The Sentinels are a fleet of European Space Agency (ESA) satellites designed to acquire measurements from multiple sensor types that provide information necessary to meet Europe's Copernicus Program objectives. 

The first mission in the series, the Sentinel-1 constellation, included twin satellites that each carried C-band synthetic aperture radar (SAR) and together provided all-weather, day-and-night imagery of Earth's surface. Sentinel-1A was launched in April 2014 and Sentinel-1B in April 2016. Their polar orbits were 180° apart, and together they had the potential to image Earth every six days.

In December 2021, an anomaly in Sentinel-1B's power supply caused the SAR instrument to stop working. Attempts to restore power to the sensor failed, and the mission officially ended on August 3, 2022.

Sentinel-1C was launched in December 2024, reestablishing a twin satellite constellation with extensive worldwide coverage and six-day repeat-pass observations.

Sentinel-1D was launched on November 4, 2025. After its commissioning phase, it will be maneuvered to replace Sentinel-1A. This is scheduled to occur during June 2026, after which Sentinel-1A operations will be terminated. Sentinel-1C and Sentinel-1D will then work in tandem and comprise the new Copernicus Sentinel-1 constellation.

The Sentinel-1 constellation supports numerous services, including monitoring Arctic sea-ice extent, routine sea-ice mapping, and surveillance of the marine environment. Applications include oil-spill monitoring and ship detection for maritime security; monitoring land surfaces for motion risks; mapping for forest, water, and soil management; and mapping to support humanitarian aid and crisis situations.

All products from the Sentinel-1 mission are available to download from NASA's Alaska Satellite Facility Distributed Active Archive Center (ASF DAAC) through the Vertex search application, programmatically using asf_search and SearchAPI, and through Earthdata Search.