Zero Tide (ZT) system

The zero tide system is the modern geodetic convention and the current international standard to handle the permanent (time-independent) effects of the Sun and Moon on the Earth’s shape and gravity field. It serves as a middle ground between the tide-free and mean-tide systems by specifically accounting for how the Earth’s mass reacts to permanent gravitational forces. In a zero tide system, the direct gravitational attraction of the Sun and Moon is removed, but the indirect permanent physical deformation of the Earth’s crust is retained.