The SIRTA (Site instrumental de Recherche par Télédétection Atmosphérique) is a ground-based platform located 25km south of Paris in France. The SIRTA observatory was created in 1999 by the French research institute IPSL (Institut Pierre Simon Laplace) to conduct research programs in order to improve the knowledge of radiative and dynamic processes in the atmosphere as well as complex interactions between clouds and aerosols. The objective is to better comprehend climate changes and evolution of environment using a suite a state-of-art active and passive remote sensing instruments.
Two ground platforms, a wooden tower, a roof platform and a building (where the lidar operates) are the main facilities of SIRTA. The project team is composed of six persons to ensure the station operations from instrument deployment, maintenance, data transfer and preliminary data analysis.
The SIRTA infrastructure enables to conduct many research activities that involve the cloud and aerosol lidar. Some of them will be discussed: the development of the STRAT (Structure of the Atmosphere) algorithm dedicated to automatically discriminate atmospheric layers and retrieve geophysical parameters from lidar profiles, and the CALIPSO validation using the dual-channel backscatter lidar deployed at SIRTA.
The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Program had a worldwide, ongoing ocean color data collection program, as well as an operational data processing and analysis capability. SIMBIOS data collection takes place via the SIMBIOS Science Team. In addition, SIMBIOS had a calibration and product validation component (Project Office). The primary purpose of these calibration and product validation activities were to (1) reduce measurement error by identifying and characterizing true error sources, such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical and atmospheric correction algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternate methods when possible, and provides the results to the appropriate project office.
Sun photometers are used to characterize the radiative properties of the atmosphere. They measure both the incident solar irradiance as well as the sky radiance (from scattered incident flux). Global networks of sun photometers provide data products such as aerosol optical thickness derived from these measurements. Instruments are typically calibrated for irradiance responsivity by cross-calibration against a primary reference sun photometer and for radiance responsivity using a lamp-illuminated integrating sphere source. A laser-based facility for Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) has been developed at the National Institute of Standards and Technology. Sensors can be calibrated in this facility for absolute spectral irradiance and radiance responsivity with combined expanded (k = 2) uncertainties ranging from 0.15% to 0.25%. Two multi-channel filter radiometers used in the Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) program of the National Aeronautics and Space Administration (NASA) at the Goddard Space Flight Center (GSFC) were calibrated for radiance and irradiance responsivity using conventional approaches and using laser-illuminated integrating spheres on SIRCUS. The different calibration methods are compared, the uncertainties are evaluated, and the impact on remote sensing applications is discussed.
Satellite ocean-color algorithms generally use aerosol-mixture models to estimate and remove the atmospheric contribution to the measured signal. These models, based on aerosol samples, may or may not be realistic. In atmospheric correction, we are more interested in the optical behavior of the aerosols through the entire atmosphere. Comparisons of SeaWiFS-derived and measured aerosol optical thickness have revealed a systematic underestimation of the Angstrom coefficient, suggesting that the reference models may not be representative of actual conditions. To investigate the adequacy of the models and ultimately to improve atmospheric correction, we analyze atmospheric optics data collected by the AERONET project under a wide range of aerosol conditions at coastal and island sites. Using non-supervised classification techniques (self-organized mapping, hierarchical clustering), we determine the natural distribution of retreived aerosol properties of the total atmospheric column, i.e., the volume size distribution function and the refractive index, and more importantly identify clusters in this distribution. These clusters may be used as new aerosols mixtures in radiative transfer algorithms. We compare the clusters with the SeaWiFS reference models and, through application examples, conclude about their potential to improve atmospheric correction of satellite ocean color.
KEYWORDS: Radiometry, Calibration, Temperature metrology, Black bodies, Sensors, Mirrors, Signal detection, Transmittance, Remote sensing, Signal to noise ratio
The radiometer CLIMAT is a highly sensitive field instrument designed for multispectral thermal infrared measurements.
Ground-based measurements can be performed. but the instrument has capabilities for operating from aircraft or balloon.
The optics consist of an objective lens and a condenser mounted according to the Koehler principle to provide uniform irradiation
over the detector surface. The radiometric signal is treated by a fast thermopile detector characterized by a low noise and a very
weak temperature dependence of its responsivity. The managing system allows either manual or automated measurements. The
energy consumption of the instrument is optimized for a maximum autonomy.
The optical and electrical units of the instrument are described. Different experimental studies for measuring the sensitivity
accuracy, spectral characteristics, thermal behavior and, field of view of the instrument are described.
The instrument is dedicated to ground and vegetation on the one hand. and on the other hand, clouds and atmospheric soundings.
The radiometer is also designed for calibrations or analyses of satellite radiometry data.
Some atmospheric measurements obtained with a prototype are presented.
Prospects are the development and the qualification of a narrow field-of-view instrument adapted to inhomogeneous targets
such as cirrus clouds. A 3.7-tim channel and an internal blackbody are under study.
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