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Europe's Galileo satellites herald new era for Earth science

Third global fleet will soon be joined by Asian counterparts, setting the atmosphere abuzz with scientifically-useful radio-wave signals.

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Thomas Nylen/UNAVCO

This antenna uses signals from navigation satellites to monitor the uplift of the ground beneath Greenland's ice sheet.

After soaring costs and years of delays, Europe’s global satellite navigation system, Galileo, finally began beaming its first signals to receivers in smartphones and cars on 15 December.

The 18-strong fleet of satellites promises travellers another way to accurately locate their position on Earth, ending Europe’s dependence on the US Global Positioning System (GPS) and Russia’s GLONASS.

But Galileo, which was first proposed in 1999, is a big deal for science too, says Richard Langley, an expert in navigation-satellite systems at the University of New Brunswick in Fredericton, Canada.  What most excites scientists is the prospect of combining signals from multiple satellite networks, which should enable new kinds of atmospheric and Earth-sciences research.

Galileo’s constellation of satellites should reach its its full complement of 30 in 2020, by which time China’s BeiDou system, comprising 35 satellites, is scheduled to enter service. Japan and India are also building regional systems. Altogether, the number of global navigation satellites encircling the Earth is set to rise from around 90 today to at least 130 over the next decade, estimates Oliver Montenbruck, a physicist at the German Aerospace Center in Oberpfaffenhofen, Germany. At the same time, existing satellite fleets such as GPS and GLONASS will be modernized to carry higher-precision clocks and transmit more advanced signals.

Earth’s atmosphere will then be streaming with many more kinds of radio-wave signals at a greater variety of frequencies — each carrying information about the time and the position of the satellite that sent it. Sat-nav receivers rely on information from multiple satellites to pinpoint their own position. So simply having more satellites overhead will help stop irritating signal loss, particularly in areas where high-rise buildings or mountains interfere with reception, and will provide more accurate position fixes for both travellers and scientists, says Langley.

“The more satellites you have, the greater the precision,” adds Tonie Van Dam, an Earth scientist at the University of Luxembourg who studies how climate change is affecting how water circulates between the ocean, atmosphere and land. She uses receivers mounted on bedrock to monitor how Earth’s crust deforms and rebounds in response to a shifting weight of water or ice. Cross-checking data from several satellite constellations is the only way to correct errors in Earth-system models, she adds.

Mapping the sea and the sky

Skies increasingly crowded with radio waves will also benefit researchers who use the refraction of navigation-satellite signals in the Earth’s atmosphere to make measurements of atmospheric temperature, pressure, density and water-vapour content — data used for weather forecasting and climate research.

And the signals can similarly be exploited to measure electron density in Earth’s ionosphere, the electrically charged layer in the upper atmosphere. Such data are used to track space weather and also to monitor tsunamis and earthquakes, notes Philippe Lognonné, a geophysicist at the Institute of Earth Physics of Paris. These events disturb the air around them so violently that they send acoustic and gravity waves up to the ionosphere where they perturb electrons. With fully operational Galileo and BeiDou systems, researchers should be better able to estimate tsunami heights, Lognonné says.

Stephane Corvaja - ESA

Over the past five years, 18 Galileo satellites have been launched into orbit. Another 12 are due to be launched by 2020.

Scientists also plan to use multiple navigation-satellite constellations to vastly improve measurements of ocean wind speeds, sea-surface roughness and the height of waves, says Jens Wickert, a scientist at the GFZ German Research Centre for Geosciences in Potsdam. Today’s remote-observation maps of the oceans are built largely by bouncing radar waves off the sea from aircraft or spacecraft, and combining those data with information from other satellite-borne instruments. The best current maps have a spatial resolution of around 80 kilometres and are updated every 10 days. Wickert aims to improve on that by taking measurements using orbiting receivers for navigation-satellite signals.

A European experiment called GEROS-ISS, which Wickert is leading, aims to fly a receiver on the International Space Station in 2019. The experiment would measure navigation-satellite signals as they reflect off the sea, and by combining data from Galileo, BeiDou, GPS and GLONASS could map the oceans at spatial scales down to a few kilometres every four days or less. Many ocean phenomena, such as eddies, occur at these scales, so better maps would help to improve weather and climate-change models.

A fleet of receivers in space could provide even finer resolution. In a step in that direction, NASA on 15 December launched its own ocean-reflection research mission, the Cyclone Global Navigation Satellite System. The fleet of eight microsatellites, each carrying four navigation-satellite receivers, will measure wind speeds and ocean roughness in the eyes of storms and hurricanes at unprecedented resolutions of a few kilometres every few hours, with the goal of improving forecasts. Chris Ruf, Cyclone's principal investigator and a remote-sensing scientist at the University of Michigan in Ann Arbor, says that the first mission will use GPS only, but he is keen to integrate data from Galileo and BeiDou in follow-ups.

Much research on how to usefully fuse signals from different global navigation-satellite systems is taking place under the auspices of a voluntary federation of more than 200 agencies, universities and research centres in more than 100 countries. Over the past four years, researchers within the federation have been testing signals from GPS, GLONASS, Galileo and other systems, to develop the algorithms and software that will be needed for scientists to combine data from multiple satellite constellations in their research.

Montenbruck, who heads this effort, cautions that it may take more than five years after Galileo and BeiDou enter full service before scientists can fully exploit their possibilities. "Today's use of GPS benefits from 30 years of experience and an excellent understanding and characterization of all the dirty details," he says. "All that still needs to be carried out for Galileo and BeiDou."

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