GNSS

OpenMANET includes integrated GPS/GNSS support for position tracking, location sharing, and ATAK integration. When enabled, nodes automatically share their position across the mesh and stream NMEA data to connected end-user devices (EUDs).

Overview

The GNSS module connected or included with your mesh radio provides:

  • Automatic position tracking from TPV (Time-Position-Velocity) reports
  • NMEA GGA sentence streaming to connected EUD clients
  • Position data in mesh announcements for topology visualization
  • Position data in the API for extendable functionality

Supported Hardware

Seeed WM1302 + Wio-WM6108 The GPS receiver is automatically enabled on OpenMANET hardware. The device appears at /dev/ttyACM0. Please Note:

  • The GPS antenna that comes with the Wio-WM6108 is not that great. Consider getting a better antenna.
  • Do not expect to get any or much signal indoors. GNSS signals are fairly week unless you have a really good antenna.

Any u-blox based USB GPS receiver that works with gpsd should also function with OpenMANET. This may require manual configuration of /etc/config/gpsd.

How It Works

Connection & Monitoring

The GPS service in OpenMANET establishes a persistent connection to the GPS an generates both position reports and can stream NMEA data:

  • Latitude/Longitude (decimal degrees)
  • Altitude (meters above sea level)
  • Speed (meters per second)
  • Track/Course (degrees)
  • GPS fix mode (2 = 2D fix, 3 = 3D fix)
  • Timestamp (UTC)

The service maintains automatic reconnection with exponential backoff (up to 3 attempts) if the GPS daemon becomes unavailable.

Position Validity

A position is considered valid when:

  • GPS fix mode is 2 (2D) or 3 (3D)
  • Position data has been received from gpsd

Invalid positions (mode 0 or 1) are not shared with EUDs or included in mesh announcements.

NMEA Streaming to EUDs

When a valid GPS position is available, the service automatically:

  1. Converts the position to NMEA GGA format (Global Positioning System Fix Data)
  2. Queries current DHCP leases to find connected EUD clients
  3. Sends the NMEA sentence via UDP to each EUD on port 4349 (Standard Port in ATAK)

NMEA GGA Format Example:

$GPGGA,123045.00,3746.4946,N,12225.1640,W,1,08,1.0,50.0,M,0.0,M,,*47

This allows tactical applications like ATAK to receive real-time GPS position from the mesh node.

Position Sharing in Mesh Announcements

When GPS is enabled and a valid position is available, nodes include their position in node data announcements sent periodically across the mesh.

This allows:

  • Web UI map visualization of node locations
  • Topology tracking with geographic context
  • Range testing with position logging

The node data worker checks GPS validity before including position data (mode > 1) and logs the current position at the debug level.

Automatic Reconnection

The GPS service includes robust connection handling:

  • Initial connection: Connects to gpsd on startup
  • Automatic reconnection: Retries up to 3 times with a 5-second delay between attempts
  • Connection loss recovery: Automatically detects connection drops and re-establishes the session
  • Graceful shutdown: Closes connections cleanly when the service stops

If the maximum reconnection attempts are reached, the service gives up and logs an error. Restarting openmanetd will reset the reconnection counter.

Integration with Other Services

Range Testing

The built-in range testing script (/usr/bin/rangetest.sh) logs GPS position alongside signal strength metrics when GPS is enabled, allowing you to correlate range performance with geographic location.

Web UI

Future releases may include a map view showing node positions on the topology page when GPS data is available.

Troubleshooting

GPS not acquiring fix

  • Ensure the GPS receiver has a clear view of the sky
  • Wait 30-60 seconds for cold start (first fix after power-on)
  • Check that gpsd is running: ps | grep gpsd
  • Verify the GPS device is detected: ls /dev/ttyUSB*

No position data in mesh

  • Confirm GPS is enabled in /etc/openmanet/config.yaml
  • Check logs: logread | grep gps
  • Verify gpsd connection: telnet localhost 2947 (type ?WATCH={"enable":true,"json":true})
  • Restart openmanetd: /etc/init.d/openmanetd restart

EUD not receiving NMEA data

  • Confirm the EUD has a DHCP lease: cat /tmp/dhcp.leases
  • Verify the EUD application is listening for NMEA on the correct port

ATAK not showing node position

  • Verify multicast is working across the mesh
  • Ensure ATAK is subscribed to the SA multicast group 239.2.3.1:6969

Notes

  • NMEA streaming uses UDP, so delivery is not guaranteed if network conditions are poor
  • CoT multicast requires multicast routing to be enabled across the mesh (BATMAN-V handles this automatically)
  • Position updates are sent whenever the GPS service receives a new TPV report with a valid fix
  • Checksums are automatically calculated for NMEA sentences to ensure data integrity