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What Is GPS?
GPS or Global Positioning System is a satellite-based navigation system made up of a network of approximately 18-24 satellites which are placed in orbit. These satellites are constantly orbiting the earth using small rocket boosters to keep them at the correct path. They make two complete orbits in a day 12,000 miles above us and at speeds of roughly 7,500 miles an hour. They are powered by solar energy only and they have some backup batteries on board to keep them running in the event of a solar eclipse. GPS does not cost anything and works regardless of the weather conditions.

GPS satellites transmit signal information to earth. The GPS receiver receives this information and compares the time the signal was transmitted with the time it was received. This time difference determines the distance of the satellite from the location of the GPS. By calculating the distance from a few of the other satellites, the GPS receiver can determine the location of the user and display the information on the unit’s electronic map.

In order to calculate a 2d position (latitude, longitude) a GPS receiver needs to lock on to the signal of three satellites. For the calculation of a 3d position (latitude, longitude, altitude) the receiver needs four or more satellites. When the GPS determines the position of the user it can also calculate:
  • Speed
  • Trip distance
  • Sunrise/Sunset time
  • Track
  • Distance to destination
  • Other information
GPS receivers are extremely accurate nowadays due to parallel multi-channel design. Garmin GPS receivers use 12 parallel channels and they lock quickly onto satellites as soon as they are turned on. They maintain strong locks no matter where the user is positioned. Certain atmospheric factors and other sources of error can affect their accuracy. They are accurate to 15 metres on average. Newer Garmin GPS receivers are equipped with WAAS (Wide Area Augmentation System) capability which can improve the accuracy to less than three meters on average. Users can also get even better accuracy with Differential GPS (DGPS) which correct the GPS signals within an average of three to five meters.

The following are possible sources of GPS signal errors:
  • Orbital errors: These are inaccuracies of the satellite's reported location and they are known as ephemeris errors.
  • Receiver clock errors: A GPS receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites.
  • Ionosphere and troposphere delays: The satellite signal slows as it passes through the atmosphere.
  • Satellite geometry/shading: When the satellites are located at wide angles relative to each other, the satellite geometry is ideal. When they are located in a line, it is considered to be poor.
  • Signal multipath: It is likely to occur when the GPS signal is reflected off objects such as tall buildings.
  • Number of satellites visible: The more satellites a GPS receiver sees the more accurate it is.
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