I do not have a handle on the precise reasoning for both SIL=0 and SDA=0 being requirements for uncert GPS ADS-B Out. I am trying to find out. I understand that CAT TCAS will ignore traffic broadcasting SIL=0/SDA=0. I don't know whether CAT TCAS will respond differently to SIL=0/SDA=2 traffic broadcasts.
Steve,
I emailed Ade Price at NATS (who was administering volunteers for their uncertified GPS trial) and posed your question to him how airborne or ground based systems might treat aircraft differently that broadcast SIL=0 and an SDA =0 or an SDA=2?
He replied "It would probably make no difference to the ground receivers if you had SIL 0 and SDA 2 but there will be a difference to some airborne equipment. Aircraft Surveillance Applications (ASA) that are compliant with TSO-C195B will display a target with SDA 1 or higher but will not show a target with SDA 0.
This means the permissible SIL and SDA values for non-certified GPS sources prevent that data from being used on approved Cockpit Display of Traffic Information (CDTI) devices. I would not be surprised if you found this situation bizarre, particularly because EASA have approved the installation of PowerFLARM in AC23 type aircraft and PowerFLARM does not have a certified GPS source.
IMHO if you are not required to have a TSO-C195B compliant device installed in your aircraft, then you are free to use equipment that may not be approved to display ADS-B targets with non-certified GPS sources."
I'm not sure what CDTI devices are, and whether the definition includes TCAS. I googled "AC23 aircraft", and at this late hour, I was none wiser!
BTW, Ade also said it's a shame that Becker have not set the SDA to zero but perhaps they can be prevailed upon to change their data output. I have a feeling I've read somewhere that its pretty widespread practice across parts of mainland Europe (particularly amongst glider pilots) to connect uncertified GPS to their transponders. If this is true it could provide a basis, together with the NATS and LAA MOD 14 requirement, to request Becker to do just that! (Unless of course they have a vested interest in CDTI devices.)
Rgds,
Chris