Thierry Tremas

Copernicus Sentinel-2A Calibration and Products Validation Status

As part of the Copernicus programme of the European Commission (EC), the European Space Agency (ESA) has developed and is currently operating the Sentinel-2 mission that is acquiring high spatial resolution optical imagery. This article provides a description of the calibration activities and the status of the mission products validation activities after one year in orbit. Measured performances, from the validation activities, cover both Top-Of-Atmosphere (TOA) and Bottom-Of-Atmosphere (BOA) products. The presented results show the good quality of the mission products both in terms of radiometry and geometry and provide an overview on next mission steps related to data quality aspects.

Thermal infrared radiance simulation with aggregation modeling (TITAN): an infrared radiative transfer model for heterogeneous three-dimensional surface--application over urban areas

The thermal infrared radiance simulation with aggregation modeling (TITAN) model, presented here, is an innovative transfer radiative code in the infrared domain (3-14 microm). It takes into account the three-dimensional (3D) structure of the landscape and simulates all the radiative components introduced by this 3D structure, which are due to the reflection and emission of walls and sloping roofs. Examples are given to illustrate the new opportunities offered by TITAN over urban areas. First, a phenomenological study is conducted at four wavelengths analyzing the relative effect of all the radiative contributors to the total signal. The same analysis is performed at bottom of atmosphere, which reveals an error occurring when a flat assumption is made (between 1% and 5%). In a second example, the directional effects at sensor level are simulated and show that the radiative temperature can vary by up to 10 K.

Radiometric and geometric scarab-megha-tropiques ground calibration comparison with first in orbit calibration

The Scarab-3 instrument, part of the Indian-French Megha-Tropiques mission [1], has been launched in october 2011. It is a radiometer dedicated to earth radiation budget. CNES is prime of the development of this instrument. Last year, CNES conducted the final integration of the instrument and the radiometric/geometric calibrations. Two main spectral bands are measured by this instrument: A shortwave (SW) channel dedicated to solar fluxes and a Total (T) channel for (total) fluxes combining the infra-red earth radiance and the albedo. The earth long wave (LW) radiance is computed by subtracting the SW channel to the Total channel. Thus is defined a 3rd (virtual) LW channel. To obtain a good radiometric accuracy, intensive tests have been conducted using integrating sphere, black body calibration, vacuum tests and collimator test. After a brief sum up of the instrument, the results of the calibration will be presented and compared with the first in orbit calibration.

ScaRaB ground calibration

Scarab Flight Model is a four channels scanning radiometer, launched in February 1994, with the same ERBE scientific mission. It worked perfectly during one year, aboard a Russian satellite, METEOR 3 N degrees 7. Data wee very consistent with ERBE results. Calibration of FM2 and Spare Model is described. The calibration comprises three phases: solar/diffuser short wave source; blackbodies long wave sources under vacuum; integrating sphere short wave source. Results are described and a comparison between the tow last methods is established.

MACCS-ATCOR joint algorithm (MAJA)

MACCS is a Multi-Mission Atmospheric Correction and Cloud Screening software. This tool has been developed by CNES. It is based on a multi-temporal algorithm that makes an optimized use of image time series to characterize the atmosphere and detect clouds. We have generated level-2 Sentinel-2 products on various targets over Europe but also over deserts or urban areas with high aerosol optical thickness (AOT). The results are validated by comparison to in-situ measurements from AERONET for AOT and water vapor. We also directly validate ground reflectance using CNES Lacrau photometer. Then, the joint effort of CNES and DLR to merge their algorithms MACCS and ATCOR into a so-called MAJA processing chain will be detailed, together with the future development and validation plan. Finally, the sentinel-2 level-2 production plan will be presented in the context of THEIA land data center.

Sentinel-2: presentation of the CAL/VAL commissioning phase

In partnership with the European Commission and in the frame of the Copernicus program, the European Space Agency (ESA) has developed the Sentinel-2 optical imaging mission devoted to the operational monitoring of land and coastal areas. The Sentinel-2 mission is based on a satellites constellation deployed in polar sun-synchronous orbits. Sentinel-2 will offer a unique combination of global coverage with a wide field of view (290km), a high revisit (5 days with two satellites), a high resolution (10m, 20m and 60m) and multi-spectral imagery (13 spectral bands in visible and shortwave infra-red domains). The first sentinel 2A has been launched on June 22nd, 2015, from Kourou, French Guyana. In this context, the Centre National d’Etudes Spatiales (CNES) supports ESA to insure the cal/val commissioning phase, for Image Quality aspects. This paper provides first, an overview of the Sentinel-2 system after the launch. Then the articles focuses on the means implemented and activated in CNES to perform the In Orbit Commissioning, the availability and performances of the different devices involved in the ground segment : the GPP in charge of producing the level 1 files, the “radiometric unit” that processes sensitivity parameters, the “geometric unit” in charge of fitting the images on a reference map, MACCS that will produce Level 2A files (computing reflectances at the Bottom of Atmosphere) and the TEC-S2 that will coordinate all the previous software and drive a database in which will be gather the incoming Level 0 files and the processed Level 1 files.

ScaRaB: first results of the scanner for radiative budget on board the Indo-French satellite Megha-Tropiques

ScaRaB (SCAnner for RAdiation Budget) is the name of three radiometers whose two first flight models have been launched in 1994 and 1997. The instruments were mounted on-board Russian satellites, METEOR and RESURS. On October 12th, a last model has been launched from the Indian site of Sriharikota. ScaRaB is a passenger of MEGHATROPIQUES, an Indo-French joint Satellite Mission for studying the water cycle and energy exchanges in the tropics. The orbit is circular inclined 20deg. ScaRaB is compatible with CERES mission. Two main spectral bands are measured by the radiometer: A short-wave (SW) channel (0.2 – 4 μm) dedicated to solar fluxes and a Total (Tot) channel (0.2 – 200 μm) for (total) fluxes combining the infrared earth radiance and the albedo. The earth long-wave (LW) radiance is isolated by subtracting the SW channel to the Total channel. Thus is defined a supplemental (virtual) channel.

On orbit ScaRaB flight model 2 calibration procedure

According to the results of SCARAB Flight Model 1 on orbit, some modifications will be introduced in the on board calibration procedure of Flight Model 2. After having described briefly the instrument and the mission, this paper will detail: the on board procedure for the first flight model (FM1); the different problems encountered during the FM1 mission; the proposed new on board calibration procedure; the global error budget.

Calibration of SCARAB-FM1: Methods and first results

Abstract The first flight model of SCARAB (SCAnner for RAdiation Budget), has been launched in February 94. Due to failures of on-board lamps just after the launch, the in-flight calibration strategy has been changed during the mission. Thanks to this new method, one year of data has been collected. This alternate method of calibration is so performant that it will become the main in-flight calibration strategy for the next model (that will be launched during the second trimester of 97). In this paper is firstly explained the original in-flight calibration method planned to fulfill the mission. The behaviour of the calibration module after the launch is analysed and a diagnosis of the failures is set. A synthesis of remaining resourses is made up. Ascertaining that some parameters are very stable during all the mission, the new calibration strategy is built. This method is based on the accuracy of the law linking gains to the temperature of the instrument, and also on the very high reliability of the on-board black body simulators. Performances of both methods are compared. Points to strengthen for the future model are also raised.

Sentinel-2B image quality commissioning phase results and Sentinel2 constellation performances

In the frame of the Copernicus program of the European Commission, Sentinel-2 is a constellation of 2 satellites on a polar sun-synchronous orbit with a revisit time of 5 days (with both satellites), a high field of view - 290km, 13 spectral bands in visible and shortwave infrared, and high spatial resolution - 10m, 20m and 60m. The Sentinel-2 mission offers a global coverage over terrestrial surfaces. The satellites acquire systematically terrestrial surfaces under the same viewing conditions in order to have temporal images stacks. The first satellite was launched in June 2015 and the second in March 2017. In cooperation with the European Space Agency (ESA), the French space agency (CNES) is in charge of the image quality of the project, and so ensured the CAL/VAL commissioning phase during the months following the launch. This cooperation is also extended to routine phase as CNES supports European Space Research Institute (ESRIN) and the Sentinel-2 Mission performance Centre (MPC) for validation in geometric and radiometric image quality aspects, and in Sentinel-2 Global Reference Image (GRI) geolocation performance assessment. This paper points on geometric image quality on Sentinel-2B commissioning phase. It relates to the methods and the performances obtained, as well as the comparison between S2A and S2B. This deals with geolocation and multispectral registration. A small focus is also done on the Sentinel-2 GRI which is a set of S2A images at 10m resolution covering the whole world with a good and consistent geolocation. This ground reference leads to ensure an accurate multi-temporal registration -on refined Sentinel-2 products over GRI- which is also presented in this paper.