The Global Positioning System (GPS) is a satellite-based Global Navigation Satellite System (GNSS) made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense (USDOD). The Global Positioning System is actually a constellation of 27 Earth orbiting satellites (24 in operation and three extra in case one fails). GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.


GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites “visible” in the sky. A GPS receiver’s job is to locate four or more of these satellites, figure out the distanc­e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called Trilateration.


GPS consists of three parts:

  • The Space segment,
  • The Control segment, and the
  • The User segment.


The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit.

The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations.

The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service.


Basic concept of GPS:

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include….

  • The time the message was transmitted
  • Precise orbital information (the ephemeris)
  • The general system health and rough orbits of all GPS satellites (the almanac).


The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites’ locations are used with the possible aid of “Trilateration”, depending on the algorithm is used, to compute the position of the receiver.

Three satellites might seem enough to solve for position because space has three dimensions and a position near the Earth’s surface can be assumed. However, even a very small clock error multiplied by the very large speed of light results in a large positional error. Therefore receivers use four or more satellites to solve for the receiver’s location and time. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation (altitude).



GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use Trilateration to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user’s 3D position (latitude, longitude and altitude). Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination etc.,


The GPS receivers can be used by attaching it to any vehicle we use may it be a car or a bike…

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