The GPS signal includes ephemeris data about where the satellites are located. As Wikipedia puts it, "In addition to the PRN ranging codes, a receiver needs to

The GPS signal includes ephemeris data about where the satellites are located. As Wikipedia puts it, "In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite." Yes, if there's a very small change - and I don't know what the limits are - then it wouldn't be detectable. FWIW, 1ppm is about 1.2 cm at GPS orbital distances. My point is still that the speed of light is being measured all the time, not that every measurement is continually improving upon the accuracy of that measurement. To further complicate things, as far as we can tell the speed of gravity is the speed of light. Should the speed of light change, then either GPS orbits change (due to the change in gravity) or they are not at the same speed.