system
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human
ELI5: Difference between GPS, GLONASS, Galileo and other navigation systems

With India's successful launch of its last satellite for IRNSS, why is there a need for so many navigational systems and how as a user, can you switch between them?
gpt
These systems are so useful, that many governments don't want to be restricted to something that is controlled by a foreign government.

For example, GPS is a military system run by the US Navy. They could, if they wanted to, shut it off or degrade it. The Russians weren't happy with relying on a US military system, given that there is a lot of disagreement between Russia and America, so they built their own (GLONASS).

In Europe, the EU government were concerned that they were relying on a foreign military system for a lot of commercial and government work. They were also annoyed that GPS only made a crude signal available for the public and law enforcement, but the military got a much higher precision signal.

So, the EU government decided to build their own system (Galileo) which would give an improved accuracy free public signal, and a premium signal (with paid subscription) which would give the same accuracy as the GPS military signal. It would also be optimized for use in Europe. GPS satellites don't really orbit over the Arctic region, so GPS reception in places like North Norway or Northern Sweden is not very reliable. The Galileo satellites are intended to go into orbits which go much further North, but the disadvantage is that you need more satellites to cover the increased amount of land.

Same sort of thing with IRNSS (India) and BeiDou (China). 

In general, all these systems you broadly the same technology but on slightly different frequencies. This means your receiver needs to have an antenna and circuits capable of receiving all the different systems. Dual system GPS/GLONASS receivers have been available for a while - they weren't terribly popular initially because the GLONASS system has been very unreliable with satellites dying or malfunctioning and not being replaced.  However, the Russians have repaired and updated their GLONASS satellites, and it's now a good quality modern system, so most latest generation GPS receivers do actually receive the GLONASS signal as well.

One of the advantages of a receiver which supports both systems is that it is possible to get a more accurate position because you can combine the measurements from both systems when the receiver does the final position calculation. 

The Galileo system isn't complete, in fact, it isn't really functional, as it doesn't have enough satellites to work 24/7. However, many new GPS receivers are "Galileo ready" which means that they have the antenna and circuits in place, but the firmware to process the signals hasn't been finalized. Once enough satellites are up and working, the manufacturer of the receiver will be able to test their firmware, and then once tested and working will be make it available for download.