  According to the general relativity theory, the geopotential difference can
be determined by gravity frequency shift between two clocks. Here we report on
the experiments to determine the geopotential difference between two remote
sites by hydrogen masers based on precise point positioning time-frequency
transfer technique. The experiments include the remote clock comparison and the
local clock comparison using two CH1-95 active hydrogen masers linked with
global navigation satellite system time-frequency receivers. The frequency
difference between two hydrogen masers at two sites is derived from the time
difference series resolved by the above-mentioned technique. Considering the
local clock comparison as calibration, the determined geopotential difference
by our experiments is 12,142.3 (112.4) m^2/s^2, quite close to the value
12,153.3 (2.3) m^2/s^2 computed by the EIGEN-6C4 model. Results show that the
proposed approach here for determining geopotential difference is feasible,
operable, and promising for applications in various fields.
