Thats a useful reference. Thankyou
It does still raise the question, why is SIL = Null and SDA = Null is not treated the same as SIL = 0 and SDA = 0. This would make both older commercial traffic and older GA ADSB useable with no modification.
It is interesting to see that in the table on P3 of the document it says that SIL and SDA are "Required" with values of >= 2 and >=3 in Europe.
I think the early adopters of Mode S, whilst being able to Squitter, are now being penalised because their Txpdrs do not assign the SIL and SDA field without an expensive upgrade.
What pressure can be bought on Funke to modify their early products to be compatible with the requirements, I wonder.
John