diff --git a/pi/backend/README.md b/pi/backend/README.md
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+++ b/pi/backend/README.md
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+# Backend
+
+Python GPS service for Smart Serow. Connects to `gpsd`, buffers positions, exposes HTTP API.
+
+## Setup
+
+```bash
+# Install uv if you haven't
+curl -LsSf https://astral.sh/uv/install.sh | sh
+
+# Install dependencies
+cd pi/backend
+uv sync
+```
+
+## Run
+
+```bash
+uv run python main.py
+```
+
+Or for development:
+```bash
+uv run flask --app main run --host 0.0.0.0 --port 5000 --reload
+```
+
+## API
+
+| Endpoint | Description |
+|----------|-------------|
+| `GET /health` | Health check, shows gpsd connection status |
+| `GET /gps` | Latest GPS fix (lat, lon, alt, speed, track) |
+| `GET /gps/history` | Last 100 buffered positions |
+
+## Test from SSH
+
+```bash
+curl http://localhost:5000/health
+curl http://localhost:5000/gps
+curl http://localhost:5000/gps/history | jq
+```
+
+## gpsd Setup (Pi)
+
+```bash
+# Install
+sudo apt install gpsd gpsd-clients
+
+# Configure (edit DEVICES to match your GPS serial port)
+sudo nano /etc/default/gpsd
+
+# Example /etc/default/gpsd:
+# DEVICES="/dev/ttyUSB0"
+# GPSD_OPTIONS="-n"
+# START_DAEMON="true"
+
+# Restart
+sudo systemctl restart gpsd
+
+# Test gpsd directly
+gpsmon
+cgps -s
+```
+
+## Stub Mode
+
+If `gpsdclient` isn't installed or gpsd isn't running, the service generates fake GPS data for UI testing.
+
+## Deploy
+
+TODO: Add to `scripts/deploy.py` as second target + systemd service.
diff --git a/pi/backend/gps_service.py b/pi/backend/gps_service.py
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+++ b/pi/backend/gps_service.py
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+"""GPS service - connects to gpsd, buffers data, handles reconnection."""
+
+import threading
+import time
+from collections import deque
+from typing import Any
+
+# gpsdclient is a modern, simple gpsd client
+# Install gpsd on Pi: sudo apt install gpsd gpsd-clients
+# Configure: sudo nano /etc/default/gpsd (set DEVICES="/dev/ttyUSB0" or similar)
+try:
+    from gpsdclient import GPSDClient
+except ImportError:
+    GPSDClient = None  # Allow import without gpsd for testing structure
+
+
+class GPSService:
+    """Threaded GPS reader with buffering and auto-reconnect."""
+
+    def __init__(self, host: str = "127.0.0.1", port: int = 2947, buffer_size: int = 100):
+        self.host = host
+        self.port = port
+        self.buffer_size = buffer_size
+
+        self._buffer: deque[dict[str, Any]] = deque(maxlen=buffer_size)
+        self._latest: dict[str, Any] = {}
+        self._connected = False
+        self._running = False
+        self._thread: threading.Thread | None = None
+        self._lock = threading.Lock()
+
+    @property
+    def connected(self) -> bool:
+        return self._connected
+
+    def get_latest(self) -> dict[str, Any]:
+        """Get most recent GPS fix."""
+        with self._lock:
+            return self._latest.copy() if self._latest else {"error": "no data"}
+
+    def get_buffer(self) -> list[dict[str, Any]]:
+        """Get buffered GPS history."""
+        with self._lock:
+            return list(self._buffer)
+
+    def start(self):
+        """Start background GPS reader thread."""
+        if self._running:
+            return
+        self._running = True
+        self._thread = threading.Thread(target=self._reader_loop, daemon=True)
+        self._thread.start()
+
+    def stop(self):
+        """Stop background reader."""
+        self._running = False
+        if self._thread:
+            self._thread.join(timeout=2.0)
+
+    def _reader_loop(self):
+        """Main reader loop with reconnection logic."""
+        while self._running:
+            try:
+                self._connect_and_read()
+            except Exception as e:
+                self._connected = False
+                print(f"[GPS] Connection error: {e}, retrying in 5s...")
+                time.sleep(5)
+
+    def _connect_and_read(self):
+        """Connect to gpsd and read data."""
+        if GPSDClient is None:
+            # Stub mode - no gpsd client installed
+            print("[GPS] gpsdclient not installed, running in stub mode")
+            self._stub_mode()
+            return
+
+        with GPSDClient(host=self.host, port=self.port) as client:
+            self._connected = True
+            print(f"[GPS] Connected to gpsd at {self.host}:{self.port}")
+
+            for result in client.dict_stream(filter=["TPV"]):
+                if not self._running:
+                    break
+
+                # TPV = Time-Position-Velocity report
+                fix = {
+                    "time": result.get("time"),
+                    "lat": result.get("lat"),
+                    "lon": result.get("lon"),
+                    "alt": result.get("alt"),
+                    "speed": result.get("speed"),  # m/s
+                    "track": result.get("track"),  # heading in degrees
+                    "mode": result.get("mode"),    # 0=no fix, 2=2D, 3=3D
+                }
+
+                with self._lock:
+                    self._latest = fix
+                    if fix.get("lat") is not None:
+                        self._buffer.append(fix)
+
+    def _stub_mode(self):
+        """Fake data for testing without gpsd."""
+        import random
+
+        while self._running:
+            self._connected = True
+            fix = {
+                "time": time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime()),
+                "lat": 35.6762 + random.uniform(-0.001, 0.001),
+                "lon": 139.6503 + random.uniform(-0.001, 0.001),
+                "alt": 40.0 + random.uniform(-5, 5),
+                "speed": random.uniform(0, 30),
+                "track": random.uniform(0, 360),
+                "mode": 3,
+            }
+            with self._lock:
+                self._latest = fix
+                self._buffer.append(fix)
+            time.sleep(1)
diff --git a/pi/backend/main.py b/pi/backend/main.py
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+++ b/pi/backend/main.py
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+"""Smart Serow Backend - GPS service with HTTP API."""
+
+from flask import Flask, jsonify
+from gps_service import GPSService
+
+app = Flask(__name__)
+gps = GPSService()
+
+
+@app.route("/health")
+def health():
+    """Health check endpoint."""
+    return jsonify({"status": "ok", "gps_connected": gps.connected})
+
+
+@app.route("/gps")
+def gps_data():
+    """Current GPS data."""
+    return jsonify(gps.get_latest())
+
+
+@app.route("/gps/history")
+def gps_history():
+    """Buffered GPS history."""
+    return jsonify(gps.get_buffer())
+
+
+def main():
+    """Entry point."""
+    gps.start()
+    try:
+        # Host 0.0.0.0 for access from Flutter app
+        app.run(host="0.0.0.0", port=5000, debug=False)
+    finally:
+        gps.stop()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/pi/backend/pyproject.toml b/pi/backend/pyproject.toml
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+++ b/pi/backend/pyproject.toml
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+[project]
+name = "smartserow-backend"
+version = "0.1.0"
+description = "GPS service for Smart Serow"
+requires-python = ">=3.11"
+dependencies = [
+    "flask>=3.0",
+    "gpsdclient>=1.3",
+]
+
+[project.optional-dependencies]
+dev = [
+    "ruff",
+]
+
+[project.scripts]
+smartserow-backend = "main:main"
+
+[tool.ruff]
+line-length = 100