News of the WeekSATELLITE NAVIGATION

Europe's Answer to GPS Could Be a Boon for Research

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Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1893
DOI: 10.1126/science.310.5756.1893

CAMBRIDGE, UNITED KINGDOM— On 26 December, a European satellite is set to lift off from Baikonur cosmodrome in Kazakhstan and, once in orbit 23,000 kilometers above Earth's surface, start transmitting time signals. Although small—roughly the size of a freezer—the satellite GIOVE-A is the start of something big.

The craft is the first test bed for Europe's answer to the U.S. Global Positioning System (GPS) satellites. Dubbed Galileo, the European system, like GPS, will consist of a constellation of satellites carrying atomic clocks. A receiver can use their signals to calculate its position to an accuracy of a few meters. Combining Galileo with GPS will double the number of transmitters, and with Galileo's updated technology, researchers expect it to bring a sharp improvement in quality and reliability, which in turn will enable new studies of the atmosphere and oceans. The system might even provide a way of watching for tsunamis.

Global upgrade.

Galileo will have enhanced capabilities.

CREDIT: J. HUART/ESA

Satellite navigation is simple in principle: The spacecraft (24 for GPS, 27 for Galileo when it is fully operational around 2010) transmit regular signals that give each craft's identity and the precise time of transmission. A receiver which can pick up signals from four different craft is able to calculate its position in three dimensions.

GPS receivers have become so cheap that they're widely used by hikers and drivers. But GPS remains a military system, and the Pentagon can degrade or even turn off the signal in times of crisis. Galileo, in contrast, has been designed with business in mind. “Guarantee of service is the basic difference,” says Dominique Detain of the European Space Agency, which is developing Galileo jointly with the European Union.

GPS receivers have already become a common research tool, providing position data points in survey work and monitoring movement of tectonic fault lines. In the late 1980s, atmospheric researchers realized they could use GPS signals to probe Earth's atmosphere. A GPS signal that passes through the atmosphere as it travels from a GPS satellite to a satellite equipped with a receiver will be refracted. This refraction gives a detailed vertical profile of the atmosphere between the two craft, revealing temperature and pressure.

This information is “really very valuable for climate benchmarking,” says physicist James Zumberge of NASA's Jet Propulsion Laboratory in Pasadena, California, which has pioneered the technique. It would be highly valued by weather forecasters, too, except that they need continuous and global coverage. A single receiver in low Earth orbit is only in the right configuration to pick up a signal passing through the atmosphere a few times per day. Galileo, however, will double the number of signal sources, and a joint U.S./Taiwanese project called Cosmic, which will launch next spring, will add six GPS-receiving satellites.

Researchers are also excited about a technique that detects satellite navigation signals bounced off the ocean surface. A team from the University of Surrey in Guildford, U.K., demonstrated the technique earlier this year, deriving sea surface roughness from reflected GPS signals. But the Galileo signal has extra features that may also allow researchers to measure wave height and the height of the ocean surface. Radar satellites can already make such measurements, but they are large, expensive, and narrowly focused. A satellite with a navigation receiver, in contrast, could weigh just 10 kilograms. “You could put up a whole load of them and get global coverage at low cost,” says Martin Unwin of the Surrey team. Such a constellation could even provide an efficient tsunami early warning system. “People are looking into it,” says Zumberge.

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