Showing papers by "Steven Delwart published in 2006"

Meris instrument calibration

Abstract: The earth is imaged at a spatial resolution of 300 m (at nadir). The reduced resolution data (1200 m) is computed by the on-board combination of four adjacent samples across track over four successive lines. The low rate reduced resolution (RR) data will be acquired systematically, while the high rate full resolution (FR) data will be recorded in parallel according to user requests for a maximum duration of 20 minutes per orbit. The paper intends to describe the operational processing of the MERIS Radiometric Calibration and its present status. An overview of the instrument, the principles of its radiometric calibration and an outline of the calibration processing chain are presented. The various models used within the calibration processing are described and discussed. A status of the error budgets and uncertainties of on-ground and in-flight measurements, of models performances, and finally of the expected radiometric accuracy is given.

MERIS In-Flight Spectral Calibration

Abstract: This paper will describe the spectral calibration activities conducted during the MERIS commissioning phase and during operation since orbit 12000. MERIS is a medium resolution (300-1200m) push-broom imaging spectrometer covering the spectral domain 390-1040nm with 15 bands, programmable in position and width down to steps of 1.25nm. The onboard spectral calibration hardware is based on the use of an Erbium doped-diffuser panel presenting well-defined absorption peaks. In the spectral calibration mode, MERIS is configured with narrow bands centred on an Erbium absorption feature (two are used). The first orbit, the instrument is calibrated by viewing the "white" radiometric diffuser plate and the following orbit the "pink" Erbium diffuser plate is deployed. This method allows each of the MERIS detectors involved to be characterized in wavelength. The Fraunhofer absorption lines were used to complement these results by providing additional measurements in the violet and near infrared part of the spectrum. For this method, MERIS was configured both for Earth and diffuser observations and acquired data for only a limited number of orbits. This procedure was repeated for different band settings covering a number of Fraunhofer absorption lines. Finally, using Oxygen (O2A) absorption Earth observation data, two different approaches were developed, one based on the retrieval of surface pressure and one based on the shape of the O2A absorption band. Both methods were developed for clear sky land observations, but their performances are improved over bright land targets. Both methods agree to within an accuracy of 0.02 nm. The results from the different methods are analyzed in order to propose a spectral model for the MERIS instrument. Preliminary results of the spectral variation with time are reported. Except camera 4, the instrument is quite stable with time. Camera 4 needs further investigations to better understand its behaviour. Except for the use of the MERIS oxygen band, the spectral characterization of the other MERIS bands is achieved within the nominal accuracy (1 nm).

Meris full resolution products, geometry aspects

Abstract: This paper presents results regarding the geometry control of MERIS Full Resolution product geometry assessment. The first part will be dedicated to the results of the specific tool AMARGOS aiming to improve the geolocation of the MERIS Full Resolution (FR) products while the second part will report on the geolocation accuracy of the standard MERIS FR products. Validation of the geometric accuracy is performed on a MERIS camera basis. A processing chain that encompasses preprocessing and geometric recalibration, automatic correlation procedure and statistic analysis has been set up. Validation criteria are based on Circular Error (90) and Root Mean Square (RMS) results. Camera orientations are also checked through analysis of along and across track residual errors. Since ENVISAT launch, geodetic location accuracy of FR 1B products is monitored. This paper aims at providing geolocation results and qualitatively analyzing time series of some key results

Enhancement of diffusers BSDF accuracy: spectral features effect

Abstract: This paper reports the activities performed in the framework of the ESA contract 18432/04/NL/AR: Enhancement of diffusers BSDF Accuracy. This study was conducted to investigate properties of various diffusers. Diffusers are widely used in space instruments as part of the on-board absolute calibration. Knowledge of the behavior of the diffuser is therefore most important. From measurements of launched instruments in-orbit it has been discovered that when a diffuser is used in the vacuum of space the BSDF (Bi-directional Scattering Distribution Function) can change with respect to the one in ambient conditions. This is called the air/vacuum effect and has been simulated in this study by measuring the BSDF in a laboratory in ambient as well as vacuum conditions, results of this part of the study will be reported. Another effect on the BSDF is not related to the air/vacuum effect, but to the design parameters of the optical system and the scattering properties of the diffuser. The effect is called Spectral Features and is a noise like structure superimposed on the BSDF. To observe this effect, spectral and spatial (partially) coherence light is needed. High-resolution spectrometers provide the spectral coherence and a narrow field of view provides the spatial coherence. Modern space spectrometers have high spectral resolution and/or a small field of view (high spatial resolution). Different diffusers create different speckle patterns leading to different Spectral Features amplitudes. Therefore the choice of diffuser can be very critical with respect to the required absolute radiometric calibration of an instrument. Even if the Spectral Features are small it can influence the error budget of the retrieval algorithms for the level 2 products. In this presentation diffuser trade-off results are presented and the Spectral Features model applied to the optical configuration of the MERIS instrument is compared to in-flight measurements of MERIS. The introduction describes the use of diffusers in earth-observation satellites and why they cause spectral features. In Sec.2 the physical background of the spectral features, speckles, is discussed. Section 3 shows the results from air/vacuum effect measurements (SubSec.3.1), spectral features amplitude measurements on our in-house setup and simulations in a single graphical display (SubSec.3.2), and the measured and simulated results for the MERIS instrument (SubSec.3.3). The following sections deal with a description of the measuring setup and the model that has been made to do the simulations. Finally, in the discussion, topics like what is the best diffuser and what can be done to minimize the amplitude of the spectral features will be dealt with.