Light transmission on the surface of the rotating Earth is examined using the Langevin metric of general relativity. It is shown that this metric which yields the rigorously tested and verified algorithm for clock synchronization in the Global Positioning System (GPS) also predicts east-west light speed anisotropy that is inconsistent with light speed isotropy postulated to hold at a local level on the rotating Earth. This light speed anisotropy is further demonstrated using experimentally confirmed light travel time in a simple speed calculation as well as the classical method of light speed relative to a terrestrial observer rotating with the Earth for light travel in the Earth-Centred Inertial (ECI) frame. These theoretical results are confirmed by experimental findings from GPS measurements and lead to the re-interpretation of the Sagnac effect normally treated as a time correction in GPS operation.