I use GPS almost every day of my life.
At work, on my ship we use multiple receivers, not always linked together, and the accuracy is very very good. Within 10m.
We have professional units, 2 independent units, we also have units in our comms systems, and speed logs, as well as in our echo sounder, and distress systems. All work just fine, ALL the time.
Our units also detect RAIM, and will track upwards of 12 satellites, giving azimuth and elevation, signal strength and health of each sat.
As water covers just about 70% of the planet, and i have been to an awful lot of it, (except high arctic) I find it works extremely well over water, and by definition that is over the vast majority of the planet. I would be interested to know where over particular places it does not work well?
Considering we can be thousands of miles from the nearest cell phone tower, I am pretty confident that they do not use cell phone signals to provide a position, and guess what? My cell phone (with weeks of not receiving a signal) with no WiFi or data or internet can tell me where i am on the ocean pretty well as well, so i do not dispute its accuracy.
We use ECDIS (electronic charts) with Primary and secondary means of position fixing as GPS, we do not carry Loran, Decca, ( in fact they were decommissioned years ago) but we can put in manual positions from radar fixes (only good if you are within 24 miles or so of land) and celestial observations.
Millions of ships use the same systems.
If they are not accurate why do we not go aground or hit things more often?
Also how do we find our position in the middle of the ocean?
I have yet to see radio beacons in the ocean, and am pretty certain we would keep on bumping into them if they were there, and uncharted and unlit at night. Yes we do navigate at night, and No we dont have headlights.......
The question remains from the OP,
HOW DOES GPS WORK?