Airbus Defence and Space

SMOS: Tracking the world’s water patterns

The Earth’s water cycle is one of the most important processes on our planet – sustaining life and controlling climate – but the fundamental system is comparatively poorly understood. ESA’s Soil Moisture and Ocean Salinity (SMOS) mission, launched on 2 November 2009 and carrying Airbus Defence and Space Spain’s innovative MIRAS instrument, is a direct response to the pressing need to know more about how climate changes may be affecting global water cycle patterns.

One of the highest priorities of environmental policy issues today is to understand the potential consequences of modification in the Earth’s water cycle due to climate change. Inadequate understanding of the factors that drive and alter the water cycle is a key source of uncertainty in climate and weather prediction and climate change projections. The link between vegetation, hydrology and climate change has implications for how societies choose to manage their ecological resources in a context of increasing population pressure. Improved understanding is essential for developing options to respond to the consequences of water cycle variability. 


A space breakthrough for Earth science

Now, after more than 10 years of research and development by engineers and scientists from over 20 European companies and universities – led by Airbus Defence and Space Spain – a real technological marvel is now in orbit which will, for the first time, provide the scientific community with global and periodic measurements of two determinant variables of the water cycle. The Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) flying on board the SMOS satellite is ingeniously contrived to be able, in a single instrument and using a technique never before employed in space, to measure both wetness in soil and saltiness in seawater. This is far from the whimsical association of two disparate areas of study that it may perhaps appear, since soil moisture and ocean salinity are both intrinsically linked in the Earth’s water cycle. Data for both these parameters is currently very limited, with relatively few precise in-situ readings for soil moisture and only a small fraction of the world’s oceans sampled for saline levels on a regular basis. 


Integration of the MIRAS instrument at Airbus Defence and Space Spain's Barajas site. (© Airbus Defence and Space)

Damp earth and salty water in the world’s water processes

Soil contains only 0.001% of the world’s water budget, but this soil moisture is crucial for regulating water and energy exchanges between the land and the lower atmosphere in the form of evaporation and infiltration processes. Data from SMOS will give global soil moisture maps at least every three days. Coupled with numerical modelling techniques, this will result in estimates of water content in soil down to a depth of one to two metres – the ‘root zone’, the reservoir from which plants can draw water. Being able to estimate moisture content in this layer is paramount for improving short- and medium-term meteorological forecasting, hydrological modelling, monitoring of photosynthesis and plant growth and thus the terrestrial carbon cycle. Soil moisture data are also a significant factor in forecasting of floods, droughts and heat waves.

Ocean salinity, together with surface temperature, determines seawater density, a central factor in driving ocean currents around the globe. Ocean circulation regulates weather and climate, transporting heat from the equator to the poles – and provokes phenomena causing in extreme cases flooding and droughts. Sea-surface salinity is largely controlled by net evaporation/precipitation levels, an important indicator of climate change. Since the balance of evaporation and precipitation is difficult to measure accurately over the oceans using ground-based means, satellite maps of ocean salinity will constitute a vital tool for more precise estimates on a global scale, leading to better atmospheric forecasts. SMOS’ data will generate ocean salinity maps at least every 30 days.

The SMOS mission will deliver this eagerly-awaited data by using the first polar-orbiting 2D interferometric radiometer, MIRAS, which will record differences in the electromagnetic radiation emitted from the Earth’s surface.

“Climate change is a fact,” says the SMOS Principal Investigator for ESA Yann Kerr, “but its impact on precipitation, evaporation, surface run-off and flood risks is still uncertain.” By making it possible to collect data on soil moisture and ocean salinity from space, SMOS will help to fill in the gaps in our knowledge and play a key role in the monitoring of climate change on a global scale. 


Deployment of the SMOS satellite's Y-shaped instrument MIRAS. (© Airbus Defence and Space/ESA)

MIRAS: the Y is how

All matter emits electromagnetic radiation to a greater or lesser extent depending on its electrical properties – this is known as its ‘emissivity’. Moisture and salinity decrease the emissivity of soil and water, respectively – damp soil and saline water emit lower levels of radiation than dry soil and pure water. MIRAS will take periodic readings – every 1.2 seconds – of the radiation, in the microwave frequency band, emitted from the Earth’s surface, to record fluctuations in the soil/moisture and salt/water mixtures at a global level over time. Since these measurements need to be made to a fairly large probing depth, through vegetation, in all weathers, it was decided that MIRAS would operate in L-band (frequency of 1.4 GHz/wavelength 21 cm) which offers minimal disturbance from vegetation cover, the weather and the atmosphere. Another advantage of this frequency is that it is reserved by international radio regulations for astronomy, which ensures that there is no interference from man-made emissions from the Earth’s surface. But low frequencies/long wavelengths such as L-band normally require a very big antenna, in the order of tens of metres across, to achieve adequate coverage and spatial resolution. Which is simply not feasible for a satellite mission. 

This is where the instrument’s designers have really demonstrated outstanding ingenuity, finding an elegant solution by synthesising the antenna aperture through 69 small antenna receivers equally distributed over a Y-shaped structure of three arms and central hub. Observations from all the receivers are transmitted to a central unit which performs interferometry cross-correlations of the signals between all possible combinations of receiver pairs. This process of clustering a multitude of small receivers produces an image resolution equivalent to that of a very large ‘phantom’ single antenna. 

The first MIRAS data was received towards the end of November 2009 at ESA’s
European Space Astronomy Centre (ESAC), in Villafranca, Spain. (© ESA)

MIRAS’ clever designers have taken a technology used in star-gazing telescopes, found a way of adapting it to a small satellite, and turned its focus round to look down at planet Earth.