The Special Relativity Theory (SRT) and the General Relativity Theory (GRT) sometimes exhibit clock effects of equal magnitude which cancel and sometimes exhibit clock effects of equal magnitude which are additive. This cannot be coincidence, yet there is nothing within the two disjoint relativity theories to suggest an underlying mechanism. The effects appear to be related to energy, but the SRT treats energy as relative and the GRT treats an orbiting body as following a force-free trajectory. Thus an alternative to the Einstein theories seems to be required. I have proposed what I call a Modified Lorentz Ether Theory (MLET) which extends the Lorentz ether concepts to cover gravitational phenomena. Following a brief review of MLET concepts, it is contrasted with SRT concepts. Current data available from a number of modern experiments are evaluated with regard to both MLET and SRT. In general, MLET provides a more coherent and consistent explanation of the data. In the principal section of the paper it is shown that in the earth-centered inertial (ECI) frame Global Positioning System (GPS) clocks must not be adjusted for the gradient of the sun's gravitational potential. MLET shows that the differential effect of the sun's gravitational potential is absorbed into the clock bias which converts the Selleri transformation into an apparent Lorentz transformation. By contrast, there is no valid explanation for this phenomenon which is consistent with SRT/GRT and they are thereby refuted. This is very strong evidence that some form of Lorentz ether theory is valid and that Einstein's relativity theories are invalid.
