Choosing GPS or MSF as a Timing Reference for NTP Servers

All PC's and networking devices use clocks to maintain an internal system time. These clocks, called Real Time Clock chips (RTC), provide time and date information. They are battery backed so that even during power outages, they can maintain time. However, personal computers are not designed to be perfect clocks - their design has been optimized for mass production and low-cost rather than maintaining an accurate time.

NTP servers (Network Time Protocol) use a single time reference to synchronise all machines on the network to a time reference. This time reference can be either relative (a computer's internal clock or the time on a wrist-watch perhaps) or absolute such as a UTC (Universal Coordinated Time) clock source like an atomic clock that is as accurate as is humanely possible.

For some applications a relative time source is sufficient, however in many environments, such as airlines and the stock exchange it is essential for time to be absolute. Imagine buying an airline seat only to be told at the airport that the ticket was sold twice because it was purchased afterwards on a computer that had a slower clock!

Atomic clocks are the most absolute time-keeping devices. They work on the principle that the atom, caesium-133, has an exact number of cycles of radiation every second (9,192,631,770). This has proved so accurate the International System of Units (SI) has now defined the second as the duration of 9,192,631,770 cycles of radiation of the caesium-133 atom and the development of UTC (Coordinated Universal Time) now means computers all over the workld can be synchronized to the same time.

However, atomic clocks are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP servers can synchronise networks to an atomic clock by using either the Global Positioning system (GPS) network or specialist radio transmissions (MTF in the UK). It must be noted that Microsoft and others strongly recommend that external based timing should be used rather than Internet based, as these can't be authenticated. Specialist NTP servers are available that can synchronise time on networks using either the MSF (or equivalent) or GPS time server signal.

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 being 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.

There are also a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom's national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77).

A radio based NTP server usually consists of a rack-mountable time server, and an antenna, consisting of a ferrite bar inside a plastic enclosure, which receives the radio time and frequency broadcast. It should always be mounted horizontally at a right angle toward the transmission for optimum signal strength. Data is sent in pulses, 60 a second. These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

Both a GPS NTP server and MSF time server can provide an affordable and efficient way to accurately synchronise computer networks using NTP.