  Here we provide an alternative approach to determine the Earth's external
gravitational potential field based on low-orbit target satellite (TS),
geostationary satellites (GS), and microwave signal links between them. By
emitting and receiving frequency signals controlled by precise clocks between
TS and GS, we can determine the gravitational potential (GP) at the TS orbit.
We set the TS with polar orbits, altitude of around 500 km above ground, and
three evenly distributed GSs with equatorial orbits, altitudes of around 35000
km from the Earth's center. In this case, at any time the TS can be observed
via frequency signal links by at least one GS. In this way we may determine a
potential distribution over the TS-defined sphere (TDS), which is a sphere that
best fits the TS' orbits. Then, based on the potential distribution over the
TDS, an Earth's external gravitational field can be determined. Simulation
results show that the accuracy of the potential filed established based on
30-days observations can achieve decimeter level if optical atomic clocks with
instability of $1\times 10^{-17}\tau^{-1/2}$ are available. The formulation
proposed in this study may enrich the approachs for determining the Earth's
external gravity field.
