Originally Posted by
underfire
After all of that, the ac systems uses a lookup feature from the internal database for altitude. Depending on the manufacturer, that methodology is vastly different. That is then compared with the rad alt and baro data, and viola, that is your altitude as reported. (ohhh the algorithms) You want it to figure out the descent profile, in a turn, (a 3d spiral) using a lookup feature to an internal database with an 8 second delay? oh yea.
ADS-B almost always sends baro altitude not GPS height, so none of the above is relevant. Once you're above the transition altitude, what it sends will be the same, give or take the 25 feet, as you are seeing on your altimeter.
No satellites or databases involved, just a bit of aneroid magic.
You can easily verify that if you visit your local airport with an ADS-B receiver and watch aircraft taking off. You will see the altitude readout changing within a second or so of the wheels leaving the runway.
Not that any of this is relevant to the WestJet - we know the data that it sent, we know the QNH correction factor, so unless you're suggesting that the positional and altitude data were out-of-sync with each other then we can be pretty confident that the flightpath plots we've seen represent what actually happened.