What are the major differences between GPS, GLONASS, COMPASS and Galileo?
Some differences, which comes to my mind now: - they use slightly different frequencies (thus are (may be) incompatible, but EU and Russia are working now on making them compatible). Also chip manufactures announced they will produce single chip capable of receiving GPS/Galileo (maybe also GLONASS) - Galileo will offer 5 different services (open access navigation, commercial navigation, safety of life navigation, public regulated navigation, search and rescue) - Galileo should offer higher accuracy for every user (GPS has high accuracy signal restricted to military/government agencies)
But as George said, they all works on the same physics and has the same problems to overcome.
GPS is the American satellite system. GLONASS and others are systems created by other countries.
GPS is the oldest and tested and true system. Others are in their infancy, or there wasn't enough funding to complete the system.
GPS is older, more mature and have many more consumer products made for it.
GLONASS and Galileo are still in the development phase, as what I know. But they are much newer and has higher accuracy.
Galileo is not made mainly for military systems and are not controlled by a single country.
First of all: they all work according to the same principles.
GNSS Systems are composed of a cloud of known position satellites (geostationary) and a receiver.
Each satellite and receiver have an atomic clock on it's system. The receiver, when it's turned on, tries to locate as many satellites as possible. As each one is located, the receiver, sends out a radio signal on a very specific frequency and counts the number of computer clocks until the signal is received back. You know how fast the radio wave goes, and time. From it you can calculate distance.
From time spent between send from receiver and signal coming back, the handheld estimates distance to that satellite. When you have 4 or more satellites receiving and sending you signals, the GNSS software is able to triangulate your position from theirs.
That is why GNSS systems suffers way too much from atmosphere, buildings surrounding, weather, number of satellites available, etc.
The GPS system, a few years ago had a "signal scrambler" that altered randomly the delivery of the satellite message, estimating your position with a few tens of meters in error. That was called SA (Selective Availability), but that was disabled (in 1995 if I'm not mistaken).
The precision of a receiver is directly related to your earth models (ellipsoid/geoid/etc) and if your GNSS device can be post-processed. Precision GNSS devices have very specific geometries and they have accurate phase-centers, which help determine the actual location where the signal is being received.
If it can be post-processed, you can get accurate measures of satellite orbits and a huge collection of points (or signals received and processed).
From those, and a few minutes with a device steady in one position, one can enhance it's precision, by using that collection of points in a interval and achieve best positioning. These are complicated algorithms and most precision devices have specific softwares to process them.
A normal GNSS device cannot be post-processed because the device does not store the collection of points, only shows you the current processed point (it's possible with some hacks)
I'm not sure if all that applies to all GNSS systems, but the physics are the same for all of them.