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What Is GPS? |
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Written by Mark Hamilton
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What is GPS? GPS stands for Global Positioning System. It is a satellite radio navigation system built by the United States Department of Defense. Research on the system began in the 60’s and the first GPS satellite was launched in 1978. It took a while for the entire system to come online but finally on April 27, 1995 it became fully operational. At first, the GPS network was only for military use but in the 80's the United States government authorized the general public to make use of it too.
The GPS System is composed of:
- Satellites
- Earth Monitoring Stations
- GPS Receivers
What Is a GPS Satellite?
There are currently 24 satellites orbiting the earth in 6 distinct trajectories. Each satellite completes two orbits around the earth each day, traveling at a speed of about 7,000 MPH. At the highest point in their orbit they are more than 12,000 miles above the earth. From this height, the satellite signals can be received over a large percentage of the surface of the earth. A GPS unit works by receiving multiple satellite signals and calculating the receiver's position based on triangulation of the different signals. Accuracy is increased with each additional satellite signal that is received so the orbits of the satellites are set so that at most points on the earth there are at least 4 signals that can be received. Of course, this is in theory only since the landscape around you can block some or all of the GPS signals.
What Is a GPS Earth Monitoring Station?
There are four unmanned monitoring stations in the US and one manned
master station which is located in Colorado Springs, Colorado. The four
unmanned stations relay position data from the satellites to the master
station. The master station uses the relayed information to correct the
position of the sat tiles and sends the corrected information back to
the satellites. This synchronizes the data that each satellite
transmits to your GPS receiver and ensures that your receiver is always
working with accurate information.
What Is a GPS Receiver?
GPS satellites transmit low power radio signals that can only be picked
up if your GPS unit has a direct "line of sight" to the satellite. The
signals can penetrate things like glass, plastic and clouds but not
solid structures like buildings or rocks. A GPS unit works by
triangulating your location based on the position of at least three
satellites in the GPS system. It compares the time it takes to receive
a signal from each of the satellites in range and calculates your
location based on the data.
If you are only able to pick up three satellite signals, your GPS unit
can report your location in latitude and longitude. However, when the
GPS unit picks up signals from at least four satellites it also
calculates your altitude. Now it can report your trip distance,
distance to a destination, travel speed and even your altitude if your
GPS receiver features an altimeter.
How accurate a GPS unit is depends on its ability to find and lock onto
a satellite’s signal. As you might expect, you get what you pay for and
the more expensive GPS units are generally more accurate. The best
consumer units claim accuracy to within a few feet while the older or
less expensive units can be off up to 30-40 feet or more. Where you
plan to use your GPS receiver could determine if it's worth shelling
out the extra money for increased accuracy. If you're using your GPS
unit in flatter terrain with a good open view of the sky a less
expensive unit could perform nearly as well as the top of the line
receiver. However, if you plan to use your receiver in dense tree
cover, while hiking in ravines or narrow valleys, or among tall
buildings you'll gain much better accuracy with a more higher end GPS
unit.
Now you know the basics about how GPS works and can answer the question "What is GPS?".
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