Geometry of the Sphere - Spatial Mathematics: Theory and Practice through Mapping - page 16

Figure 1.13 A general satellite configuration of six orbits, four satellite slots in each
orbit. Source of image: NOAA, http://www.gps.gov/systems/gps/space/ . Link, using
the QR code, to an animation to see how satellite configurations change. Source
of link embedded in QR code: Public domain, released by author El pak, 2007.
http://en.wikipedia.org/wiki/File:ConstellationGPS.gif
satellite. If two satellites are sensed, then the GPS receiver knows it is at one of
two points where the two spheres intersect. If three satellites are sensed, then
the GPS receiver can determine the position on the ground typically within
10 meters of horizontal accuracy. If four or more satellites are sensed, a recre-
ational GPS receiver can determine a position within three or four meters, and
with mathematical differential correction, a high end professional receiver can
pinpoint the user's location to within centimeters. Vertical accuracy is usually
an order of magnitude coarser than the horizontal accuracy, so if the horizon-
tal accuracy is within three meters, the vertical accuracy in terms of the eleva-
tion of the user may only be within 30 meters of the true elevation.
Three satellites might seem enough to solve for position since space has three
dimensions and a position near the Earth's surface can be assumed. However,
even a very small clock error multiplied by the very large speed of light results
in a large positional error. Furthermore, GPS signals do not travel through
most solid objects, and therefore, accuracy will suffer if the user is standing
in heavy tree cover or in or near tall buildings. In smartphones, most of which
have GPS capability, the position from GPS is enhanced by triangulation off of