Incorrect ADS-B Position Reports

FlightAware ADS-B Flight Tracking
1

Can anyone explain what causes this:

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Clearly this UPS aircraft did not land on top of the terminals. It did indeed land on the runway a couple hundred feet to the south of that path. And once on the ground, it continued to report a position that was offset a bit and was a bit “sketchy”. The real taxiway he exited on is actually off that image to the right.

This happens to about 0.5-1% of the flights I see.

And it’s not MLAT (in my config, the lat/lon in the selected box would be italicized if this were an MLAT position).

2

Semi-constant position offsets like this usually mean that the ADS-B position reported is being derived from an inertial system (which drifts over time) not GPS

3

That was my guess. But I don’t understand why an ADS-B unit would be set to rely on an IRS over GPS. I thought in today’s world IRS was the backup.

4

There’s probably a switch somewhere to switch between the two but as ADS-B isn’t really used much it probably doesn’t matter where it is set.