||When the receiver has determined the distance to four
satellites, it is able to calculate it’s three-dimension
position of latitude, longitude and elevation by the
principle of trilateration. When the GPS receiver
determines the distance to one satellite, it “knows”
that it must be located on the surface of a sphere
with the satellite at it’s center and the distance
as the radius.
When it determines the
distance to another satellite, it’s possible locations
are narrowed down as it can only be located at the
points where the spheres intersect. When three distances
are known along with their corresponding spheres, only
two points of intersection are possible – and one is far out
in space and can be discarded. The other point of
intersection is the location of the GPS receiver, which
has now achieved a 2D position fix! To achieve a 3D
position fix, four visible satellites are required.
GPS has a wide range of uses. GPS receivers are
useful for hikers and outdoor enthusiasts. Aircraft and
boats use GPS as navigation aids. New cars often come
equipped with a GPS-based navigation system to assist
motorists in unfamiliar areas. GPS can even be a game -
GeoCaching is a worldwide 'treasure hunt' in which
people set up small 'caches' and then post the
coordinates of the cache to a central website. Other
people then download the coordinates to their GPS
receiver and try to find the cache and the 'treasure'
inside. Cellular phones of the future will incorporate
GPS receivers so that emergency services can accurately
locate the owner in case of an emergency.
positioning is affected by several variables that can
lower its accuracy. These include
Ionospheric/tropospheric delays, multipath distortion,
ephemeris errors and timing errors.
Ionospheric and Tropospheric Delays
As the signal from the satellite passes through the
atmosphere, it slows down. Most GPS receivers attempt to
compensate for this with a built-in model, but it is
only partially possible without other correction methods
(more later in this piece).
Before the signal reaches the receiver, it might have
been reflected off tall buildings, rock faces and
Occasionally GPS satellites report an incorrect
The quartz clocks used in the GPS receivers are
not as accurate as the atomic clocks used onboard the
GPS satellites. Small errors can be introduced this way