Page last updated at 12:28 GMT, Friday, 30 October 2009

Mapping the 'great wheel of water'

By Jonathan Amos
Science reporter, BBC News


Jonathan Amos takes a look around the Smos satellite

You'd be forgiven for thinking that Europe's new Earth observation satellite was some kind of "space helicopter".

Satellites have a reputation for being dull, functional boxes. However, the Soil Moisture and Ocean Salinity (Smos) spacecraft is anything but routine.

Its looks, of course, are a consequence of the job it has been asked to do.

Smos will map of the variation in the wetness of the land and of the quantity of salts dissolved in sea water.

These are key parameters that can tell scientists how water is cycling around the planet - from the surface to the atmosphere and back again.

"These two variables have never been measured from space before; that means never measured globally," says Dr Yann Kerr, a lead investigator on the mission from the Center for the Study of the Biosphere from Space (Cesbio), Toulouse, France.

Dish problem

Evaporation, transpiration, precipitation - these are all stages on the grand wheel of water.

Cloud and rain (NCAR)
Scientists need better numbers on the hydrological cycle

As the Sun heats the ocean and water vapour rises skyward, the sea surface becomes a little more saline as salts are left behind. When the clouds move across the land and it rains, the precipitation will soak the earth.

Smos will see these changes; and putting some more precise numbers on the variations over time will assist scientists as they seek to make better weather forecasts and understand key features of the climate system.

The European Space Agency (Esa) satellite works by measuring the natural emission of microwaves coming up off the planet's surface.

Variations in the sogginess of the soil or the saltiness of the ocean will modify this signal.

But this is long wavelength radiation, in the L-band.

Its detection demands a big antenna; and to get the sort of wide and timely coverage scientists want over both the land and the ocean would ordinarily have required a moveable dish some 20m across.

There is no rocket in existence that can put such an object in space.

Radio astronomy has pioneered interferometry to study long wavelength light

So, the Smos team has borrowed a trick from radio astronomy.

For decades, researchers in that field have been linking together arrays of smaller telescopes to synthesise the type of image that might be acquired using a single, giant receiver.

Spanish engineers have spent 14 years, at cost of 70m euros, turning this concept into a practical space-borne instrument to study the Earth rather than the stars.

The result is Miras (Microwave Imaging Radiometer with Aperture Synthesis).

Sixty-nine small antennas are positioned on a central box and along the lengths of three extending arms.

The whole mechanism is folded for launch to fit inside a modest Russian Rockot vehicle.

Soil moisture and ocean salinity explainer (BBC)
The amount of water retained in soils varies between about 5% and 50%
This will cover most conditions from 'bone dry' to 'mud bath'
Smos sees the entire range with an accuracy of 4% at the 50km scale
Natural salinity in water covers the range from near zero to 30%
Drinking water might be one extreme; salt lakes would be the other extreme
Smos is seeking sea waters which are typically in the 3-3.5% range
This needs high accuracy (0.01-0.02%). Maps are at the 200km scale

The spacecraft is then hurled into orbit, where the 3.5m-long arms can be unpacked to give Smos its rotor appearance. The arms do not turn, however.

The satellite merely sweeps around the Earth, using its Y-shaped instrument to map a 1,000km-wide swath, or track. Critically, the antenna arrays can synthesise images that have detail at a scale down to 50km for soil moisture; and at 200km for ocean salinity.

"With our technology, we get a global picture with reasonable spatial resolution within three days everywhere on the Earth," explains Achim Hahne, the Esa Smos project manager.

The wetness of soils is really one of those must-have components in temperature, humidity and precipitation forecasts. Meteorologists see huge value in the data to improve their short and medium-term outlooks.

Modelling done following the great European heatwave in 2003 showed that if more had been known about the relative dryness of soils in the springtime, meteorologists would have been in a much better position to predict the extreme conditions that followed a few months later.

"Likewise, for hydrologists: if you see that your soils are already very wet and there is a forecast for precipitation, you know there is a potential danger of flooding because the soils are already saturated and they will not be able to take up any more water," explains Smos mission scientist Dr Matthias Drusch.

Pulled together

Ocean salinity on the other hand is a key variable, along with temperature, in driving global ocean circulation patterns.

The amount of salt in the water will help determine its buoyancy - the degree to which it tends to float or sink relative to other water around it.

The phenomenon is particularly important at high latitudes.

"This is the push-me, pull-you of the global thermohaline circulation," says Dr Mark Drinkwater, who leads Esa's mission science division at its technical centre in Noordwijk, Holland.

Smos artist's impression (Esa)
1 - Solar-array panels for power
2 - Antennas for communications
3 - Thrusters for positioning
4 - GPS antenna for location info
5 - Sun sensors for orientation
6 - Interface with launch rocket
7 - Star trackers for positioning
8 - Antenna for data downlink
9 - Payload central structure

"Dense water masses that dive into the abyssal ocean drive this large-scale circulation and this helps to moderate the climate on a global basis. We'd like to understand better how salt and the availability of fresh water helps regulate this circulation."

Smos only sees the top few centimetres of soils and ocean water. There is quite a bit of noise introduced into the signal as microwaves scatter across rough ground or choppy waters.

This means the information sent down from the spacecraft will need a lot of processing. Additional datasets, such as those from floating buoys in the case of ocean salinity, will be required to understand what might be happening below those top few centimetres.

Smos is scheduled to launch on its Rockot vehicle from the Plesetsk Cosmodrome in Russia at 0450 local time (0150 GMT) on Monday.

Although an Esa mission, Smos is a major venture for France and Spain. The instrument was built by EADS Casa Espacio in Madrid; the spacecraft itself was assembled by Thales Alenia Space in Cannes.

L-band explainer (BBC)
Both moisture and salinity strongly affect the electrical properties of matter
All matter emits energy in the form of electromagnetic radiation
The Smos signal is detected in the microwave portion of the spectrum
Long wavelength reception generally requires large antenna set-ups

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