These acronyms are all types of satellite navigation systems. They provide geo-spatial positioning across the earth, using a system of satellites – smart phones and sat navs use them as do surveying instruments. As with most things the more you pay the better information you receive, so a GNSS fix from a £20 000 surveying instrument will locate your level and position on earth better than a £50 sat nav fix.
A system with global coverage is termed a global navigation satellite system (GNSS).
As of 2013 only two GNSS’s exist. These are the USA’s NAVSTAR global positioning system (GPS) and the Russian GLONASS. China are expecting their Compass navigation system to be a global system, by 2020 and the European Galileo positioning system should go global shortly after this.
GNSS is therefore a generic term for systems such as GPS and GLONASS. In order to provide the most accurate fix in the shortest possible time surveying equipment should be able to use all GNSS networks.
DORIS by the way is the Doppler Orbitography and Radio positioning integrated by satellite system, which is a French navigation system.