  The realization of International Height Reference System (IHRS) is one of the
major tasks of the International Association of Geodesy (IAG). A main component
of the IHRS realization is the global vertical datum unification, which
requires the connection of the existing local vertical height reference systems
(VHS). However, it is difficult to estimate the offsets between two local
height systems by conventional approaches when they are far apart. In this
paper, we formulate a framework for connecting two local VHSs using
ultra-precise frequency signal transmission links between satellites and ground
stations, which is referred to as satellite frequency signal transmission
(SFST) approach. The SFST approach can directly determine the geopotential
difference between two ground datum stations without location restrictions, and
consequently determine the height difference of the two VHSs. Simulation
results show that the China's VHS and the US's VHS can be unified at the
accuracy of several centimeters, provided that the stability of atomic clocks
used on board the satellite and on ground datum stations reach the highest
level of current technology, about $4.8\times 10^{-17}/\sqrt{\tau}$ for an
averaging time {\tau} (in seconds). The SFST approach is promising to unify the
global vertical height datum in centimeter level in future, and it also provide
a new way for the IHRS realization.
