  Spin 1/2 particles in geodesic trajectories experience no gravitational
potential but they still have non-zero couplings to the curvature tensor. The
effect of space time curvature on fermions can be parameterized by a vector and
a pseudo-vector potential. These apparent CPT violating terms can be measured
with satellite based spin-polarized torsion balance and clock comparison
experiments. The Earth's curvature effect is of the order of 10^{-37} Gev which
is not far from the present bounds of ~10^{-29} Gev on such CPT violating
couplings.
