time variables

[popi613]

If I were to consider three elements (or factors) of a system (per se) and one was the Royal Observatory at Greenwich, England, and a second was the CS2 Platform located in Braunschweig, Germany, what would be the third?

[popi613]

No.1 UTC/GMT  No.2 Cs2  No.3 International Telecommunications Union out of  Geneva, Switzerland