when the speed of light was first measured, it was done by using the light from two stars that were occulted by Jupiter. THEN, using the highly complex equation of D=R Xt (Distance equals rate times time). and then , by observing the star light occultation from a known occultation clock (set up by several years of observation) and solving fro the rate,the speed of light was first established and ,based upon the orbital accuracy, the time was determined at a fairly accurate rate.
Thi is developed quite nicely in a book entitled E=Mc^^, An analysis of the world's most famous equation
There are so many thgings that depend upon c and that includes extremely accurate (sub centimeter) GPS mapping. Its mostly a function of having c be a constant. Other things include synchronized world-wide news (ever notice how theres a slight delay between responses in satellite interviews? WELL that has nothing to do with c--but setting up a very accurate clock of the transmission is dependent upon a constant c)