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. The vicarious absolute calibration of electro-optical sensors dedicated to the Earth observation includes the definition of a reference surface from which radiometric measurements taken from the ground are compared to the effective radiance measured by the sensor in orbit. In order to facilitate the surface radiometric characterization process and consequently the sensor radiometric calibration, it is desirable that the surface presents, besides additional characteristics, temporal reflectance stability. This study aimed to evaluate the temporal stability of two potential reference surfaces for radiometric calibration of orbital electro-optical sensors located at: Tuz Golu Salar in Turkey and Atacama Desert in Chile. Therefore, a temporal analysis of the radiometric properties of these two surfaces using cloud free images of TM/Landsat 5 sensor, acquired from 2003 to 2011, was performed. It was concluded, based on statistical criteria, that both reference surfaces do not presented temporal stability. Nevertheless, both surfaces may still be used for sensor calibration purposes if they were submitted to further spectral characterization with higher frequency and/or if the surfaces were considered stable “enough” within a certain limit of variation in reflectance. Taking that into account, according to the results of this work, it can be stated that Tuz G¨ol¨u surface reflectance has temporal stability within a range of 3-14% and the Atacama Desert better than 6%. Keywords: Earth observation sensors, radiometric calibration, reflectance, TM/Landsat 5. RESUMO. A primeira etapa para a realizacao da calibracao absoluta de sensores de observacao da Terra e a definicao de uma superficie de referencia. Um dos metodos mais comuns de calibracao apos o lancamento do sensor utiliza medicoes radiometricas de areas localizadas na superficie terrestre. Para facilitar o processode caracterizacao da superficie e consequentemente o processo de calibracao radiometrica, e desejavel que a superficie apresente, entre outras caracteristicas, estabilidade temporal. Assim, este trabalho teve como objetivo avaliar a estabilidade temporal de duas superficies de referencia potenciais para a calibracao radiometrica de sistemas sensores eletro-opticos: o salar de Tuz Golu na Turquia e o deserto de Atacama no Chile. Para tanto, foi realizada uma analise temporal do comportamento espectral das duas superficies por meio de imagens do sensor TMabordo do Landsat 5 livres de nuvens adquiridas nos anos de 2003 a 2011.De acordo com os resultados obtidos foi possivel concluir, segundo os criterios estatisticos, que as duas superficies de referencia nao apresentam estabilidade temporal. Apesar disso, as duas superficies ainda podem ser utilizadas para calibracao de sensores. Nesse caso, deve-se caracterizar espectralmente as duas areas com maior frequencia e/ou considerar a superficie como sendo “suficientemente” estavel se a variacao na reflectância ao longo do tempo for menor do que um determinado valor. Se esta consideracao for feita pode-se afirmar, segundo o resultado desse trabalho, que Tuz Golu tem estabilidade temporal entre 3 a 14% e o deserto de Atacama melhor do que 6%. Palavras-chave: sensores de observacao da Terra, calibracao radiometrica, reflectância, TM/Landsat 5.

TEMPORAL STABILITY OF TUZ GÖLÜ AND ATACAMA DESERT REFERENCE SURFACES FOR ABSOLUTE CALIBRATION OF ORBITAL SENSORS