gzipped_tar writes: Whether the fundamental constants really stay the same is always a question worth asking. In particular, the constancy of Planck's Constant is something that cannot be simply ignored owing to its universal importance in linking the quantum and classical pictures of our world. Using publicly available GPS data and terrestrial clocks, researchers form the California State University were able to verify that the value of h indeed stays the same across different positions in the vicinity of our Earth. Their result says the local position invariance of h is satisfied within a limit of 0.007. The paper is published in the journal Physical Review Letters (paywalled), and a free-to-read preprint is available on arXiv. tl;dr version for slashdotters: by the well-known formula E = h * f, a hypothetical variation on h induces changes in f, the transition frequency that keeps the time in atomic clocks, both on earth and aboard the satellites. When taking account of other time variations such as general relativistic time dilation, and assuming the invariance of E (atomic transition energy) on physical grounds, we can figure out an upper bound on the variation of h reflected in the measured variation in f.