This is a new one! Thanks for bringing it to my attention. Looks like a voluntary, GPS-based (i.e., not secondary surveillance) system. While the serial protocol appears to be NMEA-like, the protocol "under the hood" at the PHY layer could be basically anything, and the FLARM consortium appears to have elected to keep specs hidden. Reading the dataport specification gives few insights as to the PHY layer, but at least it's good to know the protocol has "future-proof" support for all aircraft types.