1996 IEEE TRANSACTIONS ONGEOSCIENCE AND REMOTE SENSING, VOL. 45, NO. 7, JULY 2007 Surface Topography and Mixed-Pixel Effects on the Simulated L-Band Brightness Temperatures Marco Talone, Student Member, IEEE, Adriano Camps, Senior Member, IEEE, Alessandra Monerris, Student Member, IEEE, Mercè Vall-llossera, Member, IEEE, Paolo Ferrazzoli, Member, IEEE, and María Piles, Student Member, IEEE Abstract—The impact of topography and mixed pixels on L-band radiometric observations over land needs to be quan- tified to improve the accuracy of soil moisture retrievals. For this purpose, a series of simulations has been performed with an improved version of the Soil Moisture and Ocean Salinity (SMOS) End-to-End Performance Simulator (SEPS). The bright- ness temperature (T B ) generator of SEPS has been modified to include a 100-m-resolution land cover map and a 30-m-resolution digital elevation map of Catalonia (northeast of Spain). This high- resolution T B generator allows the assessment of the errors in soil moisture retrieval algorithms due to limited spatial resolution and provides a basis for the development of pixel disaggregation techniques. Variation of the local incidence angle, shadowing, and atmospheric effects (up- and downwelling radiation) due to surface topography has been analyzed. Results are compared to brightness temperatures that are computed under the assumption of an ellipsoidal Earth. Index Terms—L-band radiometry, mixed pixel, simulator, SMOS End-to-End Performance Simulator (SEPS), Soil Moisture and Ocean Salinity (SMOS), topography. I. I NTRODUCTION T HE AVAILABILITY of high-resolution brightness tem- perature (T B ) maps at L-band is crucial to analyze im- portant issues dealing with bare and vegetation-covered land emission and to develop inversion algorithms in preparation for real Soil Moisture and Ocean Salinity (SMOS) mission data. Mixed-pixel, coastlines, shadowing, and topography effects on the measured brightness temperatures need further study, but the lack of global geophysical data at sufficient temporal and spatial resolution and the large amount of data involved in the generation of high-resolution T B maps on a global basis complicate the issue. In fact, in spite of the existence of global digital elevation models with sufficient spatial resolution, accu- rate land cover data do not exist for most parts of the world. To address these issues, a series of simulations has been performed with an improved version of the SMOS End-to-End Performance Simulator (SEPS) [1], [2], in which, to date, all points on Earth have been assumed to be at sea level. The study Manuscript received May 30, 2006; revised February 14, 2007. This study has been supported by the CICYT under Project TEC2005-06863-C02-01. M. Talone, A. Camps, A. Monerris, M. Vall-llossera, and M. Piles are with the Remote Sensing Laboratory, Department Teoria del Senyal i Comunica- cions, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain. P. Ferrazzoli is with Tor Vergata University, Ingegneria, DISP, 00133 Roma, Italy. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2007.898254 has been done over the region of Catalonia, on the northeastern coast of Spain, because of its many different land cover types, topography, and the presence of a coastline. A 30-m-resolution digital elevation map [3] and a 100-m-resolution land coverage map of Catalonia [4] have been used as inputs, and SEPS has been conveniently modified to generate high-resolution T B maps of this area. A variety of soil and land cover types (crops, bushes, marshes, etc.) have been parameterized using the values obtained from field experiments and literature [5]–[10], [12]. II. GENERATION OF HIGH-RESOLUTION BRIGHTNESS TEMPERATURE MAPS AT L-BAND A. Description of the Auxiliary Data A digital elevation model [3] and a land cover map [4] of Catalonia (northeast Spain; Fig. 1) have been used as inputs to the high-resolution T B generator. The digital elevation model of Catalonia (DEMC) has 30 × 30 m resolution and is referred to the official reference system European Datum 1950. Real and estimated values from 4957 test points were compared, and a root mean-square error (RMSE) of 2.6 m for the first version and 1.4 m for the second version were obtained. The first version of DEMC is available for the whole region, whereas the second version (with a lower RMSE) is available only for two thirds of Catalonia. The DEMC was used to account for the effect of topography and shadowing on the radiometric signal. On the other hand, the land coverage map of Catalonia [Mapa de Cobertes del Sòl de Catalunya (MCSC), Fig. 1] has 100-m resolution and includes 21 different land cover types. In the latest version, these categories are divided into sub- categories, which better characterize the environment, but this finer classification is only available for one third of Catalonia. The classification follows the legend proposed by the European Environmental Agency, CORINE, but with a 50 times better resolution. High-resolution T B maps are generated combining the DEMC and the MCSC information and are composed of an area with the top left corner at (42 ◦ 55 ′ N, 0 ◦ 8 ′ E) and with the bottom right corner at (40 ◦ 28 ′ N, 3 ◦ 25 ′ E). B. Brightness Temperature Maps Generator: Input Parameters By taking into account the auxiliary data, a local incidence angle at each DEMC 30 × 30 m pixel has been computed, and a set of geophysical parameters has been associated with each category of the MCSC. The geophysical parameters 0196-2892/$25.00 © 2007 IEEE