The GP33 is its own GPS unit via the provided antenna. It doesn't use alternative sources. The short list of PGN data it will accept is listed on page 9-8 of the operations manual. If looking for a unit like this without a GPS antenna, the RD33 (remote display) would have been an option because it will show TDs and use GPS from the bus. The RD33 was not designed to be it's own GPS like the GP33 is.