GPS: Explained

Have you ever wondered how your smartphone can tell your exact position on the globe? The answer is GPS, a space-based satellite navigation system that works in all weather conditions.
In 1973 US began the project to overcome the limitation of previous navigation systems, integrating ideas from several predecessors, including a number of engineering design studies from 1960s. The first system used 24 satellites and it became fully operational in 1995.
Advances in technology and new demands led to efforts to modernize The System, and the implementation of IIIA satellites.
The whole concept is based on time. The satellites carry very accurate atomic clocks that are synchronised to each other and to ground clocks. Any drift from true time maintained on the ground is corrected daily. A satellite continuously transmits its time and position. A GPS receiver monitors multiple satellites and solves equations to determine the exact position and its deviation from true time. At a minimum 3 satellites need to be in range for a latitude and longitude coordinates. An additional satellite is needed for altitude calculation.
The system was not all the time opened for casual users. It all started as an army technology, and it took some time and Soviet Union to shoot down a civilian aircraft that accidentally entered it`s air space to let the slightly tore down version of the GPS to be.
The problem with GPS is that you need a direct sight to the the 4 satellites. This is almost impossible in big cities or if you are underground.Also, the average accuracy is 100 nanoseconds, which means that you do not see where the satellite was when it sent the message, but you see where it was 100 nanoseconds earlier. This is not a big problem, as for travel needs it`s no big difference.

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