Satellite navigation systems, including GPS. Questions should be related either to amateur radio operations (e.g. APRS tracking) or the theory of positioning using radio signals; questions merely about using GPS receivers are not on-topic.
GNSS or "global navigation satellite system" is the generic term for a collection of satellites that may be used for determining position. The most well known system, whose name is often used interchangeably for it and the technology in general, is the United States' Global Positioning System (GPS).
In amateur radio, the most common use of GNSS / GPS technology is to report the location of a station over aprs. It may also be used to obtain accurate time information to coordinate operations between stations, such as agreed-upon transmission times or locating a transmitter by precisely measuring the time of reception at multiple receivers.