What are the requirements on the GNSS source?
The requirements are described in AMC 20-24 and CS-ACNS.
- Position and velocity information must come from the same source.
- ETSO-C129A is the minimum equipage qualification in Europe.
- For compliance with CS-ACNS, the applicable ETSOs (C129a/196/145/146) alone does not guarantee that the unit is suitable as an ADS-B position source. Further demonstration is required (see in particular AMC1 ACNS.D.ADSB.070 and .080).
- The System Design Assurance (SDA) parameter can be set to 2 without analysis if the GPS and ADS-B unit are both qualified (ETSO’d) and directly connected (i.e. direct wiring from GPS to receiver with no routing through other equipment). Aircraft with other architectures require a system safety analysis to set the SDA.