And if the GPS receivers of two planes say they are separated by 1000 feet of altitude?
Then they are most probably separated by 1000ft. But to this date, nobody can (or wants to) guarantee that GPS will be awailable one second from now, contrary to the atmosphere which will be here until our sun evolves into a red giant.
Jan_Olieslagers wrote:
And if the GPS receivers of two planes say they are separated by 1000 feet of altitude?
If they are both using the same satellites, they would be, but there is no way to guarantee that.
I don’t for a moment think that altitude accuracy is the reason we aren’t using GPS altitude… it is all the other reasons, starting with simplicity.
1000ft is about 50 times more than the altitude accuracy of even a crappy GPS, assuming geoid correction is done (as in all IFR certified units).
Airborne_Again wrote:
they are both using the same satellites, they would be, but there is no way to guarantee that.
That’s like saying “the reported latitude and longitude positions depend on the satellites used”.
what_next wrote:
But to this date, nobody can (or wants to) guarantee that GPS will be awailable one second from now, contrary to the atmosphere which will be here until our sun evolves into a red giant.
Except the same reasoning isn’t applied to lateral navigation which is done more and more GPS/GPS (or GPS/INS for those who afford it). So if we trust it one second from now to separate horizontally, why don’t we trust it one second for now for vertical separation?
Jan_Olieslagers wrote:
And if the GPS receivers of two planes say they are separated by 1000 feet of altitude?
Likely, but if one user reports his altitude based on a barimetric altitude (say a flight level, so that it’s allowed to not to coincide at all with real altitude) and the other one from a GPS, then you might end up with something very different than 1000ft
Shorrick_Mk2 wrote:
That’s like saying “the reported latitude and longitude positions depend on the satellites used”.
But they do! The satellite geometry affects the precision.
Not much though, in this context. Even a crappy old GPS like a KLN94 returns an altitude within 10-30ft.
Yesterday I was given a regional pressure setting of 1021. That was 20nm of Shoreham whose QNH was 1028. That is “only” 210ft out… OK; we all know the RPS is a load of crap and nobody else in the world uses it, but really to pretend that barometric altimetry is somehow specially accurate is like pretending the proverbial king is wearing any clothes. Baro altimetry is used for other reasons, and always will be.
Jan_Olieslagers wrote:
why do we still rely on barometric altimeters at all, in this era of GPS?
I guess a part of it is legacy. It’s legal to fly without GPS and mixing those two isn’t trivial.
What’s wrong with using barometric altitude with a standard pressure setting? It’s cheap, reliable, everybody got it and it works perfectly fine.
In Europe we generally have a low transition altitude which is very sensible because we use barometric altitude for separation where relative altitude counts — not to know how far above the ground we are in cruise flight. GPS is much better for that.
I think there is just no reason to consider any alternatives to barometric altitude — nothing to be gained!
