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GPS Systems for NTP Time Servers and Timing Applications

Categories: GPS Articles | February 28th, 2009 | by Mike Tucker | no comments

The Global Positioning System (GPS) is most widely known as a system for vehicle and marine navigation. However, each GPS satellite orbiting the earth also has an integrated precise atomic clock that can be used as a reference clock for NTP time servers and for general computer timing applications. This article describes the components required to utilise GPS for computer timing applications. It also discusses how the GPS system can be used for computer network timing by using the Network Time Protocol (NTP) and NTP time servers.

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. In order to provide very precise time, each satellite has an integrated highly accurate atomic clock onboard. 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.

In order to receive GPS timing and positioning information some relatively low-cost equipment is required. 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. Rugged GPS marine antennas screw down on to a rigid threaded pole mount able to withstand the worst of the weather. 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. GPS receivers are small modules that have a connection to the GPS antenna to receive amplified GPS signals. The receiver decodes these signal and provides a computer readable output usually via a RS232 or USB interface. Typically, information provided by the receiver includes time and positioning information, number of satellites in use, number of visible satellites and whether a signal lock has been acquired.

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.

The author, David Evans, is a technical author in the field of computer network timing systems. David has a number of years experience of writing articles and white papers about NTP and GPS time synchronisation. Please visit our web-site if you would like more information on NTP Time Server systems: http://www.timetools.co.uk/ntp-servers/ntp-s5500.htm

Article Source: GPS Systems for NTP Time Servers and Timing Applications - ArticleSpan.com

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Use of GPS for Computer Timing Applications

Categories: GPS Articles | February 25th, 2009 | by Mike Tucker | no comments

The Global Positioning System (GPS) is now a very familiar tool in helping motorists to navigate but GPS has more uses than merely triangulating a position for direction finding, it can also 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 accurately.

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 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.

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 to be 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 GPS 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 Network Time Protocol (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 an external reference.

For more information on GPS timing applications please see http://www.galsys.co.uk

Article Source: Use of GPS for Computer Timing Applications - ArticleSpan.com

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Using GPS to Synchronise Computer Systems

Categories: GPS Articles | February 23rd, 2009 | by Mike Tucker | no comments

The Global Positioning System (GPS) is a US military system intended for global navigation and positioning. GPS is a satellite based system that consists of 24 earth orbiting satellites, each transmitting a weak radio signal that can be used to provide accurate positioning information anywhere on the globe. Each satellite has an integrated synchronised highly accurate atomic clock that can be utilised by ground based timing and synchronisation equipment.

This article describes how the GPS system can be used to provide an accurate time and frequency reference for NTP servers and computer network time synchronisation.

GPS timing information is continuously broadcast by each GPS satellite. GPS time is not affected by leap seconds. Therefore, GPS time is currently about 14 seconds ahead of UTC time. However, offset information is available to readily convert GPS time to UTC time, which is used in NTP server and computer time server systems.

Each GPS satellite transmits information as very low-power radio frequency transmission. Two frequencies are used, one for civilian use coded L1, and one for military use coded L2. The civilian L1 frequency is the most widely used and is transmitted at 1575 MHz. The GPS radio transmissions can easily pass through plastic, glass and clouds but is blocked by metal and brick structures.

An antenna is utilised to boost the GPS signal and pass the signal along a coax cable. For consistent operation a GPS antenna needs to have a view of the sky. Ideally, it should be located outdoors on a rooftop with a good 360-degree un-obscured view of the sky.

GPS antennas are very sensitive to cable types and distances. The maximum cable length that a GPS antenna can utilise depends on the quality of coax utilised and the gain of the GPS antenna. Higher quality coax cable generally have much lower attenuation figures and hence provide much longer cable runs between the receiver and antenna. Typically, a low-quality coax can run to 20-30m, while a high quality coax can be used in excess of 100m cable runs. To increase these distances still further, a GPS amplifier can be used to amplify and boost the received GPS signal.

GPS timing systems also utilise a GPS receiver in order to decode the received GPS transmission and convert it to a computer readable format. The receiver decodes the GPS time and positioning information and converts it into a regular serial transmission that can easily be read. GPS NTP servers and computer time servers may also utilise a precise pulse per second output generated by the GPS receiver to provide ultra-precise timing. The timing pulse output can typically provide synchronisation down to a few hundred nanoseconds.

To summarise, the GPS system is ideal for synchronising NTP server and computer time servers. Accuracies of a few hundred nanoseconds can be reasonably achieved with low-cost GPS receiving equipment.

David Evans is a technical author with many years experience in the time and frequency industry. David has written a number of technical articles providing information on NTP servers. For more information on computer time server systems, please visit: http://www.timetools.co.uk/data/time-server.htm

Article Source: Using GPS to Synchronise Computer Systems - ArticleSpan.com

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