sensors2nmea

Turn a Windows laptop's built-in GNSS receiver into a network NMEA 0183 source for OpenCPN, Navionics, qtVlm or any other chart plotter — no serial cable, no virtual COM port, no driver hacks.

Many modern laptops carry a GNSS receiver inside their WWAN (mobile broadband) module. Windows exposes it through the Sensors and Geolocation APIs, but chart plotters expect NMEA 0183 on a serial port or TCP socket. sensors2nmea bridges the two: it reads Windows.Devices.Geolocation / Windows.Devices.Sensors and broadcasts standard NMEA sentences over TCP to every connected client.

It can also replay a recorded GPX track instead of reading the sensors, which is useful for testing a plotter setup at your desk.

┌──────────────────────┐      ┌──────────────┐      ┌──────────────────┐
│ Windows Sensors API  │      │              │      │ OpenCPN          │
│  Geolocator (GNSS)   │─────▶│ sensors2nmea │─────▶│ Navionics        │
│  Compass             │      │   TCP :10110 │      │ qtVlm            │
│  Inclinometer        │      │              │      │ ...many clients  │
└──────────────────────┘      └──────────────┘      └──────────────────┘
         or  ──▶ GPX track playback ──▶

This is not a certified navigation device. See Disclaimer before you rely on it for anything. Seriously — read it.

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Table of contents

• Features
• Requirements
• Installation
• Running
• Connecting OpenCPN
• Command-line options
• What the data actually means
• GPX track playback
• Reliability
• Troubleshooting
• Reference setup
• Known limitations
• Disclaimer
• License and attribution

---

Features

• GNSS position → GPGGA, GPRMC with real fix quality, not invented values
• Time from the receiver, not the PC clock — sentences carry the actual fix timestamp, so one fix is transmitted exactly once
• UTC + local zone → GPZDA
• Course and speed over ground → GPVTG (many plotters prefer this over RMC)
• Compass heading → HCHDM, HCHDT, HCHDG, with mounting-offset and magnetic-variation correction
• Pitch and roll → YXXDR
• GPX playback of tracks or routes, with speed multiplier and looping
• Honest fix quality: a WiFi- or IP-derived position is flagged invalid instead of masquerading as a satellite fix
• Multi-client TCP server — plotter, logger and a debug telnet at once
• Self-healing: every task is supervised and restarted with backoff; sensors that disappear are re-acquired automatically
• No dependencies for --simulate and --gpx modes — those run on plain Python, including on Linux and macOS

Requirements

OS	Windows 10 or 11 for sensor mode; any OS for --simulate / --gpx
Python	3.9 or newer
Hardware	a GNSS receiver Windows recognises as a location sensor
Permissions	Location access enabled for desktop apps

Check whether Windows sees a usable receiver:

Get-PnpDevice -Class Sensor | Select-Object FriendlyName, Status

If that lists nothing, Windows has no location sensor and only --simulate and --gpx will work. A USB GPS dongle that appears as a COM port is not a Windows location sensor — feed it to your plotter directly instead; you do not need this tool for that.

Installation

From PyPI (recommended)

py -m venv .venv
.\.venv\Scripts\Activate.ps1
pip install "sensors2nmea[sensors]"

This installs a sensors2nmea command on your PATH. The [sensors] extra pulls in the WinRT bindings needed to read the real hardware.

If you only want simulation or GPX playback, drop the extra — the core has no dependencies at all and runs on Linux and macOS too:

pip install sensors2nmea

From source

git clone https://github.com/BTL00/sensors2nmea.git
cd sensors2nmea
py -m venv .venv
.\.venv\Scripts\Activate.ps1
pip install -e ".[sensors]"

Manual, no packaging

The program is a single file. Download sensors2nmea.py, install the WinRT bindings, and run it:

pip install winrt-runtime winrt-Windows.Devices.Sensors `
            winrt-Windows.Devices.Geolocation winrt-Windows.Foundation
python sensors2nmea.py

The older monolithic winsdk package also works — the code tries winrt-* first and falls back automatically.

Enabling location access

Settings → Privacy & security → Location, and turn on both:

• Location services
• Let desktop apps access your location

Without this, Geolocator.request_access_async() is denied and the program says so explicitly at startup.

Running

# defaults: port 10110, all sentences, 5 Hz, listening on every interface
sensors2nmea

# a specific port, and echo sentences to the console so you can see them
sensors2nmea --port 15555 -v

# bind to localhost only
sensors2nmea --host 127.0.0.1 --port 10110

# synthetic data, no sensors needed
sensors2nmea --simulate

# replay a recorded track in real time
sensors2nmea --gpx examples/sample_route.gpx --sim-as-real

# replay 10x faster, looping forever
sensors2nmea --gpx examples/sample_route.gpx --sim-as-real --gpx-speed 10 --gpx-loop

Stop it with Ctrl+C.

Verify the stream without a plotter:

# PowerShell one-liner: dump whatever is being broadcast
$c = New-Object Net.Sockets.TcpClient('127.0.0.1', 10110)
$r = New-Object IO.StreamReader($c.GetStream())
1..10 | ForEach-Object { $r.ReadLine() }
$c.Close()

Connecting OpenCPN

1. Options → Connections → Add Connection
2. Type: Network, Protocol: TCP
3. Address: 127.0.0.1 (same machine) or the LAN IP of the machine running it
4. Data port: 10110, or whatever you passed to --port
5. Leave input filtering off until you know it works

Two things that cost real debugging time, so they are worth stating plainly:

GPX playback needs --sim-as-real. By default, replayed and simulated data are honestly flagged as simulator output (GGA quality 8, FAA mode S). OpenCPN deliberately ignores such data: the ship icon will not move and the Dashboard shows --- for SOG and COG even though the values are present in the sentences. Adding --sim-as-real presents the data as an ordinary GPS fix (GGA quality 1, mode A) and everything appears. Do not use that flag with real sensors — there, honest source flagging matters and works correctly.

OpenCPN ignores the local-zone fields of GPZDA. Its local time comes from its own setting, not from the NMEA stream: Dashboard preferences → Appearance → Local offset from UTC. Leave it at 00:00 to follow the computer's time zone. Also note that the Dashboard has three different clocks — GPS clock shows raw UTC, Local GPS clock applies that offset, Local CPU clock reads the operating system. Pick the one you actually want.

Command-line options

Network and output

Option	Default	Meaning
--host ADDR	0.0.0.0	Listen address. 0.0.0.0 exposes every interface
--port N	10110	TCP port. 10110 is the IANA port for NMEA 0183
--rate HZ	5	Broadcast frequency
--sentences LIST	all	Comma-separated: gps,zda,vtg,hdg,att
--gps-repeat	off	Repeat the last fix at --rate instead of once per fix
--stale SEC	5	Stop sending after this long without a fresh reading
-v, --verbose	off	Also print sentences to stdout
--status-interval S	60	Status heartbeat in the log; 0 disables
--log-file PATH	–	Append diagnostics to a file as well as stderr
--version		Print version and exit

Heading correction

Option	Default	Meaning
--offset DEG	0	Added to the compass reading: mounting error, deviation
--variation DEG	–	Magnetic variation, east positive. Used only when Windows does not supply a true heading

Simulation and playback

Option	Default	Meaning
--simulate	off	Synthetic data instead of sensors
--gpx FILE	–	Replay a GPX track or route. Implies simulation mode
--gpx-speed X	1	Playback rate multiplier
--gpx-knots KN	5	Speed used only when the GPX has no <time> stamps
--gpx-loop	off	Restart from the beginning at the end of the track
--sim-as-real	off	Present simulated data as a genuine GPS fix. Needed for OpenCPN

What the data actually means

This tool tries hard not to invent numbers. That occasionally makes the output look emptier than other NMEA sources — deliberately.

The satellite-count field in GPGGA is always empty. Windows does not expose how many satellites were used, nor a list of satellites in view. A number there would be fiction, and GPGSV cannot be generated honestly at all.

HDOP comes from the receiver (GeocoordinateSatelliteData) or is left empty. It is not derived from the accuracy radius by some invented factor.

Fix quality follows the actual position source:

position_source	GGA	RMC	FAA mode	Usable for navigation
SATELLITE	1	A	A	yes
WI_FI, CELLULAR, IP_ADDRESS	0	V	N	no
DEFAULT, OBFUSCATED, UNKNOWN	0	V	N	no
--simulate / --gpx	8	A	S	no — simulation
…plus --sim-as-real	1	A	A	no — deliberately disguised

Windows will silently fall back to WiFi or IP geolocation when satellites are unavailable, and such a position can be tens of kilometres off. Those fixes are marked invalid rather than passed off as GPS. Every change of source is logged:

[2026-10-03T22:43:31] INFO  Geolocator: zrodlo pozycji = SATELLITE (fix nawigacyjny)
[2026-10-03T22:51:07] WARN  Geolocator: zrodlo pozycji = WI_FI (NIE nadaje sie do nawigacji -> GGA 0 / RMC V)

Timestamps come from the fix, not from the moment of transmission. The receiver reports roughly once per second while the default broadcast rate is 5 Hz, so stamping with the PC clock would send the same measurement five times under five invented timestamps. Instead each fix is transmitted once with its own timestamp, and a dropped fix produces silence rather than a stale position with a fresh time. If your consumer needs an uninterrupted stream, use --gps-repeat.

GPX track playback

Accepts tracks (<trkpt>), routes (<rtept>) and loose waypoints (<wpt>), with or without XML namespaces. Position is interpolated linearly between points, so output is smooth at any --rate. COG is the bearing of the current leg — with linear interpolation that is the exact direction of travel, not an approximation. SOG is leg length divided by leg duration.

Real-world GPX files are messy, and three specific hazards are handled:

• Nonsense <ele>. Garmin exports write 1e25 to mean "no elevation". Values outside −1000…10000 m are rejected and the altitude field is left empty; the count of rejects is logged.
• Relative timestamps. Files from gpx.studio carry dates of 1970-01-01. Stamps are used only as intervals between points; sentences get the current UTC time. Taking them literally would put 010170 in the RMC date field.
• Duplicate points. Zero-duration and zero-length legs are common. SOG and COG carry over from the last meaningful leg, otherwise speed would drop to zero mid-motion and the course would jump randomly.

--gpx-speed scales the playback rate but not the reported SOG. At x30 the vessel crosses the chart thirty times faster while still reporting the recorded 2.7 kn. This is intentional: the alternative, 81 kn, is a fiction some plotters reject outright. At --gpx-speed 1 the question does not arise.

In GPX mode the compass is fed the current COG, so HCHDM and HCHDG work. There is no source for attitude, so YXXDR is not emitted.

Reliability

Intended to be left running for a whole passage, so failures are contained rather than fatal:

• Every long-running task is supervised. A crash is logged with a full traceback and the task restarts with exponential backoff (1 s doubling to 30 s). The backoff resets once a task has survived a minute, so a task that dies instantly cannot spin.
• Unrecoverable problems stop immediately with a readable message instead of retrying forever: a missing or malformed GPX file, absent WinRT libraries, a port already in use. The GPX file is validated before the port is bound, so a bad file never leaves a socket behind.
• Sensors are re-acquired. A compass or receiver missing at startup is retried every 15 seconds — USB hardware may appear later. A sensor that starts throwing errors is released after 10 consecutive failures and acquired again from scratch.
• Repeated errors are rate-limited so a failing sensor cannot flood the log.
• Sentence-building errors cannot stop the broadcast; they are logged and the loop continues.
• Receiver status is reported. PositionStatus transitions are logged, so NO_DATA or DISABLED is visible rather than silent.
• A heartbeat line every --status-interval seconds shows client count, sentences sent, fix age and position source.
• Diagnostics go to stderr, keeping stdout clean for -v sentence output so you can pipe one without losing the other.

Exit codes: 0 normal, 1 unrecoverable error, 2 bad command line, 130 interrupted with Ctrl+C.

Troubleshooting

Plotter connects but shows nothing, or shows obviously wrong data. Check for a port conflict. Windows permits two processes to listen on the same port if one binds 127.0.0.1 and the other 0.0.0.0 — and a client connecting to localhost reaches the first one, not necessarily yours:

Get-NetTCPConnection -LocalPort 10110 | Select-Object LocalAddress, State, OwningProcess
Get-Process -Id <OwningProcess>

Ship icon points north and Dashboard shows ---. Either heading sentences are not being sent — do not restrict --sentences, the default includes hdg and vtg — or you are replaying GPX without --sim-as-real. See Connecting OpenCPN.

No GNSS time in the Dashboard. Add the GPS clock instrument; Local CPU clock shows the computer's clock instead. Confirm GPRMC or GPZDA is actually arriving with -v.

Position is far from where you are. Look at the logged position source. WI_FI or IP_ADDRESS means Windows has no satellite fix — go outdoors or near a window, and give the receiver a few minutes for a cold start. Such fixes are correctly flagged invalid.

Geolocator: brak dostepu / access denied. Location is off, or desktop apps are blocked. See Enabling location access.

brak bibliotek WinRT. Install the extras: pip install -e ".[sensors]", or run --simulate / --gpx, which need nothing.

Empty SOG and COG fields in the sentences. Expected when the fix carries no motion measurement — Windows returns NaN for speed and heading on WiFi and IP fixes. An empty field is correct NMEA; the literal text nan would not be.

Reference setup

The configuration this was developed and verified on, for comparison if yours behaves differently:

Laptop	Lenovo ThinkPad T480s (20L8S4PR0U)
OS	Windows 11 Pro 10.0.22631 (build 22631)
Python	3.10.11
GNSS	Fibocom GNSS Sensor — the GNSS half of a Fibocom L850-GL WWAN module (Intel XMM7360-P)
Compass / inclinometer	none — the Sensor device class contains only the GNSS sensor
Chart plotter	OpenCPN 5.12.4-0+37fd0cd

The receiver is worth a note, because it is what makes this approach work: the GNSS is integrated into the mobile-broadband card and is presented to Windows as a location sensor, not as a serial port. There is no COM port to open, which is precisely why going through the Sensors API is necessary rather than merely convenient.

Measured values from that receiver, for a sense of what to expect: horizontal accuracy 4.0 m, HDOP 0.5, PDOP 1.0, VDOP 0.75, with GeometricDilutionOfPrecision and TimeDilutionOfPrecision both unsupported (None). Altitude is geoid-referenced (AltitudeReferenceSystem.GEOID), so placing it in the mean-sea-level field of GPGGA with an empty geoid-separation field is correct.

Since the test machine has no compass, the heading sentences are exercised mainly through GPX playback and simulation. Reports from hardware with a real compass are very welcome.

Known limitations

• No GPGSV and no satellite count — Windows does not expose either
• No GPGSA, although PDOP and VDOP are available and could populate it
• The magnetic-course field of GPVTG is left empty; only true course is known
• No GPGST: Windows reports an accuracy radius rather than per-axis deviations, and altitude_accuracy is frequently None
• The TCP server has no authentication and no client limit. It is meant for a trusted local network; --host 0.0.0.0 exposes it on every interface
• Sensor mode is Windows-only by nature. --simulate and --gpx are portable

Disclaimer

THIS SOFTWARE IS NOT A CERTIFIED NAVIGATION DEVICE AND MUST NOT BE USED AS A PRIMARY MEANS OF NAVIGATION.

sensors2nmea converts readings from consumer-grade hardware and operating system APIs into NMEA 0183 sentences. The accuracy, availability, continuity and integrity of that data are not guaranteed. In particular:

• positions may be missing, stale, or silently derived from WiFi or IP geolocation and wrong by tens of kilometres;
• the host operating system may suspend, throttle or deny sensor access at any moment, without warning;
• heading, speed and course are only as good as the consumer sensors behind them, and no integrity monitoring of any kind is performed;
• the GPX playback and simulation modes emit entirely synthetic data by design, and --sim-as-real makes that synthetic data deliberately indistinguishable from a real fix.

The author accepts no responsibility and no liability for any grounding, collision, allision, stranding, loss of or damage to any vessel, cargo or property, nor for any injury or loss of life, arising from use of or reliance upon this software or the data it produces, whether in whole or in part, however caused.

You, the mariner, remain fully and solely responsible for the safe navigation of your vessel and for maintaining a proper lookout at all times. Navigate using official charts and type-approved equipment, and always cross-check position against independent sources.

This software is provided "AS IS", without warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose and non-infringement. See sections 7 and 8 of the Apache License 2.0 for the governing terms.

License and attribution

Licensed under the Apache License, Version 2.0 — see LICENSE.

This is genuine open source: use it, modify it, redistribute it, build a commercial product on it. One condition — you must credit it.

Section 4(d) of the licence requires that the NOTICE file be reproduced in any derivative work you distribute. Concretely:

• Redistributing sensors2nmea, in source or binary form, modified or not, requires including a readable copy of NOTICE in at least one of: your source distribution, your documentation, or a display generated by your software such as an "About" or "Credits" screen.
• This applies to commercial and closed-source products too. Selling software that incorporates sensors2nmea is fine; removing the attribution is not.
• The name sensors2nmea and the author's name may not be used to endorse or promote your product without separate written permission.

If the Apache terms do not suit your use case, get in touch — other arrangements are possible.

Contributing

Issues and pull requests are welcome. Useful contributions in particular:

• reports from hardware that actually has a compass or inclinometer
• GPGSA and GPGST support built on the DOP values already available
• testing against plotters other than OpenCPN