The GPS system is a satellite navigation system operated by the US Department of Defence. It essentially provides a subscription-free civilian timing and navigation service. GPS is primarily intended as a highly accurate global positioning and navigation reference. However, a by-product of accurate positioning is that each orbiting satellite also has an integrated highly precise atomic clock unit synchronised by the US Naval Observatory (USNO). It is therefore ideal for providing a high accuracy time reference for computer timing systems, such as NTP. This article attempts to describe simply, how network time servers can obtain accurate timing information to provide a precise reference for computer time synchronisation. It also discusses how NTP servers can utilise the atomic time generated by GPS to provide a timing reference for computer networks.

The GPS system is a constellation of 24 orbiting satellites, each with highly precise atomic clock timing systems onboard. The system provides a global service, which can be received anywhere on the face of the planet. Accurate time data is transmitted continuously from each satellite. The time information can easily be received with modern low-cost antenna and receiver equipment.

Time is broadcast from each satellite as Universal Coordinated Time (UTC), which is virtually identical to Greenwich Mean Time (GMT). UTC time is the same worldwide; it does not vary with time zone or daylight saving time. UTC is a high precision atomic time standard maintained by atomic clocks located in national standards laboratories. UTC has uniform seconds, as defined by the International Atomic Time (TAI) institute.

Each satellite broadcasts time and positioning information as a very low-power radio frequency transmission. The US Military designates two frequencies, one for civilian use coded L1, and one for military use coded L2. The L1 frequency is broadcast at 1575 MHz (1.575 GHz). These weak radio transmissions can easily pass through less dense materials, such as plastics and glass, but are blocked by denser materials, such as metal and brick. However, newer more sensitive receivers are being developed for indoor operation.

A GPS antenna needs to have a 'line-of-sight' view of satellites. Therefore, a roof-mounted antenna is ideal, with a full 360-degree view of the sky. Quite often however, an antenna located on the side of a building is adequate, provided the horizon is not too obscured by trees or buildings. The antenna is essentially a signal amplifier; it boosts the received signals for transmission along a cable to a receiver. Fifty-ohm coax cable is generally used to transfer signal information between the antenna and receiver.

The receiver decodes the signals received by the antenna into a useable, easily readable format. The most common protocol used by receivers is NMEA, which consists of a number of transmitted sentences. Each NMEA sentence provides a packet of information consisting of time, date and positioning information. The protocol also provides additional information such as visible satellites and satellite orientation in the sky.

NTP servers utilise a specific receiver tailored for timing applications. Timing receivers have additional functions and circuitry to ensure a highly precise reference time. They also perform an automated site-survey to compare satellite clocks for synchronicity. Time servers also utilise an accurate pulse per second (PPS) output generated by the receiver. A PPS output provides a highly accurate reference trigger for timing purposes.

The GPS system provides an ideal solution for computer network timing applications. Using the 'free-to-air' system, NTP time servers can synchronise to within a few microseconds of UTC.