sevenperforce

Rich.

You have two variables: time and distance.

If you have three spacecraft which are all measuring the distance and relay time of each other, then you can check their internal timekeeping against each other to determine whether there is variance in electromagnetic behavior.

Sigh.

You clearly fail to grasp even the most basic principles of physics -- seriously, triangulation is EASY to figure out -- and stubbornly insist you know otherwise. This is indeed hopeless.

Neither of the issues you raise apply to the information I provided you. Nor did I fail to answer or avoid your question in any way. My explanation (triangulation, multiple relay, and the use of multiple solar system bodies to create a gigantic Michelson-Morley interferometer) fully and completely precludes the sort of confounding issues you raised.

You apparently think you understand the topics being discussed, when in fact you do not. Since it seems unlikely that you will recognize your lack of subject matter familiarity or accept any new information, I don't see what use it is to attempt further education.

Sorry, @KAL 9000. Not much else I can do.

CC @Darnok @KAL 9000

There are currently five satellites orbiting Mars, plus multiple ground probes in communication with the satellites. They use a variety of different solid-state timekeeping devices with multiple layers of redundancy.

But why does it matter? If someone uses a match to light a cigarette, you don't have to know the exact chemical composition of the matchhead to accept that the cigarette was indeed lit. The probes are all tests of each other in communication with Earth. They all triangulate each other. If their clocks were wrong, they wouldn't match predictions.

And while you're at it, the clocks AREN'T actually in perfect lockstep with Earth, or with each other. Rather, they have all deviated based on the spacetime distortion encountered between Earth and Mars and extended Martian orbits. Just as anticipated.

Simplest example:

There are multiple probes orbiting Mars. We communicate with all of them, and they communicate with each other. They all have clocks on them. We have clocks too.

If the speed of light was not invariant, then the signal we receive when a probe is moving away from us in its Martian orbit would be different than when it is moving toward us in its Martian orbit. Specifically, the photons coming toward us would be moving slower in the first case than in the second case. This doesn't happen.

Not only does this prove lightspeed invariance on Mars, but it proves lightspeed invariance across the entire path between here and Mars. By extension, then, we can use this relay to test lightspeed invariance across the entirety of the inner solar system, since the path between Mars and Earth sweeps out the entire inner solar system every sidereal year. You can even do the experiment at opposition to measure how the sun's gravity distorts light geodesics. 

And lest you think there is some other anomaly that just mimics lightspeed invariance, remember that there are multiple probes spinning around Mars. Enough to exactly triangulate each other's positions in real time and precisely measure lightspeed relay. 

The same thing is true for the moon, though I'm not sure off the top of my head whether we have multiple lunar satellites operating simultaneously right now. We certainly have in the past.

Another way to test lightspeed invariance on the moon is to use the reflectors on the lunar surface left by Apollo. Amateur stargazers can send multiple laser pulses to multiple reflectors at once, at moonrise and at moonset, both at the lunar perigee and at the lunar apogee. Each instance precisely tests relativity and lightspeed invariance, effectively using the entire Earth-moon system as a gigantic Michelson-Morley interferometer. The predictions of relativity exactly match the results every single time.

So yeah. Lightspeed invariance and relativity are not only provable but are fully proven across the solar system. 

While you're at it, we can observe lightspeed invariance and relativity all the way out to the edge of the universe. When a supernova in a distant galaxy explodes, we can use the redshift of the hydrogen lines in surrounding stars to precisely measure local system velocity, then measure redshift within the expanding nebula to measure local velocity. We can then observe the multiple layers of reflection and excitation in the surrounding medium to again confirm that the speed of light is exactly the same there as it is here.