The GPS (Global Positioning System) is a Global Navigational Satellite System (GNSS) controlled and run by the United States of America.
GNSS systems work by using satellites several thousand miles above the Earth’s surface that beam timing information down to a GNSS receiver (like the satellite navigation unit in our cars). It is this information that is used by the GPS receiver to triangulate an exact position. They can only do this by having onboard their own highly accurate atomic clock as the distance the satellites are away from the Earth, even an inaccuracy of a second or two could mean a sat navigation’s location could be miles out.
As a consequence of having this accurate time sources, GPS and the new breed of GNSS systems can all be used to receive an absolute or UTC (Universal Coordinated Time) time source. This time source can be used by computer networks running a NTP server (Network Time Protocol) to synchronise all machines and devices to the same time.
NTP is a protocol designed to synchronise computers and network devices to an external timing reference.
GPS is an ideal time and frequency reference because it can provide highly accurate time anywhere in the world using relatively cheap components. Each GPS satellite transmits in two frequencies L2 for the military use and L1 for use by civilians transmitted at 1575 MHz, Low-cost GPS antennas and receivers are now widely available and dedicated NTP GPS servers are now relatively low cost.
The radio signal transmitted by the satellite can pass through windows but can be blocked by buildings so the ideal location for a GPS antenna is on a rooftop with a good view of the sky. The more satellites it can receive from the better the signal. However, roof-mounted antennas can be prone to lighting strikes or other voltage surges so a suppressor is highly recommend being installed inline on the GPS cable.
A NTP GPS Server is ideal in providing NTP time servers or stand-alone computers with a highly accurate external reference for synchronisation. Even with relatively low cost equipment, accuracy of hundred nanoseconds (a nanosecond = a billionth of a second) can be reasonably achieved using GPS as an external reference.