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Nowadays GPS is most commonly known for vehicle navigation solutions. However, the GPS system can be used for many other useful purposes. For instance, many computer network time synchronisation solutions, such as NTP time servers, use the atomic clocks on board each GPS satellite for accurate timing. The GPS system can be used as an accurate reference clock for synchronising time-critical applications on computers and networks. This article describes how computer systems can utilise GPS time and discusses what equipment is required to provide a precision timing reference.

The GPS system is a United States military system intended for global navigation. The GPS system is a space-based system, consisting of a network of 24 orbiting satellites. Precise navigation and positioning is achieved by utilising precision timing signals and triangulation between multiple satellites. Each orbiting satellite has an integrated atomic clock timing reference to provide ultra high precision timing. Each satellite broadcasts timing information to within a few nanoseconds of the correct time to provide positioning to an accuracy of better than 10m. This precise timing information can be used by NTP time servers and other computer timing equipment as a highly accurate time reference. The great thing about GPS is that it works anywhere in the world and is entirely free to air service.

Each GPS satellite transmits a low-power radio signal down to the surface of the Earth. Two separate frequencies are utilised, designated L1 and L2. The L1 band is the civilian signal; transmitted at 1575.42 MHz. L2 is the military frequency, intended for ultra-high precision positioning. The radio signals are transmitted from the satellites by line of sight. The transmitted signal is easily powerful enough to pass through clouds, glass and most plastics but is absorbed by more substantial material such as brick, roofing materials and metals. The ideal location for a GPS antenna tends to be outdoors and ideally on a rooftop with the best possible view of the sky.

With the advent of GPS navigation equipment for the mass market, GPS antennas and receivers can be obtained at very reasonable cost. A GPS antenna is utilised to receive and amplify the GPS radio signals. While a GPS receiver decodes the information and presents it in a computer readable format. Many antennas are available with integrated receivers but these have the drawback of only providing a few meters of cabling to the host NTP time server or computer system. Marine antennas are designed to withstand the worst of the weather, they typically screw onto a threaded pole to provide a secure mounting system. These antennas provide the best solution for static timing applications.

The GPS receiver is generally embedded into the NTP time server or encapsulated within an enclosure. A GPS receiver is a small elecronic device that receives amplified GPS signals from an antenna via a coaxial cable. The receiver decodes these signal and provides a computer readable output usually via a RS232 or USB interface. Most receivers provide decoded information such as: position, visible satellites, precise time and an accurate pulse output timing reference.

Coaxial cable is utilised to provide a connection between the GPS antenna and receiver. The cable distance can be critical. The antenna must amplify the GPS signal sufficiently to overcome any cable losses associated with the cable run utilised. Better quality antennas generally provide more amplification (gain) additionally higher quality coax cables can be used with much smaller signal losses per meter allowing for relatively long cable runs between a antenna and receiver. In order to extend cable runs still further GPS amplifiers can be used to boost the signal transmitted down the coax cable. Typically, a GPS amplifier sits in-line on the coax cable and is powered from the receiver. Multiple amplifiers can be used for extremely long cable runs.

A roof mounted GPS antenna can be prone to lightning strikes and voltage surges. It is recommended that surge suppressors are utilised to protect expensive NTP time server and timing systems from potential damage. A surge suppressor is essentially a barrier that sits in-line on the coax cable between the antenna and receiver that protects the receiver from any potential damage due to voltage surges.

To summarise, the GPS system provides a highly precise timing reference for NTP time servers and other computer timing equipment. It is a reliable free to air service that can be used anywhere in the world. Additionally, with the boom in vehicle navigation systems the technology required to receive GPS broadcasts is continually falling in price.

Copyright (c) 2007 Galleon Systems

This article explains how GPS Time server Systems achieves exact timing information to give a highly accurate timing reference for Network Computer time.

GPS System is a navigation system operated by the Military but also can be used by the civilian and does not need any subscription. The GPS System is known as an extremely accurate global positioning system. Each GPS satellite has an onboard a highly accurate atomic clock, ideal for supplying the correct time for computer network time synchronization.

GPS Time Servers: The GPS System exist of a constellation of 24 orbiting satellites, each has a precise atomic clock timing reference and covers the entire globe. The exact time information can be continuously broadcast from each of the satellites. You can receive the broadcast timing information with a relatively low cost GPS antenna and receiver.

GPS time is transmitted as Universal Coordinated Time (UTC), which is similar to Greenwich Mean Time (GMT). The UTC time is the same worldwide and does not vary regarding the time zone or daylight saving time. UTC is a very accurate atomic time standard sustained by atomic clocks located in national standards laboratories. UTC has consistent seconds distinct by the International Atomic Time (TAI) Institute.

GPS Radio Transmission: The GPS satellites broadcast information as very low-power radio frequency transmission. There are two designated frequency that are used, one for civilian use coded L1 and the Military use coded L2.The civilian L1 frequency is transmitted at 1575MHz. It can easiliy pass through plastic and glass but is blocked by metal and brick.

GPS Antenna Location: The precondition to receive transmission from as many satellites as possible is a good clear view to the sky. Ideally a roof mounted antenna is best with a full 360-degree view of the sky. Conversely, an antenna can be located on the side of the building with a 180-degree view of the sky if the horizon is not too obscured.

GPS Antenna Types: The GPS antenna is essentially a signal amplifier. The antenna improves the received GPS signals for transmission along a cable to a receiver for decoding. A Coax Cable is normally used to transfer signal information between the GPS antenna and receiver. These antennas can be provided in different shapes and sizes. The general antennas being pole-mounted are dome shaped antennas and small patch type antennas. The pole-mounting antenna screws onto the threaded pole for mounting. Whereas the patch type antenna is a small flat bottomed device ideal for mounting on a windowsill.

GPS Receivers: GPS Receivers decipher the GPS transmission received from the antenna into a useable format. NMEA is the most common protocol used by the GPS receiver. This protocol insists a number of sentences which provide a packet on information confirm of time, date and positioning information. The NMEA protocol quotes information indicating visible satellites and satellite location.

GPS time servers can use a specific GPS timing receiver. These timing receivers have additional functions to provide a highly accurate reference time. The timing receiver can also perform an automated survey and compare satellite atomic clocks to check for synchronicity. GPS time servers may also use an accurate pulse per second (PPS) output, which provides a highly accurate reference trigger for GPS timing, generated by the receiver.

GPS Antenna Installation: The gain of the antenna and the coax is important for the maximum cable distance that can be used by a GPS antenna and receiver. An antenna used with a GPS time server may have 35dB gain. RG58 coax has an attenuation of 0.64dB/m at 1575MHz. Therefore a cable length of 35/0.64=55m could be used. Higher quality coax has a much lower attentuation value allowing longer cable runs. Also GPS amplifiers and updown converters can be utilized to increase the cable length. Surge lightening arrestors are also recommended on externally mounted GPS antenna to protect expensive network equipment from potential damage by lighting strikes