The 100-Million-Mile NetworkBy David F. Carr Print
Think your network is hard to manage? Try remote diagnosis and repair when you're relying on radio signals from Mars.
Listening anxiously for any sign of life were navigators at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. They had to fix a broken interplanetary communications link that reached more than 100 million miles (and counting-the distance keeps growing as the orbits of Earth and Mars draw apart).
"The most difficult thing is to know how to talk to the spacecraft when you're getting no response from it," says Douglas J. Mudgway, a former National Aeronautics and Space Administration (NASA) engineer who managed communications with the Viking landers in the 1970s and helped save the Galileo mission in the early 1990s.
Spirit was exploring the Gusev crater on Mars on Jan. 21, and was already sending back spectacular photographic images. The wandering robot had rolled out of its landing nets and had approached a rock to take measurements using an appendage called the Rock Abrasion Tool.
Diagnosing what was wrong with Spirit depended on interpreting squawks, tones and other sounds traveling along a conduit dubbed the Deep Space Network.
Operators of this interplanetary signaling system send commands to and listen for data from "nodes" such as Spirit and its twin rover, Opportunity, using three facilities spaced roughly one-third of the way around Earth apart from each other. These communications complexes are in Goldstone, Calif.; near Madrid, Spain; and near Canberra, Australia.
This geographic separation means that, as the Earth rotates, at least one of these listening posts will be able to point its antennae toward the spacecraft being tracked at any given moment. Designed much like radio telescopes, the antennae are parabolic dishes as large as 70 meters in diameter (although the trend for the future is to use arrays of smaller antennas).
During normal operations, the rovers communicate directly with Earth when receiving instructions or sending back diagnostic information. They send back the bulk of their scientific data and photographs by using NASA's Mars Odyssey and Mars Global Surveyor probes as relay stations. These unmanned craft orbit the red planet carrying cameras, high-gain and ultra-high-frequency (UHF) antennae along with other scientific instruments.
The omnidirectional mast antenna sticking up from each rover's top like a dorsal fin knows when to transmit by listening for a signal that one of the orbiters is passing overhead. The orbiter then uses its more-powerful antenna to send as many as one million bits of data per second back to Earth. While fairly fast for an attenuated radio connection, that's only about a tenth of the speed of a cable-modem connection for the average home-computer user.
The rover-to-orbiter link uses UHF radio-the same basic technology used for broadcasting channels 14 and higher to television sets in the United States-while long-haul communications to Earth use X-band radio, which is a higher frequency (about 8 gigahertz) and easier to focus into a tight beam.
For critical commands, the rovers do communicate directly with Earth over X-band. Each rover has directional antennae that provide relatively strong signals that make it easier for the ground stations on Earth to filter out space noise and terrestrial interference. The omnidirectional antenna can also send and receive X-band when the directional one is not aimed properly.
Despite all this radio power, it's not unusual for a connection to be lost, at least temporarily. When Spirit landed the night of Jan. 3, the cheering in the JPL control room-over a series of simple radio tones indicating the lander had survived its fiery descent and dropped to the surface within a protective cluster of airbags-abruptly ended with the announcement, "We currently do not have signal from the spacecraft."
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