GPS is increasingly becoming the standard time reference for the synchronization of computers and computer networks. The system itself is an US military satellite system, primarily intended for global positioning and navigation. The system consists of a constellation of 24 orbiting satellites, each carefully positioned to provide global signal coverage. Each satellite has an on-board atomic clock, and continually broadcasts a weak radio signal to the surface of the Earth containing highly precise timing information. This information can be used for timing applications as well as the more normal location detection and navigation.
A GPS-reference NTP server has a number of fundamental advantages over other precise sources of time. The primary advantage is that GPS signals can be received anywhere in the world. Unlike radio broadcasts, such as MSF, DCF-77 and WWVB – it is not limited by transmission range or national boundaries. Additionally, the same equipment can be utilized in any location. Whereas national radio time signals vary in frequency and data format so that different receiving equipment and software is required to receive signals in different localities. GPS also has an unsurpassed, very high, degree of precision.
Traditionally, GPS antennas required line-of-sight view of the sky in order to obtain a consistent signal lock. Ideally, they would be located on a rooftop with an all-round view of the sky. However, modern timing receivers can operate down to a single satellite in view, rather than the traditional four. This allows receivers to maintain a signal lock with a much-reduced view of the sky – maybe from a windowsill location.
Modern receivers are also becoming much more sensitive and can often operate with antennas located indoor, with no line-of-sight view to the sky. This feature can often provide a substantial cost saving over traditional receivers, since the expense of cabling and installation of an antenna on a rooftop is often unnecessary.
A GPS time server generally contains a GPS receiver enclosed within the unit. A TNC or BNC type coax connector is often used to connect the device to an antenna using RG58 or similar 50-ohm coax cable. The antenna is powered from the time server, so that no additional power outlet is required at the antenna location.
If the antenna is located outdoors, it is generally accepted that some form of surge suppression be used. A surge suppressor protects the base unit and attached network from voltage surges generated by potential lightning strikes. They generally take the form of either small gas-filled barrier devices or solid-state electronic surge diverting devices. The suppressor generally sits in-line on the coax cable and is located where the cable enters the building.
For very long cable runs between an antenna and base unit, an amplifier can be used to boost signals to overcome cable losses. Alternatively, a media converter can be used, such as an optical fibre converter, to transmit signals over extremely long distances, even kilometres.
The NTP server appliance will use the GPS signals to synchronise and calibrate it’s internal time. Typically, they can synchronise to within a few microseconds of the correct time. The server will provide an indication of signal lock as well as the number of satellites in use at any one time along with the signal to noise ratio (SNR) of communication with each satellite. This information is very useful for debugging and proving antenna location suitability.
Galileo is the name given to the civilian global navigation satellite system (GNSS) being constructed by the European Union. Galileo aims to provide a reliable GNSS system on which the European nations can rely and which is independent of other similar systems.
The system will be based around a constellation of 30 orbiting satellites, 27 will be operational at any given time and 3 will be used as spares. The service will provide both horizontal and vertical positioning information to a precision of 1 metre. Basic, low-precision, positioning services will be free-to-air, while a higher-precision service will be available for commercial organisations, which will require a subscription fee.
The first two satellites were launched in October 2011, followed by a further two in October 2012. The initial four-satellite system allows system testing to be carried out while the other satellites are being constructed and launched. It is hoped that a complete system of 30 satellites will be operational by 2019.
The Galileo GNSS system should provide an additional source of precise time which will be available globally and can be received without charge. It will provide an alternative source of time for applications whose reliance on the current US military service is sensitive.
To summarize, GPS is currently the preferred means of providing computer networks time synchronization. They can operate anywhere in the world and provide a highly accurate time resource for computer networks and other timing applications. Galileo will provide an additional source of precise time based on a civil rather than military service.
About the Author.
Andy Shinton has spent his entire career within the IT industry, mainly in the Time and Frequency sector. Since 2002, he has headed TimeTools Research and Development Division. Andy regularly writes white-papers and articles about NTP and Network Timing Solutions.[/box]