By Geoff Brumfiel of Nature magazine

Galileo, the largest program ever launched by the European Space Agency (ESA) will enter its latest phase with the launch of two navigation satellites on 20 October. They will join two test satellites already in orbit, allowing the first tests of the Galileo network, a rival to the US Global Positioning System (GPS).

Scientists in Europe are following the program's progress closely, hoping that the satellite constellation will enable new or improved research. Nature lays out what the latest launches mean for Galileo and the scientific community.

How does satellite navigation work?

It is all in the timing. Navigation satellites carry ultra-precise clocks that regularly transmit the time and their orbital positions to Earth. The signals travel at the speed of light, arriving on Earth's surface after a very small delay. By comparing the arrival times of signals from different satellites, a receiver can work out how far away it is from each, and deduce its own position on Earth.

What is Galileo?

Galileo is the European satellite navigation system. The network, costing more than €5-billion (US$6.93-billion), will eventually consist of up to 27 operational satellites and three spares in three orbital planes. The test satellites being launched tomorrow will for the first time allow operators to test the system, including its ground stations.

How is Galileo different from existing systems, such as the GPS?

The short answer is that Galileo is not all that different from the GPS or the Russian system, GLONASS. But it does have a few tricks up its sleeve. Galileo will broadcast on a wider range of frequencies than other systems, and the first satellites will carry clocks based on hydrogen masers--devices that exploit an ultra-stable transition in hydrogen atoms to achieve accuracies up to five times better than the current GPS standard.

How will scientists use Galileo?

Satellite navigation systems are enormously important to science, says Bertram Arbesser-Rastburg, the head of the electromagnetics and space-environment division at ESA. Researchers use the GPS and GLONASS for everything from tracking wildlife to studying the motion of tectonic plates; scientists everywhere will use Galileo for similar measurements.

In many fields, Galileo will strengthen existing measurements, says Seth Gutman, an atmospheric scientist at the Earth System Research Laboratory of the US National Oceanic and Atmospheric Administration in Boulder, Colorado. The signals from navigation satellites are often used to study the atmosphere, and the ability to monitor atmospheric changes "strongly depends on how many satellites are transmitting signals, where they are in space and time, and how many receivers there are on Earth tracking these signals", says Gutman. Galileo will augment information from the GPS and GLONASS.

Will Galileo be able to do anything that the GPS can't?

For certain applications, Galileo does stand out. For example, scientists are interested in using navigation satellites to measure features of Earth's surface. By watching for the reflected satellite signal shining off water or land, researchers can make specific measurements of things such as sea level and soil moisture. Because Galileo operates over a wider bandwidth than the GPS, it will provide more accurate reflection measurements.

What other scientific potential does Galileo hold?

Galileo's clocks will be able to test some alternative theories of gravity, according Jorge Páramos, a physicist at the Higher Technical Institute in Lisbon. Clocks many times more accurate as those on current Galileo satellites would allow for fundamental checks of Einstein's predictions of how Earth's mass warps space-time. However, it remains unclear whether clocks that accurate will ever be developed for Galileo, because they would be of little commercial value.

What happens now?

Two further satellites will be launched in 2012, and by 2015 or so, enough satellites should be in orbit to provide some initial services. ESA is aiming to have the full 30-satellite constellation in orbit by the end of the decade

This article is reproduced with permission from the magazine Nature. The article was first published on October 19, 2011.