Global Positioning System (GPS)
The Global Positioning System (GPS) is a network of 24 satellites
orbiting earth at 7,000 miles an hour. The satellites were launched and
are maintained by United States Department of Defense for the benefit of
military applications. The Satellites orbit the earth twice a day at about
12,000 miles above the planet. Considering there are no obstructions, no
matter where you are on the planet earth there are at least 4 GPS satellite
signals visible to GPS receivers at all times making it accurate and a huge
benefit for people who enjoy the outdoors.
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The idea of GPS originated in the 1960’s, but the first satellite was not
put into orbit until 1978. The 24th satellite was launched in 1994
completing the mission and enabling the military to calculate positioning
around the world extremely accurately. The Satellites are designed to last
about 10 years, so there isconstant upkeep and launching of new satellites
regularly.
How Does GPS Work?
The Global Positioning System transmits signals from the
satellites down to earth. A GPS receiver then picks up that transmission
and analyzes the data that was sent. Mostly, the GPS receiver gathers the
exact position of the satellite and determines the time it took for the
signal to reach the receiver. The signal travels at the speed of light
(186,282 miles per second) so we are talking about a very short amount
of time. With the distance measurements of several different satellite
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signals, the GPS receiver then uses the mathematical concept of
triangulation to compute the exact location of the user.
To accurately determine 2D information (longitude and latitude) a GPS
receiver needs to be receiving signals from at least 3 satellites. To
determine 3D information, the receiver needs to be picking up signals
from 4 or more satellites which will enable it to determine more
information such as exact location, elevation, altitude, distance traveled,
etc.
the GPS receiver to "see" a satellite, the satellite must be above the
horizon and unobstructed by buildings, mountains, etc. At any given
moment at any point on the planet there are between 6 and 9 satellites
above the horizon. During the process of acquiring the satellites, the GPS
display will look something like this:
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On this screen, the larger circle represents the horizon and smaller circle
represents 45 degrees. The dot in the center is straight overhead. The
numbers within the circles represent satellites that are visible, and the bar
chart on the right represents the relative strength of the signals from the
different satellites. Once the GPS receiver has locked on to 3 satellites, it
can display your longitude and latitude to about 100 foot accuracy. If the
receiver can see 4 satellites it can also tell you your altitude. With this
information you know exactly where you are.
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Most modern GPS receivers are able to store your track. As you move,
the GPS periodically stores your position in its internal memory. It can
then show you the path you have followed on the display so that you can
see exactly where you have been. Tacks also make backtracking
Most modern GPS receivers also support the concept of waypoints and
routes. A waypoint is a specific point (longitude and latitude) that you
have stored in memory. A route is a series of waypoints connected
together to form a path from one point to another. A hiker might store
different landmarks or resting points along a trail as waypoints. You can
then string a collection of waypoints together into a route. At the start of a
trip you tell the GPS receiver which route you want to follow. The
receiver will then tell you the heading you need to take to get to the next
waypoint in the route. As you pass each waypoint, the GPS receiver gives
you the heading to the next waypoint on the route. Some of the newer
receivers, like the GPS III shown above, have road maps stored in
memory. The unit is therefore able to show you exactly where you are on
a map of the area. Internal maps usually show major highways.
