Olivier Hagolle

PARASOL in-flight calibration and performance

Since 18 December 2004, the PARASOL satellite is a member of the so-called A-train atmospheric orbital observatory, flying together with Aqua, Aura, CALIPSO, CLOUDSAT, and OCO satellites. These satellites combine for the first time a full suite of instruments for observing aerosols and clouds, using passive radiometer complementarily with active lidar and radar sounders. The PARASOL payload is extensively derived from the instrument developed for the POLDER programs that performs measurements of bidirectionality and polarization for a very wide field-of-view and for a visible/near-infrared spectral range. An overview of the results obtained during the commissioning phase and the reevaluation after one year in orbit is presented. In-flight calibration methods are briefly described, and radiometric and geometric performances are both evaluated. All algorithms are based on a panel of methods using mainly natural targets previously developed for POLDER missions and adapted or redeveloped in the PARASOL context. Regarding performances, all mission requirements are met except for band 443 (not recommended for use). After one year in orbit, a perfect geometrical stability was found while a slight decrease of the radiometric sensitivity was observed and corrected through an innovative multitemporal algorithm based on observations of bright and scattered convective clouds. The scientific exploitation of PARASOL has now begun, particularly by coupling these specific observations with other A-train sensor measurements.

Absolute calibration of VEGETATION derived from an interband method based on the Sun glint over ocean

Absolute radiometric calibration is one of the main elements that contribute to the quality of measurements obtained with optical remote sensing instruments, but maintaining a good calibration accuracy during the whole life of an instrument is a difficult task. Since the sensitivity of an instrument generally changes after launch and degrades with time, many sensors have been equipped with onboard calibration devices. But these devices being not perfectly reliable, independent calibration methods based on natural targets are necessary to validate the results. The Sun glint calibration method is an interband calibration method that uses the specular reflection of the Sun on the ocean surface to transfer the absolute calibration of one reference spectral band to other spectral bands, from visible to short wave infrared wavelengths. Despite the drawback of relying on the absolute calibration of a reference spectral band, this method is one of the rare methods that can provide accurate calibration results for near-infrared spectral bands up to 1650 nm, without requiring costly in situ measurements simultaneously to the satellite overpass. This paper details the Sun glint calibration method and its error budget, and gives the results obtained with the VEGETATION instrument that was recently launched onboard the Systeme Pour lObservation de la Terre 5 (SPOT-5) satellite. These results compare very well with the results of other calibration methods.

Remote sensing data respository for in-flight calibration of optical sensors over terrestrial targets

The present study is part of an investigation aimed at optimizing the use of desertic sites for absolute or relative calibration of satellite visible sensors. This effort includes characterization of the surface, gathering of climatology or atmospheric data sets, ground- and air- based measurements as well as result of calibration of various sensors over these sites. All these measurements and estimates are stored in a repository and made available to various methods for calibration. Post-launch degradation and relative sensitivity of various sensor have been estimated using north african desertic sites as radiometrically stable targets. The selected area have first been characterize in terms of bidirectional and spectral reflectances by making use of POLDER capabilities, then to cross-calibrate SeaWifs, VEGETATION on-board SPOT4 and AVHRR on-board NOAA-14 by reference to POLDER. Results are compared with absolute and relative calibration issued from other sources. Extensive period of time are spanned to assess the ability of this method to monitor long term trends in sensor evolutions. Results of this cross calibration will be presented. The method developed for this study will be presented as well, in order to make it applicable to other sensor. A sensitivity study has also been realized, considering synthetic data, allowing to evalute the main contributions to the error budget. The need for aerosol optical thickness is then evidenced, and will lead to the set up of a sun photometer on one of the selected sites in 1999.

Automatic Registration of Optical Images, a Stake for Future Missions: Application to Ortho-Rectification, Time Series and Mosaic Products

Today, new earth observation missions are designed from satellite to ground segment to best fit the end-user needs while reducing the overall costs and complexity. Furthermore, an efficient use of high resolution image data is only possible if its location is accurate enough. The accuracy of the image location model depends on the knowledge of platform and orbital parameters (instruments calibration, satellite orbit and pointing) and also of the ground elevation (Digital Elevation Model). Good accuracy can be ensured by stringent requirements on onboard equipments (GPS, gyro, ...) but also by appropriate ground processing which is easier and less expensive to setup. This paper describes and evaluates an automatic method to improve location models during ground processing. This method has been implemented and tested at CNES for current and new missions (such as SPOT/FORMOSAT image ortho-rectification, Pleiades-HR images mosaicking or Venmus images time-series generation). These results must be considered for future missions designing.

Calibration of SPOT4 HRVIR and Vegetation cameras over Rayleigh scattering

The Rayleigh scattering over a clear ocean is a target which radiance is very well modeled and which enables to calibrate the short wavelengths of remote sensing instruments. But the quality of the calibration strongly depends on the evaluation of the other contributors to the observed Top Of Atmosphere radiance i. e. aerosol scattering and reflection over the sea surface (water color, foam, glint...). However these contributors can be reduced by appropriate viewing conditions. This technique is used to calibrate B1 (051-0.59 µm) and B2 (0.61-0.68µm) channels of HRVIR camera, and B0 (0.4-0.5µm) and B2 channels of VEGETATION camera both of which are aboard SPOT4. This article presents the calibration results obtained during the satellite two years in orbit. The results are compared to: - pre-flight results (integrating sphere) - in-flight results. The in-flight results are provided by: - on board calibration system (lamp and sun sensor) - vicarious calibration over test sites (White Sands, La Crau) - calibration over stable deserts - calibration over the sun glint The analysis of the sensitivity of the calibration to the different parameters used to model the TOA radiance shows the accuracy of such a technique.

Inter-calibration using desertic sites as a reference target

The present study is part of an investigation aimed at optimizing the use of desertic sites for absolute or relative calibration of satellite optical sensors. This effort includes characterization of the surface, gathering of climatology or atmospheric data sets, ground- and air-based measurements as well as results of calibration of various sensors over these sites. Post-launch degradation and relative sensitivity of various sensors have been estimated using north African desertic sites as radiometrically stable targets. The selected areas have first been used to cross-calibrate SeaWiFS sensor and POLDER, and then to cross-calibrate AVHRR and POLDER. Extension to other sensors will be presented.

POLDER multiangular calibration using desert sites: method and performances

The multiangular calibration is used to estimate the sensitivity changes in the different points of the wide field of view of an optical instrument equipped with linear or array detectors. The baseline method consists in having the instrument looking at a spatially uniform landscape. For a wide field of view instrument, continuous uniform landscape does not exist, so we propose a new method using several desert sites to simulate a spatially known landscape. Desert areas are already good candidates for the assessment of multitemporal calibration of optical satellite sensors. This requires that the sites be well characterized in terms of directional variations of their top of atmosphere reflectances, to account for variations in the solar or viewing configurations between each measurement. A ground campaign has been done to evaluate the bidirectional reflectances of different sites which are then used as reference. POLDER instrument is the first instrument using these references for the multiangular calibration. First, this paper describes the multiangular calibration method used on POLDER based on the knowledge of these desert sites. The site selection criteria and the method developed to localize these desert sites are remembered. Then the results are presented in different spectral bands and the performances of this calibration estimated.

Measurements and computations of polarized marine reflectance

In this study, we try to improve the understanding of the polarization of the signal reflected by the water body for which an accurate knowledge is necessary in remote-sensing applications. We only detail the Opol factor defined as the ratio between the vertically polarized to the total (unpolarized) marine reflectance. In the validation plans of many ocean color sensors, some above-water measurements of the diffuse marine reflectance, such as the SIMBAD measurements, use polarized observation of the sea which reduce the skylight reflection effects. Consequently, to access the marine reflectance, a correction by the Opol factor is made and for which an accuracy better than 2% is required. Another goal is to better consider polarization in the computation of the Top of the Atmosphere signal in the absolute calibration of satellite sensors over Rayleigh scattering (POLDER for instance). Measurements were collected for various conditions by REFPOL and SIMBAD radiometers. The results were confronted to computations performed using two different transfer radiative codes, a Monte-Carlo code and a Successive Order of Scattering code. The radiative transfer into the sea is computed considering the water index, the concentration and polarized phase function for the phytoplankton. As the upwelling signal observed just above the sea surface depends strongly to the radiance downwelling onto the sea surface, the presence of molecules and aerosol in the atmosphere which induce a polarized signature, have to be considered.