In uk.transport.london message Pine.LNX.4.64.0802150035150.18870@urchin
..earth.li, Fri, 15 Feb 2008 00:39:03, Tom Anderson
posted:

Am i right in thinking that you couldn't calculate height if the
satellites were all equidistant from you?
Yes, if they are co-planar; not, I think, otherwise.

But then you wouldn't be able to calculate position at all.
No. For example, if more than two satellites are equidistant from you
and randomly placed the surface of a sphere, then you must be at the
centre of the sphere.

Is there a configuration where you can get a fix in XY but not Z?



S1 .. S2








___________________TA______________ - You X -






If the satellites are co-planar, you cannot determine the sign of your
height measured from that plane; and if you are either very near to or
very far from that plane your height measurement becomes inaccurate.

In terrestrial use : At sea, the altitude is already known to within a
few metres. On land, the altitude is already known to within about
+-5km. In the air, the altitude is already known to within about
+-15km. In those cases, the usable satellites are always above a
horizontal plane through the receiver.

AIUI, commercially-available GPS receivers will refuse to give an answer
if above some specific altitude; that is to annoy D-I-Y ICBM makers.
There could also be a speed limit.

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