PRISMA is the hyperspectral mission from the Italian Space Agency (ASI) launched in 2019. It samples the solar irradiance reflected and diffused by the earth-atmosphere system between 400 nm and 2500 nm with a spectral distance better than 11nm and a 30m Ground Sampling Distance. To answer the demanding need of hyperspectral applications, a high absolute radiometric accuracy is required and reached through the combination of on board and natural targets based calibration.
This paper describes PRISMA mission and focuses on the natural targets based calibration methods used to assess the instrument sensitivity. Two methods are used:
- PICS (Pseudo Invariant Calibration Sites) allow to cross-calibrate PRISMA with SENTINEL-2 and 3 ESA missions.
- Gobabeb and La Crau Instrumented sites known as RadCalNet sites which provide a BOA spectral BRDF through a dedicated acquisition protocol and processing as well as atmospheric parameters simultaneously to the satellite pass.
The adaptation of these methods to hyperspectral sensors calibration is presented. The calibrations results which show the very good temporal stability of PRISMA instrument are discussed as well as the methods and in situ instrumentation evolutions planned to improve the calibration of hyperspectral sensors using natural targets.
The TRISHNA program marks a step further in the fruitful cooperation built between CNES and ISRO since many years, through a new Earth observation mission dedicated to the improvement of water cycle understanding and water resource management. Thanks to its unprecedented high spatial resolution in the thermal infrared domain, together with a high revisit frequency, TRISHNA mission will significantly contribute to the detection of ecosystem stress and to the optimization of water use in agriculture in a context of global climate change. The TRISHNA payload is composed of two principal instruments: the VNIR-SWIR imager provided by ISRO, and the TIR (Thermal InfraRed) imager. CNES is responsible for the TIR instrument development with Airbus Defence and Space as a prime contractor. The targeted launch date for TRISHNA satellite is 2025, being then positioned as a precursor of the LSTM Copernicus mission from ESA. This paper presents a status of the TIR instrument development, currently in phase C after a successful Preliminary Design Review in 2021. The equipment development status is detailed, and the progress of validation activities at Airbus level is addressed, focusing on the tests at detection laboratory with a full detection chain including a Development Model (DM) detector, and the preparation of the extensive test campaign to be done on an Engineering Model (EM) of an equipped cryostat (including EM detector, filters, and cryocoolers). An overview of the instrument predicted radiometric, spectral and geometric performances is also presented, as well as some measured elementary performances already available on FM optics.
The Multi-viewing, Multi-channel, Multi-polarization Imager (3MI) of the EUMETSAT Polar System - Second Generation (EPS-SG) is a radiometer dedicated to aerosol characterization for climate monitoring, air quality forecasting, and Numerical Weather Prediction (NWP). The 3MI aims to provide multi-spectral, multipolarization, and multi-angular images of the Earth upward radiances at the top of the atmosphere (TOA). This particular design, inspired from the POLarisation and Directionality of the Earth's Reflectances (POLDER) instrument, will lead to a better understanding of the microphysical properties of aerosols and clouds. In terms of design, the instrument's calibration will be done using natural targets, exclusively. This particular constraint requires to further study the already existing calibration techniques that were used on the predecessors of 3MI. The main reason is that the instrument will provide spectral bands (mainly in the short-wave infrared) that will require a reliable characterization. In this paper, we present the results based on measurements obtained with the Observing System Including PolaRisation in the Solar Infrared Spectrum (OSIRIS) instrument, a 3MI airborne simulator developed at the Laboratoire d'Optique Atmosphérique.
KEYWORDS: Sensors, Reflectivity, Device simulation, Short wave infrared radiation, Polarization, Instrument modeling, Picture Archiving and Communication System, Satellites, Ray tracing, Signal to noise ratio
The 3MI instrument is a multi-directional spectro-polarimeter to fly on-board the Metop-SG platform to be launched in 2022, as part of EUMETSAT’s EPS-SG system space segment. As for most of radiometers devoted to Earth Observing from space, the radiometric budget is a system budget which includes many contributors, from the raw detector signal-to-noise to the artefact introduced by the ground correction in level-1 processing. The straylight happening in the optics is one of these contributors which could sometimes become dominant in the system budget. In this context, it is required to constrain the instrumental design as well as the ground characterization and its correction by processing. This is supported by maintaining accurate understanding of the straylight and its behaviour from early on in the project in order to check its radiometric level and quantify the potential impact on the products. This paper will present the analysis done to understand and model the physical behaviour of 3MI’s straylight. Based on initial inputs from the instrument builders and assuming some simplification, a physical straylight model was derived. This model allows an easy implementation in a simulator able to add this straylight contribution to any type of images, particularly on very realistic scenes. Our physical model shows advantageous complementarity to the builder’s simulations in the sense it allows a better quantification on a wide variety of realistic images while specifications usually assume theoretical targets such as a knife-edge transition. Such a capability is needed for 3MI system activities, in particular to cope with any deviation to the performance requirements. This paper presents examples of 3MI straylight-impacted images simulated at EUMETSAT using as input 1/ a simulation of the straylight provided by the instrument builder (ESA and Leonardo), and 2/ a realistic test data set based on PARASOL and MODIS acquisitions from the A-train observatory generated by ICARE/LOA. This simulator will be used during the development of the 3MI instrument and its ground characterization in order to monitor the impact on products.
For Venµs satellite, launched August 2 nd, 2017, the calibration and performance assessment activities of the commissioning phase took place in the second semester of 2017. In particular, the radiometric calibration includes fine tuning of the dynamic, identification of defective pixels, equalization of the detectors, absolute calibration, viewing parameters tuning. A special focus is made on specificities of the mission concerning level-1 radiometric calibration. As far as equalization is concerned: an unusual behavior has been observed on numerous pixels, called “radiometric spikes”, on Venµs images. An improvement of the Venµs radiometric model has been implemented to remove these spikes. Absolute calibration using the moon consists in comparing the global irradiance of the Venµs moon image with the reference from ROLO radiometric model. Absolute calibration using simultaneous nadir observations with Sentinel-2 is also a specificity of the mission: since Sentinel-2A and Sentinel-2B are currently in orbit with very similar spectral bands, it is possible to use Simultaneous Nadir Observations of Venµs and Sentinel-2. The cross-calibration results will be presented and compared to other vicarious calibration methods. About stray-light modelling and correction: the optical stray-light level exceeds the requirements by far. Two types of stray-light are observed: local ghost and cross-talk ghost. Local stray-light model is computed as a global system MTF, whereas the estimated cross-talk contribution is subtracted from the image. The specific correction model and the in-flight results will be presented.
Sentinel-2 is an optical imaging mission devoted to the operational monitoring of land and coastal areas. It is developed in partnership between the European Commission and the European Space Agency. The Sentinel-2 mission is based on a satellites constellation deployed in polar sun-synchronous orbit. It 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). CNES is involved in the instrument commissioning in collaboration with ESA. This paper reviews all the techniques that will be used to insure an absolute calibration of the 13 spectral bands better than 5% (target 3%), and will present the first results if available. First, the nominal calibration technique, based on an on-board sun diffuser, is detailed. Then, we show how vicarious calibration methods based on acquisitions over natural targets (oceans, deserts, and Antarctica during winter) will be used to check and improve the accuracy of the absolute calibration coefficients. Finally, the verification scheme, exploiting photometer in-situ measurements over Lacrau plain, is described. A synthesis, including spectral coherence, inter-methods agreement and temporal evolution, will conclude the paper.
A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellite sensors is currently under development, focussing on the visible and near infra-red parts of the spectrum. The site will become a European contribution to the Committee on Earth Observation Satellites (CEOS) initiative RadCalNet (Radiometric Calibration Network). The exact location of the permanent monitoring instrumentation will be defined following the initial site characterisation. The new ESA/CNES RadCalNet site will have a robust uncertainty budget and its data fully SI traceable through detailed characterisation and calibration by NPL of the instruments and artefacts to be used on the site. This includes a CIMEL sun photometer (the permanent instrumentation) an ASD FieldSpec spectroradiometer, Gonio Radiometric Spectrometer System (GRASS), and reference reflectance standards.
In partnership with the European Commission and in the frame of the Copernicus program, the European Space Agency (ESA) 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 satellites constellation deployed in polar sun-synchronous orbit. Sentinel-2 offers a unique combination of global coverage with a wide field of view (290km), a high revisit (5 days with two satellites), a high spatial resolution (10m, 20m and 60m) and multi-spectral imagery (13 spectral bands in visible and shortwave infrared domains). The first satellite, Sentinel-2A, has been launched in June 2015. The Sentinel-2A Commissioning Phase starts immediately after the Launch and Early Orbit Phase and continues until the In-Orbit Commissioning Review which is planned three months after the launch. The Centre National d’Etudes Spatiales (CNES) supports ESA/ESTEC to insure the Calibration/Validation commissioning phase during the first three months in flight. This paper provides first an overview of the Sentinel-2 system and a description of the products delivered by the ground segment associated to the main radiometric specifications to achieve. Then the paper focuses on the preliminary radiometric results obtained during the in-flight commissioning phase. The radiometric methods and calibration sites used in the CNES image quality center to reach the specifications of the sensor are described. A status of the Sentinel-2A radiometric performances at the end of the first three months after the launch is presented. We will particularly address in this paper the results in term of absolute calibration, pixel to pixel relative sensitivity and MTF estimation.
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