APXP is short for “APEX Protocol” which itself is short for “Automatic Packet EXchange Protocol”, usually refered to as simply “APEX” for short. It is a new protocol designed to be a next-generation replacement for APRS, as well as completely interoperable with the current APRS network.
APEX defines both a new protocol and a new paradigm. Since much of the existing APRS network will not support APXP extensions we also have developed a Ruby based reference implementation, called Digipex, which runs with most existing KISS based TNC as well as DireWolf. Since the APXP definition is currently a work in progress we encourage contributors to work with this reference implementation until a first release of the protocol is made and software can be designed to follow the finalized new specificiation.
The Digipex reference implementation can be found here: Digipex Git page.