Zenith Total Delay (Marine GPS)

Zenith total delay (ZTD) is the propagation delay a GNSS signal accumulates passing vertically through the neutral atmosphere, the sum of the zenith hydrostatic delay (about 2.3 m at sea level, modeled from surface pressure) and the zenith wet delay (a few to tens of centimeters from water vapor). On a marine or coastal GNSS station it is estimated together with the position by applying mapping functions to slant observations. The wet part yields precipitable water vapor, so shipborne and coastal GNSS feed GNSS meteorology, while accurate ZTD modeling is also needed to reach centimeter heights in RTK and tide-gauge geodetic positioning.