Comment on "Global Positioning System Test of the Local Position   Invariance of Planck's Constant"
  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. 
J. C. Berengut and V. V. Flambaum
gr-qc astro-ph.CO hep-ph physics.atom-ph physics.space-ph