How does GPS work?

TomTom Education

The science behind GPS

Atmosphere-induced error & Multipath error

Atmosphere-induced error
A real GPS signal, passes through charged particles in the ionosphere and then water vapour in the troposphere, both of which slow it down. So along with their signal, the satellites also send the navigation device information on the ‘thickness’ of the ionosphere at that moment. From which the navigation device can correct the signal error.

I = Interference by particles in the ionosphere

II = Interference by water vapour in troposphere

III = Interference by bouncing off buildings & big constructions

Result = possible errors in satellite signal

Multi-path error
Error can also arise in the last phase of the GPS signal’s journey from the satellite to the Navigation device, as the signals can bounce off local obstructions, such as mountains, before reaching your Navigation device. So the signal takes longer to reach your receiver than if it had travelled in a direct path and the receiver therefore assumes the satellite is farther away than it actually is. Alternatively, one signal may bounce off local obstructions while another travels direct to your Navigation device, so that the signals are not in synch with the Navigation device. Resulting in what is known as ‘multi-path error’.

Fortunately, good navigation devices can for a large part correct this error, so that for civilian navigational purposes it is not an issue.


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