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 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.
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.