Global Positioning System (GPS) is used for determining precise location of a GPS receiver. There are two types of GPS service:
Global Positioning System (GPS) is used for determining precise location of a GPS receiver.
* Precise Positioning Service (PPS) used by military. Its accuracy is 22m horizontally, 27.7m vertically and 200ns of time
* Standard Positioning Service (SPS) used by civil users worldwide without charge or restrictions except that SPS accuracy is intentionally degraded to 100m horizontally, 156m vertically and 340ns of time
GPS system is based on 24 satellites rotating on 6 different orbital planes with 12h orbital period. It makes that at least 5, but usually 6 or more satellites are visible at any time anywhere on the Earth. GPS receiver calculates more or less precise position (latitude, longitude and altitude) and time based on signals received from 4 satellites (three are used to determine position and fourth is used to correct time), which are broadcasting their current positions and UTC time.
Precise time is mainly intended to be used by built-in NTP server, which can use it as a time source without any additional configuration if GPS is configured to set system time.
