What do you do if you want to throw a javelin or a cricket ball really quickly? You take a run-up. Because if you're moving quickly to begin with, anything you throw in front of you will move more quickly as a result. Relativity begins by understanding that this isn't true for light.
Light always goes at the same speed. Whether a satellite is coming towards you or going away, however fast it's going, the signal it transmits always gets to you at the same speed. This has been confirmed in many experiments and is a fundamental truth built into the very fabric of the universe.
Here's why this is strange:
Imagine you're standing on a platform with a train steadily moving past. When the train is in the middle of the platform, it emits two beams of light, one going forward and one going backwards. As we've said, the original speed of the train doesn't matter: these two beams of light both travel at the same speed, the speed of light. And as you can see, the beams reach the ends of the platform simultaneously.
Simple, isn't it? Now click here to see things from the point of view of an observer on the train...