Communication

As the rovers collect data, they need to send it back to Earth. Data includes photos, spectrometer information, system-status information and so on. In addition, scientists and engineers on Earth want to send things like commands and software updates to the rover. The rover has three different radios to handle communications.

Photo courtesy NASA You can see each of the rover's three antennas in this image: UHF, Low Gain and High Gain.

The first radio is a low-power, slow UHF radio. This link uses a low-gain, omni-directional antenna. It does not require any aiming, and it transmits back to Earth or to a satellite at a low data rate. It is an "if all else fails" way to communicate.

The second radio is a high-speed UHF radio, and it communicates with two satellites already in orbit around Mars: The Mars Odyssey satellite and the Mars Global Surveyor satellite. When a satellite appears overhead and signals the rover, the rover can dump data to the satellite at high speed for perhaps eight minutes on each pass. The rover can send data at 128 kilobits per second when the satellite is overhead, using a radio that consumes 15 watts. The satellite can then forward the information to Earth when it comes into view using its 2.5-meter (2.7-yard) antenna and 100-watt radio. This is how most image data gets back to Earth. Perhaps 10 megabytes of data per day can get back to Earth through these channels.

Photo courtesy NASA Overhead view of Spirit on the surface of Mars

Finally, there is a 1-foot-diameter (.3 meter) directional (high-gain) antenna on the rover. When the Earth is visible to the rover, the rover's antenna tracks the Earth and can communicate directly to scientists and engineers. There is a 20-minute round-trip delay because of the 200-million-mile (322-million-km) distance between Earth and Mars. The rover uses a 40-watt radio, and it transmits at only 12 kilobits per second over this link. Because it is a direct link, NASA uses it to send commands to the rover and to get critical data back. This link is only available for about three hours per day because of the alignment of the planets and the power requirements of the radio.