To add to what Goldierocks wrote
GPS - Is the American Satellite Global Positioning System. It consists of 24 operational satellites and 6 spares, with 4 satellites in 6 orbital planes (with a spare in each plane)
GLONASS - Is the Russian Globalnaya Navigazionnaya Sputnikovaya Sistema, equivalent. It consists of 24 operational satellites distributed over 3 orbital planes. So a totalof 48 satellites. Other countries either have or are developing their own, but not necessarily available in Oz
United States (GPS) ...
Japan (QZSS) ...
Russia (GLONASS) ...
China Peoples Republic of China (BeiDou Navigation Satellite System) ...
European Union (Galileo) ...
India (IRNSS)
Many later model GPS devices i.e. the Garmin Etrex use both GPS and GLONASS concurrently, giving a far more accurate and faster position fix.
To fix position you need a minimum of 3 satellites, albeit less accurate. To more accurately fix your position and at the same time your altitude you need a minimum of 4.
The other thing many users may not be aware of is; If you have not used your GPS recently (i.e. months) and or have moved several 100 kms from when last used, your device can take several minutes to locate.
Whereas if you for example, have located a waypoint at your car, turned it off, gone detecting for a few hours and then switched it back on, to get back to the car, it can only take several seconds to get a fix.
I had an old Garmin handheld, 15 or so years ago, that was not used for over a year, it refused to "lock on". I phoned the local Garmin service agent who walked me through a code process to input for factory reset, to get it to work again. In simple terms, it had forgotten what it was
I also found that Garmin also produce a GPS that has a message capability, but like it, I think all of them require some sort of annual subscription cost to be used.......
Some use Global Star, others the Iridium system and I'm sure others to be useful.