Comment Information Week's editing is shocking. (Score 1) 287
Greenwich Mean Time is a known source of reliable time, as is the US Naval Observatory. Their time is based on the solar day -- the time it takes for the earth to complete a rotation in its orbit. NTP consults UTC or Universal Coordinated Time, which is Greenwich Mean Time expressed in the military's 24:00:00 hours terms.
On a daily basis, NTP also consults atomic clocks, which tick off precise seconds based on radioactive Cesium-133 decomposition. A GPS receiver can be tied into an NTP server, and use the transmission of a GPS satellite to get the correct atomic time. A GPS satellite has three atomic clocks, so if one falls out of synch, the other two can overrule it and keep the system on track. For GPS time to be off by a billionth of a second means its answer to a location query will be off by a foot. So GPS relies on precisely counted time, not the solar day.
Wow, that's so bad I'm not sure where to start; "Greenwich Mean Time" is a) a timezone still used by the UK when "British Summer Time" is not in effect, and is similar but not the same as UTC "Universal Coordinated Time" timezone, c) based upon the mean solar time at the Royal Observatory in Greenwich, London, UK.
UTC "Universal Coordinated Time" is the present day global standard time reference (yes damnit that is the correct English name, in French "temps universel coordonné" or unofficially "Universel Temps Coordonné" with an unofficial English name of "Universal Time, Coordinated" to keep the abbreviation similar to UT0, UT1, etc.).
The "military time" (i.e. 24-hour clock) reference is nonsense, and ignore 24-hour clock usage in civilian European life, and as well as being standard in anything time oriented.
NTP is references to UTC, but UTC is in fact itself coordinated globally by about 80 national labs that operate their own national time references (typically 3 or more Cesium based time references, larger labs include hydrogen masers) which is coordinated by BIPM (International Bureau of Weights and Measures located in France). They work with International Astronomical Union (IAU) for things like determining when leap seconds are necessary to keep errors minimal. The largest contributors (by clock sources) are the US National Institute of Standards and Technology (NIST), US Naval Observatory, and the UK National Physics Laboratory (NPL) as I recall. The UK NPL and US NIST being pioneers in Cesium (Caesium) clocks.
GPS has become the dominate, and preferable means of professional time synchronizations over distance due to the presence of rubidium or caesium references on board the GPS satellites themselves, and the proliferation of low-cost, widely-available GPS receiver modules including time-synchronisation models with 10s of nanosecond or better accuracy (uncertainty). This means GPS has also become the preferred means of high quality synchronization of NTP "masters" or low stratum references. -- The under-noted point that GPS's geo-location functionality requires a high precision time synchronization between the multiple satellites to determine a position with any amount of accuracy (bounded uncertainty).
