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Last Updated: Tuesday, 31 July 2007, 14:47 GMT 15:47 UK
Measuring sea level rise from space
By Mark Kinver
Science and nature reporter, BBC News

Artist's impression of Jason-2 (Image: Cnes)

Meteorologists and climate modellers are eagerly awaiting the launch of a satellite that will be able to measure sea level rise to an unprecedented degree of precision.

Jason-2, scientists hope, will help shed light on the oceans' dynamics by measuring the topography - the "hills" and "valleys" - of the world's seas every 10 days.

The satellite's radar altimeter, Poseidon-3, is designed to measure the sea level height to within a few centimetres. It will do this from its orbit more than 1,300km above the Earth.

Data collected by Jason-2's instruments will help researchers develop more precise forecasts, improve hurricane path projections and reveal how climate change is affecting ocean currents.

Diagram of Jason-2 (BBC)
1. Advance Microwave Radiometer (AMR) - measures signal delay caused by water vapour
2. GPS antennas - ensures precise orbit path
3. Poseidon-3 altimeter- measures sea level
4. Doris antenna - tracking and positioning control
5. Laser Retroreflector Array (LRA) - tracks and calibrates measurements
Mass: 525kg (1,155lb)
Power generation: 511 watts
Height: 3m (9ft 8in)
Orbit: 1,338km (831 miles)
(Source: Eumetsat, Cnes, Nasa)

"There is more to the dynamics of sea level rise than just a single, global rise," explained Mikael Rattenborg, director of operations for the European Organisation for the Exploitation of Meteorological Satellites (Eumetsat).

"Although we have seen, overall, global sea level rise, there are areas that have decreased for long periods, followed by an increase.

"We can only analyse the significance of regional variability of sea level rise if we have altimetry data available to us," he added. "Jason-2 will help us model and explain this evolution."

The satellite will be able to map 95% of the world's ice-free oceans every 10 days, something that would be impossible using survey vessels on the surface of the planet.

As well as observing variations in sea levels, Mr Rattenborg said the mission would also help researchers map seasonal and inter-annual ocean patterns, such as the Pacific's El Nino effect.

"This has a profound impact on the weather, not only in that region but globally. We can study this phenomenon in much greater detail with the altimetry data.

"All of these processes are coupled to climate analysis, which is the key reason why Eumetsat is interested in altimetry."

Storm tracking

Eumetsat operates and collects data from satellites on behalf of Europe's national meteorological agencies, such as the UK's Met Office, to compile forecasts and climate models.

Mr Rattenborg said the sea surface topography recorded by Poseidon-3 would also reveal tell-tale signs that would help predict the path and intensity of hurricanes.

Map showing sea level variations

He used Hurricane Katrina, which devastated the US Gulf coast in 2005, as an illustration.

"It passed over the Florida peninsula into the Gulf of Mexico as a strong hurricane (category three), but not an intense one.

"But suddenly, about 24 hours before it hit New Orleans, it developed into a category five hurricane.

"If you look at the sea surface temperature in the Gulf at the time Katrina passed over, it is fairly homogenous, so it does not explain why the system developed so rapidly."

Mr Rattenborg said the answer could be found in something called the Tropical Cyclone Heat Potential (TCHP).

"It is a measurement of the heat energy available in the deep layer of the ocean," he explained.

"Altimetry provides us with a measurement of this potential, because the (sea) surface topography reacts to the changes to the heat content beneath the ocean.

"In the area of the Gulf, south of New Orleans where Katrina passed, there was a sea-surface height anomaly, which corresponds to a very deep layer of very warm water.

"This clearly shows that by looking into the ocean, we can monitor the availability of heat energy."

But it is not only the thermal energy stored deep within the oceans that causes the variation in sea level, gravity also has an influence.

The subterranean geology is not uniform, some regions are more dense than others. This causes a subtle but significant shift in the Earth's gravitational force.

To measure the influence of gravity and its impact on ocean topography and currents, the European Space Agency (Esa) plans to launch an arrow-like satellite called the Gravity field and steady-state Ocean Circulation Explorer (Goce).

Goce (Esa)

"If we want to improve our climate models then we need to improve our knowledge of how the oceans move, and Goce will help us do that," mission scientist Dr Mark Drinkwater, from Esa, told BBC News.

By combining the data gathered by Goce and Jason-2, meteorologists and climate scientists will advance their understanding of the physical factors influencing the oceans and atmosphere.

Jason-2 is the latest addition to a series of satellites fitted with altimeters to map the sea surface.

The first, Topex/Poseidon, was launched in 1992 as an experiment to assess the effectiveness of high-accuracy altimeters to measure ocean dynamics from space.

Its success paved the way for the Jason-1/Poseidon-2 mission, launched in 2001.

Lessons learned from the previous missions have allowed the team building the Poseidon altimeter instrument for Jason-2 to improve its accuracy and reduce the margin of uncertainty to within 2.5cm.

All systems go

Because the satellite's orbit of 1,338km above the Earth's surface takes it through a harsh radiation environment, the craft's operational life is listed as five years.

Scientists involved in the project say Jason-1 is already using a number of back-up systems and it is only a matter of time before these fail, too; hence the need for Jason-2 to replace it.

As with the original Jason, this satellite and altimeter is being built by Thales Alenia Space in Cannes, France.

Scientist working on Jason-2 (Image: ThalesAlenia)
Jason-2 is on schedule for a June 2008 launch, say scientists

It forms the space segment of a mission called Ocean Surface Topography Mission (OSTM) - a joint initiative between the French space agency (Cnes), Nasa, and meteorological bodies Eumetsat and Noaa.

"This mission is dedicated to several objectives, including the precise, continuous and global measurement of the sea surface," said Pascale Ultre-Guerard, head of Cnes' Earth observation programme.

Thierry Huiban, Jason-2 project manager, said preparations were on schedule and the team was confident the satellite would be ready for its launch in June.

"We have completed the integration of the payload module with the [satellite's] platform.

"We are now carrying out tests to ensure the satellite qualifies for the space environment."

These tests include exposing the craft to the kinds of forces it will experience during its journey into space and while it is in orbit.

If all goes according to plan, Jason-2 is scheduled to be moved from southern France to its launch site at the Vandenberg Air Force Base, California, in February 2008, ahead of a June blast-off onboard a Delta II rocket.

Diagram showing how Jason-2 measures sea levels from space (BBC)
1. Jason-2 satellite: From its 1,338km-high circular orbit, the craft maps 95% of the world's ice-free oceans' topography every 10 days
2, GPS satellites: The system is used to track Jason-2's position, ensuring very precise sea level height measurements
3. Sea height measurement via Poseidon-3 altimeter: The dual frequency radar signals are able to measure sea level height, wave height and surface wind speed
4. Sea surface topography: Variations in the height of the sea surface, when combined with measurements from other satellites and in-situ instruments, will allow scientists to improve weather and climate system models
5. Doppler Orbitography and Radiopositioning Integrated by Satellite (Doris) and laser ranging beacon: Ground stations ensure the precise positioning of Jason-2, which enables researchers to gather meaningful data from the satellite

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