Hi guys,
we recently added support [1] for parsing SBS-1 messages from an ADS-B [2] receiver.
You can use cheap SDR soluitons together with hardware like a DVB-T stick, hackrf or rad1o to receive ADS-B and publish the information as "INTRUDERS" in paparazzi.
These external aircrafts will be displayed in the GCS and the information also sent to paparazzi aircrafts via ACINFO (for e.g. TCAS).