What is GPS?

  What is GPS?

  In 1978 the first Global Positioning System (GPS) satellite was launched into space by the U.S. Department   of Defense as a part of a satellite navigational system created, funded, and operated as a tool for the U.S.   military. Although begun under the direction of the military, the GPS program was opened to civilian use not   long after inception. Civilian use of the GPS system has grown so much so that recent figures estimate   civilian GPS receivers outnumber military units about 10 to 1!

  The GPS system is comprised of three basic parts, the ground based tracking and control stations, the   network of GPS satellites orbiting the earth, and the GPS receivers that interpret the satellite data. Each of   these components works together in precise unison to create a hyper-accurate means of plotting a location   anywhere on the planet. Sophisticated processors and surface map overlays built into GPS receivers visually   interpret the satellite data for use in navigation or tracking applications.

  The U.S. military employs a network of tracking stations located around the world that monitor signals from   the GPS satellites and track the precise orbit of each satellite. Although the orbit of a satellite is very   predictable, forces such as the gravitational field of the moon, and solar radiation can have a slight affect on   the speed and path of the satellite. These forces create minute deviations from a satellite's planned orbit   and are called “ephemeris” errors. The ephemeris errors are processed by the GPS Master Control station in   Colorado, which then uploads minute corrections to the GPS data and atomic clocks of each satellite for
  re-transmission to GPS receivers.

  Navigation is made possible by the constellation of 24 GPS satellites orbiting the planet every 12 hours from   a distance of about 12,000 miles, and transmitting radio signals from space that allow earth-based GPS   receivers to calculate their X/Y/Z axis position, relative speed, and time. Each GPS satellite is equipped with 4   atomic clocks that keep pace with the atomic Universal Time clock on earth in order to accurately record the   length of time it takes a signal to reach the earth from space. This measurement is particularly important   since the length of time it takes a signal to reach the earth is the key element in calculating the satellite's   distance from earth. In addition to transmitting the code used for tracking the satellite (known as
  “pseudo-random code”) the satellites also send GPS time and ephemeris data used to update the receiver's   internal almanac, which precisely details where each satellite should be at any given time.

  From any point on the planet, there are normally 5 to 8 satellites visible to an earth-bound GPS receiver   allowing it to pinpoint its three dimensional position in space. The receiver determines accurate location by   locking onto the signals of at least four satellites, reading their clock data, and measuring the distance to   each. Normally, a point in space can be defined by measuring its distance from three known points, in a   process known as “triangulation.” In theory, using three satellites as reference points and calculating the   amount of time it takes their signals to reach the receiver then dividing by the speed of light (rate at which   radio signals travel, equivalent to about 186,000 miles per second) should yield the exact distance from each   of the three points, thereby pinpointing the location of the receiver. However, forces of the earth's   atmosphere (which is constantly changing) slightly delay signals sent by the satellites, causing an error in   distance calculations. Since a delay of only one-thousandth of second can mean a distance error of almost   200 miles, an accurate plot cannot be obtained using the slightly inaccurate data from the three satellites,   and the information from a fourth satellite is required. The information from the fourth satellite enables the   use of a simple error correction technique that yields a corrected signal-timing model for each satellite and   ensures distance accuracy. Tracking the GPS time information from several satellites also allows the receiver   to update its own clock and record time with atomic accuracy, without having an expensive on-board atomic   clock.

  Each satellite broadcasts in data on two different frequencies that penetrate the Earth's atmosphere at   different speeds, allowing a more sophisticated means of measuring distance between the satellite and   receiver by comparing signal arrival times. The processed dual frequency GPS data is so accurate, that with   the use of specialized military equipment, physical position can actually be determined to within a   centimeter. However, for National Security reasons the Department of Defense reserves access to the second   GPS carrier frequency for military use only. The primary GPS frequency still provides civilian receivers accuracy   inside of 100 feet, or better.

  GPS technology is continually growing, and is now used worldwide for everything from commercial   navigation, to agriculture. GPS technology applications are becoming so diverse that it is even used in the   banking industry as a Universal Coordinated Time (UTC) reference in coordination of transactions across the   globe. In just a short period of time, GPS has completely changed the way we measure distance, locate   objects, and navigate the planet.