Sentinel-1
ESA's C-band Synthetic Aperture Radar (SAR) satellite constellation providing all-weather, day-and-night imaging. Used for flood mapping, subsidence monitoring, ice monitoring, and ship detection regardless of cloud cover or lighting conditions.
Overview
Sentinel-1 is ESA's C-band Synthetic Aperture Radar (SAR) mission under the Copernicus programme. Unlike optical sensors, Sentinel-1 actively transmits microwave pulses at 5.405 GHz and measures returns, enabling all-weather, day-night imaging. The constellation originally comprised Sentinel-1A (April 2014) and 1B (April 2016). After 1B suffered an unrecoverable anomaly in December 2021, Sentinel-1C launched December 5, 2024 to restore the constellation. Free and open data policy makes it one of the most widely used SAR datasets globally.
How It Works
In its primary Interferometric Wide Swath (IW) mode, Sentinel-1 uses TOPSAR — the antenna beam is electronically steered across three sub-swaths, producing a 250 km swath at 5×20 m resolution with homogeneous image quality. The constellation orbits at 693 km with a 12-day repeat per satellite (6-day with two). SAR measures backscatter intensity (surface reflectance strength) and phase (enabling InSAR deformation measurement at millimeter precision). The instrument supports dual polarization (VV+VH or HH+HV).
Key Facts
- C-band SAR at 5.405 GHz with dual polarization (VV+VH or HH+HV).
- IW mode: 5 × 20 m resolution, 250 km swath.
- 6-day revisit with two-satellite constellation.
- Free and open data under Copernicus.
- Sentinel-1C (launched December 5, 2024) includes an AIS antenna for maritime surveillance.
- Builds on European SAR heritage from ERS-1 (1991), ERS-2 (1995), and Envisat ASAR (2002–2012).
Applications
Flood Mapping
SAR backscatter drops sharply over open water, allowing rapid flood mapping within hours — even through storm clouds that block optical sensors.
Ground Deformation Monitoring
InSAR measures phase differences between passes to detect millimeter-scale displacement from subsidence, mining, tectonics, or volcanic inflation.
Deforestation Detection
C-band radar interacts with forest canopy structure, detecting logging in tropical regions where cloud cover limits optical monitoring.
Sea Ice and Maritime
Maps sea ice extent, classifies ice types, and detects ships. Sentinel-1C adds an AIS antenna for enhanced vessel tracking.
Limitations & Considerations
C-band has limited canopy penetration compared to L-band SAR (ALOS-2, NISAR), reducing biomass estimation capability. Speckle noise requires filtering that trades resolution for quality. Geometric distortions (foreshortening, layover, shadow) affect mountainous and urban areas. SAR processing requires specialized expertise beyond typical optical imagery workflows. The 2021–2025 gap between 1B failure and 1C operations disrupted many time-series analyses.
History & Background
Sentinel-1A launched April 3, 2014, becoming the first Copernicus satellite. Sentinel-1B followed April 25, 2016. Built by Thales Alenia Space with Airbus providing the C-SAR instrument. Sentinel-1B suffered an anomaly December 23, 2021 and was decommissioned August 2022. ESA accelerated Sentinel-1C, which launched December 5, 2024 on Vega-C from Kourou, restoring the constellation.
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