General GPS Question

[Matthias Camenzind]

I thought right now near same like you. I thought about the speed of satellites,
But also right now i had another idea. With the signal form two satellites, and a  
known high over sea it should be possible to locate you on north or south of
earth (only two possibilities).
This would be nice if you go on a mountain and you are standing in front of a
"signpost" (google translated) with meters over Sea value to get faster your first fix. A simple
question "Are you in the North of earth?" would be enough to get only one possibility.

BTW: Agreed with Abdelrazak Younes (Mail entered while writing). 
----------------------------------------
> Date: Thu, 30 Oct 2008 18:24:26 +0000
> From: taw27 at cam.ac.uk
> To: community at lists.openmoko.org
> Subject: Re: General GPS Question
> 
> On Thu, 30 Oct 2008 18:09:47 +0000
> Matthias Camenzind  wrote:
> 
>> The satellites are moving around. So on a different time the position
>> is different.
> 
> There is a number called the "dilution of precision" which quantifies
> the factor by which the accuracy of the GPS reading is reduced by the
> positions of the satellites.  For instance, if a reading were from three
> satellites all positioned close to the zenith, the accuracy would be
> reduced compared to how it would be if they were widely spaced apart.
> 
> To see how this works, visualise how the GPS correlator calculates your
> position by the intersection of spheres centered on the satellites, and
> consider how this would be affected by an inexact measurement of the
> radii of the spheres.  Then consider how that would in turn be affected
> by the positions of the satellites.
> 
> While in theory your suggestion would work, the geometrical dilution of
> precision would be HUGE, and so the measurement would overall be very
> inaccurate.
> 
> Tom
> 
> -- 
> 
> Thomas White
> Department of Materials Science and Metallurgy
> Electron Microscopy Group (PhD Student)
> University of Cambridge / Downing College
> 
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