The European Space Agency (ESA), under the European Union’s Copernicus program, is developing a series of seven earth observation missions of each two satellites that will provide free data for global level monitoring of the earth’s resources. ESA has so far (at start of 2017) launched four of these satellites. These are Sentinel-1A in April 2014, 1B in April 2016, Sentinel-2A in June 2015 and Sentinel-3A in February 2016. The launch of Sentinel-2B is foreseen for March 2017. The missions are expected to feed into various observational networks and provide spatially explicit physical, biogeophysical and biological variables required for ocean, cryosphere and land research activities (Malenovský et al., 2012). Data can be downloaded free-of-charge from the Sentinel scientific hub.
Sentinel-1 is a radar imaging system that provides all-weather, day and night images for land- and water-related research (Torres et al., 2012). The mission ensures continuity of ESA’s previous C-band SAR instruments of ERS and ENVISAT, although compared the previous missions, Sentinel-1 presents vast improvements especially in the areas of reliability, revisit time, geographical coverage and rapid data dissemination (Torres et al., 2012). Possible application areas include flood mapping, snow cover monitoring, soil moisture, surface topography, marine surveillance, sea ice classification, vegetation analysis and agricultural applications (Malenovský et al., 2012; Paloscia et al., 2013; Torres et al., 2012). With respect to agricultural applications, radar data have been shown to be an important addition to optical images in crop mapping (McNairn et al., 2009). Thus, the free data policy of ESA regarding Sentinel-1 data is expected to improve crop mapping activities in cloud-affected areas where lack of useful images during the cropping season often hinders mapping the spatial distribution of crops.
Sentinel-1 data are delivered within an hour of reception for Near Real-Time (NRT) emergency response, within three hours for NRT priority areas and within a day for systematically archived data. A user can look at planned acquisitions for his/her area of interest by inspecting the detailed observation scenarios available here.
Data are acquired in different modes and delivered at different processing levels with varying resolutions. All data are delivered in the Standard Archive Format for Europe (SAFE) format. To facilitate processing, ESA has developed the Sentinel toolbox for processing Sentinel data. More details about Sentinel-1, the different acquisition modes, product types and characteristics can be found here.
The Sentinel-2A satellite was launched in June 2015. It is the first optical satellite from the Sentinel range launched in the Copernicus program. The mission ensures continuity of the multispectral imagery provided by the SPOT (Satellite Pour l'Observation de la Terre) series of satellites. The Sentinel-2 satellite is in a sun-synchronous orbit at an altitude of 786 km. It provides multi-spectral images in thirteen spectral bands at different spatial resolutions. These include four visible and near-infrared bands at 10 m resolution, six red-edge and shortwave infrared bands at 20 m resolution and three atmospheric correction bands at 60 m resolution (Drusch et al., 2012). With a swath width of 290 km, Sentinel-2 aims at providing cloud-free images of global land surfaces including major islands, with a high revisit frequency over Europe and Africa.
The range of spectral bands makes Sentinel-2 data suitable for a wide range of applications including agriculture, forestry, land use/land cover analysis, risk and disaster mapping, coastal applications and retrieval of critical and vegetation and biophysical variables such as leaf area index, normalized difference vegetation index, fraction of photosynthetically active radiation, leaf chlorophyll content and leaf water content. Inglada et al. (2015) already noted the potential of Sentinel-2 data in implementing an operational and automated crop type mapping system by simulating Sentinel-2 time-series from SPOT4 and Landsat 8 data. Compared to SPOT4/Landsat8, Sentinel-2 will have a higher spatial resolution and better spectral sensitivity, which will permit even better results to be obtained. Other studies have also demonstrated the potential of Sentinel-2 data for the retrieval of three biophysical parameters namely leaf chlorophyll content, leaf area index and fraction vegetation cover (Richter et al., 2012; Verrelst et al., 2012) and for assessing rangeland quality (Ramoelo et al., 2015).
The Sentinel-3A satellite was launched in February 2016. It focuses mainly on the provision of high accuracy and reliable information on sea surface topography, sea and land surface temperature, forest cover mapping, fire detection, and ocean and land surface colour to support ocean forecasting systems, environmental monitoring and climate monitoring. The satellite will use multiple sensing instruments such as the Sea and Land Surface Temperature Radiometer (SLSTR), Ocean and Land Colour Instrument (OLCI) and Microwave Radiometer (MWR) to accomplish its objectives. Sentinel-3 has been designed to provide data continuity for some previous missions including ERS and Envisat. Data can be downloaded free-of-charge from the Sentinel scientific hub. More information on Sentinel-3 can be found here.