How do satellites talk to Earth from thousands of kilometers away?

Most satellites communicate with Earth using radio frequencies because radio waves travel well through space and through the atmosphere. A satellite’s transmitter encodes information into a signal that is received by ground stations equipped with large antennas or by relay satellites such as NASA’s Tracking and Data Relay Satellite System. The signal moves at the speed of light, so even deep-space communications are delayed only by the distance traveled, not by the transmission method itself. Deep-space missions often use highly directional antennas to send weak but tightly focused beams back to Earth. Accurate timing and frequency control are essential because signals can be very faint when they arrive.