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 network is composed of:
Satellites Earth Monitoring Stations GPS receivers
What is a GPS Satellite?
There are currently 24 satellites orbiting the earth is 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 over 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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