Originally Posted by
slats11
An interesting question. What altitude would you like tracked? FL as per altimeter? GPS derived altitude (when available)? Both?
Imagine if we had both and there was a sudden substantial discrepancy. Imagine how useful it would be in a case when it was going to takie time to get the recorders (deep mid pcean).
As an aside, how often is GPS coverage sufficient to permit altitude to be calculated.
This is an interesting question.
GPS provides a 3D position, it is just that only the 2D position is used normally.
GPS is used (as one would expect) by GPS Landing Systems that provide the vertical as well as lateral guidance. These are made more accurate by ground based augmentation systems (GBAS) but the accuracy is of the order of 8 meters in all directions without GBAS but with wide area augmentation systems (WAAS).
If everyone flew on GPS altitude then the huge (and they are worryingly huge) number of errors caused by mis-set altimeters would go away.
Currently aircraft in cruise follow a pressure level based on 29.92 / 1013 which means as they fly into a low pressure area they descend and in high pressure areas they climb. Crossing some fronts will mean a descent followed by a climb. However, this is keeping them at the same pressure level so might be better for engine economy but that may be lost if the engines reduce thrust in the decent toward a front then increase thrust to climb away from the front.
Some aircraft are using GPS to provide backup speed information by continually cross referring GPS calculated ground speed to airspeed so if the airspeed indicators drop out the GPS system can provide a reasonable airspeed figure.
So expect to see satellite based altitude and speed information continue to creep into aircraft systems.