Long-time Monitoring of L-Band Emission at Dome-C Antarctica: the DOMEX-3 Experiment
Macelloni, Giovanni1; Brogioni, Marco1; Pettinato, Simone1; Monti, Fabiano2; Casal, Tania3
1IFAC - CNR, ITALY; 2CVA-ARPAV, ITALY; 3ESA - ESTEC, NETHERLANDS

Along with the development of SMOS [1] and Aquarius [2] missions at L-band (and with the future SMAP mission), which feature large antennas, the need has arisen to find stable areas for the external calibration of the satellite instruments. The usual observation of deep space and calm ocean regions provide only "cold" calibrations measurements (i.e. few Kelvin and around 100 K respectively). Among the possible selections for a high temperature target, the East Antarctic Plateau (EAS), in particular the Dome-C area (75°06' S, 123°24' E) near the Italian-French base of Concordia, was investigated in recent years as a potential candidate. The reason for this interest lies in its geographical location (it can be seen several times a day by polar-orbiting satellites), as well as in the size, structure, spatial homogeneity, and thermal stability of this area. This region is particularly interesting for low-frequency microwave radiometers since, due to the low extinction of dry snow at L-band, the upper ice sheet layer which experienced seasonal variations is almost transparent and the brightness temperature variability is therefore extremely small. In order to verify the temporal stability of microwave emission in a restricted area of the Dome-C site, since 2004 two dedicated experiments supported by ESA have been carried out there [3],[4]. These experiments, called DOMEX-1 and DOMEX-2, featured a ground-based L-band radiometer operating 24 hours per day for one month and for around two years, respectively. The data collected during these experiments helped to confirm that the microwave emission of the Dome C site at L-band can be considered stable (especially at V polarization) for periods several months long. These results have been used in order to monitoring the performances of SMOS [5]. With the aim of keeping on monitoring the Dome-C region for the entire lifetime of SMOS, a new experiment called DOMEX-3 started in December 2013 and will last for 3 more years. The instrument used in the campaign is an improved version of the Radomex radiometer built by the Institute of Applied Physics in Florence, Italy, which was used successfully in the 20082010 period. The newly-designed Radomex radiometer is able to operate autonomously and remotely transmitting data daily to Europe. The instrument was accurately tested and calibrated before the shipment using standard procedures [3]. The instrument was installed in the US tower of Concordia station and acquisition started in mid of December 2012.
Preliminary data analysis confirms the results obtained in previous years. A first study was conducted over the angular trend of brightness temperature and compared to the measurements collected during DOMEX-2 experiment. Simultaneously to the microwave measurements, several snow profiles were performed in the investigated area. The most important snow physical characteristics (e.g. snow density, grain size and grain type) will be used to improve the measurements interpretation.
Microwave DOMEX-3 time-series data acquired from the beginning of the experiment (and they will be acquired in the forthcoming months) are compared to SMOS and Aquarius data collected in the pixel where the instrument is located. A comparison of Brightness Temperature versus incidence angle for the three sensors is also presented.
Moreover satellites data were also analyzed with the purpose of evaluating the spatial and temporal Tb distribution in a larger region surrounding Concordia station. In this case, we have used the SMOS data acquired from July 2010 to December 2012. The L1C data was transferred from Antenna to Top of Atmosphere (TOA) level (XY to HV) by applying Faraday and geometric rotations and the brightness temperatures were averaged within an incidence angle of between 37.5 and 47.5 degrees (only for nodes within the alias-free part of the SMOS field of view [1]). The Aquarius dataset consisted of Level-2 data (version 1.3) acquired from August 2011 to December 2012. Brightness temperature data without land correction collected at 45.6 deg of incidence angle were used. The analysis pointed out that the whole region is almost stable in time showing for each pixel a variability of the mean value of around 1 K at V polarization in the considered time period. In the study, a triangular-shape area was found 200 Km far from the station in the west direction. The area of the triangle was very large (approx. 30000 Km2), containing several SMOS pixels.. From the data, we computed that the mean difference of the Tbv time series between the triangular area and Concordia base was 2.6 K ( is higher over Concordia). Moreover, this difference remained stable in time (the temporal standard deviation was 0.24K ), and it was thus possible to correlate the measurements collected at Concordia to those acquired over this large area.
Aquarius data presents the same triangular pattern, which has dimensions and shape similar to the one individuated by SMOS. In this case the difference between the triangle and Concordia was 2 K at V polarization.

REFERENCES
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