TT – Terrestrial Time

Terrestrial Time (TT) is a theoretical time scale defined as the independent variable for geocentric ephemerides and astronomical calculations. It is a uniform time scale derived from International Atomic Time (TAI), with a constant offset of 32.184 seconds (TT = TAI + 32.184 s). TT serves as the practical realization of time at the Earth’s surface for high-precision scientific applications.

In geodesy, TT is essential for precise satellite orbit determination, Earth rotation modeling, and the transformation between terrestrial and celestial reference frames. Consistent use of TT ensures temporal coherence in space-geodetic techniques such as GNSS, VLBI, and SLR, supporting accurate positioning and reference frame realization.