CarCode..
A GNSS receiver capable to use several GNSS systems and following the NMEA0183 specification will use a TalkerID referred to into the linked document herein. Normally such a device will use the best of what's available in the sky and thus use the TalkerID "GN" for messages like "GNRMC".
How any device is set to do that and which of available system(s) to use for the reported fix is the manufacturer's responsibility and nothing OCPN can control or choose to select from.
It's could be some devices letting the user control which
satellite system to use. I don't know. But that's nothing OCPN can or will take care of. But to my imagination a proper GNSS receiver program will sort out deviant positions before reporting a fix. There are a lot more satellites available these day's so it would be no problem.
My device normally sees about 40 satellites from five system, lat 58 N, lon 11 E, and use to
work with about 14 for the fix.
Link:
https://www.nmea.org/Assets/20190303...0mnemonics.pdf