Untitled :: NmeaGenerator Plugin Manual

Author

Plugin created by ©Kupofty 2026

Overview

NmeaGenerator is a plugin for OpenCPN which generates and transmits NMEA0183 sentences.

The plugin can be used as a lightweight ship simulator to reproduce realistic navigation behavior inside OpenCPN or for testing UI, other plugins, instruments, etc.

Capabilities

Generates standard NMEA0183 sentences
Produces real-time data streams
Simulates vessel movement (position, speed, heading)
Supports AIS target simulation

Use cases

Testing OpenCPN UI with live data
Validating instruments and dashboards
Developing and debugging NMEA-based plugins
Simulating navigation scenarios without hardware

Links

Sources:

Discussions:

Installation

Method 1: Plugin Manager (Recommended)

Open OpenCPN
Go to Options → Plugins
Update catalog and search for the NmeaGenerator plugin
Click Install then Enable

Method 2: Manual Install (Cloudsmith)

Go to Cloudsmith
Select the correct tarball for your OS
Download the .tar.gz archive (not the .xml)
Click on Options → Plugins → Import plugin…
Enable plugin in Options → Plugins

Method 3: Build from Source (Github)

Clone the repository:

git clone https://github.com/Kupofty/NmeaGenerator_pi.git

Create the build directory:

mkdir build
cd build

Linux

cmake ..
make tarball

Windows

cmake .. -A Win32
cmake --build . --config Release --target tarball

Import the tarball in OpenCPN via Options → Plugins → Import plugin… and enable it.
For more information, see the INSTALL.md file in the GitHub repository.

User Interface

To open the plugin, click on the toolbar icon :

The plugin is divided into multiple tabs:

Manual Input

The "Manual Input" tab lets you type and send any NMEA sentence directly to OpenCPN, making it ideal for testing specific messages independently of the simulation engine, or for validating and debugging data flows.

There is no validation on the content — any sentence will be forwarded as-is. Sentences must start with $ (NMEA) or ! (AIS/VDM). The checksum is optional.

Checksum

The "Add automatic checksum" option computes and appends the correct checksum to your sentence automatically.

When enabled, you can paste a sentence that already includes a checksum and visually compare it with the generated one — the manually written checksum will be stripped before sending.

When disabled, a status message is displayed below the input field indicating whether the checksum is absent, correct, or invalid.

Controls

Send	Sends the sentence once to OpenCPN.
Copy	Copies the current input to the clipboard.
Clear	Clears the input field.
Automatic send	Sends the sentence continuously at the specified frequency. The rate can be adjusted at any time.

Sentence Builder

The "Sentence Builder" tab provides quick access to the most common NMEA sentences, each with editable fields. Multiple sentences can be sent simultaneously and automatically.

Use the search box to filter sentences by ID, and the vertical scrollbar to browse the full list.
For text input fields, values must follow the official NMEA field format to be interpreted correctly by OpenCPN or external hardware/software.

Controls

Send	Sends the selected sentence once to OpenCPN.
Auto	Adds the sentence to the automatic send list.
Check all / Uncheck all	Quickly adds or removes all sentences from the automatic send list.
Automatic send	Sends all checked sentences continuously at the specified frequency. Must be enabled for automatic sending to take effect.
Help	Opens external references on NMEA and AIS sentence structures, field definitions, and units.

Simulation

The Simulation tab generates continuous NMEA data streams to mimic own ship and/or AIS target.

Vessel Selection And Data Outputs

Control vessel	Selects which vessel is currently being controlled (own ship or AIS target). The other vessel retains its last speed and rudder angle.
AIS type	Sets the AIS target type: Class A, Class B, or ARPA radar target.
NMEA output	Controls which data is transmitted. Current sends data for the vessel selected in Control vessel only; Both sends data for both own ship and AIS target simultaneously, with both vessels continuously updated in position and heading.

Vessel Controls

Initial position	Set latitude and longitude manually, or right-click on the map to update the position directly.
Throttle	Speed control slider.
Rudder angle	Turn rate slider.
Reset	Returns slider to zero.

Simulation

START/STOP

Starts or stops sending simulation data at the specified rate (adjustable).

The status labels in the centre of the tab (position, heading, COG, speed) reflect the live data of the vessel currently selected in "Control vessel".

Settings

The plugin can be accessed via Options → Plugins → NmeaGenerator in the OpenCPN toolbar.

Click on "Preferences" to open the settings window.
Remember to click "OK" or "Apply" before closing to save any changes.

Display

Default tab	Tab opened by default when the plugin window is launched (only applies if Restore last tab is disabled).
Restore last tab	Reopens the last active tab when launching the plugin, overriding the default tab setting.
Restore last window position	Restores the plugin window to its last position before it was closed.
Restore last window size	Restores the plugin window to its last size before it was closed.

Options

Keep transmitting when window is closed	Continues sending data when the plugin window is closed, allowing use of other plugins without interrupting an active simulation or auto-send.
Copy sentence to clipboard on manual send	Automatically copies the sent sentence to the clipboard when clicking Send in the Sentence Builder tab.

AIS

Customize the MMSI used for AIS simulation in the Sentence Builder tab (VDM/VDO sentences) and for the AIS target in the Simulation tab. Can be used to mimic a vessel or SART targets (MOB, EPIRB, etc).

Notes

Connections

NMEA sentences are sent to OpenCPN’s internal data bus. They will be received by any plugin or instrument listening on that bus, including Dashboard, AIS display or third-party plugins.
The plugin does not create a serial or network output by default. To forward sentences to external software or hardware, use OpenCPN’s connection settings to create an output connection.

General

Multiple NMEA sources may cause conflicts in OpenCPN: ensure sensor inputs and generated data do not overlap.
Do not use the plugin during real navigation, or do so at your own risk — it may interfere with critical navigation data and affect system reliability.
The outputs of all three tabs (in auto-send mode) are automatically stopped when the plugin window is closed, unless Keep transmitting when window is closed is enabled in Settings.
Output can be monitored in the OpenCPN Data Monitor window.
Plugin sentences are prefixed with a "virtual" tag, making them easy to identify.

When simulating AIS targets, make sure the MMSI used does not match any real vessel to avoid confusion in AIS displays.
Sentence fields left empty or filled with invalid values may be silently ignored or cause unexpected behavior in receiving instruments — always refer to the NMEA reference when in doubt.