GPS Broadcaster¶

GPS broadcasters send UDP messages on port 5354 — the same port disco uses for host discovery. Two message types are produced:

Three implementations send TIME_ANNOUNCE: the Go binary (built as part of this project), an Arduino sketch, and an ESPHome component. The Heltec Wireless Tracker (esp32-timesyncd) sends both message types.

Go binary (Raspberry Pi, Linux)¶

Build¶

make                                              # builds disco-gps-broadcaster for the current platform

Cross-compile for Raspberry Pi Zero 2W (arm64):

CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build \
  -o build/bin/disco-gps-broadcaster cmd/gps-broadcaster/main.go

Run¶

disco-gps-broadcaster -device /dev/ttyACM0

With options:

disco-gps-broadcaster \
  -device /dev/ttyACM0 \
  -broadcast 255.255.255.255:5354 \
  -id gps-pi-01 \
  -interval 16s \
  -interfaces eth0,wlan0 \
  -v

systemd service¶

Create /etc/systemd/system/disco-gps-broadcaster.service:

[Unit]
Description=Disco GPS time broadcaster
After=network.target

[Service]
ExecStart=/usr/local/bin/disco-gps-broadcaster -device /dev/ttyACM0 -id gps-pi-01
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

sudo systemctl enable --now disco-gps-broadcaster

Arduino¶

The Arduino sketch is in gps-broadcaster/arduino/. It reads NMEA data from a GPS module over a serial connection and broadcasts TIME_ANNOUNCE messages via UDP.

Hardware¶

Connect a GPS module to the Arduino's hardware serial pins. The sketch reads from Serial1 by default.

Flash¶

Open gps-broadcaster/arduino/gps_time_broadcaster.ino in the Arduino IDE (or use PlatformIO with gps-broadcaster/arduino/platformio.ini). Configure your network settings in the sketch before uploading:

// Broadcast interval (milliseconds)
const unsigned long BROADCAST_INTERVAL = 16000;

ESPHome (ESP32 / ESP8266)¶

The ESPHome component is in gps-broadcaster/esphome/. Copy both files to your ESPHome configuration directory.

Hardware (ESP32)¶

Configure and flash¶

Edit gps-broadcaster.yaml to set your Wi-Fi credentials and source ID, then flash:

esphome run gps-broadcaster.yaml

Tune the broadcaster in gps-broadcaster.yaml:

custom_component:
  - lambda: |-
      auto broadcaster = new GPSBroadcasterComponent();
      broadcaster->set_source_id("gps-esphome-01");
      broadcaster->set_interval(16000);    # ms between broadcasts
      broadcaster->set_port(5354);
      App.register_component(broadcaster);
      return {broadcaster};

Heltec Wireless Tracker (esp32-timesyncd)¶

The esp32-timesyncd firmware runs on a Heltec Wireless Tracker and sends both message types:

• TIME_ANNOUNCE — every 16 seconds when a GPS fix is held
• LOCATION_ANNOUNCE — every 60 seconds when a GPS fix is held

The location broadcast payload:

{
  "type": "LOCATION_ANNOUNCE",
  "message_id": "gps-timeserver-<millis>",
  "timestamp": 1718000000,
  "source_id": "gps-timeserver",
  "location": {
    "latitude": 52.370216,
    "longitude": 4.895168,
    "altitude": 3.5,
    "fix": true,
    "satellites": 8
  }
}

The daemon stores the latest fix per source_id. Query it with:

disco location
disco location show gps-timeserver
disco location --json

Verify¶

On any node running disco-daemon with time_sync.enabled: true, listen for broadcasts:

sudo tcpdump -i any udp port 5354 -A

Check time sync:

disco time

Sources: 1 (or more) confirms the daemon received a TIME_ANNOUNCE message.

Check location data:

disco location

A row in the output confirms the daemon received a LOCATION_ANNOUNCE message.