Special and General principle of relativity
Special relativity determs that all uniform motion are relative and motion must be considered relative motion compared to other object. Without a reference to something else we can not say if there is motion. There is no such thing like Isaac Newton's Absolute space nor Absolute time. Time and distance are relative but the speed of light is always constant. James Clerk Maxwell (1831-1879) unifyi in 1865 in his partial theories the forces of electricity and magnetism. In Maxwells electromagnetism theory where magnetism and light are considered to be same kind of wave motion. 
Maxwell showed mathematically that electric and magnetic forcies do not arise from particles acting directly on each other; rather, every electric charge and current creates a field in the surrounding space that exerts a force on every other charge and current located within space. He found that a single field carries electric and magnetic forces; electricity and magnetism are inseparable aspects of the same force. He called that force the electromagnetic force, and the field that carries it the electromagnetic field. 
Fundamental pospulate of special relativity stated that the laws of science should be same for all freely moving observers, no matter what speed they were moving at. General relativity the priciple of eqivalence extends this to those observers who are not freely moving but are under the influence of gravitional field. 
If the gravitional field is not uniform, you must apply the principle separately to series of small, overlapping patches. In small enought regions of space, it's impossible to tell if object is at rest in gravitional field or uniformly accelerating in empty space. 
We propose for looking for the smallest region on which we can measure objects motion under gravitation and acceleration.
This way we will control motion most accurate, energy efficient and control of motion will be easy even in changing conditions of space.
The speed of light is the fastest method to transfer information. Light travels always with the same speed compared to any observer dispite of the observers own speed. The speed of ligth is invariant.
Measuring accuracy can be adjusted by changing simultaneous number of received optical light waves, colors and changing optic magnification.