  In their Letter, Kentosh and Mohageg [Phys. Rev. Lett. 108, 110801 (2012)]
seek to use data from clocks aboard global positioning system (GPS) satellites
to place limits on local position invariance (LPI) violations of Planck's
constant, h. It is the purpose of this comment to show that discussing limits
on variation of dimensional constants (such as h) is not meaningful; that even
within a correct framework it is not possible to extract limits on variation of
fundamental constants from a single type of clock aboard GPS satellites; and to
correct an important misconception in the authors' interpretation of previous
Earth-based LPI experiments.
