Skip to content

Recording and replaying a log

In this tutorial you record a session to a log file, look inside it, replay it with pause, step and seek, and replay a plain NMEA log captured by another program. It takes about ten minutes and needs only the desktop application.

1. Record a session

Start NMEA Simulator X with any profile and press Start. Choose File → Record log... (Ctrl+R) and accept the suggested file name, something like nmeasim-20260923-101500.log in your documents folder. The status bar reports Recording to ... and the action stays ticked. Steer around with the arrow keys for half a minute, change the depth override, then press Stop.

Open the file in a text editor. It starts with a short header and then holds one line per sentence, each prefixed with the exact UTC time it was sent:

# NMEA Simulator X log 1
# recorded: 2026-09-23T10:15:00.000Z
# profile: Default
2026-09-23T10:15:00.104Z $GPRMC,101500.10,A,3759.0281,N,02343.6502,E,6.5,45.0,230926,4.6,E,A*0D

Untick File → Record log... to close the recording. Leaving it ticked would continue the same file at the next Start.

2. Replay it

Choose File → Open log for replay... (Ctrl+L) and pick the file you just recorded. The profile switches to replay mode, the status bar reads (replay ...) and the seek slider shows the length of the recording. Press Start: the sentences are sent again, byte for byte, on the cadence they were recorded, and the dashboard and the map follow them because every replayed sentence is decoded into the vessel state. The moves you made with the arrow keys happen again at the same moments.

3. Pause, step and seek

Press Pause (F6). The stream stops where it is; the outputs stay open. Press Step (F7) a few times: each press sends exactly one recorded sentence and the console shows it. Drag the seek slider back to the start: the instruments show the values the log had at that point, without any sentence being sent, and untick Pause to continue from there.

When the log runs out the run ends by itself and the status bar reports End of the track or log reached. To loop instead, open File → Settings... and tick Start again at the end in the Log replay group.

4. Replay a log from another program

Any plain NMEA log works too: a file saved by a plotter, a serial logger or a nc session, with or without timestamps. Open it with File → Open log for replay... as before. Lines without a timestamp are timed from the UTC time fields of the RMC, GGA, GLL and ZDA sentences they contain, so a log with one fix per second replays at one fix per second. A log without any time information, for example a depth-only capture, plays at the Interval without times set in the settings dialog, 100 ms by default.

The log file reference lists every line shape the reader accepts and which sentences update the dashboard.

5. Do the same headless

Both halves work from the command line:

nmeasim run --record monday.log --tcp-server 10110 --duration 600
nmeasim run --replay monday.log --tcp-server 10110

Where next