NTP time servers are devices that distribute a single time source amongst a computer network to ensure that all machines and devices are synchronised to the same time.
Most time servers use the protocol NTP (Network Time Protocol) which distributes a single time source across the network. Most time servers receive a UTC time source (coordinated universal time) which is a global timescale based on the time told by atomic clocks.
An inaccurate time stamp, or a computer network that is not synchronised will lead to all sorts of problems including failing of time sensitive applications, emails arriving before they have been sent and even leaving a system open to fraud.
A time server is therefore an essential piece of hardware as it is the only means of keeping a computer network running accurate and synchronised time.
There are two methods in which a NTP time server can receive a secure and accurate UTC time source. The first is via specialist national time and frequency transmission broadcast in long-wave by several national physics laboratories such as MIT (Massachusetts Institute of Technology) in the US who broadcast the WWVB radio time signal or NPL (National Physical Laboratory) in the UK who broadcast the MSF time signal.
The second method is to use the signal from the GPS network (Global Positioning System). All GPS satellites transmit a timing signal from their onboard atomic clocks, it is this signal that is utilised by satellite navigation devices and can be received by GPS time servers as a timing reference.
Dedicated NTP time servers are therefore either designed to receive the radio signal or the GPS network, although some high quality devices are duel systems, designed to receive both GPS and radio to ensure more accurate synchronisation, reliability and are more failsafe in case one signal fails.