Elsevier

Remote Sensing of Environment

Volume 120, 15 May 2012, Pages 25-36
Remote Sensing of Environment

Sentinel-2: ESA's Optical High-Resolution Mission for GMES Operational Services

https://doi.org/10.1016/j.rse.2011.11.026Get rights and content

Abstract

Global Monitoring for Environment and Security (GMES) is a joint initiative of the European Commission (EC) and the European Space Agency (ESA), designed to establish a European capacity for the provision and use of operational monitoring information for environment and security applications. ESA's role in GMES is to provide the definition and the development of the space- and ground-related system elements. GMES Sentinel-2 mission provides continuity to services relying on multi-spectral high-resolution optical observations over global terrestrial surfaces. The key mission objectives for Sentinel-2 are: (1) To provide systematic global acquisitions of high-resolution multi-spectral imagery with a high revisit frequency, (2) to provide enhanced continuity of multi-spectral imagery provided by the SPOT (Satellite Pour l'Observation de la Terre) series of satellites, and (3) to provide observations for the next generation of operational products such as land-cover maps, land change detection maps, and geophysical variables. Consequently, Sentinel-2 will directly contribute to the Land Monitoring, Emergency Response, and Security services. The corresponding user requirements have driven the design toward a dependable multi-spectral Earth-observation system featuring the Multi Spectral Instrument (MSI) with 13 spectral bands spanning from the visible and the near infrared to the short wave infrared. The spatial resolution varies from 10 m to 60 m depending on the spectral band with a 290 km field of view. This unique combination of high spatial resolution, wide field of view and spectral coverage will represent a major step forward compared to current multi-spectral missions. The mission foresees a series of satellites, each having a 7.25-year lifetime over a 15-year period starting with the launch of Sentinel-2A foreseen in 2013. During full operations two identical satellites will be maintained in the same orbit with a phase delay of 180° providing a revisit time of five days at the equator. This paper provides an overview of the GMES Sentinel-2 mission including a technical system concept overview, image quality, Level 1 data processing and operational applications.

Highlights

► Sentinel-2 provides multi-spectral high-resolution optical observations. ► Systematic global acquisitions over land with high revisit frequency. ► Observations in 13 spectral bands with a resolution of 10 m to 60 m. ► Production of land cover and land change detection maps and geophysical parameters.

Introduction

Global Monitoring for Environment and Security (GMES) is a European Union (EU) led initiative designed to establish a European capacity for the provision and use of operational monitoring information for environment and security applications. This capacity is seen to be composed of three modules, which together constitute the functional GMES ‘system’: (1) The production and dissemination of information in support of EU policies for Environment and Security; (2) the mechanisms needed to ensure a permanent dialog between all stakeholders and in particular between providers and users; and (3) the legal, financial, organizational and institutional framework to ensure the functioning of the system and its evolution.

Many elements of the modules already exist but have been conceived, designed and managed in isolation, thus limiting interoperability and production of relevant information. The coherence, efficiency and sustainability of a shared information system for Europe will be the added value of GMES. Developing compatibility between the existing elements, establishing cooperation between the organizations and filling the gaps where necessary will achieve this goal.

Within the GMES program, ESA is responsible for the development of the Space Component, a fully operational space-based capability to supply earth-observation data to sustain environmental information services in Europe, namely Geoland2, SAFER (Services and Applications For Emergency Response) and G-MOSAIC (GMES services for Management of Operations, Situation Awareness and Intelligence for regional Crises). These Services, implemented in parallel by the European Commission, will provide value-added data and services to the GMES end-users; the European Environmental Agency (EEA) and the Member States are responsible for the in-situ component.

The Sentinel missions (Donlon et al., 2012-this issue, Ingman et al., 2012-this issue, Torres et al., 2012-this issue) are GMES dedicated Earth Observation missions composing the essential elements of the Space Component. In the global GMES framework, they are complemented by other satellites made available by third-parties or by ESA and coordinated in the synergistic system through the GMES Data-Access system (see http://gmesdata.esa.int) versus the Services.

The GMES Sentinel-2 mission provides continuity to services relying on multi-spectral high spatial resolution optical observations over global terrestrial surfaces (Martimort et al., 2007). Sentinel-2 will capitalize on the technology and the vast experience acquired in Europe and the United States to sustain the operational supply of data for services such as Risk Management (floods and forest fires, subsidence and landslides), European Land Use/Land Cover State and Changes, Forest Monitoring, Food Security/Early Warning Systems, Water Management and Soil Protection, Urban Mapping, Natural Hazards, and Terrestrial Mapping for Humanitarian Aid and Development. The design of the Sentinel-2 mission aims at an operational multi-spectral Earth-observation system that complements the Landsat and SPOT (Satellite Pour l'Observation de la Terre) observations and improves data availability for users.

The SPOT remote sensing program was initiated in 1978 by France in partnership with Belgium and Sweden. SPOT 1 was launched with Ariane 2 on February 22, 1986. SPOT 2 joined SPOT 1 in orbit on January 22, 1990 and SPOT 3 followed on September 26, 1993. The satellite payloads included two identical HRV (High Resolution Visible) imaging instruments that were able to operate in two modes, either simultaneously or individually. The two spectral modes are panchromatic and multispectral. The panchromatic band has a resolution of 10 m, and the three multispectral bands have resolutions of 20 m with scene sizes of 3600 km2 and a revisit interval of one to four days, depending on the latitude. Since the deorbitation of SPOT 2 in 2009, after almost 20 years of service, satellites SPOT 4 and 5 together ensure the provision of high-resolution SPOT images and of VEGETATION global images. Spot 4 offers an additional band in the short wave infra-red when compared against SPOT 1, 2, 3; SPOT 5 features an increased spatial resolution of 5 m to 20 m and higher absolute location accuracy compared to its predecessors. The continuity of the SPOT program is planned with the development of the Pleiades system, as well as Spot 6 and 7. Spot 6 and SPOT 7 will form a constellation of Earth imaging satellites providing high resolution wide-swath data up to 2023 (http://www.spotimage.com/web/en/3319-spot-6-and-spot-7-extending-spot-continuity-to-high-resolution-wide-swath-imagery.php) with scheduled launches in 2012 and 2014, respectively (http://eoedu.belspo.be/en/satellites/spot.htm). SPOT 6 and 7 will acquire observations in five bands: A panchromatic band with 1.5 m spatial resolution, blue, red and green bands, and a near-infrared band; all obtained at 6 m spatial resolution. The imaging swath will be 60 km. SPOT observations have been used primarily for cartography (e.g. Gastellu-Etchegorry, 1989, Giles and Franklin, 1996), land cover classification (e.g. Gong et al., 1992, Kanellopoulos et al., 1992) and land change detection (e.g. Jensen et al., 1995, Lu et al., 2004).

The Landsat Program started in 1972 with the launch of the first satellite. Since then Landsat data have become key observations for monitoring global change and have been a primary source of medium spatial resolution Earth observations for a wide range of applications (e.g. Special Issue on 25th Anniversary of Landsat, 1997, Special Issue on Landsat 4, 1984, Special Issue on Landsat 7, 2001, Special Issue on Landsat Image Data Quality Analysis (LIDQA), 1985, Special Issue on Landsat Operations: Past, Present and Future, 2006, Special Issue on Landsat Sensor Performance Characterization, 2004, Special Issue on Synergistic Utilization of Landsat 7, 2003). Landsat satellites can be classified into three groups, based on sensor and platform characteristics (Chander et al., 2009). The first group consists of Landsat 1, Landsat 2 and Landsat 3 carrying the Multispectral Scanner (MSS) sensor and the Return Beam Vidicon (RBV) camera. The second group comprises Landsat 4 and Landsat 5 with the MSS and the Thematic Mapper (TM). Landsat 7 includes the Enhanced Thematic Mapper Plus (ETM +). The Landsat Data Continuity Mission (LDCM)/Landsat 8 will continue and advance the collection of Landsat data with a two-sensor payload: The Operational Land Imager (OLI) will collect image data for nine visible, near infrared and shortwave infra red bands with a 30 m spatial resolution (15 m for the panchromatic band); the Thermal Infrared Sensor (TIRS) will collect data for two longwave thermal bands with 100 m resolution. Both instruments are push broom sensors with a 185 km cross-track field of view. For more details the reader is referred to http://ldcm.nasa.gov/mission_details.html.

The Sentinel-2 mission will offer an unprecedented combination of systematic global coverage of land surfaces, a high revisit of five days at the equator under the same viewing conditions, high spatial resolution (i.e. Landsat-type), and a wide field of view for multi-spectral observations from 13 bands in the visible, near infra-red and short wave infra-red part of the electromagnetic spectrum. The spectral band coverage and the corresponding spatial resolutions are shown in Fig. 1.

In this paper we present the current status of the Sentinel-2 mission after the completion of the satellite's Critical Design Review (CDR), which is held at the end of phase C to judge the readiness of the project to move into phase D by assessing the qualification and validation status of the critical processes. We address the space component including the platform and the instrument, the ground segment, data and processing and the future applications. The information is based on various ESA documents, for example the Mission Requirements Document (MRD, ESA, 2010a) or the Products Definition Document (ESA, 2010b). Additional information can be found under http://www.esa.int/esaLP.

Section snippets

Mission overview

Frequent revisits of five days at the equator require two identical Sentinel-2 satellites operating simultaneously favoring a small, cost-effective and low-risk satellite. The orbit is Sun-synchronous at 786 km altitude (14 + 3/10 revolutions per day) with a 10:30 a.m. descending node. This local time was selected as the best compromise between minimizing cloud cover and ensuring suitable sun illumination. It is close to the Landsat local overpass time and matches SPOT's, allowing the combination

System concept overview

The Sentinel-2 end-to-end system will comprise two segments: The space segment with the two orbiting satellites including their payload instrument, and the ground segment. The ground segment shall facilitate the data acquisition from the space segment, the data processing, the archiving and dissemination as well as the control of the mission as a whole.

Image quality

The Sentinel-2 products will take advantage of the stringent radiometric and geometric image quality requirements. These requirements constrain the stability of the platform and the instrument, the ground processing and the in orbit calibration. The spectral bands, their characteristics and the required Signal to Noise Ratio (SNR) for the reference radiances (Lref) defined for the mission are shown in Table 3. The accurate knowledge of the band equivalent wavelength is very important as an

Level 1 products and processing

As the Sentinel-2 mission objectives emphasize the potential of data time series, the basic Level 1 products must be geometrically registered and radiometrically calibrated. This led to the following product definition: The Level 0 and Level 1A products provide raw compressed and uncompressed data, respectively. The Level 1B data are radiometrically corrected radiances. The physical geometric model is refined using available GCPs and appended to the product but not applied. The Level 1C product

Cloud screening and atmospheric corrections

The PDGS will offer additional data-processing options, through a software toolbox on user side, to derive Bottom-Of-Atmosphere (BOA) reflectance (Level 2A) and enhanced cloud masks from the TOA reflectance (Level-1C). The Sentinel-2 atmospheric correction is being developed based on algorithms proposed in the Atmospheric/Topographic Correction for Satellite Imagery (ATCOR, Richter & Schlaepfer, 2011). The method performs atmospheric correction based on the libRadtran radiative transfer model (

Operational applications

Observations from the Sentinel-2 mission will be used by three GMES main service elements, namely Geoland2, SAFER and G-MOSAIC. The mission has been designed to fulfill their user requirements for a number of operational applications, which are summarized in this section. For potential scientific applications the reader is referred to Berger et al. (2012-this issue).

The pre-operational land service of GMES is currently provided through Geoland2 (www.gmes-geoland.info/home.html). The project

Calibration and validation

Calibration and validation (Cal/Val) corresponds to the process of updating and validating on-board and on-ground configuration parameters and algorithms to ensure that the product data quality requirements are met. To meet the baseline product quality requirements, a well-defined Cal/Val plan will be systematically applied. Cal/Val activities will be carried out in coordination and cooperation with other CEOS (Committee on Earth Observation Satellites) partners and in line with its quality

Campaigns

Since one of the most important aspects of developing an Earth-observation mission is to ensure that the eventual data meets the users' exacting requirements, efforts are put into pre-flight campaigns to evaluate the future performance of a mission. In order to meet this objective, airborne and ground measurements have to be acquired so that the final data products can be simulated and evaluated. In order to support the development of the Sentinel-2 mission the CarboEurope, FLEX and Sentinel-2

Summary

Sentinel-2 has been designed to support GMES Land, Emergency and Security applications, namely Geoland2, SAFER, and G-MOSAIC. The mission will provide enhanced continuity to the SPOT 4/5 missions and will complement the Landsat series multi-spectral observations. The unique key features are the revisit time of five days, a relatively wide swath width of 290 km, the 13 spectral bands providing high radiometric and geometric image quality and its global coverage — contributing to the fulfillment

Acknowledgment

The authors acknowledge the contributions from the ESA S2 Project Team and the industrial teams from Astrium.

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