Richard N Williams

Use of GPS for Computer Timing Applications

This article describes how the Global Positioning System (GPS) can be used to receive time source information.

The Global Positioning System (GPS) is now a familiar tool in helping motorists to navigate but GPS has more uses than merely triangulating a position for direction finding, it can be utilized to provide time and frequency information worldwide.

Developed by the United States military, GPS incorporates at least 24 communication satellites in high orbit, all of which contain precise timing equipment to enable the satellite to triangulate positions with accuracy.

However, each satellite’s highly accurate atomic clock timing reference can also be used by NTP (Network Time Protocol) servers to synchronise computer networks using the highly accurate GPS time signal as an external reference. 

GPS is an ideal time and frequency source 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, see here for more information.

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 recommend; installed inline on the GPS cable.

The cable between the GPS antenna and receiver is also critical. The maximum distance that a cable can run is normally only 20-30 metres but a high quality coax cable combined with a GPS amplifier placed in-line to boost the gain of the antenna can allow in excess of 100 metre cable runs.

A GPS receiver then decodes the signal sent from the antenna to a computer readable protocol which can be utilised by most time servers and operating systems including, Windows, LINUX and UNIX.

The GPS receiver also outputs a precise pulse every second that GPS NTP servers and computer time servers may utilise to provide ultra-precise timing. The pulse-per-second timing on most receivers is accurate to within 0.001 of a second of UTC (Coordinated Universal Time or Temps Universel Coordonné).

GPS 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.

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