Europe's Copernicus Radar Constellation Complete: Sentinel-1D Now Operational

The Copernicus Sentinel-1 radar mission has reached a major milestone with the full operational status of its fourth satellite, Sentinel-1D. Launched in November of the previous year, the satellite has successfully passed its in-orbit commissioning phase. This achievement completes the quartet of Sentinel-1 satellites, a journey that began over a decade ago and strengthens Europe’s capabilities in Earth observation. Below, we answer key questions about this development.

What is Sentinel-1D and why is it important?

Sentinel-1D is the fourth and final satellite of the European Copernicus Sentinel-1 radar mission. It was launched in November of the previous year and has now completed its in-orbit commissioning phase, meaning all systems are verified and it is ready for routine operations. This satellite is crucial because its deployment completes the full four-satellite constellation, ensuring continuous, reliable radar imagery for global monitoring. The importance lies in redundancy and revisit frequency: with all four satellites operational, the mission can provide more frequent observations and maintain coverage even if one satellite faces an issue. Sentinel-1D carries a C-band synthetic aperture radar (SAR) capable of imaging Earth day and night through clouds, making it indispensable for applications like sea-ice monitoring, oil spill detection, land subsidence tracking, and emergency response during natural disasters.

When was Sentinel-1D launched and what happened after?

Sentinel-1D was launched in November of the previous year aboard a Vega rocket from Europe’s spaceport in Kourou, French Guiana. Following a successful separation from the launcher, the satellite entered its planned polar orbit at an altitude of about 693 km. The subsequent months were dedicated to the in-orbit commissioning phase, a standard procedure for new satellites. During this period, engineers performed thorough checks and calibrations of the satellite’s systems, including the synthetic aperture radar, power systems, communication links, and data handling units. The commissioning phase concluded with the satellite declared fully operational, joining its siblings – Sentinel-1A, Sentinel-1B, and Sentinel-1C – to form a complete constellation.

What does the 'commissioning phase' involve for a radar satellite?

The commissioning phase is a critical period after launch where the satellite is tested and calibrated to ensure all instruments and subsystems function as intended. For Sentinel-1D, this involved several steps: first, the satellite’s attitude control and orbit positioning were verified. Then the synthetic aperture radar (SAR) antenna was deployed and its performance measured. Engineers executed a series of calibration maneuvers – switching the radar between different imaging modes (Interferometric Wide swath, Wave, Stripmap, and Extra Wide swath) to adjust gain, phase, and timing. Data downlink to ground stations was tested to confirm data integrity. Thermal control and power generation from solar arrays were assessed. Only after all checks passed and the radar’s image quality met stringent specifications – including radiometric and geometric accuracy – was the satellite declared operational. This process typically takes several months.

Why is the completion of all four Sentinel-1 satellites a milestone?

Having all four Sentinel-1 satellites operational is a major milestone because it marks the fulfillment of the original constellation design envisioned over a decade ago when the Copernicus program was conceived. The quartet provides unmatched redundancy and temporal resolution. With four satellites phased in orbit, the repeat cycle for interferometric applications is shortened, improving the capability to detect ground deformations and monitor dynamic processes like glacier movement and volcanic activity. This milestone also ensures continuity of data supply for Copernicus services. The journey – from the launch of Sentinel-1A in 2014 through the subsequent launches of 1B, 1C, and now 1D – has overcome technical challenges and delays. Completing the constellation paves the way for future Earth observation innovations, such as the planned Sentinel-1 Next Generation satellites.

How does the Sentinel-1 mission contribute to Earth observation?

Sentinel-1 is a cornerstone of the Copernicus program, providing free and open radar data for environmental monitoring and security. Its C-band synthetic aperture radar can acquire images regardless of weather conditions or daylight, making it ideal for consistent surveillance. Key applications include: Marine monitoring – detecting oil spills, tracking sea ice, mapping ship traffic, and monitoring ocean winds; Land monitoring – mapping ground deformation for earthquake and volcanic risk assessment, monitoring subsidence due to mining or groundwater extraction, and tracking changes in agricultural land and forests; Emergency response – providing rapid imagery for floods, earthquakes, and landslides to aid disaster management; Climate change – long-term monitoring of ice sheets, glaciers, and permafrost. The data are used by scientists, government agencies, and commercial entities across Europe and worldwide.

What are the next steps for the Copernicus Sentinel-1 mission?

With Sentinel-1D now operational, the immediate focus is on routine operations and data dissemination. The constellation will continue to deliver high-quality radar imagery to users for the expected design lifetime of each satellite (7 years for 1A and 1B, extended for 1C and 1D). Meanwhile, ESA and the European Commission are already planning for the future. The Sentinel-1 Next Generation (Sentinel-1 NG) mission is in development, aiming to enhance capabilities with higher resolution and wider swaths, while also ensuring continuity beyond the current satellites. Additionally, upgrades to ground infrastructure and data processing algorithms will maximize the value of the existing constellation. Sentinel-1D’s success confirms the robustness of the design and provides confidence for the long-term sustainability of Copernicus services until the NG satellites are ready in the late 2020s or early 2030s.