Posted by: robotnews | April 10, 2006

Mars Exploration Rovers

U0205298 Li Si

RAT weighs only 1.5 pounds (0.68 kg) and uses less power (only 30 watts) than most light bulbs. It is about the size of a soda can.

The RAT occupies the turret, or “hand,” of the rover’s robotic arm, along with other science instruments for rock analysis, a microscopic imager, and Mössbauer and alpha-particle X-ray spectrometers. The agile arm, which has shoulder, elbow and wrist joints just like a human arm, presses the RAT up against a rock’s surface.

In just two hours, the RAT’s grinding wheel can shave off a disc about twice the diameter and thickness of a nickel from a hard rock surface. Two brushes sweep the resulting dust away from the hole to provide a clean surface for an up-close view.

Once the fresh surface is exposed, the imager and the spectrometers take over, peering through the abraded opening to perform a detailed analysis of the rock’s interior. In order for scientists to learn about the processes that might have weathered the rock, the rover also records temperature and current readings from the RAT’s three motors while they grind away the exterior layer.

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.

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.

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.



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