Radio navigation systems all work by comparing the time a signal leaves a transmitting station with the time it reaches a receiving station. The signal contains data to enable this process.
Land based systems included Decca ( a british system) and Loran-C (a french system). They were generally accurate to about 100m but were rather unreliable.
Decca was allowed to slide into obscurity when Trinity House (I believe) stopped maintaining it in the mid-90s.
To the best of my knowledge, Loran-C still operates.
Both systems made small boat navigation in home waters and cross-channel relatively straight-forward and meant I had no need to learn astro-navigation for weekend 'jollies' to stock the wine cellar.
Along came wonderful GPS in the guise of the american Navstar system. This uses a constellation of about 30 satellites orbiting the earth, of which 24 will be operational at any one time. Selective Availability restricted the accuracy of civilian use to 100m but, a differential correction system was developed that sent a correction from a land based station so nullifying SA. This is the point at which the americans allowed full functionality for civilian use.
Because all the satellites are continually circling the earth, your in-car sat nav would see every one of them if it was left on for a certain amount of time (probably less than 24 hours) without even moving. If you have a sat nav, there will be a page on it to show satellite status with 'bars' showing the signal strength form each 'visible' satellite. Watch it for a while and you will see satellites vary as they pass overhead.
For an accurate 3D fix, a receiver needs to see at least 4 satellites. Tom-tom and other systems only use a few more than this, hence slight inaccuracies at times.
Marine systems are designed around using, probably, 12-14 satellites when they are visible enabling greater accuracy.
More [somewhat technical] detail available here:
http://en.wikipedia.org/wiki/GPS
The european Galileo and russian GLONASS systems will be independent and complimentary to Navstar ensuring that furture political interference is reduced to a minimum.
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