GPS referenced network time servers utilise the global positioning system to obtain very precise time for computer network time synchronization purposes. The GPS system is an US military system monitored by the U.S. Naval Observatory (USNO) and is primarily used for navigation purposes. The system consists of a constellation of 24 orbiting satellites. Each satellite contains an extremely precise atomic clock that can be utilized by equipment on the surface of the Earth as an accurate time reference. Timing signals can be received by a GPS NTP server using a relatively low-cost GPS receiver and external antenna.
The use of GPS in time servers has a number of distinct advantages over other time references, such as CDMA and radio time and frequency broadcasts, that are frequently used for synchronization.
GPS transmissions can be received world-wide, the signal can be easily received anywhere on the surface of the Earth, provided that an antenna can be located with a good clear view of the sky and line of sight to the satellites. Also, it is the most accurate broadcast time reference currently available. It can provide a time signal that can be traced to within nanoseconds of UTC time. Additionally, the implementation of the European satellite navigation system Galileo promises to improve accuracy even further and should be fully compatible with existing receivers. GPS signals are broadcast continuously, the signal is rarely interrupted by the requirement of maintenance.
CDMA and Radio Broadcasts
In contrast, CDMA and radio time and frequency broadcasts have a finite transmission range from a terrestrial based transmitters. Radio time broadcasts are defined by national boundaries. Time broadcasts from various national transmitters are incompatible with each other and generally operate at different frequencies to avoid interference. An antenna tuned to the correct frequency is required to receive the required broadcast. Radio time references generally provide far less accuracy than GPS, usually in the range of 1 to 50 milliseconds. Additionally, regional radio time broadcasts can be switched off periodically for maintenance leaving timing equipment unsynchronized until the signal re instigated.
One disadvantage of GPS when compared with CDMA and radio broadcasts is that a GPS antenna needs to have a line of sight to the satellites. Ideally, the antenna should be mounted on a rooftop with a full 360 degree view of the sky. CDMA and radio broadcasts generally penetrate ordinary brick-built buildings. Therefore, a good signal can often be received with an indoor antenna located close to the time server. However, metal-work such as steel frames and cladding can act as a Faraday cage blocking out radio signals. Under these circumstances, a the antenna would need to be located outdoors offering no significant advantage over GPS. Also, the topology of the surrounding land can severely impact radio reception. Antennas located in valleys or the other side of hills from the transmitter can suffer reception problems.
Modern GPS receivers are getting ever more sensitive and now often have the ability to receive signals indoors. Additionally, they can often provide timing information from a single satellite in view thus providing much more resilience in challenging environments with restricted line of sight to satellites. GPS NTP network time server solutions have overtaken radio based systems as the synchronization reference of choice.
A Typical GPS Antenna Installation
An externally roof-mounted antenna can be highly susceptible to electrical surges from lightning strikes. A direct strike on an antenna can destroy expensive equipment utilizing the antenna. Additionally, surges can jump across air gaps to other cables in close proximity and damage other equipment. This can be easily remedied using a surge suppressor, which simply sits in-line on the coax cable between the antenna and receiver. A suppressor that can operate in the correct frequency range is required (1575MHz). There are two basic types of surge suppressor – gas discharge and electronic suppressors. Gas discharge suppressors act very much like a fuse and are destroyed when activated by a surge, but protect any equipment the clean side of the suppressor. Electronic suppressors work by channeling any surge to a low-impedance ground, away from the protected equipment. Electronic suppressors have the advantage of having multi-strike capability and of being maintenance free.
GPS systems generally utilize coax cable between the antenna and receiver. For distances up to 50m, typically low-cost coax , such as RG58, can be utilized. However, for longer cable distances a slighter higher-quality coax will be required. Low-cost amplifiers can be used to boost signal levels for extremely long cable runs. While GPS over Fiber systems can be used to provide completely noise and interference immune cable runs of many kilometers between antenna and receiver. GPS splitters can provide additional flexibility and reduce installation costs by sharing a single antenna between multiple time servers or other receivers.
To summarise, GPS NTP Server systems can be more costly to install than CDMA and radio solutions. Often requiring cabling to an external roof-mounted antenna, however, they generally provide a much more accurate and stable source of time for network time synchronization. Also, with an extensive range of hardware a solution can be found for even the most challenging environments.
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