Keeping an eye on the ocean surface

Climate scientists, weather services and coastal protection officials need to now how high sea levels across the world really are. But it's pretty hard to determine the precise movement of waves and the depth of the water accurately. Sea levels are being influenced by so many factors that they are hard to grasp.

They are influenced by the position of celestial objects - most noticeably the moon, which is responsible for high and low tides. Wind, volcanic or seismic activities, movements of tectonic plates, earthquakes and even more simple factors like rain and snow or freezing sea ice in the polar regions can all influence sea levels as well.

To make matters more complicated, scientists also have to deal with the fact that Earth isn't really round. When you look at its gravitational field, you will see it's shaped more like a potato than a perfect ball. And even the gravitational patterns may change over time, triggered by movements deep under the Earth's surface.

"Normal zero" isn't normal

Furthermore, the Earth's rotation also has an effect on the sea level. The result: it does not have the same distance from the geocenter, everywhere. For example at the equator, the ocean surface is more than 4,700 meters farther away from the planet's center than at the 30th degree of northern latitude. This means that the highest mountain on earth is actually not Mount Everest - when calculating its distance from the geocenter.

The peak of the Chimborazo in Ecuador is 6,384,557 meters above the center of the Earth, while the peak of Mount Everest stands only 6,382,414 meters tall. However, when looking at height from the ocean surface - which is the traditional way of measuring mountains - Mount Everest with its 8,848 meters stands higher than the 6,267 meter tall Chimborazo, of course.

Satellites can handle gravitational influences very well by calibrating the data through computing. They are virtually immune against earthly factors such as tectonic movements or weather since they're running on rather stable orbits and are therefore incorruptible.

The newest satellite for measuring the ocean surface is Jason-3 - a joint project of the US space agency NASA and its French counterpart CNES. The US weather service NOAA and the European Meteorological satellite service Eumetsat are also involved in the project.

A complete picture once every ten days

Jason-3 circles Earth at 1,330 kilometers and is scanning 95 percent of the ice-free ocean surface every ten days. The precision of the measurements reaches up to four centimeters - considerably less than normal waves generate.

To ensure that the orbit of the satellite is known and that the collected data is correct, Jason-3 uses three different instruments to determine its own orbit down to a centimeter: the satellite navigation system GPS, an instrument called the "Doppler Orbitography and Radiopositioning Integrated by Satellite" (DORIS) and a Laser Retroflector Array which can measure the altitude of the satellite.

Early detection of tropical storms

The satellite is also able to measure water temperature. This way it can identify pools of warm water from which tropical storms such as typhoons or hurricanes can develop. And the quality of the reflected radar signals gives meteorologists information about wind speeds and the height of waves.

Jason-3 was launched on 17 January from the Vandenberg Airforce Base near Lompoc, California. NASA has passed the satellite control over to NOAA on 3 June after a six-month installation period. Since then, the meteorologists have been able to compare and calibrate the data that Jason-3 delivers with that from the predecessor satellite Jason-2.

Learn more at a life-briefing

In early October Jason-3 is supposed to take over his new role as the official "source of reference for global measurements of the sea level from space." Then, weather services worldwide will be able to use the data.

And for everybody who wants to learn more about Jason-3, there is an interactive opportunity: On September 19th starting at 16:00 Central European daylight savings time, Eumetsat is organizing a life-briefing, here - and on twitter: #askjason3.