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OpenSky Network
OpenSky is available as a tracking data source to everyone and immediately
As of Nov-2023 OpenSky Network is unreliable or at time completely down and has not yet indicated when it will be back online in a reliable fashion. That means there currently is no out-of-the-box option available that would cover essential parts of world-wide air traffic.
See this forum post for background and updates.
Another free option is ADS-B Exchange's Basic plan, but it requires you to set up an account and obtain an API key, see here.
And see the full list of channels for even more options.
OpenSky works out of the box as a flight data tracking source and is enabled by default in the Settings > Input Channels:

Settings > Input Channels > Open Sky Network (no user)
However, since July 2022 it is advisable to register a user account with OpenSky Network. then enter those credentials here in LiveTraffic and click [Save and Try]. Without a user account you are only allowed a very limited number of requests per day, with an account you are allowed a lot more (details). Your credentials work well when you see a much higher number of "requests left today" displayed some time (refresh period) after saving new credentials:

Settings > Input Channels > Open Sky Network
The fields in detail:
Setting | Default | Description |
---|---|---|
OpenSky Network | Enabled | Enables usage of OpenSky Network as a live flight data channel. As this channel provides positional information only, OpenSky Master Data (see next row) is forced to be enabled, too, as long as OpenSky Network is enabled.
If interested find API documentation here. |
OpenSky Explorer | | |
OpenSky Network Master Data | Enabled | For fetching master data on the involved aircraft (aircraft type, registration, operator, ...) OpenSky requires this additional channel, but it is also queried for flights read from other channels. So generally, it is a good idea to keep it enabled. |
Registration | | |
Username
Password | | Credentials of your OpenSky account.
To save new entries click the [Save and Try] button. |
Connection Status | | Displays current status info, including number of remaining requests today. The same info is displayed on the Status / About page. |
Since July 2022 OpenSky Network restricts the number of requests per user, see here for current limits. If you use a registered user account then the request budget is significantly higher.
You control the number of requests LiveTraffic sends per minute via the Advanced Setting Live Data Refresh. The default of one request per 20s is totally sufficient for fluent aircraft display and means that you use just 3 requests per minute and the 1,000 requests limit lasts for 5.5 hours.
To further save on your request budget, you can separately define a higher refresh period in the Advanced Settings > Aircraft Selection while you are flying your user aircraft higher than a configurable height above ground (default: 60s when flying higher than 10,000ft).
LiveTraffic displays the remaining requests per day in OpenSky's connection status, see screenshots above for examples. Please note that OpenSky might run in a different timezone than you are in, so the end of an OpenSky day might not be your local midnight.
LiveTraffic displays a warning once the number of remaining requests reach 50 and once again when it reaches 10.
In case you run out of requests OpenSky sends an information when budget is available again. LiveTraffic displays this as local time in the Connection Status:

OpenSKy: Retry information
Last modified 10d ago