GPS Spotlight
 

Satellites, Controllers, Users

GPS is made up of three components: a constellation of satellites, controllers on the ground, and users.

Satellite Segment

GPS satellites have some special characteristics:

  • They are orbiting the Earth ~20,000 km above the Earth's surface. That means they orbit the Earth twice a day.
  • They transmit coded signals for both civilians and military users.
  • Originally GPS used two frequencies so that users could reduce the effects of the ionosphere. Recently GPS added a third frequency.
  • They carry their own clocks so that they can keep synchronized with their ground controllers.
  • The GPS signal also includes information about where the satellite is.

Although initially designed for 24 satellites, the Global Positioning System currently has more than 30 satellites. You can check the GPS constellation status by using this Coast Guard site.


Figure 1.This animation depicts the GPS constellation as viewed by a receiver (blue circle) at a latitude of 40 degrees. Satellites are small dots. Satellites that can be viewed by the receiver are black and connected with dashed green lines (Credit: Wikipedia.)

Figure 2.Official logo of GPS. (Credit: Wikipedia.)

Figure 3.A GPS satellite. The big rectangles are the solar panels needed to generate power for the system.


Control Segment

The U.S. Air Force keeps track of the GPS satellites once they're launched, and communicates with them in case there are any problems or updates. It also computes orbits and clock corrections for each satellite so that users can easily calculate their position. In addition to the master control station at Schriever Air Force Base in Colorado, the U.S. operates a global network of tracking stations.


Figure 4.The GPS control segment consists of a global network of ground facilities that track the GPS satellites, monitor their transmissions, perform analyses, and send commands and data to the constellation. The NGA is a U.S. government-sponsored mapping agency. The AFSCN is operated by the Air Force. (Credit: gps.gov)


Ground Segment

The third component of GPS, users, is sometimes called the ground segment. In order to access GPS, you need:

  • An antenna to receive the satellite signals.
  • A clock to time the satellite signals.
  • Electronics to process the signals.
  • A computer to calculate position.

GPS receivers come in all sizes and shapes. Scientists use the biggest and most expensive GPS receivers. They mount the antenna to bedrock so that it is very stable. Surveyors need almost the same accuracy as scientists, but they usually want the receiver and antenna to be portable. Environmental sensing such as snow depth is a new application for GPS and uses the same receiver as geophysicists. Farmers that use GPS in their tractors don't need to worry about portability, but they do require high accuracy. GPS receivers are routinely used by airplanes for navigation. Animal tracking requires very light GPS units.


 

Last modified: 2019-12-26  16:24:59  America/Denver  

 

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