A Navigation System typically gives you position, altitude and time. For that, 4 satellites are necessary.
Clock mistakes can happen during a fast displacement. To refine accuracy, you can use the differential GPS, or geodesics recievers.
What is the altitude of the 24 GPS satellites ? (tick the right answer):
With 2 points, you typically (BONUS!) have a vector. If you give a point of
arrival, the reciever can calculate many things.
If you have a winding trip, you can place waypoints or even upload
maps in the reciever.
Geodesic recievers are a lot more precise, but also very expansive (10.000 euros).
The US Department of Defense can typically decide to
stop completely the GPS at
any moment, or to degrade precision.
GPS has some technical limitations and does not provide a
service garantee.
GLONASS was created by the Russian Army. There are only 5 or 6 working satellites remaining, because they do not have enough money to maintain them.
Galileo has no military purposes. It only has civil purposes. It is developped by the UE in close cooperation with the European Space Agency. Galileo includes an integrity message, so that you can detect errors.
GLONASS stands for (tick the right answer):