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Feeding OpenCPN and a Signal K server at once

In this tutorial you build one profile with two outputs: NMEA 0183 sentences for the OpenCPN chart plotter and Signal K deltas for a Signal K server, both from the same simulated vessel at the same time. You then watch the vessel in both programs, steer it and see both follow. It takes about twenty minutes.

You need NMEA Simulator X (install it), OpenCPN 5.8 or later and a Signal K server. Everything can run on one computer; the steps say where addresses change when it does not.

1. Start a Signal K server

If you do not have one yet, the quickest way is the official container image:

docker run -d --name signalk -p 3000:3000 signalk/signalk-server

or, with Node.js 20 or later installed, npm install -g signalk-server followed by signalk-server. Open http://localhost:3000 in a browser and create the administrator account when asked. The server's own port is 3000, so the simulator must not use it.

2. Build the profile

The profile needs two outputs, each with its own encoding:

Output Type Port Encoding Consumer
1 TCP server 10110 NMEA 0183 OpenCPN
2 TCP server 8375 Signal K, every 500 ms Signal K server
  1. Start NMEA Simulator X. The default profile already has the first output, a TCP server on port 10110 carrying NMEA 0183.
  2. Open File → Settings... and the Outputs tab. Click Add, choose TCP server, set Port to 8375, Encoding to Signal K deltas and Period to 500 ms. Leave the Signal K context on the vessel with the AIS MMSI.
  3. On the Simulation tab set Name to OpenCPN and Signal K, then click OK.
  4. Save the profile with File → Save profile as..., for example as opencpn-and-signal-k.json.

--encoding applies to every output given on the command line, so two outputs with different encodings need a profile. Create the default one and replace its outputs:

nmeasim profile init opencpn-and-signal-k.json

"name": "OpenCPN and Signal K",
"outputs": [
  { "type": "tcp-server", "port": 10110 },
  { "type": "tcp-server", "port": 8375, "encoding": "signalk", "period_ms": 500 }
],

Check it with nmeasim profile show opencpn-and-signal-k.json, which prints the profile with every default filled in, or an error that names the wrong key.

3. Start the stream

Press Start (F5). The Outputs panel lists both TCP servers as open with no clients yet.

nmeasim run --profile opencpn-and-signal-k.json

The tool prints one line per output and keeps running until Ctrl+C:

output: TCP server on 0.0.0.0:10110
output: TCP server on 0.0.0.0:8375
running profile 'OpenCPN and Signal K' with a 100 ms tick; press Ctrl+C to stop

If OpenCPN or the Signal K server runs on another computer, note this computer's address with nmeasim interfaces and allow the two ports through its firewall.

4. Connect OpenCPN

  1. In OpenCPN open Options → Connections and click Add Connection.
  2. Choose Network, protocol TCP, address localhost (or this computer's address) and port 10110. Leave the input filter empty and click OK.
  3. Close the options. Within a second the own-ship icon appears off Athens, the GPS status in the toolbar turns green and the heading and course lines point north-east.

The Outputs panel, or the table in the desktop application, now shows one client on port 10110.

5. Connect the Signal K server

  1. In the Signal K administration page open Server → Data Connections and click Add.
  2. Set Data Type to Signal K, Signal K Source to TCP, Host to localhost (or this computer's address) and Port to 8375. Give it an ID such as nmeasim and click Apply, then restart the server when it asks.
  3. Open Data Browser. The simulated vessel appears under the context vessels.urn:mrn:imo:mmsi:239000001, with navigation.position, navigation.speedOverGround, environment.depth.belowTransducer, the wind and the two engines under propulsion.port and propulsion.starboard, updated twice a second.

The deltas belong to the MMSI of the profile's AIS data, 239000001 by default. To make the simulated vessel the server's own vessel, so that plugins and the instrument panel use it, enter the same MMSI under Server → Settings → Vessel.

6. Steer and watch both follow

Click the dashboard of the desktop application and press Right a few times: the heading override rises by one degree per press. OpenCPN's own ship turns, and in the Data Browser navigation.headingTrue and navigation.courseOverGroundTrue change with it. Press Up to speed up and watch navigation.speedOverGround follow. Both consumers read the same simulated vessel; only the encoding differs.

To try the autopilot paths, hold Shift and click the map to set a destination. OpenCPN then receives APB, RMB and XTE, and the server gets navigation.courseRhumbline.nextPoint and navigation.courseRhumbline.crossTrackError.

7. Filter what each consumer gets

Each output has its own filter. On the Outputs tab, select the Signal K output and enter navigation, environment.depth as Sentence filter: the server now receives only the navigation and depth paths. The filter of the NMEA 0183 output takes sentence ids instead, for example RMC, GGA, HDT, DPT for a plotter that only needs a fix, heading and depth.

What you built

One profile that feeds two different kinds of consumer from the same simulation. The same pattern adds a third output, for example a UDP broadcast for tablets or a ViewSync output for Google Earth, without affecting the other two.

Where next