How does GPS Work?

What is GPS?

The Global Positioning System (GPS) is a network of 31 satellites orbiting the Earth at an altitude of 20,000 km. Of these 31, at least 24 are always in operation. Although initially developed by the US Govt for military navigation, nowadays, it is a commonly used navigation system worldwide. All that is needed, is a GPS device, be it a SatNav, a mobile phone or a hand-held GPS unit.

Global Positioning System : Working Principle

• The basic principle is transmission of radio signals with satellites. Let’s see what exactly it means. To determine your location using GPS, your device needs to determine its exact distance from 4 GPS satellites, and their exact location when they send a scheduled signal.
• GPS satellites are line of sight instruments. Hence, wherever you are on the planet, at least four GPS satellites are ‘visible’ at any time. Each one constantly transmits information about its exact position and the current time at regular intervals.
• The receiver in your device can identify individual satellites, knows the schedule for sending these signals, and can determine how far away each satellite is based on how long it took for the messages to arrive. Since the signals travel at the speed of light, that time delay can be converted to distance.
• Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration.
• Let’s see what trilateration means-  Suppose you are somewhere on the Earth with three satellites in the sky above you. If you know how far away you are from satellite A, then you know you must be located somewhere on the ‘A’ circle.
• Doing the same for satellites B and C gives you an area where the three circles intersect. This is where you are. Your GPS receiver does the same thing, using intersecting spheres instead of circles. So the greater the number of ‘visible’ satellites above the horizon, the greater the accuracy with which your GPS device can pin point where you are.

Facts about GPS Satellites

The GPS Satellite system is made up of 31 satellites that constantly orbit the earth at an altitude of 20,194 km. Each satellite completes two revolutions around the earth in one day, following a precise orbit. The earth completes one spin in that duration, so in effect, each satellites completes one sweep of the earth in one day.

Although the GPS system requires precise time reference, leap seconds and other time anomalies cause a lag of up to 14 seconds. GPS satellites last for 10 years, after which they are replaced by a new one. The defunct satellites are then sent into graveyard orbit.

GPS is sure is amazing isn’t it?

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