Page last updated at 21:51 GMT, Wednesday, 30 July 2008 22:51 UK

Ocean mission delivers first maps

By Jennifer Carpenter
Science reporter, BBC News

Map (Nasa)
Jason-2 maps ocean shape (reds indicate higher waters; blues are lower waters)

Less than a month after it was put in orbit, the ocean-mapper Jason-2 has returned its first pictures to Earth.

From an altitude of more than 1,300km, the spacecraft is now feeding back data covering nearly the entire globe.

Jason-2 is set to become the primary means of measuring the shape of the world's oceans, taking readings with an accuracy of better than 4cm.

The information will be crucial to our understanding of both sea level rise and changing ocean currents.

The satellite is now flying in tandem with its predecessor, Jason-1.

The spacecraft, only 55 seconds apart, are making simultaneous measurements of the oceans' "hills" and "valleys", to allow precise calibration of Jason-2's instruments.

Jason-2 (CNES)
The Jason Ocean Surface Topography Mission is led by the US and France
With everything in line, data collected with every circumnavigation of the Earth will help weather and climate agencies make better forecasts.

Jason-2, built by Thales Alenia Space, represents the joint efforts of the US and French space agencies (Nasa and CNES), and the US and European organisations dedicated to studying weather and climate from orbit (Noaa and Eumetsat).

Its key instrument is the Poseidon 3 solid-state altimeter. It constantly bounces microwave pulses off the sea surface.

By timing how long the signal takes to make the return trip, it can determine sea surface height. Additionally, the signal can indicate the height of waves and wind speed.

Elevation is a critical parameter for oceanographers. Just as surface air pressure reveals what the atmosphere is doing above, so ocean height will betray details about the behaviour of water down below.

Jason data gives clues to temperature and salinity. When combined with gravity information, it will also indicate current direction and speed.

The Jason lineage shows that mean sea level has been rising by about 3mm a year since 1993.

Jason-1 will continue to operate for as long as it is returning reliable data.

A map showing atmospheric water vapour (red greatest). Water slows the altimeter's signal and must be taken into account to make sense of Jason's data

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