On 12 Feb, 14:10, "John Rowland"
wrote:
It doesn't give height, though. Is it hard for GPS to be used to determine
height above sea level?

A GPS receiver works by narrowing down your position in three-
dimensional space, so figuring out your altitude and figuring out your
location are inseparable.

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

Mr Thant wrote:
On 12 Feb, 14:10, "John Rowland"
wrote:
It doesn't give height, though. Is it hard for GPS to be used to
determine height above sea level?

A GPS receiver works by narrowing down your position in three-
dimensional space, so figuring out your altitude and figuring out your
location are inseparable.

If only three satellites are visible, the locus of possible locations is a
straight line in space which intersects the earth's surface at two points.
Knowing which satellites are visible enables you to eliminate one of these
points, but it doesn't give you the height. Even if quite a few satellites
are visible which were all roughly in a plane, there would be low precision
on the height, and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

On 13 Feb, 00:21, "John Rowland"
wrote:
If only three satellites are visible, the locus of possible locations is a
straight line in space which intersects the earth's surface at two points.
Knowing which satellites are visible enables you to eliminate one of these
points, but it doesn't give you the height.

Unless the line is vertical you need to know (or guess, I suppose) the
altitude to provide an accurate lat/long.

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

In uk.transport.london message ,
Wed, 13 Feb 2008 00:21:44, John Rowland
n.co.uk posted:
and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

Not so. Consider two satellites at the same height above a flat earth,
for high and low satellites, and emitting pulses simultaneously.


S1 S2









s1 s2
_____JR______________ - You X -


The delay between hearing S1 & S2 is clearly less than that between
hearing s1 & s2. JR can therefore tell, if the X-positions of the
satellites are known, how far below the satellite line he is.

--
(c) John Stockton, nr London UK.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Correct = 4-line sig. separator as above, a line precisely "-- " (SoRFC1036)
Do not Mail News to me. Before a reply, quote with "" or " " (SoRFC1036)

On Thu, 14 Feb 2008, Dr J R Stockton wrote:

In uk.transport.london message ,
Wed, 13 Feb 2008 00:21:44, John Rowland
n.co.uk posted:
and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

Not so. Consider two satellites at the same height above a flat earth,
for high and low satellites, and emitting pulses simultaneously.


S1 S2









s1 s2
_____JR______________ - You X -


The delay between hearing S1 & S2 is clearly less than that between
hearing s1 & s2. JR can therefore tell, if the X-positions of the
satellites are known, how far below the satellite line he is.

True. Although now John doesn't have any reason to shout "I want these
motherf****ing satellites OFF the motherf****ing plane!", which is a
shame.

Am i right in thinking that you couldn't calculate height if the
satellites were all equidistant from you? But then you wouldn't be able to
calculate position at all. Is there a configuration where you can get a
fix in XY but not Z?

tom

--
Work alone does not suffice: the efforts must be intelligent. -- Charles
B. Rogers

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.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Proper = 4-line sig. separator as above, a line exactly "-- " (SonOfRFC1036)
Do not Mail News to me. Before a reply, quote with "" or " " (SonOfRFC1036)