| The GPS system is a satellite navigation system | | | | transmission. The US Military designates two |
| operated by the US Department of Defence. It | | | | frequencies, one for civilian use coded L1, and one |
| essentially provides a subscription-free civilian timing | | | | for military use coded L2. The L1 frequency is |
| and navigation service. GPS is primarily intended as a | | | | broadcast at 1575 MHz (1.575 GHz). These weak |
| highly accurate global positioning and navigation | | | | radio transmissions can easily pass through less dense |
| reference. However, a by-product of accurate | | | | materials, such as plastics and glass, but are blocked |
| positioning is that each orbiting satellite also has an | | | | by denser materials, such as metal and brick. |
| integrated highly precise atomic clock unit | | | | However, newer more sensitive receivers are being |
| synchronised by the US Naval Observatory (USNO). | | | | developed for indoor operation. |
| It is therefore ideal for providing a high accuracy time | | | | A GPS antenna needs to have a 'line-of-sight' view of |
| reference for computer timing systems, such as NTP. | | | | satellites. Therefore, a roof-mounted antenna is ideal, |
| This article attempts to describe simply, how | | | | with a full 360-degree view of the sky. Quite often |
| network time servers can obtain accurate timing | | | | however, an antenna located on the side of a building |
| information to provide a precise reference for | | | | is adequate, provided the horizon is not too obscured |
| computer time synchronisation. It also discusses how | | | | by trees or buildings. The antenna is essentially a |
| NTP servers can utilise the atomic time generated by | | | | signal amplifier; it boosts the received signals for |
| GPS to provide a timing reference for computer | | | | transmission along a cable to a receiver. Fifty-ohm |
| networks. | | | | coax cable is generally used to transfer signal |
| The GPS system is a constellation of 24 orbiting | | | | information between the antenna and receiver. |
| satellites, each with highly precise atomic clock timing | | | | The receiver decodes the signals received by the |
| systems onboard. The system provides a global | | | | antenna into a useable, easily readable format. The |
| service, which can be received anywhere on the face | | | | most common protocol used by receivers is NMEA, |
| of the planet. Accurate time data is transmitted | | | | which consists of a number of transmitted sentences. |
| continuously from each satellite. The time information | | | | Each NMEA sentence provides a packet of |
| can easily be received with modern low-cost antenna | | | | information consisting of time, date and positioning |
| and receiver equipment. | | | | information. The protocol also provides additional |
| Time is broadcast from each satellite as Universal | | | | information such as visible satellites and satellite |
| Coordinated Time (UTC), which is virtually identical to | | | | orientation in the sky. |
| Greenwich Mean Time (GMT). UTC time is the same | | | | NTP servers utilise a specific receiver tailored for |
| worldwide; it does not vary with time zone or | | | | timing applications. Timing receivers have additional |
| daylight saving time. UTC is a high precision atomic | | | | functions and circuitry to ensure a highly precise |
| time standard maintained by atomic clocks located in | | | | reference time. They also perform an automated |
| national standards laboratories. UTC has uniform | | | | site-survey to compare satellite clocks for |
| seconds, as defined by the International Atomic Time | | | | synchronicity. Time servers also utilise an accurate |
| (TAI) institute. | | | | pulse per second (PPS) output generated by the |
| Each satellite broadcasts time and positioning | | | | receiver. A PPS output provides a highly accurate |
| information as a very low-power radio frequency | | | | reference trigger for timing purposes. |