Launched in November 2009, the first space-borne passive L-band microwave (1.4 GHz) radiometer is the European Space Agency’s (ESA) Soil Moisture and Ocean Salinity (SMOS) mission. Operating at the preferred frequency, a multi-angle, fully polarimetric system on board the satellite offers unprecedented possibilities for retrieving surface soil moisture data (~5 cm depth) of global coverage every 2-3 days at a spatial resolution of ~44 km. Such data is urgently needed in a wide range of applications from hydrologic, weather forecast and climate modeling to agriculture, water management and flood monitoring.
An important step in any spaceborne mission is the calibration and validation (Cal/Val) of the retrieval algorithm and the associated data by ground measurements across a range of climatic regions. Based thereupon the algorithm is adjusted in order to continuously enhance data quality.
One of the SMOS Cal/Val sites has been established in the Skjern River Catchment in the framework of HOBE. Two complementary validation approaches have been used: (1) a short-term (2 weeks) campaign where soil moisture was densely measured within selected patches, while the area was simultaneously covered by airborne radiometer measurements, and (2) the installation of a permanent soil moisture network with several distributed stations within the satellite footprint. The advantage of the airborne campaigns is the possibility to bridge the spatial scales stepwise, from ground via airborne to spaceborne measurements. Meanwhile soil moisture networks provide long-term series at high temporal resolution.
More information on the SMOS mission can be found under
In the context of SMOS Calibration/Validation activities, an airborne campaign took place in the Skjern River Catchment, Denmark. Between April 26 and May 10, 2010, 4 flights with the L-band radiometer EMIRAD-2 built at the Technical University of Denmark (DTU) and concurrent ground sampling within three patches were carried out.
At the beginning of each flight external calibration took place over open water in the Ringkøbing fjord, subsequently, 10 tracks were flown over the SMOS pixel. EMIRAD-2 has two antennas measuring at 0 and 40 degree incidence angle and horizontal and vertical polarizations, respectively. The 2000 m flight altitude resulted in an approximate EMIRAD footprint size of 1.4 km.
The ground sampling patches had sizes of 2x2km and were selected as homogeneously as possible and to cover the prevailing land cover and soil conditions, namely agriculture, heath and forest and very sandy soils. Theta probe moisture readings of the mineral soil were taken on 6 lines at regular spacing. In case of forest and heath, readings were also taken in the organic surface layers, thus reducing the total number of points measured. Additionally, several soil and organic samples were taken on each sampling day.
At the beginning and towards the end of the campaign auxiliary measurements were taken. Surface roughness was measured on a flat barley field and on a potato field with distinct row structure to span the entire range of expected conditions. Destructive vegetation sampling was carried out within the agriculture and heath patches for the estimation of the vegetation water content. During each sampling, land cover information was noted at each measurement point.
During the two weeks time window an alternating Danish and international crew of highly motivated people contributed significantly to the success of this campaign.
SkyTEM is a high resolution airborne time domain electromagnetic instrument (TDEM) carried by a helicopter, which facilitates rapid coverage of large areas and provides maximum operational flexibility. A TDEM instrument maps the electrical conductivity of the subsurface, which historically has been used mainly for locating highly conductive targets in mineral exploration. SkyTEM was developed at Aarhus University with the specific purpose of resolving more subtle near surface structures that were impossible to chart only 10 years ago. Today, the SkyTEM instrument is often regarded the worlds best for high resolution near surface EM surveying, eg. hydrogeological studies. The instrument itself along with methodology for processing and modeling the data is constantly being improved and refined, which constitutes a large part of the agenda for the HGG group at Aarhus University.
In August 2009 a 3 day SkyTEM survey was flown in the eastern part of the HOBE catchment, covering a lagoon type area of shallow saline surface water. Resolving details of a setting covered by such highly conductive material is a very difficult task for any TEM instrument, however, the study ended up providing a very detailed picture of the salinity distribution in the area. Large parts of the data processing and inversion techniques facilitating this result were developed within the HOBE framework.