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SPOT-6 is an optical imaging satellite built by ASTRIUM (an EADS company), successfully launched on September 9, 2012. It is capable of imaging the Earth with a resolution of 1.5 meter Panchromatic and 6 meter Multispectral (Blue, Green, Red, Near-IR). SPOT6 offers imaging products to customers in defence, agriculture, deforestation, environmental monitoring, coastal surveillance, engineering, oil, gas and mining industries. The constellation SPOT-6 and SPOT-7 (to be launched at end of 2013) will actually provide a daily revisit everywhere on Earth with a total coverage of 6 million km² per day. SPOT-6 and 7 will ensure continuity of service provided by SPOT-4 and SPOT-5 during the next 10 years. Since many applications are concerned about possible radiometric bias between these sensors, radiometric absolute calibration and cross-calibration are crucial needs in order to guarantee or quantify the consistency of the measurements. Three different vicarious calibration methods have been used during SPOT-6 in-orbit commissioning phase. The presentation will detail the methods used, and the adaptations and tunings performed in order to deal with the sensor’s and mission’s specificities. Six desert sites selected amongst the 20 sites used by CNES as radiometric targets are revisited. The site selection process and the cross-calibration method over desert sites will be briefly explained: starting from measurements by a reference sensor, surface reflectances are computed through an atmospheric correction step. A spectral interpolation is then performed to derive the surface reflectance for spectral bands of the sensor to calibrate. Then the atmospheric contribution is added to compute top-of-atmosphere reflectance. The comparison to reflectance acquired by the sensor to calibrate provides an estimation of the cross-calibration with the reference sensor. ROSAS method, which uses a photometer implemented in the site of LaCrau, in South East of France, consists of a characterization of the optical properties of the atmosphere through measurements of the solar extinction, sky radiance and upwelling radiance of the ground, simultaneously to the satellite overpass. The data are automatically processed, in order to calibrate the photometer and estimate the bidirectional reflectance distribution function of the site. This computation gives access to the top-of-atmosphere radiance in the geometric conditions of any satellite over-passing the site. Absolute calibration using Rayleigh scattering takes advantage of the fact that the top-of-atmosphere signal measured by a satellite optical sensor observing oceanic targets is in large proportion due to scattering of the incident solar irradiance by atmospheric components. This Rayleigh scattering process can be accurately predicted and computing using surface pressure, knowing the instrument spectral response. We will see how this method has to be adapted to take into account the SPOT-6 instrumental concept: two cameras are implemented in the same instrument to achieve the requested 60km swath, in a very compact concept based on a Korsch combination. Finally, the results of the different vicarious calibration methods used will be combined, providing an accurate absolute and cross-calibration of the SPOT-6 products, allowing it to fulfill its mission to continue sustainable wide-swath high-resolution observation services as currently provided by the previous SPOT missions. |
