Page "Terrestrial Time" Paragraph 33

The present definition of TT is a linear scaling of Geocentric Coordinate Time ( TCG ), which is the proper time of a notional observer who is infinitely far away ( so not affected by gravitational time dilation ) and at rest relative to the Earth.

TCG is used so far mainly for theoretical purposes in astronomy.

From the point of view of an observer on the Earth's surface the second of TCG passes in slightly less than the observer's SI second.

The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster.