How does GPS work?

TomTom Education


Alternative satellite systems

GLONASS (Global Orbiting Navigation Satellite System) is the Russian satellite system, which became fully operational in December 1995.  Like GPS, GLONASS also uses 24 satellites and though slightly more accurate than GPS, its big drawback was that the satellites only lasted about 3 years.

With a Russian economic crisis in the late 90’s, the satellites were not always replaced, so that the system gradually lost its effectiveness. By 2000, only 8 of the 24 satellites were still operational. However, for the last few years, Russia has been working hard on a GLONASS comeback and hopes to have the system fully operational again in 2012.

GPS BLOCK III is the new version of GPS. The biggest improvement on the current GPS system is that GPS III will send out a more powerful signal. It will also follow a different orbit, so countries on higher latitude, such as in Scandinavia, will now get better coverage. Another major advantage is its ability to operate accurately together with GALILEO, the new European satellite system.

GALILEO is the new satellite system of the European Union. One of the key reasons for the EU to develop its own satellite system was so that it would no longer be dependent on GPS.
GALILEO will provide greater accuracy and coverage than GPS and is mainly intended for civilian use. It is due to be completed in 2008/9 and the first satellite was launched in December 2005. It will eventually consist of a constellation of 30 satellites orbiting at some 14700 miles/ 23,600 kilometres above Earth.

Click here to read all about GALILEO.

Despite the impressive accuracy of GPS and GLONASS, two further systems were launched to make them even more accurate. WAAS (Wide Area Augmentation System) for the American continent and EGNOS (European Geostationary Navigation Overlay System) for the European continent.
Each consists of three satellites that send out signals to receivers. Measuring stations then calculate if the satellite signal has a discrepancy and sends any correction to two of the three ‘geostationary’ satellites. These geostationary satellites beam the correction signal back to Earth, where WAAS/EGNOS -enabled GPS receivers apply that correction to their computed GPS position.

However, the new GPS III and GALILEO systems will not work together with WAAS or EGNOS, as they’ll be able to measure and correct their own inaccuracies.


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