Sentinel-2
Sentinel-2 is an Earth observation satellite mission developed by the European Space Agency (ESA) as part of the Copernicus programme, featuring a constellation of satellites that acquire high-resolution multispectral imagery of Earth's land surfaces, coastal zones, and inland waterways to support environmental monitoring and land management applications.[1] The mission comprises multiple satellites, including Sentinel-2A, launched on 23 June 2015, Sentinel-2B, launched on 7 March 2017, and Sentinel-2C, launched on 5 September 2024, all deployed from the Guiana Space Centre in Kourou, French Guiana, using Vega rockets.[1] As of 2025, the operational constellation includes Sentinel-2A (in extended operations), -2B, and -2C. These satellites operate in a sun-synchronous orbit at an altitude of 786 km, with a 10:30 a.m. local solar time at the descending node. The current three-satellite configuration, with Sentinel-2B and -2C positioned 180 degrees apart and Sentinel-2A shifted approximately 36 degrees from Sentinel-2B, achieves a combined revisit time of approximately 3 days at the equator for global coverage of landmasses, large islands, and coastal areas.[2][3][4][5] Each Sentinel-2 satellite is equipped with the Multispectral Instrument (MSI), an innovative wide-swath imager that captures data across 13 spectral bands in the visible, near-infrared, and shortwave infrared regions, offering spatial resolutions of 10 meters for four bands in the visible and near-infrared (used for high-precision applications such as RGB imaging and vegetation indices), 20 meters for six bands (including the vegetation red edge and shortwave infrared), and 60 meters for three atmospheric correction bands, with a swath width of 290 km.[2] Key applications of Sentinel-2 data include agricultural and forestry monitoring, such as assessing crop health through vegetation indices like leaf area index and chlorophyll content; tracking land cover changes and soil properties; mapping natural disasters like floods, volcanic eruptions, and landslides; detecting inland and coastal water quality; and supporting food security, pollution monitoring, and climate change impact assessments.[2] The open-access nature of the data, processed into products like Level-1C (top-of-atmosphere reflectance) and Level-2A (bottom-of-atmosphere reflectance), enables widespread use in scientific research, policy-making, and operational services under the Copernicus framework.[1]Mission Overview
Objectives and Scope
The Sentinel-2 mission, as a cornerstone of the European Union's Copernicus Earth observation programme, is designed to deliver high-resolution multispectral optical imagery for the systematic monitoring of land surfaces, coastal zones, and inland waters.[7] Its primary objectives focus on providing continuous global coverage to support environmental and land management services, including the observation of vegetation, soil, water cover, and changes in land use and cover.[7] This enables the detection of biophysical variables such as leaf area index, leaf chlorophyll content, and land surface reflectivity, contributing to applications in agriculture, forestry, and ecosystem assessment. The mission's scope emphasizes operational support for EU policy implementation, particularly the Common Agricultural Policy (CAP) through parcel monitoring and yield estimation, as well as broader climate change initiatives via long-term environmental trend analysis.[8] Coverage extends to all continental landmasses between 56°S and 83°N latitude, including coastal waters up to 20 km offshore, major islands, and select closed seas like the Mediterranean, with provisions for targeted acquisitions in areas such as Antarctica.[7][9] To achieve these goals, Sentinel-2 offers spatial resolutions of 10 m for four key bands, 20 m for six bands, and 60 m for three bands, combined with a 5-day revisit frequency at the equator when both primary satellites are operational.[7] The system's 290 km swath width and 13 spectral bands—from visible and near-infrared (443–865 nm) to shortwave infrared (945–2190 nm)—facilitate comprehensive Earth surface observation.[7] Historically, the Sentinel-2 programme was initiated under the Global Monitoring for Environment and Security (GMES) initiative, with the satellite development contract signed in 2008 to ensure continuity of high-resolution land imaging capabilities building on predecessors like Landsat and SPOT.[10] The programme was renamed Copernicus in 2012 to honor the astronomer Nicolaus Copernicus and reflect its expanded focus on user-driven services, achieving full operational status in 2014 through EU Regulation (EU) No 377/2014.Constellation Design
The Sentinel-2 mission employs a constellation of identical satellites in sun-synchronous orbits to achieve high-frequency global monitoring of land surfaces. Initially designed as a twin-satellite system, Sentinel-2A and Sentinel-2B operate in the same orbital plane at an altitude of 786 km, phased 180 degrees apart, enabling a combined revisit time of 5 days at the equator for areas between 56°S and 83°N latitude. This configuration covers approximately 99% of the Earth's landmasses, including inland and coastal waters, while excluding southern polar regions south of 56°S (with targeted acquisitions for areas like Antarctica) and northern polar regions north of 83°N, as well as very small islands less than 100 km².[11][12][9] To ensure operational redundancy and continuity, the satellites are built to identical specifications by Airbus Defence and Space, allowing seamless replacement in case of failure and minimizing data gaps during the mission's 7.25-year baseline lifetime. The 180-degree phasing optimizes overlap in swath coverage—each satellite provides a 290 km swath—reducing the effective revisit interval from the single-satellite 10-day cycle to 5 days, with denser observations at higher latitudes due to orbital convergence. This redundancy strategy supports uninterrupted service for Copernicus applications, such as land cover mapping and vegetation monitoring.[1][13] In September 2024, Sentinel-2C was launched as a replacement for the aging Sentinel-2A, joining Sentinel-2B to maintain the nominal two-satellite 5-day revisit cycle post-2024. However, to enhance coverage temporarily, Sentinel-2A received a one-year extension starting in March 2025, during which it was maneuvered to a position 36 degrees ahead of Sentinel-2B, while Sentinel-2C occupies the 180-degree offset from Sentinel-2B. This three-satellite arrangement, operational through at least March 2026, improves the average revisit to approximately 2.5-3 days at mid-latitudes, boosting data volume without requiring changes to ground processing infrastructure.[14][15][16] The constellation's design rationale prioritizes a balance between spatial resolution (up to 10 m), wide swath width, and frequent revisits while constraining onboard power consumption to under 2 kW and downlink data rates to manageable levels via the European Data Relay Satellite system. By limiting the number of satellites to two (with redundancy via identical units and a third for succession), the architecture avoids excessive complexity and cost, ensuring sustainable high-resolution optical observations tailored to environmental and security needs.[11][12]Development and Launches
Program History
The Sentinel-2 program emerged as a key component of the European Union's Global Monitoring for Environment and Security (GMES) initiative, with foundational concept studies initiated in 2003 by the French space agency CNES, focusing on high-resolution optical imaging for land monitoring. The European Space Agency (ESA) formalized the mission's definition phase from 2005 to 2006, followed by Phase A/B feasibility and preliminary design studies spanning 2006 to 2008, which refined user requirements and system architecture. These early efforts built on prior European Earth observation heritage, such as the SPOT and Envisat missions, to address needs for systematic, high-revisit coverage of terrestrial surfaces.[17][18] In April 2008, ESA awarded the prime development contract valued at €195 million to Astrium (now Airbus Defence and Space) for the first satellite, Sentinel-2A, initiating the implementation phase in October 2007. A follow-on contract for the identical Sentinel-2B satellite was signed on March 31, 2010, ensuring the constellation's dual-satellite design for enhanced temporal resolution. The Critical Design Review, a pivotal milestone confirming the maturity of the satellite and instrument designs, was successfully completed in 2011. In December 2012, the overarching GMES program was rebranded as Copernicus, reflecting its expanded role in Earth observation services and honoring the astronomer Nicolaus Copernicus.[10][12][19][20] Funding for the Sentinel-2 program is provided by the European Union through the Copernicus budget, with ESA managing the space segment implementation on behalf of EU member states. Key partnerships involve Airbus Defence and Space as the lead integrator, alongside contributions from entities like CNES for ground processing algorithms and the German Aerospace Center (DLR) for optical communication systems. International collaborations emphasize open data sharing under Copernicus, enabling global access and joint applications in environmental monitoring.[1][12] The program's development encountered challenges, including funding uncertainties that delayed EU commitment to the space component until 2012, shifting the original 2012 launch target to 2015. Technical hurdles encompassed achieving precise pointing accuracy for the multispectral instrument and managing the substantial data volume generated per orbit, approximately 1.6 terabytes, which required innovative processing solutions. These issues were resolved through iterative testing and refinements, paving the way for operational readiness.[18][12]Satellite Deployments
The Sentinel-2 constellation began with the launch of Sentinel-2A on 23 June 2015 aboard a Vega rocket from Europe's Spaceport in Kourou, French Guiana.[1] This was followed by Sentinel-2B on 7 March 2017, also using a Vega launcher from the same site.[1] The third satellite, Sentinel-2C, lifted off on 5 September 2024 aboard a Vega rocket from Europe's Spaceport in Kourou, French Guiana, marking the final flight of the original Vega launcher after 12 years of service and enhancing the constellation's redundancy.[21] Following each launch, the satellites underwent a commissioning phase lasting approximately 3 to 6 months, during which in-orbit testing verified system performance.[22] This period included detailed instrument calibration to ensure radiometric accuracy across spectral bands, geometric alignment for precise geolocation, and validation of data products against ground references to confirm the multispectral imager's operational integrity.[7] For Sentinel-2C, commissioning concluded successfully by early 2025, enabling its integration into routine operations.[23] As of November 2025, Sentinel-2B and Sentinel-2C remain in nominal operations, providing the core 5-day revisit coverage over land surfaces.[7] Sentinel-2A, after transferring primary imaging duties to Sentinel-2C on 21 January 2025, entered an exceptional one-year extension campaign beginning 13 March 2025, repositioned 36 degrees from Sentinel-2B to augment data acquisition and support user needs until at least March 2026.[24][25] The constellation's cumulative data archive, encompassing Level-1 and Level-2 products, exceeds 10 petabytes, reflecting a decade of high-volume multispectral observations. End-of-life management for Sentinel-2 satellites follows ESA's space debris mitigation guidelines, emphasizing controlled re-entry into Earth's atmosphere to prevent long-term orbital debris.[26] Each satellite is designed with sufficient propulsion for deorbit maneuvers at the conclusion of its extended mission lifetime, targeting disposal within 25 years post-mission to comply with international standards.[7]Spacecraft and Orbit
Platform Specifications
The Sentinel-2 satellites are constructed by Airbus Defence and Space using the AstroBus-L platform, a modular low Earth orbit bus designed for high stability and reliability in Earth observation missions. Each satellite has a launch mass of 1,140 kg and features a compact structure measuring 3.4 m in length, 1.8 m in width, and 2.35 m in height, built on an aluminum frame with aluminum-core honeycomb panels for lightweight strength and thermal stability. The platform is engineered for a nominal lifespan of 7.25 years, with redundancy and fault tolerance to ensure one-failure operation throughout the mission.[22][12] The power subsystem relies on a single deployable solar array spanning approximately 7.1 m², generating 2,300 W at the beginning of life (BOL) and sustaining 1,700 W during operations, supplemented by lithium-ion batteries with a 87 Ah end-of-life capacity for eclipse periods and peak loads. Thermal management includes dedicated radiators to maintain the platform and payload within operational temperature ranges, preventing overheating from solar exposure and internal heat dissipation. Data handling and communications are supported by an X-band downlink capable of 560 Mbit/s for high-volume image transmission, along with S-band for telemetry, tracking, and command at 2 Mbit/s downlink and 64 kbit/s uplink.[22][12][27] Attitude and orbit control is achieved through a three-axis stabilization system, incorporating multi-head star trackers and fiber optic gyroscopes for precise pointing, a GNSS receiver for position data, four reaction wheels for fine adjustments, magnetic torquers for momentum dumping, and 1 N hydrazine thrusters for coarse control and cross-track steering. The propulsion subsystem uses a monopropellant hydrazine setup with 120 kg of propellant, enabling orbit maintenance, safe mode recovery, debris avoidance maneuvers, and end-of-life deorbiting to comply with space debris mitigation guidelines. This configuration supports geolocation accuracy of 20 m without ground control points, ensuring stable Earth-oriented attitudes across all operational modes.[22][12]Orbital Parameters
As of 2025, the Sentinel-2 constellation comprises three operational satellites: Sentinel-2A (launched 2015, operations extended), Sentinel-2B (launched 2017), and Sentinel-2C (launched September 2024, operational since January 2025). Sentinel-2 satellites follow a sun-synchronous, near-polar orbit characterized by an inclination of 98.62° and a mean altitude of 786 km.[9] This configuration ensures repeatable ground tracks and consistent observation conditions across global land surfaces from 56°S to 84°N latitude.[12] The orbital period measures 100.6 minutes, enabling approximately 14.3 orbits per day.[28] The descending node crosses the equator at a mean local solar time of 10:30, which optimizes solar illumination angles for imaging and minimizes variations in lighting across acquisitions.[29] The mission's coverage geometry features a 290 km swath width achieved through nadir pointing of the satellite platform.[9] To address potential gaps in the observation grid, particularly during the initial single-satellite phase, the system incorporates off-nadir steering capabilities up to ±11° for targeted acquisitions.[12] Orbital maintenance involves periodic adjustments to the semi-major axis to counteract perturbations from atmospheric drag, with maneuvers executed approximately monthly to preserve the required altitude and phasing.[30] The satellites are positioned such that Sentinel-2B and Sentinel-2C are phased 180° apart, with Sentinel-2A offset by approximately 36° from Sentinel-2B, achieving an average revisit time of approximately 2.5 days at the equator.[14][9]Instrumentation
Multispectral Imager Design
The Multispectral Imager (MSI) on Sentinel-2 is a push-broom scanner designed to acquire imagery across 13 spectral bands, utilizing separate focal plane assemblies for the visible and near-infrared (VNIR) range and the short-wave infrared (SWIR) range.[12] The instrument employs a three-mirror anastigmat (TMA) telescope configuration, with the primary (M1), secondary (M2), and tertiary (M3) mirrors constructed from silicon carbide for thermal stability and lightweight performance.[31] This optical design achieves a pupil diameter of 150 mm and a focal length of 0.60 m at an f/4 aperture, enabling a 290 km swath width while maintaining high geometrical fidelity.[32] The VNIR focal plane assembly integrates monolithic CMOS detectors covering the VNIR spectral range, while the SWIR assembly uses mercury cadmium telluride (HgCdTe) detectors hybridized on CMOS readouts for the SWIR range, with the latter passively cooled to approximately 195 K.[12] Each focal plane features 12 elementary detectors arranged in two staggered rows to span the full swath, resulting in a total of approximately 295,000 pixels across both assemblies, including redundancies for reliability.[31] Dichroic beam splitters separate the incoming light into VNIR and SWIR paths prior to detection, ensuring efficient spectral isolation without mechanical components.[33] The video electronics subsystem includes a Video and Compression Unit (VCU) that performs 12-bit analog-to-digital conversion and applies onboard lossless compression via wavelet transform, reducing data volume from an input rate of about 1.3 Gbit/s to 450 Mbit/s for downlink.[12] To compensate for the staggered detector layout and differing integration times, temporal offsets are introduced between the VNIR and SWIR acquisitions, allowing precise alignment of the multispectral data during ground processing.[31] This architecture draws heritage from prior missions like SPOT and Landsat, optimizing for wide-area, high-resolution Earth observation.[33]Spectral Bands and Resolution
The Multispectral Imager (MSI) aboard Sentinel-2 satellites acquires imagery across 13 spectral bands in the visible/near-infrared (VNIR) and shortwave infrared (SWIR) regions, ranging from 443 nm to 2190 nm, to support high-resolution Earth observation for land, coastal, and atmospheric monitoring. These bands are designed with specific central wavelengths, bandwidths, and spatial resolutions tailored to key environmental applications, such as aerosol correction, vegetation analysis, and water quality assessment. The following table summarizes the band specifications:| Band | Purpose | Central Wavelength (nm) | Bandwidth (nm) | Spatial Resolution (m) |
|---|---|---|---|---|
| B1 | Aerosol correction (coastal) | 443 | 20 | 60 |
| B2 | Blue (vegetation, water) | 490 | 65 | 10 |
| B3 | Green (vegetation, soil) | 560 | 35 | 10 |
| B4 | Red (vegetation) | 665 | 30 | 10 |
| B5 | Vegetation red edge | 705 | 15 | 20 |
| B6 | Vegetation red edge | 740 | 15 | 20 |
| B7 | Vegetation red edge | 783 | 20 | 20 |
| B8 | Near-infrared (vegetation) | 842 | 115 | 10 |
| B8A | Narrow NIR (vegetation) | 865 | 20 | 20 |
| B9 | Water vapor correction | 945 | 20 | 60 |
| B10 | Cirrus detection (SWIR) | 1375 | 30 | 60 |
| B11 | SWIR (soil, vegetation) | 1610 | 90 | 20 |
| B12 | SWIR (soil, geology) | 2190 | 180 | 20 |