If the speed of light is constant regardless of the state of motion of the source, then doesn't this imply that it is possible to calculate the velocity of a reference frame by measuring the time it takes for light to traverse some known distance in that frame. For instance if our frame is moving in the positive x direction with velocity v and a pulse of light is emitted from the origin, shouldn't it be possible to calculate v by considering the time it takes for the pulse to reach some position x? The actual distance travelled by the light beam will be the measured distance x plus the amount our reference frame has travelled in the requisite time it took for the light to reach x. In maths, ct=x+vt, and therefore v=c-x/t, where x is the apparent distance travelled by the light as measured in our reference frame, t is the amount of time it took for the light to make that trip, and v is the velocity of our reference frame. In sum, since the speed of light is constant, the amount of time it takes for light to travel some fixed distance will depend on one variable only, and that is the state of motion of the frame in which the time is measured, and thus it should be possible to deduce the state of motion of a reference frame, simply by measuring how long it takes for light to travel a fixed distance. My question is, doesn't something have to be wrong here, since according to relativity it should be impossible for an inertial observer to deduce his own state of motion by observing local phenomena?