Prof. Ouahid Aznay Profile

Prof. Ouahid Aznay

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Publications (6)

Proceedings Article | 14 September 2016 Paper

ScaRaB and CERES-Terra: results of the inter-comparison campaigns

Thierry Trémas, Ouahid Aznay, Olivier Chomette

Proceedings Volume 9973, 99730B (2016) https://doi.org/10.1117/12.2237591

KEYWORDS: Calibration, Black bodies, Infrared radiation, Satellites, Radiometry, Lamps, Sensors, Clouds, Reflectivity, Earth's atmosphere

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ScaRaB (SCAnner for RAdiation Budget) is an Indo-French satellite onboard MEGHA-TROPIQUES launched on October 12th 2011. This radiometer has been designed to fill the gap between the ERBS and CERES missions to study the water cycle and energy exchanges in the tropics. ScaRaB is fit with four parallel and independent channels: channel- 2 and channel-3 being considered as the main ones, channel-1 is dedicated to measure solar radiance while channel-4 is an infrared window. The absolute calibration of ScaRaB is achieved by internal calibration sources (black bodies and a lamp for channel-1). The radiometric properties of deserts sites and more especially their stable spectral response over time made them very good candidates to perform temporal monitoring of ScaRaB channel-1. This paper deals with the corresponding results. High altitude clouds are observed by ScaRaB to survey the balance between channel 2 and channel 3: the earth longwave radiance is isolated by subtracting the short-wave channel to the total channel. Radiometric cross calibration of Earth observation sensors is a crucial need to guarantee or quantify the consistency of measurements from different sensors. CERES and ScaRaB Earth Radiation Budget missions have the same specification: to provide an accuracy of ~1% in the measurement of short-wave and long-wave radiances and an estimation of the short-wave and long-wave fluxes less than 10 W/m². Taking advantage of the “equatorial” orbit of Megha-Tropiques, NASA proceeded to manoeuvers on CERES-Terra in order to ease an inter-comparison between both instruments over common targets. Actually, The CERES PAPS mode was used to align its swath scan in order to increase the collocated pixels between the two instruments. The experience lasted 3 months from March 22th and May 31st 2015. A previous similar campaign has already been led in 2012. This article presents the results of these inter-comparisons, providing an indication on the temporal stability of the calibration between 2012 and 2015.

Proceedings Article | 15 October 2015 Paper

ScaRaB: first results of absolute and cross calibration

Thierry Trémas, Ouahid Aznay, Olivier Chomette

Proceedings Volume 9643, 964304 (2015) https://doi.org/10.1117/12.2194866

KEYWORDS: Calibration, Sensors, Satellites, Clouds, Lamps, Radiometry, Black bodies, Reflectivity, Aerosols, Sensor calibration

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ScaRaB (SCAnner for RAdiation Budget) is the name of three radiometers whose two first flight models have been launched in 1994 and 1997. The instruments were mounted on-board Russian satellites, METEOR and RESURS. On October 12^th 2011, a last model has been launched from the Indian site of Sriharikota. ScaRaB is a passenger of MEGHA-TROPIQUES, an Indo-French joint Satellite Mission for studying the water cycle and energy exchanges in the tropics. ScaRaB is composed of four parallel and independent channels. Channel-2 and channel-3 are considered as the main ones. Channel-1 is dedicated to measure solar radiance (0.5 to 0.7 μm) while channel-4 (10 to 13 μm) is an infrared window. The absolute calibration of ScaRab is assured by internal calibration sources (black bodies and a lamp for channel-1). However, during the commissioning phase, the lamp used for the absolute calibration of channel-1 revealed to be inaccurate. We propose here an alternative calibration method based on terrestrial targets. Due to the spectral range of channel-1, only calibration over desert sites (temporal monitoring) and clouds (cross band) is suitable.

Desert sites have been widely used for sensor calibration since they have a stable spectral response over time. Because of their high reflectances, the atmospheric effect on the upward radiance is relatively minimal. In addition, they are spatially uniform. Their temporal instability without atmospheric correction has been determined to be less than 1-2% over a year. Very-high-altitude (10 km) bright clouds are good validation targets in the visible and near-infrared spectra because of their high spectrally consistent reflectance. If the clouds are very high, there is no need to correct aerosol scattering and water vapor absorption as both aerosol and water vapor are distributed near the surface. Only Rayleigh scattering and ozone absorption need to be considered. This method has been found to give a 4% uncertainty.

Radiometric cross calibration of Earth observation sensors is a crucial need to guarantee or quantify the consistency of measurements from different sensors. ScaRaB is compatible with CERES mission. Two main spectral bands are measured by the radiometer: A short-wave channel (0.2 to 4 μm) dedicated to solar fluxes and a Total channel (0.2 to 200 μm) for fluxes combining the infrared earth radiance and the albedo. The earth long-wave radiance is isolated by subtracting the short-wave channel to the Total channel.

Both Earth Radiation Budget missions (CERES and ScaRaB) have the same specification: to provide an accuracy of ~1% in the measurement of short-wave and long-wave radiances and an estimation of the short-wave and long-wave fluxes less than 10 W/m². We use the CERES PAPS and Cross-Track SSF datasets for direct radiances and fluxes comparisons during two validation phases. The first one occurred during April 17^th to June 8^th (51 days) in 2012 and the second one occurred between March 22^th and May 31^st 2015. The first validation campaign has been held with the CERES team using the Terra FM2 data. The CERES PAPS mode was used to align the swath scan, in order to increase the collocated pixels between the two instruments. This campaign allowed us to validate the ScaRaB radiances and to refine the error budget. The second validation campaign aims to provide a temporal monitoring of ScaRab calibration.

Proceedings Article | 12 October 2015 Paper

Comparison of S-NPP VIIRS and PLEIADES lunar observations

Xiaoxiong Xiong, Sophie Lachérade, Ouahid Aznay, Bertrand Fougnie, Jon Fulbright, Zhipeng Wang

Proceedings Volume 9639, 96390Y (2015) https://doi.org/10.1117/12.2193158

KEYWORDS: Calibration, Sensors, Satellites, Reflectivity, MODIS, Spatial resolution, Sensor calibration, Diffusers, Photonic integrated circuits, Near infrared

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Proceedings Article | 11 November 2014 Paper

Comparison of MODIS and PLEIADES Lunar observations

Xiaoxiong Xiong, Sophie Lachérade, Laurent Lebègue, Bertrand Fougnie, Amit Angal, Zhipeng Wang, Ouahid Aznay

Proceedings Volume 9241, 924111 (2014) https://doi.org/10.1117/12.2067442

KEYWORDS: MODIS, Calibration, Sensors, Reflectivity, Satellites, Space operations, Scanning laser ophthalmoscopy, Near infrared, Aerospace engineering, Photonic integrated circuits

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Proceedings Article | 7 October 2014 Paper

POLO: a unique dataset to derive the phase angle dependence of the Moon irradiance

S. Lachérade, O. Aznay, B. Fougnie, L. Lebègue

Proceedings Volume 9241, 924112 (2014) https://doi.org/10.1117/12.2067283

KEYWORDS: Calibration, Spatial resolution, Satellites, Sensors, Data modeling, Near infrared, Satellite imaging, Image resolution, Visible radiation, Detector arrays

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Proceedings Article | 15 October 2012 Paper

ScaRaB: first results of the scanner for radiative budget on board the Indo-French satellite Megha-Tropiques

Thierry Trémas, Nadia Karouche, Alain Rosak, Aimé Meygret, Ouahid Aznay, Emmanuel Hillairet

Proceedings Volume 8510, 851002 (2012) https://doi.org/10.1117/12.928293

KEYWORDS: Calibration, Radiometry, Satellites, Black bodies, Lamps, Optical filters, Clouds, Sensors, Scanners, Matrices

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Showing 5 of 6 publications

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