http://education-portal.com/academy/lesson/light-and-relativity-breakdown-of-classical-relativity-with-light-example.html "IS ALL MOTION RELATIVE? (...) IT APPEARS THE ANSWER IS NO. (...) SPEED OF LIGHT IS NOT RELATIVE. The speed of light is c, and it's c regardless of the source of the light or the perspective of the observer. Let's look at an example. Imagine you're traveling away from the sun at a velocity that is really close to the speed of light - let's say 250,000 km/sec (that's pretty fast). Let's have fun with this and say you can see a photon of light moving away from the sun and passing you up through some window on your spaceship. Now get out your trusty radar gun and measure the speed of light as it passes by the window. What would the speed of light measure relative to your ship? Based on classical relativity, you might predict the velocity of the light relative to your ship to measure 50,000 km/sec, as your ship is already traveling 250,000 km/sec and 300,000 km/sec - 250,000 km/sec = 50,000 km/sec. However, you would measure the speed of light to be 300,000 km/sec. Don't buy a new radar gun just yet. As it turns out, your radar gun is correct. The speed of light is the same relative to your fast-moving ship or even an external stationary point."
IT APPEARS THE ANSWER IS YES:
http://www.datasync.com/~rsf1/crit/1908l.htm Walther Ritz (1908): "The only conclusion which, from then on, seems possible to me, is that (...) THE MOTION OF LIGHT IS A RELATIVE MOTION LIKE ALL THE OTHERS, that only relative velocities play a role in the laws of nature..."
http://www.worldnpa.org/pdf/abstracts/abstracts_215.pdf Herbert Dingle: "Either there is an absolute standard of rest - call it the ether as with Maxwell, or the universe as with Mach, or absolute space as with Newton, or what you will or else ALL MOTION, INCLUDING THAT WITH THE SPEED OF LIGHT, IS RELATIVE, AS WITH RITZ."
http://www.sps.ch/fr/artikel/geschichte_der_physik/walter_ritz_the_revolutionary_classical_physicist_2/ Jan Lacki: "Ritz had no time to make his theory more elaborate. He died complaining that no one, even in Göttingen, was granting his views sufficient care. His emissionist views were submitted to heavy criticism and experimental tests were later realized to show their inanity. Today, with considerable hindsight, we know the end of the story and how Einstein and Planck's views shaped our contemporary physics. While few would today contest the reality of quanta or turn their back on field theory of elementary processes, it is interesting to know that the criticisms against Ritz's conceptions were shown, since then, often wanting, if not simply incorrect. It is fair to say that if Ritz's emission theory is false, it cannot be as easily dismissed as it was thought in Ritz's times."
https://webspace.utexas.edu/aam829/1/m/Relativity.html Alberto Martinez: "Does the speed of light depend on the speed of its source? Before formulating his theory of special relativity, Albert Einstein spent a few years trying to formulate a theory in which the speed of light depends on its source, just like all material projectiles. Likewise, Walter Ritz outlined such a theory, where none of the peculiar effects of Einstein's relativity would hold. By 1913 most physicists abandoned such efforts, accepting the postulate of the constancy of the speed of light. Yet five decades later all the evidence that had been said to prove that the speed of light is independent of its source had been found to be defective."
http://a-levelphysicstutor.com/wav-doppler.php "vO is the velocity of an observer moving towards the source. This velocity is independent of the motion of the source. Hence, the velocity of waves relative to the observer is c + vO. (...) The motion of an observer does not alter the wavelength. The increase in frequency is a result of the observer encountering more wavelengths in a given time."
http://physics.bu.edu/~redner/211-sp06/class19/class19_doppler.html Sidney Redner: "The Doppler effect is the shift in frequency of a wave that occurs when the wave source, or the detector of the wave, is moving. Applications of the Doppler effect range from medical tests using ultrasound to radar detectors and astronomy (with electromagnetic waves). (...) We will focus on sound waves in describing the Doppler effect, but it works for other waves too. (...) Let's say you, the observer, now move toward the source with velocity vO. You encounter more waves per unit time than you did before. Relative to you, the waves travel at a higher speed: v'=v+vO. The frequency of the waves you detect is higher, and is given by: f'=v'/(lambda)=(v+vO)/(lambda)."
http://www.usna.edu/Users/physics/mungan/Scholarship/DopplerEffect.pdf Carl Mungan: "Consider the case where the observer moves toward the source. In this case, the observer is rushing head-long into the wavefronts, so that we expect v'>v. In fact, the wave speed is simply increased by the observer speed, as we can see by jumping into the observer's frame of reference. Thus, v'=v+v_o=v(1+v_o/v). Finally, the frequency must increase by exactly the same factor as the wave speed increased, in order to ensure that L'=L -> v'/f'=v/f. Putting everything together, we thus have: OBSERVER MOVING TOWARD SOURCE: L'=L; f'=f(1+v_o/v); v'=v+v_o."