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Ocean Reference for SMOS Zero-Baselines Based on Aquarius Brightness Temperature Simulator
[16-Apr-2013] Colliander, A., Dinnat, E., Le Vine, D., Chae, C-S., and Kainulainen, J.
Presented at the 2013 SMOS-Aquarius Science Workshop
SMOS [1] and Aquarius [2] are ESA and NASA missions, respectively, to make L-band measurements from the Low Earth Orbit. SMOS makes passive measurements whereas Aquarius measures both passively and actively. SMOS was launched in November 2009 and Aquarius in June 2011.The scientific objectives of the missions are overlapping: both missions aim at mapping the global Sea Surface Salinity (SSS). Additionally, SMOS mission produces soil moisture product (Aquarius data will eventually be used for retrieving soil moisture too). The consistency of the brightness temperature observations made by the two instruments is essential for long-term studies of SSS and soil moisture.
For ensuring the consistency, the calibration of the instruments is the key. The basis of the SMOS brightness temperature level is the measurements performed with the so-called zero-baselines [3]: SMOS employs an interferometric measurement technique which forms a brightness temperature image from several baselines constructed by combination of multiple receivers in an array and in which the zero-length baseline defines the overall brightness temperature level [4]. The basis of the Aquarius brightness temperature level is resolved from the brightness temperature simulator combined with ancillary data such as antenna patterns and environmental models [5]. Consistency between the SMOS zero-baseline measurements and the simulator output would provide a robust basis for establishing the overall comparability of the missions.
The footprints of the SMOS zero-baseline and the beams of the Aquarius are fundamentally different. This is due to the fact that the SMOS zero-baseline is designed to measure the average brightness temperature of the entire SMOS scene and, therefore, the 3-dB footprint spans over 1000 km in the cross-track direction and includes a piece of sky in the along-track direction, whereas Aquarius is a real aperture pushbroom system which has footprints of size about 70 km to 150 km on the surface of the Earth. Although the primary purpose of the Aquarius simulator is to simulate antenna temperature corresponding to Aquarius footprints, it is able to determine the brightness temperature of a wide beam such as that of the SMOS zero-baseline because it includes the entire field-of-view of the antenna including the side and back lobes of the antenna for accuracy. Accordingly the simulator has the capability to simulate the brightness temperature emitted by sea, land, atmosphere, microwave background and celestial objects over the full view angle of the antenna. The comparisons of this study will be made over ocean where the model is the most accurate (and which is most relevant for the SSS retrieval of the missions) and the largest contribution comes from the sea with partial effect from the sky.
The ability of the simulator to predict correct brightness temperature level for the SMOS zero-baseline has been verified in [6]. The Aquarius antenna pattern was replaced by the antenna pattern of a zero-baseline element of SMOS in the simulator, and simulations were carried out in the actual measurement geometry of SMOS. The level of the resultant antenna temperature was very close to the actual zero-baseline measurement. Full simulations of the SMOS zero-baseline observations are being carried out. The simulations focus on observations over the Pacific Ocean where the surface conditions are climatologically relatively constant, the full field of view is as free as possible from the effect of land, and the interference from the galaxy and the sun is minimal (but the significance of these interference sources will also be studied). The simulations include all observations carried out since the end of the SMOS commissioning phase which yields a data set of almost 3 years long. This allows investigation of seasonally driven anomalies in addition to short term and instantaneous effects.
The presentation will show the 3-year simulation of SMOS zero-baselines compared with the actual measurements. The similarities and differences will be analyzed and the implications on the consistency of the brightness temperatures of SMOS and Aquarius will be assessed. Although the comparison is not done at the retrieval resolution of SSS, the results will give important direction where corrections are potentially required for building a continuous SSS product between the missions.
[1] Y. Kerr et al., "The SMOS Mission: New Tool for Monitoring Key Elements of the Global Water Cycle", Proc. of IEEE, Vol. 98, No. 5, May 2010.
[2] D. Levine, G. Lagerloef, S. Torrusio, "Aquarius and Remote Sensing of Sea Surface Salinity from Space", Proc. of IEEE, Vol. 98, No. 5, May 2010.
[3] R. Oliva, M. Martin-Neira, I. Corbella, F. Torres, J. Kainulainen, J. Tenerelli, F. Cabot, F. Martin-Porqueras, "SMOS Calibration and Instrument Performance After One Year in Orbit", IEEE Trans. Geosci. Rem. Sens., Vol. 51, No. 1, January 2013.
[4] A. Colliander, L. Ruokokoski, J. Suomela, K. Veijola,J. Kettunen, V. Kangas, A. Aalto, M. Levander, H. Greus, M. Hallikainen, J. Lahtinen, "Development and Calibration of SMOS Reference Radiometer", IEEE Trans. Geosci. Rem. Sens., Vol. 45, No. 7, July 2007.
[5] D. Le Vine, E. Dinnat, S. Abraham, P. de Matthaeis, F. Wentz, "The Aquarius Simulator and Cold-Sky Calibration", IEEE Trans. Gesci. Rem. Sens., Vol. 49, No. 9, September 2011.
[6] A. Colliander, E. Dinnat, D. Le Vine, J. Kainulainen, "Synthesizing SMOS Zero-Baselines with Aquarius Brightness Temperature Simulator", Proc. IGARSS 2012, Munich, Germany, July 22-29, 2012.

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