Patrice Henry

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.

Assessment of Spectral Band Impact on Intercalibration Over Desert Sites Using Simulation Based on EO-1 Hyperion Data

Since the beginning of the 1990s, stable desert sites have been used for the calibration monitoring of many different sensors. Many attempts at sensor intercalibration have been also conducted using these stable desert sites. As a result, site characterization techniques and the quality of intercalibration techniques have gradually improved over the years. More recently, the Committee on Earth Observation Satellites has recommended a list of reference pseudo-invariant calibration sites for frequent image acquisition by multiple agencies. In general, intercalibration should use well-known or spectrally flat reference. The reflectance profile of desert sites, however, might not be flat or well characterized (from a fine spectral point of view). The aim of this paper is to assess the expected accuracy that can be reached when using desert sites for intercalibration. In order to have a well-mastered estimation of different errors or error sources, this study is performed with simulated data from a hyperspectral sensor. Earth Observing-1 Hyperion images are chosen to provide the simulation input data. Two different cases of intercalibration are considered, namely, Landsat 7 Enhanced Thematic Mapper Plus with Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and Environmental Satellite MEdium Resolution Imaging Spectrometer (MERIS) with Aqua MODIS. The simulation results have confirmed that intercalibration accuracy of 1% to 2% can be achieved between sensors, provided there are a sufficient number of available measurements. The simulated intercalibrations allow explaining results obtained during real intercalibration exercises and to establish some recommendations for the use of desert sites for intercalibration.

Relative and multitemporal calibration of AVHRR, SeaWiFS, and VEGETATION using POLDER characterization of desert sites

This paper presents the last results of a continuing study aiming at a better characterization of desertic sites for satellite borne optical sensors calibration. The study relies on the gathering of long term archive of satellite data and coincident atmospheric variables, and use of physical properties of the surface as extracted from POLDER measurements, over a set of carefully selected desertic sites in North Africa and Saudi Arabia. All these data sets are included in a repository specifically designed which is also presented. Results are included for AVHRR/NOAA14, SeaWiFS and VEGETATION from 1996 up to present and extension to MODIS and MISR are presented.

Underwater Radiance Distributions Measured with Miniaturized Multispectral Radiance Cameras

Miniaturized radiance cameras measuring underwater multispectral radiances in all directions at highradiometric accuracy (CE600) are presented. The camera design is described, as well as the main steps of its optical and radiometric characterization and calibration. The results show the excellent optical quality of the specifically designed fish-eye objective. They also show the low noise and excellent linearity of the complementary metal oxide semiconductor (CMOS) detector array that is used. Initial results obtained in various oceanic environments demonstrate the potential of this instrument to provide new measurements of the underwater radiance distribution from the sea surface to dimly lit layers at depth. Excellent agreement is obtainedbetweennadirradiancesmeasuredwiththecameraandcommercialradiometers. Comparisonofthe upwelling radiance distributions measured with the CE600 and those obtained with another radiance camera also shows a very close agreement. The CE600 measurements allow all apparentoptical properties (AOPs) to be determined from integration of the radiance distributions and inherent optical properties (IOPs) to be determined from inversion of the AOPs. This possibility represents a significant advance for marine optics by tying all optical properties to the radiometric standard and avoiding the deployment of complex instrument packages to collect AOPs and IOPs simultaneously (except when it comes to partitioning IOPs into their component parts).

Climatology of Oceanic Zones Suitable for In-flight Calibration of Space Sensors

One way to calibrate space sensors on the visible part of the spectrum is to use acquisitions over Rayleigh scattering for dark surface conditions. Oceanic sites are good candidates because of their behaviour in term of spatial homogeneity and temporal stability. An appropriate selection is consequently required to identify the best oceanic areas. Nevertheless, the knowledge of the surface reflectance of such sites remains a limitation while their stability (and/or homogeneity) is usually not perfect. A previous study (Fougnie et al., 2002) has defined a selection of oceanic sites using one year of SeaWiFS data and regarding their spatial homogeneity and temporal stability. A first characterization of their monthly surface reflectances was derived (seasonal cycle) and used for several years as input for in-flight calibration processes. The major oligotrophic sites are located in North/South Atlantic and Pacific oceans, in Indian ocean, and in the Mediterranean Sea while some other mesotrophic sites were also defined for example in the Gulf of Mexico or Yucatan strait. The goal of this study was to revisit the definition of these sites regarding their spatial homogeneity and to analyze the annual cycle over 9 years of L3B R-2009 SeaWiFS products. Site behaviours are accurately defined with these longer time series, hence new recommendations are drawn for all sites and an updated climatology is proposed to be used for future in-flight calibrations.

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.

Toward Consistent Satellite Calibration and Validation for GEOSS Interoperability

A significant challenge for the Global Earth Observation System of Systems (GEOSS) interoperability is the lack of consistency in the Earth observations from satellites developed, calibrated, and operated by different space agencies worldwide, and the potential for significant discrepancies among products exists. The Committee on Earth Observation Satellites (CEOS) and its Working Group on Calibration and Validation (WGCV) are taking specific steps to facilitate interoperability by developing data quality assurance strategies and conducting joint cross-calibration studies. In this study, the Antarctic Plateau Dome C site is used for cross-comparison of visible/near infrared, and microwave instruments. Observations from AVHRR, MODIS, Hyperion, AMSR-E, and other instruments were intercompared. The findings suggest that the site is stable with relatively low radiometric uncertainties, and is a good candidate for CEOS endorsed cal/val site for satellite cross-comparison to facilitate GEOSS interoperability.

Comparison of CNES spherical and NASA hemispherical large aperture integrating sources: I. Using a laboratory transfer spectroradiometer

CNES spherical and NASA hemispherical large aperture calibration sources are examined using a laboratory transfer spectroradiometer and SPOT-2 instruments. The sources, collected at Matra in France during October 1987, are compared in terms of absolute calibration, linearity, and uniformity. The laboratory transfer spectroradiometer data reveal that the calibration results correspond to within about 7 percent absolute accuracy level and the linearity of the CNES source with lamp level is good. It is observed using the satellite data that both sources have an excellent uniformity over a 4 deg field of view.

Comparison of CNES spherical and NASA hemispherical large aperture integration sources: II. Using the SPOT-2 satellite instruments

Abstract The absolute calibration, linearity, and uniformity characteristics of a CNES spherical and a NASA hemispherical large aperture integrating calibration sources were compared at MATRA, Toulouse, France, in October 1987, using the SPOT-2 satellite instruments. An important output of the experiment is the SPOT-2 absolute calibration coefficients obtained by using the two sources each with their own calibration. The paper describes the results and a tentative error budget to explain the 4–8% range discrepancies. The linearity of the two sources has been investigated by plotting as a function of radiance the apparent calibration of SPOT-2 when seen by a given source (sphere or hemisphere) with a varying number of lamps alight in the source. It is also possible to infer to some extent the SPOT-2 nonlinearity by using the data of nonlinearity of the two sources as seen by a commercial laboratory spectroradiometer. The angular uniformity of the sources in the field of view of the SPOT-2 instrument (4°) is tested by a comparison of the normalized responses of the CCD detectors in the focal plane when illuminated by the sphere and by the hemisphere. This comparison shows virtually no difference between the responses, from which it is inferred that both sources have an excellent uniformity over a 4° field of view.

SENTINEL-2 image quality and level 1 processing

In the framework of the Global Monitoring for Environment and Security (GMES) programme, the European Space Agency (ESA) in partnership with the European Commission (EC) is developing the SENTINEL-2 optical imaging mission devoted to the operational monitoring of land and coastal areas. The Sentinel-2 mission is based on a twin satellites configuration deployed in polar sun-synchronous orbit and is designed to offer a unique combination of systematic global coverage with a wide field of view (290km), a high revisit (5 days at equator with two satellites), a high spatial resolution (10m, 20m and 60 m) and multi-spectral imagery (13 bands in the visible and the short wave infrared spectrum). SENTINEL-2 will ensure data continuity of SPOT and LANDSAT multispectral sensors while accounting for future service evolution. This paper presents the main geometric and radiometric image quality requirements for the mission. The strong multi-spectral and multi-temporal registration requirements constrain the stability of the platform and the ground processing which will automatically refine the geometric physical model through correlation technics. The geolocation of the images will take benefits from a worldwide reference data set made of SENTINEL-2 data strips geolocated through a global space-triangulation. These processing are detailed through the description of the level 1C production which will provide users with ortho-images of Top of Atmosphere reflectances. The huge amount of data (1.4 Tbits per orbit) is also a challenge for the ground processing which will produce at level 1C all the acquired data. Finally we discuss the different geometric (line of sight, focal plane cartography, ...) and radiometric (relative and absolute camera sensitivity) in-flight calibration methods that will take advantage of the on-board sun diffuser and ground targets to answer the severe mission requirements.