CoSMOS: An Airborne Campaign in Support of the ESA SMOS Mission.

Significant progress in weather forecasting, climate monitoring, and extreme event forecasting, depends on a good estimation of both soil moisture and sea surface salinity on a global scale.


The concept of measuring sea salinity and soil moisture by remote sensing is presently a hot issue as both Europe and the US are preparing satellite missions for that purpose. ESA will launch its synthetic aperture radiometer SMOS in 2008, while NASA plans to launch its Aquarius push-broom radiometer in 2010. SMOS is the first synthetic aperture radiometer to be launched in space, and it is illustrated in Figure 1. A traditional 9 m diameter solid antenna reflector and one radiometer are replaced by 69 small antenna elements and 69 radiometers placed on 3 each 4.5 m long arms in a Y-shape. The outputs from all possible pairs of antennas are cross correlated, and via digital processing a ground resolution comparable with that of the 9 m solid reflector is achieved. The advantage is that the arms carrying the many elements are relatively easy to fold during launch. The disadvantage is complexity and significant digital processing. With present day technology, however, the synthetic aperture concept comes out favorably of a trade-off exercise.


An L-band radiometer can measure sea salinity and soil moisture. But the brightness temperature as seen by the radiometer depends also on a range of other geophysical parameters, like for example sea surface roughness in the sea salinity case and vegetation cover in the soil moisture case. Many of these dependencies are well understood and modeled, but there are still outstanding issues requiring ground based and airborne experiments under controlled/monitored conditions.


The CoSMOS project is a multiyear initiative featuring airborne campaigns over the North Sea and the Mediterranean Sea, in Australia, Finland, Germany, and Spain. DTU Space has developed an airborne, fully polarimetric L-band radiometer system featuring unique features. The radiometer is of the digital kind: the L-band signal is directly fed into fast A to D converters using sub-harmonic sampling. All Stokes parameters are calculated digitally in a fast FPGA. Special attention is paid to detection and mitigation of interference from external active sources. The antenna system comprises 2 large Potter horns for almost ideal antenna patterns and low loss.


Figure 2 shows the antenna horns on a Skyvan owned and operated by Helsinki University of Technology. This aircraft has been used for all European missions. An example of a flight pattern over the North Sea is seen in Figure 3. The aim of this mission was to investigate a possible variation in the measured signal as a function of look angle compared with the wind direction.