Satellite navigation
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). As of 2024, four global systems are operational: the United States 's Global Positioning System (GPS), Russia 's Global Navigation Satellite System (GLONASS), China 's BeiDou Navigation Satellite System (BDS), and the European Union's Galileo.

Satellite-based augmentation systems (SBAS), designed to enhance the accuracy of GNSS, include Japan's Quasi-Zenith Satellite System (QZSS) and the European EGNOS, both based on GPS.Stand-alone operational regional navigation satellite systems (RNSS) include earlier generations of the BeiDou navigation system and the current Indian Regional Navigation Satellite System (IRNSS) or NavIC.

Satellite navigation devices determine their location (longitude, latitude, and altitude / elevation) to high precision (within a few centimeters to meters) using time signals transmitted along a line of sight by radio from satellites. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver (satellite tracking). The signals also allow the electronic receiver to calculate the current local time to a high precision, which allows time synchronisation. These uses are collectively known as Positioning, Navigation and Timing (PNT). Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated.

Global coverage for each system is generally achieved by a satellite constellation of 18–30 medium Earth orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but all use orbital inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about 20,000 kilometres or 12,000 miles).
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