Leica Geosystems GPS Basics - Manual (Page 5)

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GPS Basics -1.0.0en System Overview
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1. What is GPS and what does it do ?
GPS is the shortened form of NAVSTAR
GPS. This is an acronym for NAVigation
System with Time And Ranging Global
Positioning System.
GPS is a solution for one of man’s
longest and most troublesome prob-
lems. It provides an answer to the
question ‘Where on earth am I ?’
One can imagine that this is an easy
question to answer. You can easily
locate yourself by looking at objects that
surround you and position yourself
relative to them. But what if you have no
objects around you ? What if you are in
the middle of the desert or in the middle
of the ocean ? For many centuries, this
problem was solved by using the sun
and stars to navigate. Also, on land,
surveyors and explorers used familiar
reference points from which to base their
measurements or find their way.
These methods worked well within
certain boundaries. Sun and stars
cannot be seen when it is cloudy. Also,
even with the most precise measure-
ments position cannot be determined
very accurately.
After the second world war, it became
apparent to the U.S. Department of
Defense that a solution had to be found
to the problem of accurate, absolute
positioning. Several projects and
experiments ran during the next 25 years
or so, including Transit, Timation, Loran,
Decca etc. All of these projects allowed
positions to be determined but were
limited in accuracy or functionality.
At the beginning of the 1970s, a new
project was proposed – GPS. This
concept promised to fulfill all the require-
ments of the US government, namely
that one should be able to determine
ones position accurately, at any point on
the earth’s surface, at any time, in any
weather conditions.
GPS is a satellite-based system that
uses a constellation of 24 satellites to
give a user an accurate position. It is
important at this point to define ’accu-
rate’. To a hiker or soldier in the desert,
accurate means about 15m. To a ship in
coastal waters, accurate means 5m. To
a land surveyor, accurate means 1cm or
less. GPS can be used to achieve all of
these accuracies in all of these applica-
tions, the difference being the type of
GPS receiver used and the technique
employed.
GPS was originally designed for military
use at any time anywhere on the surface
of the earth. Soon after the original
proposals were made, it became clear
that civilians could also use GPS, and
not only for personal positioning (as was
intended for the military). The first two
major civilian applications to emerge
were marine navigation and surveying.
Nowadays applications range from in-
car navigation through truck fleet man-
agement to automation of construction
machinery.
What is GPS and what does it do ?
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