The orthometric height system plays a key role in geodesy, and it has broad applications
in various fields and activities. On an arbitrary equigeopotential surface, there does not exist the
frequency shift of an electromagnetic wave signal. However, between arbitrary two different
equigeopotential surfaces, there exists the frequency shift of the signal. Just due to this principle
of nature, one can determine the geopotential difference as well as the orthometric height
difference between two separated points P and Q using electromagnetic wave signals,
especially the GPS signals. GPS signals with a definite frequency f are emitted and two
receivers at P and Q on ground receive the signals coming from the emitter simultaneously.
The frequencies of the signals are recorded by receivers at P and Q, and consequently the
frequency difference (shift) between the received frequencies of the signals at P and Q is
determined. Then, the geopotential difference between these two points is determined based on
the geopotential frequency shift equation, and the corresponding orthometric height difference is
further determined based on the Bruns' formula. Further, using this approach a unified world
height datum could be realized, because P and Q could be chosen quite arbitrarily, e.g., they
are located on two separated continents or islands.