General GPS Question

Thomas White taw27 at
Thu Oct 30 19:24:26 CET 2008

On Thu, 30 Oct 2008 18:09:47 +0000
Matthias Camenzind <m.camenzind at> 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



Thomas White
Department of Materials Science and Metallurgy
Electron Microscopy Group (PhD Student)
University of Cambridge / Downing College

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