content: car tracker page with a real hardware image

- content/edge/l3/edge-car-tracker.md, written from the evidence notes
- product image of the SIM7080G Cat-M module, cropped to 640px, with a caption
  naming what it is and that it is a product image
- referenced from the edge overview
- original kept in sam/CAR_GPS_TRACKER so the project repo documents its own build

content/edge/edge-overview.md

@@ -8,7 +8,7 @@ confidence: running
 period: "2020–"
 tech: [ESP32, LoRa, Zigbee, MQTT, Home Assistant, Snapcast, Raspberry Pi]
 metrics: [{label: "edge nodes", value: 20}, {label: "lighting zones", value: 9}, {label: "automations", value: 20}]
-artefacts: [{type: doc, label: "Edge architecture — power · radio · deep sleep", url: "content/edge/l3/edge-architecture.md"}, {type: doc, label: "The failures, kept in the record", url: "content/edge/l3/edge-failures.md"}, {type: doc, label: "Distributed audio — eight endpoints with ducking", url: "content/edge/l3/edge-snapcast.md"}]
+artefacts: [{type: doc, label: "Edge architecture — power · radio · deep sleep", url: "content/edge/l3/edge-architecture.md"}, {type: doc, label: "The failures, kept in the record", url: "content/edge/l3/edge-failures.md"}, {type: doc, label: "Distributed audio — eight endpoints with ducking", url: "content/edge/l3/edge-snapcast.md"}, {type: doc, label: "Vehicle tracker on LTE Cat-M", url: "content/edge/l3/edge-car-tracker.md"}]
 public: true
 last_verified: 2026-10-26
 tags: [edge, iot, esp32, lora, zigbee]

content/edge/l3/edge-car-tracker.md

@@ -0,0 +1,51 @@
+---
+id: edge-car-tracker
+area: edge
+level: 3
+title: "Artefact — Vehicle tracker on LTE Cat-M"
+claim: "A tracker that sleeps between reports · wakes on a schedule or when the car moves · and reports when it leaves a boundary set from the web interface."
+confidence: in-progress
+period: "2024–"
+tech: [ESP32, LTE Cat-M, NB-IoT, SIM7080G, GPS, LiFePO4]
+metrics: [{label: "battery packs", value: 2}]
+artefacts: []
+public: true
+last_verified: 2026-10-28
+tags: [edge, tracker, lte, low-power, artefact]
+---
+
+The build is on the bench: two LiFePO batteries, a buck converter, an ESP32 with GPS, and a
+cellular module for the uplink. The car keeps the packs charged while it is running.
+
+![Waveshare Pico SIM7080G Cat-M and NB-IoT module — the cellular link in the tracker. Product image, used to identify the part.](../../assets/images/sim7080g-module.jpg)
+
+## What it does
+
+Two behaviours, chosen by the settings rather than hard-coded. When the car is parked the
+ESP32 sits in deep sleep, wakes on a timer, checks its own configuration, and reports only
+what the settings ask for. When the car is moving it stays awake and reports on a shorter
+interval. Either way the interval and the boundary can be changed from the web interface,
+and a change can force an immediate update rather than waiting for the next wake.
+
+The geofence is the part that matters. A boundary is drawn from the web interface, and the
+device is responsible for deciding whether it has left it — which is why the settings have
+to travel to the device rather than live only in the server.
+
+## Why LTE Cat-M rather than WiFi
+
+The tracker lives in a car. WiFi coverage is a driveway, not a road. Cat-M and NB-IoT were
+chosen because they are designed for exactly this shape of traffic — small payloads, long
+sleeps, coverage beyond what 4G handsets need — and because the modules are cheap enough to
+leave installed.
+
+## What it proves
+
+A complete path rather than a demo: constrained hardware, a power budget that dictates the
+schedule, a device that holds enough state to act on its own, and a web application that
+configures it and shows where it has been.
+
+## Still open
+
+Field testing across the full battery cycle. The sleep interval that is right for a parked
+car and the interval that is right for a moving one are guesses until the discharge curve
+has been measured over a week rather than a day.