Ntp Server Reference Clocks: What's Best Radio or GPS ?

A NTP Server is a network device that provides network servers and other network infrastructure with an accurate timing reference. A NTP Server obtains precise time from an external time source, such as radio time and frequency broadcasts or GPS. This article describes the external time references that are available to synchronise NTP servers and discusses the merits of each.

There are a number of radio time and frequency broadcasts that are available in various locations. The WWVB signal is a US based reference, DCF-77 in Germany and MSF in the UK. These radio time and frequency broadcasts provide a stable and precise source of time. Additionally, an indoor located radio antenna can be generally be used to receive these signals - lowering installation costs. However, radio signals can be affected by local topography - such as hills and valleys. There is no guarantee of radio signal reception even well inside the transmission range of the signal.

The MSF radio signal broadcast in the UK by the National Physics Laboratory recently moved from Rugby in the East Midlands to Anthorn in Cumbria. Since the move, it has been widely acknowledged that signal reception in the South of England has suffered - even though it is well within the physical reception range for the signal.

Local environments can also affect radio signals. Radio antennas placed inside metal cages or metal clad buildings can experience poor signal reception. Also, antennas located in a basement or underground may fail.

GPS utilised the Global Positioning System intended for worldwide navigation. Each GPS satellite has an onboard highly precise synchronised atomic clock - an ideal reference for NTP server timing.

The GPS system is a constellation of 24 orbiting satellites each broadcasting continuous time and positioning information. This information can be received anywhere on the face of the planet, providing an antenna can be provided with line of sight to the satellites.

GPS has a number of advantages over radio-based time and frequency broadcasts. Firstly, GPS can be received anywhere in the world and is not restricted by transmitter power and radio signal range. Secondly, the GPS signal is a much more accurate reference than radio solutions. Thirdly, GPS timing information can be received by anyone who can provide an antenna with a good view of the sky. However, GPS antennas do need to have line of sight with GPS satellites. Therefore, the ideal location for a GPS antenna is on a rooftop with a full 360-degree view of the sky. Additionally, if buildings or trees obscure the horizon in anyway, it can affect reception integrity. Roof-mounted antennas can significantly add to the installation costs of a NTP server system.

To summarise, radio and GPS based time and frequency references for NTP server systems both have certain advantages and disadvantages. However, the accuracy of GPS and the guaranteed reception, albeit with roof-mounted antennas, make it the reference of choice for NTP server systems.