The global positioning system (GPS) can be used by GPS time server appliances as a source of very accurate time for the synchronization of computer networks. The GPS system has been in operation for a number of years. It is primarily used for positioning and marine navigation. However, it can also be used as a source of very accurate time. The system comprises of a constellation of orbiting satellites, each located at a critical location to provide coverage of the entire earths surface. Each satellite has an integral atomic clock, which is periodically synchronized by atomic clocks located at the US Naval Observatory (USNO) so that the system of satellites are very accurately synchronized with each other. A widely available and inexpensive GPS antenna and receiver can be used to obtain positioning and timing information from the satellites.
Optimized Satellite Orbit
The location of each satellite in orbit is crucial to provide world-wide coverage. This is a big advantage over other sources of accurate time. Radio time and frequency broadcasts, such as WWVB, MSF and DCF-77 have a finite range and are therefore regional, often defined by national boundaries. Additionally, radio broadcasts operate at different frequencies and each broadcast is encoded differently. This means that generally, different receiving hardware and decoding software is required to decode each transmission. Also, availability of the radio signals can vary according to maintenance schedules and also due to local topography. Reception near mountain ranges or in valleys can be challenging at best. GPS on the other hand is available 24 hours a day, 7 days a week, 365 days of the year. Also, being a satellite based system, reception in mountainous areas and valleys is still easily achievable. Additionally, the GPS signal is rarely affected by shutdown for maintenance.
Typical GPS Timing Accuracy
A GPS timing receiver can provide a time stamp accurate to within a few nanoseconds of UTC time. Often the timing signals provided by GPS are utilized to calibrate or ‘discipline’ a high-stability oscillator, such as a TCXO, OCXO or Rubidium device. Using a stable oscillator in this manner can provide a very accurate timing reference even when the GPS signal becomes unavailable for a period of time. This operation is often referred to as ‘holdover’. OCXO and Rubidium oscillators provide the best performance, but are very expensive. While TCXO provides an excellent price-performance compromise.
A GPS referenced time server installation will typically consist of a roof-mounted antenna connected to a surge suppressor by a length of coax cable. The surge suppressor is typically located where the cable enters the building. Therefore, cable and equipment prone to surge is on one side of the suppressor, while the indoor cable which is less prone is on the other. A further length of coax cable connects the surge suppressor to the time server. Typically, the time server is mounted in a 19″ rack or cabinet, however, compact desk mounted servers are available. Each coax connection is generally achieved using a secure and rugged TNC or N-type connector. Power to the antenna is provided by the server itself – typically, 5V is used, but 3.3V and 12V are common.
TimeTools SR Series is a Stratum 1 GPS-referenced NTP server. The device obtains accurate time from the GPS system and using the NTP protocol provides a network with an accurate clock reference. The unit is provided in a high-quality 1U high 19-inch rack-mountable aluminium enclosure. It occupies minimal rack-space. Also, with it very low power consumption it runs very cool, aiding reliability and the environment. It is available with a range of hardware reference clocks and also high-stability back-up oscillator options. Using NTP and SNTP protocols, the SR series can accurately synchronize most servers, workstations and network infrastructure to within milliseconds of the correct time. It’s reliability is demonstrated by the fact that the device is provided with an industry-leading 5-year warranty.
TimeTools is a leading manufacturer of GPS time server appliances and time references for synchronizing servers, workstations and other network devices. Since 2002, TimeTools has been supplying high-quality, great value, timing equipment to large, high-profile, companies around the world.