Two spaceships with their engines shut off and identical radio receiver-amplifier-reemitting devices are in the empty space, very far from each other and from any celestial body. The lag time from absorbing to reemitting in the device is vary small compared to the return time of the signal (2t). If you want, this delay time can be determined by the constructing farm, by checking the two devices in the same laboratory. The two devices can be synchronyzed as clocks with the atomic clock held by A, in the following way. First A send a "hello" radio signal to B and waits a 2t time for the received "hello" signal re-sent by B, and instantaneously (almost), with the same device, re-send the "hello" signal, taking note of the 2t time between the two emissions. Then A waits for another 2t' time of return from B. If 2t' is equal to 2t, then A and B are in relative rest to each other (this is their aim in travelling towards a very far galaxy). If 2t' is different from 2t A sends a radio-signal to turn on adjustment engines to reduce relative speed along the line between A and B. After some trial and errors 2t will remain constant. This means that A and B are in two different positions in the same inertial frame, and relatively motionless. Now A can continue to send hello signals every 2t pair seconds (0t, 2t, 4t...) while B re-send hello signals every 2t odd seconds (at 1t, 3t, 5t...). When A is perfectly confident they are both motionless send a radio signal "go-six" at an exact odd time (e.g. 5t). When B receives the signal at the unusual even time (i.e. 6t) he can a) set his one clock to 6t, b) start his engine at a prefixed power in the direction of the galaxy and, in the same instant, re-send the signal "go-six" to A. When A receives the t6 hello signal, he knows that B has turned the engine in that instant and does the same simultaneously. So the instantant t=6 is exactly the same instant for A and B. This is not conventional. Simultaneity makes sense in the same frame. Bea, that is Adam's wife, can send a kiss towards Adam's spaceship at any even time (t8, t10...) knowing her husband is kissing her in the same instant. So we can say "where is Mars and what's happening in Mars right in this moment?" Moreover, if they know someway their distance, along the way, they can measure the famous one way light-speed and compare with the round-way one (c).