Sentinel-1 continues the C-band SAR Earth Observation heritage of ESA's ERS-1, ERS-2 and ENVISAT, and Canada's RADARSAT-1 and RADARSAT-2.
ERS-1 and ERS-2
Figure 1: ERS-2 Satellite
European Remote Sensing (https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/ers ) satellites have achieved several milestones in Earth Observation. Launched in July 1991, ERS-1 introduced reliable and stable radar from space with its Active Microwave Instrument (AMI). ERS-2, launched in April 1995, shared the same orbital plane with ERS-1 allowing for a tandem mission in 1995 and 1996, where interferometric data was collected one day apart. ERS-1 was in operation until March 2000 while ERS-2 far exceeded its expected lifespan, acquiring images until 2011.
Figure 2: ENVISAT Satellite
https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/envisat provided 10 years of SAR data mapping from March 2002 to May 2012, five years longer than its designed mission duration. With 10 instruments aboard and at 8 tons, ENVISAT is the largest civilian Earth Observation mission put into space. ENVISAT added new capabilities over ERS with its Advanced Synthetic Aperture Radar (ASAR) sensor including wide swaths, dual polarisation and simultaneous acquisitions from its MERIS sensor.
Sentinel-1 provides enhancements over previous missions in the form of reliability, revisit time, geographical coverage and rapid data dissemination.
Copernicus Sentinel-2 heritage
The spectral band configuration of the Sentinel-2 mission arose as a result of consultation with the user community during the design phase. The existing Copernicus/GMES Service Elements (GSEs) services were developed around the use of LANDSAT and SPOT wavelengths, and the service requirements for Sentinel-2 have these at their core.
Figure 3: Comparison of Spatial Resolution and Wavelength Characteristics of Sentinel-2 MultiSpectral Instrument (MSI), the Operational Land Imager (OLI) On-Board LANDSAT-8, and SPOT 6/7 Instruments [Credits: ESA Special Publication 1322/2]
Narrowing the width of the Sentinel-2 spectral bands limits the influence of atmospheric constituents, including water vapour. The original LANDSAT Near Infra-Red (NIR) band (760-900 nm) was found to be heavily contaminated by water vapour and not sensitive enough to parameters such as soil iron oxide content. The narrowness of the 8a band at 865 nm in the NIR is designed to avoid contamination from water vapour yet still be able to represent the NIR plateau for vegetation and be sensitive to iron oxide content for soil.
Precise aerosol correction of acquired data is enabled by the inclusion of a spectral band in the blue domain at 443 nm (Band 1) in the Sentinel-2 configuration. The 443 nm band was used in previous missions: for the calculation of the ENVISAT MERIS Global Vegetation Index (MGVI), and in atmospheric corrections for NASA's MODIS sensor.
Due to its potential impact on reflectance values, its use as an indicator in weather forecasting and its role in the trapping of incoming solar radiation, the presence of cirrus cloud needs to be addressed. Adding a spectral band at 1 375 nm (band 10) enables cirrus detection. The correction of data for thin cirrus can be managed using Visible to Near Infra-Red (VNIR) band information. This band is included in the MODIS instruments as band 26, and its is used in current US multispectral missions such as LANDSAT-8 and the Visible Infrared Imaging Radiometer Suite (VIIRS).