APRS has standardized an ID series for amateur radio Oscar spacecraft -APOxxx
At the request of Juan Carlos, LU9DO, AMSAT-LU wanted a series of APRS designators for uniquely identifying AMSAT APRS applications. He suggested those beginning with the letter O for OSCARS.
ALL APRS applications include this identifier in their packets so that the source of APRS data can be known.
Full List - http://aprs.org/aprs11/tocalls.txt