PLEIADES ABSOLUTE CALIBRATION : INFLIGHT CALIBRATION SITES AND METHODOLOGY

In-flight calibration of space sensors once in orbit is a decisive step to be able to fulfil the mission objectives. This article presents the methods of the in-flight absolute calibration processed during the commissioning phase. Four In-flight calibration methods are used: absolute calibration, cross-calibration with reference sensors such as PARASOL or MERIS, multi-temporal monitoring and inter-bands calibration. These algorithms are based on acquisitions over natural targets such as African deserts, Antarctic sites, La Crau (Automatic calibration station) and Oceans (Calibration over molecular scattering) or also new extra-terrestrial sites such as the Moon and selected stars. After an overview of the instrument and a description of the calibration sites, it is pointed out how each method is able to address one or several aspects of the calibration. We focus on how these methods complete each other in their operational use, and how they help building a coherent set of information that addresses all aspects of in-orbit calibration. Finally, we present the perspectives that the high level of agility of PLEIADES offers for the improvement of its calibration and a better characterization of the calibration sites

Results from the Radiometric Absolute Calibration of Sentinel-2A

The SENTINEL-2A mission, launched in June 2015, is dedicated to the observation of Earth’s land surface and coastal zones, especially the monitoring of land cover change and vegetation. The mission provides data continuity with both LANDSAT and SPOT programs, with increased resolution (down to 10m), revisit (10-day cycle for Sentinel-2A alone, 5-day cycle with Sentinel-2B to be launched in the end of 2016) and spectral resolution (13 spectral bands from 443 to 2190 nm). The Instrument is in-flight calibrated and characterized primarily using an on-board device (diffuser). Afterward, vicarious calibration methods are used in order to validate Sentinel-2A radiometry. The calibration can be checked over dedicated natural targets such as Rayleigh scattering, desert sites, Antarctica, and tentatively deep convective clouds. Based on these methods, it is possible to provide an accurate checking of many radiometric aspects such as absolute and interband calibrations, trending correction, calibration consistency within the field-of-view, and more generally this will provide an evaluation of the radiometric consistency for various types of targets. Another important aspect will be the checking of cross-calibration between many other instruments such as MERIS, MODIS (bridge to the GSICS radiometric standard), as well as the recently launched Sentinel-3 (bridge between Sentinel missions). The results of both nominal calibration over the on-board device and validation from vicarious methods, will be presented and discussed

Outcome of the GSICS/CEOS-IVOS Lunar Calibration Workshop

In December 2014 experts from 14 different agencies and departments attended the joint GSICS CEOS/IVOS Lunar Calibration Workshop meeting organized by EUMETSAT in collaboration with USGS, CNES and NASA. This represents potentially more than 25 instruments capable of observing the Moon. The main objectives of the workshop were i) to work across agencies with the GSICS Implementation of the ROLO model (GIRO) - a common and validated implementation of the USGS lunar radiometric reference, ii) to share knowledge and expertise on lunar calibration and iii) to generate for the first time a reference dataset that could be used for validation and comparisons. This lunar calibration community endorsed the GIRO to be the established publicly-available reference for lunar calibration, directly traceable to the USGS ROLO model. However, further effort is required to reach inter-calibration between instruments, in particular for each instrument team to accurately estimate the over-sampling factor for their images of the Moon. A way forward to develop a cross-calibration algorithm and GSICS inter-calibration products is proposed. This includes key issues of fixing the GIRO calibration to an absolute scale, addressing spectral differences between instruments, and improving the existing calibration reference, which translates into future updates of the GIRO. The availability of extensive Moon observation datasets will help to further improve this reference and is expected to grow with the availability of additional lunar observations from past, current and future missions. All participants agreed on EUMETSAT pursuing its efforts in developing and maintaining the GIRO in collaboration with USGS to ensure traceability to the reference ROLO model

PLEIADES-HR IMAGE QUALITY COMMISSIONING

PLEIADES is the highest resolution civilian earth observing system ever developed in Europe. This imagery program is conducted by the French National Space Agency, CNES. It operates since 2012 a first satellite PLEIADES-HR launched on 2011 December 17th, a second one should be launched by the end of the year. Each satellite is designed to provide optical 70 cm resolution coloured images to civilian and defence users. The Image Quality requirements were defined from users studies from the different spatial imaging applications, taking into account the trade-off between on-board technological complexity and ground processing capacity. The assessment of the image quality and the calibration operation have been performed by CNES Image Quality team during the 6 month commissioning phase that followed the satellite launch. These activities cover many topics gathered in two families : radiometric and geometric image quality. The new capabilities offered by PLEIADES-HR agility allowed to imagine new methods of image calibration and performance assessment. Starting from an overview of the satellite characteristics, this paper presents all the calibration operations that were conducted during the commissioning phase and also gives the main results for every image quality performance

The Canopy and Aerosol Particles Interactions in TOulouse Urban Layer (CAPITOUL) Experiment

The CAPITOUL experiment is a joint experimental effort in urban climate, including the energetic exchanges between the surface and the atmosphere, the dynamics of the boundary layer over the city and its interactions with aerosol chemistry. The campaign took place in the city of Toulouse in southwest France, for one year, from February 2004 to February 2005. This allowed the study of both the day-to-day and seasonal variability of urban climate processes. The observational network included surface stations (meteorology, energy balance, chemistry), profilers and, during intensive observing periods, aircraft and balloons. The urban Surface Energy Balance differs between summer and winter: in summer, the solar heat stored during the previous daytime period is enough to maintain the heat release at night, but in winter, almost all the energy comes from the anthropogenic heat released by space heating. Both processes produce the well known Urban Heat Island (UHI). The city is shown to impact the entire boundary layer on specific days, when an urban breeze is observed. In wintertime, fog is found to be modified due to the vertical structure of the nocturnal boundary layer above the city (which is slightly unstable and not stable). The measurements of aerosol properties in and downwind the city permitted documentation of the urban aerosol as well as the chemical transformation of these aerosols, in particular the ageing of carbonaceous aerosols during transport. The Toulouse aerosol is mainly composed of carbonaceous particles. There is important seasonal variation in the ratio of black carbon to organic carbon, in the concentration of sulfates and nitrates and in the related radiative aerosol impacts. SF6 was released as a tracer in a suburban area of Toulouse during anticyclonic conditions with weak winds. The tracer measurements show dispersion was mainly driven by the surface sensible heat flux, and was highly sensitive to the urban heat island and also to the transport of boundary layer clouds. Modeling was fully integrated into the campaign. Surface energy balance and urban boundary layer processes have already been used to complement the analyses of the physical processes observed during the campaign. Companion papers detail most of these observation or modeling studies