by Rakesh Ravikumar

Mars is calling, we are reluctant to go.

The NASA Deep Space Network (DSN) is an international network of antennas that provide the communication links between the scientists and engineers on Earth to the Mars Exploration Rovers in space and on Mars.
The DSN consists of three deep-space communications facilities placed approximately 120 degrees apart around the world: at Goldstone, in California’s Mojave Desert; near Madrid, Spain; and near Canberra, Australia. This strategic placement permits constant observation of spacecraft as the Earth rotates on its own axis.

How the rovers can communicate through Mars-orbiting spacecraft

Not only can the rovers send messages directly to the DSN stations, but they can uplink information to other spacecraft orbiting Mars, utilizing the 2001 Mars Odyssey and Mars Global Surveyor orbiters as messengers who can pass along news to Earth for the rovers. The orbiters can also send messages to the rovers.
The benefits of using the orbiting spacecraft are that the orbiters are closer to the rovers than the DSN antennas on Earth and the orbiters have Earth in their field of view for much longer time periods than the rovers on the ground.
Because the orbiters are only 250 miles (400 kilometers) above the surface of Mars, the rovers don´t have to “yell” as loudly (or use as much energy to send a message) to the orbiters as they do to the antennas on Earth. The distance from Mars to Earth (and from the rovers to the DSN antennas) during the primary surface missions varies from 110 to 200 million miles (170 to 320 million kilometers).

X-band radio waves used by the rovers to communicate

The rovers communicate 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 the low-gain and high-gain antennas. One UHF antenna is on the rover and one is on the petal of the lander to aid in gaining information during the critical landing event. The Mars Global Surveyor will be in the appropriate location above Mars to track the landing process. (2001 Mars Odyssey will not be in the vicinity.)
When the rovers speak directly to Earth, they send messages via both the low-gain antenna (LGA) and the high-gain antenna (HGA). The low-gain antenna sends and receives information in every direction; that is, it is “omni-directional” and it transmits radio waves at a low rate to the DSN antennas on Earth. 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.
During cruise, the Mars Exploration Rover uses both the low-gain antenna (LGA) and a separate medium-gain antenna (MGA), which is used only during cruise. The antennas on the cruise stage leave with it. The medium-gain antenna on the cruise stage has a waveguide (a pipe) to the rover that is not used on the surface. The low-gain antenna on the cruise stage connects directly to the low-gain antenna on the backshell, which in turn connects to the low-gain antenna on the rover, in a Russian doll sort of design.