X-band Radio Waves
X-band radio waves used by the rover to communicate
The rover communicates with the orbiters and the DSN through radio waves. They communicate with each other through X-band, which are radio waves at a much higher frequency than radio waves used for FM stations.
The radio waves to and from the rover are sent through the orbiters using UHF antennas, which are close-range antennas that are like walkie-talkies compared to the long range of low-gain and high-gain antennas. All three orbiters active at Mars — NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express — were at positions where they could receive transmissions from the Mars Science Laboratory spacecraft during its entry, descent and landing. Only Odyssey relayed the information immediately, however. The other two orbiters recorded Mars Science Laboratory data from the Mars Science Laboratory spacecraft, holding it onboard, and sending it to Earth hours later. Mars Reconnaisance Orbiter even captured images of the spacecraft on its parachute during entry, descent and landing.
The cruise stage has two antennas that are used to communicate with the Earth. The low-gain antenna is omni-directional and is used when the spacecraft is near the Earth. Because it radiates in all directions, the low-gain antenna does not need to be pointed at the Earth to enable a communications link. The medium-gain antenna is a directional antenna that must point toward the Earth for communications, but has more power to communicate when the spacecraft is farther away from the Earth. The medium-gain antenna acts like a floodlight and can direct the energy into a tighter beam to reach Earth. Just like a floodlight directs more light into a focused area than a normal light bulb does out of a lamp, the medium-gain antenna can direct the data from the spacecraft into a tighter beam than the low-gain antenna.
When the rover speaks directly to Earth (from the surface of Mars), it sends messages via its high-gain antenna (HGA). The high-gain antenna can send a "beam" of information in a specific direction and it is steerable, so the antenna can move to point itself directly to any antenna on Earth. The benefit of having a steerable antenna is that the entire rover doesn't necessarily have to change positions to talk to Earth. Like turning your neck to talk to someone beside you rather than turning your entire body, the rover can save energy by moving only the antenna.
Watch "Mars in a Minute": Phoning Home: Communicating from Mars