Originally Posted by
chimbu warrior
I have never heard of "inertially-driven" ADS-B. My understanding (albeit that of a layman) is that it requires GPS to "broadcast" the signal to ground stations. There certainly are older aircraft that are satisfying ADS-B requirements, but I understood they had to be retrofitted with GPS.
No, you've got it back-to-front.
it requires GPS to "broadcast" the signal to ground stations
Broadcasting the data to ground stations is done via the 1090 MHz Mode S ATC transponder, GPS isn't involved in the transmission (though there are moves afoot to capture the transmissions from aircraft via satellite, but again that's satcomms, not GPS).
Where GPS is involved (in most, but not all cases - see below) is in deriving the navigational information that's then sent via ADS-B (position, groundspeed, etc).
There certainly are older aircraft that are satisfying ADS-B requirements, but I understood they had to be retrofitted with GPS.
GPS, however, is not a prerequisite for ADS-B.
As I mentioned in my previous post, there are plenty of older aircraft flying that don't have GPS, but do have ADS-B. They derive their navigational data in the same way as it's been done for the last 50 years or so - from inertial navigation systems (INS) and ground navaids. Wikipedia has a good explanation of how INS works.
Obviously that's less accurate than data derived from GPS (INS is prone to drift as a flight progresses) and so the data sent is accompanied by a parameter that indicates its likely degree of accuracy.
It's these older aircraft that won't satisfy the requirements of the FAA's NextGen, for example, as their ADS-B data can't be used for providing separation in a terminal environment. Given the age of the types involved, it's unlikely that many of them will be upgraded to GPS-powered ADS-B.