Comment Re:Altitude? Not so fast (Score 1) 290
Actually, GPS altitude data is in reality not so hot.
The triangulation calc that's carried out to work out positions doesn't extend well to produce altitude info, due to the likely positions of the satellites. To use GPS terminology, VDOP >> HDOP. Then you have the fact that GPS altitudes are actually altitudes relative to a datum Geoid, rather than the actual Sea Level height... so they themselves are an approximation.
For barometric altitudes, the big problem is the drift in the underlying barometric pressure due to weather systems as a plane flies along it's flightplan, but these are generally cancelled out by resetting the altimeter regularly to relayed ground readings when moving from one flight information region to another.
Barometric altitudes are generally more accurate by perhaps a factor of 10 than GPS altitudes (error of 30ft vs 300ft) but the drift in barometric readings due to underlying pressure changes is in itself a problem which if uncorrected can easily result in errors as large as 1000ft (30ft per mb).
So both systems have their own difficulties.
Regards,
Mike
The triangulation calc that's carried out to work out positions doesn't extend well to produce altitude info, due to the likely positions of the satellites. To use GPS terminology, VDOP >> HDOP. Then you have the fact that GPS altitudes are actually altitudes relative to a datum Geoid, rather than the actual Sea Level height... so they themselves are an approximation.
For barometric altitudes, the big problem is the drift in the underlying barometric pressure due to weather systems as a plane flies along it's flightplan, but these are generally cancelled out by resetting the altimeter regularly to relayed ground readings when moving from one flight information region to another.
Barometric altitudes are generally more accurate by perhaps a factor of 10 than GPS altitudes (error of 30ft vs 300ft) but the drift in barometric readings due to underlying pressure changes is in itself a problem which if uncorrected can easily result in errors as large as 1000ft (30ft per mb).
So both systems have their own difficulties.
Regards,
Mike
