@Michael_Oborne: The overall system of FLARM is outlined here: https://flarm.com/technology/eid/
In this short overview of UAS Electronic Identification, there is also a reference how to obtain their protocol
Additionally, they very recently announced, that " FLARM and uAvionix Collaborate to Create Electronic Conspicuity Solutions for Manned and Unmanned Aircraft".
Acc. to their website, “FLARM is the state-of-the-art traffic information, collision avoidance and remote electronic identification technology used in general aviation, installed in over 35’000 manned aircraft”
With the new EU regulations having been published recently, an increasing push towards the implementation of EIDs will be seen, I am sure.
If the Ardupilot community can proactively contribute to taking part in this development, this will certainly help this open source solution to thrive in the coming regulatory changes, both here and in the US.
P.S.: The EU regulation can be reviewed here, and states in Annex IX “Essential requirements for unmanned aircraft”, right in 1.3.: “… to mitigate risks … arising from the operation, the unmanned aircraft must have the corresponding and specific features and functionalities … According to the needs those features and functionalities must ensure easy identification of the aircraft and of the nature and purpose of the operation;…”