Position accuracy

[XPMorten]

Hi all,

I've been tracking flights on the new SBS-1 system which takes
signals fron aircraft equiped with ADS-B.
More info;
http://www.kineticavionics.co.uk/modes.php

Anyway, when tracking, I've noticed that sometimes
the ARRIVING aircraft position does not match up with the map.
They can be up to 1-2 nm off.
The strange thing is that the DEPARTING acf always seem to be
exactly where you would expect.

Not sure if the ADS-B sends the real GPS position or
if it's IRS calculated position thats being transmitted.
Not being a pilot, I'm just guessing this could maybe be
the IRS system drifting over time? Or is this just a SBS "bug".
Any thoughts?

Cheers,

M

[FCS Explorer]

i actually didn't know the position could be/is encoded into the transponder signal.
anyway, most airliners will have more than one IRS plus one or more GPS-receivers. furthermore the position calculation can be backed up by automatic radio-nav tuning. in the end u get an "composed position", some kind of average from all the values. and the longer the flight, the more "off" this position will be. as long as there is GPS mixed in, it will be pretty good (about 0.02NM). IRS-only you might expect the 2-3NM you mentioned.