"The Confrontation between General Relativity and Experiment"
Clifford M. Will 
Figure 3
Figure 3: Selected tests of local position invariance via gravitational redshift experiments, showing bounds on α, which measures degree of deviation of redshift from the formula Δ ν∕ν = ΔU ∕c2. In null redshift experiments, the bound is on the difference in α between different kinds of clocks.