what constitutes the nav msg of GPS?

[WeeJeem]

Originally Posted by A Squared

Without intending to advocate for GPS derived altitudes, b) shouldn't be much of a concern.

Nevertheless, b) should be a concern.

Since the six orbit planes have an inclination of about 55°, when one travels north of 55°N or south of 55°S one will not be able to have an SV in the overhead (the best position to minimise VDOP).

As one travels progressively further north or south respectively, the SV geometry will worsen progressively until when, you're over the pole itself, and the highest possible elevation angle to an SV is about 45°, and hence a great big 90° wide cone of nothing centred overhead you, just where you really wanted an SV.

Originally Posted by A Squared

Altitude errors which are a function of latitude would affect all aircraft at the same latitude. And you have to be at the same latitude to collide.

So, if you're happy with that, then presumably you're also quite happy to fly with barometric-based altimetry that could have the equivalent potential error of about ±150m error in its readout, regardless of the altitude you're flying at?

And, as you say, it would indeed "affect all aircraft at the same latitude"; so, presumably, you're also comfortable with all other aircraft in the vicinity potentially being a couple of hundred metres off between their intended and actual altitudes.

What's not to like? What could possible go wrong there?

[D driver]

thanks WeeJeem that clarified my doubt