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How does GPS work?

TomTom Education


The history of GPS

How it all began
In 1957 the former USSR launched the first man-made satellite: Sputnik 1. Scientists quickly realised first that by using the Doppler Effect one could work out a satellite’s orbit. Then that by turning it round, you can use satellites to work out the position of a receiver on earth.

The foundations of the modern GPS were laid during the early 1960’s by the US military. The Navy, Air Force and Army each came up with their own designs and ideas, and eventually in 1973 a design that incorporated elements from each was approved by the US government. This was to become NAVSTAR.

The first satellite for the new NAVSTAR GPS was launched in 1974 and from 1978 to 1985 another 11 were launched for testing purposes. The full constellation of 24, that today allows your navigation system to enjoy worldwide GPS coverage, was completed in 1993.

GPS for everyone
Initially GPS was only intended for military use. But then tragedy struck. On 1st September 1983, Korean Airlines flight KAL007 from Anchorage to Seoul strayed off course into USSR airspace and was shot down by a soviet Su-15 fighter jet. All 269 passengers and crew were killed.

Two weeks later, US President Reagan proposed GPS be made available for civilian use to avoid navigational error ever again leading to such a catastrophe. While by no means the only reason, the Korean Airlines disaster was certainly a major catalyst toward civilian access to GPS.

Selective Availability (SA)
Having spent some $12 billion to develop the most accurate navigation system in the world, the US government then built a function called Selective Availability (SA) into NAVSTAR that would limit its accuracy for civilian users to ensure no enemy or terrorist group could use GPS to make accurate weapons.

It worked by introducing deliberate errors into the data broadcast by each satellite. Military users could access the fully accurate system by decrypting a secured second frequency that was broadcast simultaneously.

Then during the Gulf War the US military needed many more GPS receivers than it had. It solved the problem by using civilian GPS receivers. But to increase the accuracy of these devices, the SA function had to be temporarily disabled. Then in 2000, US President Clinton announced that SA would be disabled completely. Because US government ‘threat assessments’ concluded that removing SA would have minimal impact on national security. Though in the same speech he said the US would still be able to ‘selectively deny’ GPS signals on a regional basis when national security was threatened.

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