SENTINEL-3B

Sentinel-3B is a European Earth observation satellite operated by the European Space Agency (ESA) and dedicated primarily to oceanographic and environmental monitoring. Launched on 25 April 2018, it forms a key component of the Copernicus Programme — the European Union's flagship Earth observation initiative — and represents the second satellite in the Sentinel-3 constellation. Carrying a suite of instruments designed to measure sea surface temperatures, ocean colour, sea level, and land surface conditions, Sentinel-3B works in concert with its predecessor, Sentinel-3A, to provide continuous, high-revisit-frequency data for scientists, policymakers, and operational forecasters around the world. Assigned NORAD catalog ID 43437 and international designator 2018-039A, the satellite remains operational in orbit as of this writing.

Mission and Purpose

The Sentinel-3 series was conceived within the broader Copernicus Programme to ensure systematic, long-term collection of Earth observation data across both marine and terrestrial domains. While the first satellite in the series established the baseline observation capability, Sentinel-3B was designed to complement that coverage by flying in a coordinated orbit that halves the time required to revisit any given point on Earth's surface. Together, the two satellites reduce the repeat cycle for global ocean coverage from roughly 27 days to approximately two days when operating in tandem — a critical improvement for tracking fast-evolving oceanographic and meteorological phenomena.

The satellite's primary scientific focus is the ocean. Its instruments are capable of measuring sea surface temperature with high precision, monitoring the colour of the ocean surface to infer phytoplankton concentrations and water quality, and tracking variations in sea surface topography that reveal ocean circulation patterns, heat content, and sea level trends. These measurements feed directly into operational marine forecasting services such as the Copernicus Marine Environment Monitoring Service (CMEMS), which provides data products used by shipping, fisheries management, climate research, and emergency response organisations.

Beyond its oceanographic mandate, Sentinel-3B also contributes to land surface monitoring. Its optical instruments observe vegetation health and land surface temperature, supporting agricultural monitoring, wildfire detection, and long-term tracking of land cover change. Ice sheet and sea ice extent are additionally within its observational scope, providing polar researchers with consistent time-series data that complements dedicated cryosphere missions. This multi-domain capability reflects the Copernicus Programme's philosophy of building general-purpose, long-lived infrastructure that serves a wide community of users simultaneously.

Orbit and Tracking

Sentinel-3B occupies a sun-synchronous orbit (SSO), a type of nearly polar orbit in which the satellite's orbital plane maintains a nearly constant angle relative to the Sun throughout the year. This geometry ensures that the satellite crosses any given point on Earth's surface at approximately the same local solar time on each pass — a highly desirable property for Earth observation missions, because consistent solar illumination angles make it far easier to compare images and measurements taken on different dates without correcting for lighting differences.

The satellite's orbital parameters reflect the precision required for systematic Earth observation. Its apogee stands at 808 km and its perigee at 806 km, indicating an almost perfectly circular orbit with negligible eccentricity. This near-circular profile is deliberate: an elliptical orbit would cause the satellite's altitude, and therefore its ground resolution and swath width, to vary from pass to pass in ways that would complicate the intercalibration of data. The orbital inclination is 98.6°, which is characteristic of sun-synchronous orbits at this altitude. Each orbit takes approximately 100.9 minutes to complete, meaning Sentinel-3B circles Earth roughly 14 times per day.

For observers and analysts tracking the satellite, it can be found in orbital databases under NORAD ID 43437 or COSPAR designation 2018-039A. At an altitude of roughly 806–808 km, Sentinel-3B is well above the atmospheric drag that affects low-orbit debris and small satellites, and its relatively stable orbit requires only periodic station-keeping manoeuvres to maintain the precise ground track necessary for coordinated operation with Sentinel-3A.

Design and Operator

Sentinel-3B was manufactured by Thales Alenia Space, a Franco-Italian aerospace company with extensive experience building Earth observation and telecommunications satellites for European institutions. The satellite has a launch mass of 1,200 kg, placing it in the medium-class spacecraft category — substantial enough to carry a capable multi-instrument payload while remaining compatible with a wide range of launch vehicles.

The satellite was launched on 25 April 2018 at 20:00 Eastern Daylight Time from the Plesetsk Cosmodrome in Russia aboard a Rokot launch vehicle — a converted intercontinental ballistic missile repurposed as a small orbital launcher. The launch placed Sentinel-3B into its intended sun-synchronous orbit, after which ground controllers conducted an extensive commissioning phase to verify the health of its instruments and calibrate them against known references before operational data delivery began.

ESA serves as both operator and owner of the satellite, consistent with its role as the space component agency of the Copernicus Programme. The Copernicus Programme itself is a joint initiative of the European Union and ESA, with data freely distributed to users worldwide under an open data policy. Day-to-day operations, including telemetry monitoring, orbit maintenance, and data downlink, are managed through ESA's ground segment infrastructure. Processed data products are then delivered to downstream service providers including CMEMS and the Copernicus Land Monitoring Service.

Thales Alenia Space delivered Sentinel-3B as the second of four satellites planned for the Sentinel-3 series. The constellation architecture is designed for continuity: as earlier satellites age and eventually leave service, newer Sentinel-3 units are intended to assume their roles without gaps in the long-term climate data record. This continuity is particularly important for datasets like sea level rise and sea surface temperature, where the scientific value depends on uninterrupted, consistently calibrated observations stretching across decades.

Status and Significance

As of the time of this writing, Sentinel-3B remains in orbit and continues to contribute to the operational Copernicus data stream. The satellite's clean, nearly circular orbit at approximately 807 km altitude shows no signs of imminent natural decay; objects at this altitude typically remain in orbit for centuries in the absence of deliberate deorbiting manoeuvres, so Sentinel-3B is expected to remain functional until either its consumables are exhausted or a technical failure ends its operational life.

The broader significance of Sentinel-3B lies in its role within a systematic, long-term Earth monitoring architecture. Individual satellites can and do fail, but the Copernicus Programme's strategy of launching multiple units in each series, phased to provide coverage continuity, has made the Sentinel-3 data record notably robust compared with predecessor missions. Ocean colour and sea surface temperature records assembled from the Sentinel-3 series are now long enough to contribute meaningfully to climate trend analysis, not merely short-term operational forecasting.

Sentinel-3B also exemplifies a wider shift in Earth observation philosophy during the 2010s toward open-access data. All Copernicus data is freely available without charge or licensing restrictions to any user — a policy that has driven widespread adoption in academic research, commercial applications, and governmental decision-making. The availability of consistent, well-characterised data from Sentinel-3B and its sister satellites has contributed to advances in ocean biogeochemistry, harmful algal bloom monitoring, coral reef health assessment, and coastal erosion tracking, among many other fields.

For the satellite tracking community, Sentinel-3B represents a well-characterised, stable object in a well-defined orbit. Its near-circular sun-synchronous trajectory at just over 800 km altitude means its ground track is highly predictable, and its orbital elements are routinely updated by space surveillance networks. The satellite's mass and altitude also place it in a regime where atmospheric drag is negligible on human timescales, making it a reliable long-term fixture in low Earth orbit catalogues.

How to Spot It

Sentinel-3B is a medium-sized satellite with a launch mass of 1,200 kg, and at an orbital altitude of approximately 807 km it is visible to the naked eye from dark locations under the right conditions, though it will not typically rank among the brightest objects in the sky. Like all satellites in sun-synchronous orbits, it passes over most populated latitudes and can be observed shortly after dusk or before dawn, when the ground is in darkness but the satellite itself is still illuminated by sunlight.

To find a pass prediction for your location, enter NORAD ID 43437 into the LowEarth tracking tools. The satellite's near-circular orbit and stable inclination of 98.6° mean that pass predictions are reliable several days in advance. Observers at higher latitudes will see passes more frequently due to the polar nature of its orbit. During favourable geometry — a low solar depression angle and a high-elevation pass — Sentinel-3B should be readily visible as a steadily moving point of light crossing the sky over the course of a few minutes.