A while ago, I decided to play around with trying to create something cheap that would be able to synchronise its time by using GPS instead of over the internet – essentially building my own Stratum 1 time server. I intend this to be a series of articles about NTP and time servers, but let’s start with the basics…

What is a Stratum 1 Time Server?

The lower the “Stratum” number the more accurate the time it should be able to tell. Usually Stratum 1 time servers are directly connected to an accurate source of time such as (but not limited to) Atomic, GPS or even Radio clocks. The stratum of the machine which itself is directly interfacing with the accurate time source would be “Stratum 0”. For each additional layer of NTP servers added, the stratum increases by 1.

So if the server that’s connected directly to the time source is stratum 0, it would appear to any clients who want to synchronise with it as a stratum 1 source. If someone then tries to synchronise to one of those, they would see a stratum 2 source and so on. The image on the right illustrates the top-down increase in stratum and was taken from Wikipedia.

Generally the higher the stratum number, the less-accurate its view of time is. A stratum ranges from stratum 1 all the way to stratum 15 indicating the time is getting further away from a reference clock the higher you go. Stratum 16 indicates the clock is pretty far from the reference and should be considered un-synchronised and should not be used.