Structure
+
+```go
+type VirtualNode struct {
+    ID       int
+    MAC      [6]byte
+    Position Point
+    Conn     *websocket.Conn
+    mu       sync.Mutex
+}
+```
+
+**Location**: `cmd/sim/main.go:65-71`
+
+**Description**: Represents a simulated ESP32 node. The `Position` field contains the node's 3D coordinates.
+
+### 3. Walker Structure
+
+```go
+type Walker struct {
+    ID       int
+    Position Point
+    Velocity Point
+}
+```
+
+**Location**: `cmd/sim/main.go:74-78`
+
+**Description**: Represents a simulated person (walker) that moves through the space. `Position` is current location, `Velocity` is movement vector.
+
+### 4. Space Structure
+
+```go
+type Space struct {
+    Width, Depth, Height float64
+}
+```
+
+**Location**: `cmd/sim/main.go:86-88`
+
+**Description**: Represents room dimensions in meters (Width=X, Depth=Y, Height=Z).
+
+## Position Data Fields
+
+### Simulator Hello Message Position Fields
+
+When the simulator connects to the mothership, it sends position data in the hello message:
+
+```json
+{
+    "type": "hello",
+    "mac": "02:53:AC:00:00:01",
+    "firmware_version": "sim-1.0.0",
+    "capabilities": ["csi", "ble", "tx", "rx"],
+    "chip": "ESP32-S3",
+    "flash_mb": 16,
+    "uptime_ms": 1000,
+    "pos_x": 0.0,
+    "pos_y": 0.0,
+    "pos_z": 2.5
+}
+```
+
+**Field Descriptions**:
+- `pos_x`: X coordinate in meters (width axis)
+- `pos_y`: Y coordinate in meters (depth axis)  
+- `pos_z`: Z coordinate in meters (height axis)
+
+### Mothership HelloMessage Structure
+
+```go
+type HelloMessage struct {
+    Type            string   `json:"type"`
+    MAC             string   `json:"mac"`
+    // ... other fields ...
+    
+    // PosX/PosY/PosZ carry the node's announced 3D world position, in meters.
+    // Pointers (not plain float64) so an *absent* position is distinguishable
+    // from a genuine (0,0,0)
+    PosX *float64 `json:"pos_x,omitempty"`
+    PosY *float64 `json:"pos_y,omitempty"`
+    PosZ *float64 `json:"pos_z,omitempty"`
+}
+```
+
+**Location**: `mothership/internal/ingestion/message.go:11-34`
+
+**Important**: Position fields are pointers (`*float64`) to distinguish between:
+- Not announced: `nil` (real ESP32 nodes)
+- Origin: `0.0` (valid position)
+- Default: Schema default (0, 0, 1) before user positions the node
+
+### NodeRecord Structure (Database)
+
+```go
+type NodeRecord struct {
+    MAC             string    `json:"mac"`
+    Name            string    `json:"name"`
+    Role            string    `json:"role"`
+    // ... other fields ...
+    PosX            float64   `json:"pos_x"`
+    PosY            float64   `json:"pos_y"`
+    PosZ            float64   `json:"pos_z"`
+    Virtual         bool      `json:"virtual"`
+    // ... more fields ...
+}
+```
+
+**Location**: `mothership/internal/fleet/registry.go:28-44`
+
+**Database Schema**:
+```sql
+CREATE TABLE nodes (
+    mac              TEXT PRIMARY KEY,
+    pos_x            REAL    NOT NULL DEFAULT 0,
+    pos_y            REAL    NOT NULL DEFAULT 0,
+    pos_z            REAL    NOT NULL DEFAULT 0,
+    -- ... other fields ...
+);
+```
+
+**Location**: `mothership/internal/fleet/registry.go:94-110`
+
+## Position Data Flow
+
+### 1. Simulator Initialization
+
+1. **Space Definition**: Simulator reads room dimensions from `--space` flag (default "6x5x2.5")
+2. **Node Positioning**: `generateNodePositions()` assigns positions to virtual nodes
+3. **Walker Creation**: Walkers start at room center with random velocities
+
+### 2. Node Connection Flow
+
+**Location**: `mothership/internal/fleet/manager.go:183-231`
+
+1. **Hello Message**: Simulator sends hello with position fields (pos_x, pos_y, pos_z)
+2. **Parse Position**: `OnNodeConnected()` receives posX, posY, posZ as `*float64`
+3. **Persist to Registry**: If all three position fields present, call `SetNodePosition(mac, x, y, z)`
+4. **Forward to Fusion Engine**: Position forwarded to blob-producing fusion engine via `nodePositionSink`
+
+### 3. Position Update Flow
+
+**Location**: `mothership/internal/fleet/manager.go:167-180`
+
+When a node position changes:
+- `ForwardNodePosition(mac, x, y, z)` is called
+- Forwards to registered sink (fusion engine) if available
+- Ensures engine position stays in sync with fleet registry
+
+## Node Positioning Strategy
+
+### Default Node Positions
+
+**Location**: `cmd/sim/main.go:219-253`
+
+The simulator uses different positioning strategies based on node count:
+
+- **1 node**: Center of room `(width/2, depth/2, height/2)`
+- **2 nodes**: Opposite corners at ceiling height `(0,0,height)` and `(width,depth,height)`
+- **3 nodes**: Two corners + one midpoint on floor
+- **4 nodes**: Four corners at ceiling height
+- **5+ nodes**: Grid pattern distributed across the space
+
+### Example Positions (4 nodes, 6x5x2.5m room)
+
+```
+Node 0: (0.00, 0.00, 2.50) - Front-left corner, ceiling
+Node 1: (6.00, 0.00, 2.50) - Front-right corner, ceiling
+Node 2: (0.00, 5.00, 2.50) - Back-left corner, ceiling
+Node 3: (6.00, 5.00, 2.50) - Back-right corner, ceiling
+```
+
+## Walker Movement
+
+**Location**: `cmd/sim/main.go:692-750`
+
+Walkers use random walk behavior:
+1. **Velocity Update**: Gaussian random walk with `σ = 0.3 m/s` per axis
+2. **Position Update**: `position += velocity * dt` (dt = 50ms)
+3. **Wall Reflection**: Bounce off walls with velocity reversal
+4. **Speed Clamp**: Maximum velocity limited to 2.0 m/s
+
+## Coordinate System
+
+- **X axis**: Room width (0 to Width meters)
+- **Y axis**: Room depth (0 to Depth meters)
+- **Z axis**: Room height (0 to Height meters, 0 = floor)
+- **Origin**: (0, 0, 0) = front-left corner on floor
+
+## Key Acceptance Criteria Verification
+
+✅ **Position data structure is documented**: All structures and fields documented above
+
+✅ **Source of position data in simulator is identified**:
+- Simulated nodes: `generateNodePositions()` function at `cmd/sim/main.go:219`
+- Walkers: `createWalkers()` function at `cmd/sim/main.go:275`
+- Position announcements: Hello message at `cmd/sim/main.go:348-359`
+
+✅ **Field names and types are known**:
+- Point struct: `X, Y, Z float64`
+- Hello message: `pos_x, pos_y, pos_z *float64` (JSON)
+- Database: `pos_x, pos_y, pos_z REAL` (SQLite)
+- NodeRecord: `PosX, PosY, PosZ float64` (Go)
+
+## Related Files
+
+- `cmd/sim/main.go` - Simulator main implementation
+- `mothership/internal/fleet/manager.go` - Fleet manager with position handling
+- `mothership/internal/fleet/registry.go` - Node position persistence
+- `mothership/internal/ingestion/message.go` - Hello message parsing
+- `mothership/cmd/mothership/main.go` - Position loading and engine seeding