I have learned today from an authoritative source that ADS-B messages tagged as:
- Version 0 do not support/include SIL or SDA data.
- Version 1 do not support/include SDA data but do support/include SIL
- Version 2 does support/include SIL and SDA data, AND the definition of SIL was changed between Version 1 and Version 2.
Unfortunately, I currently have no details of the version 1 to version 2 change in the definition of SIL.
Information on the ADS-B spec I have seen mentions broadcasting SIL & SDA values 'within 10 seconds of them changing'. This caters for airliners that may have multiple GPS devices that can be switched between for feeding GPS position data to their transponder. As such, even CAT traffic setup to support SIL & SDA for ADS-B may not actually broadcast SIL or SDA very often. How TCAS is supposed to be able to rely on SIL or SDA is therefore rather unclear to me. By comparison, the latest firmware for Trig transponders for GA have been coded to broadcast SIL & SDA every 2.5 seconds when in 'Air' mode and every 5 seconds when in 'Ground' mode.
In case anyone is wondering about the voracity of the new PAW Traffic display, please bear the above in mind.
Steve