Earth observation is one of the most important tasks that satellites fulfil for the benefit of mankind. To meet this challenge, the EU has launched the Copernicus program. The program aims is to create an independent infrastructure that provides high-quality data to answer environmental and security-related questions. Sentinel-1A, the first dedicated Copernicus satellite, was launched in 2014 and currently the program comprises three complete constellations of two satellites each plus a single satellite.
In total, the Sentinel family encompasses six different satellite types which, in combination with other data sources in the air, on the water and on the ground, provide six application-related core services:
1. Land monitoring
2. Surveillance of the oceans
3. Disaster and crisis management
4. Atmospheric monitoring
5. Monitoring of climate change
Already in orbit: Sentinel-1 to Sentinel-3
The two Sentinel-1 satellites carry modern radar instruments (Synthetic Aperture Radar, SAR), which make it possible to take high-quality pictures of the Earth’s surface even in dense clouds and at night. The satellites are in the same orbit but offset by 180° and map the entire Earth every six days. The collected data provides information about the distribution of sea ice, changes to landscapes due to human influence and the extent of natural disasters such as floods and earthquakes.
The Sentinel-2 constellation is equipped with high-resolution multispectral sensors that can record strips of land with a width of 290 kilometers. The two satellites in the constellation are identical in construction and also move in the same orbit around the Earth with an offset of 180°. Orbiting close to the equator, they monitor all land masses, large islands and coastal waters every five days; at higher latitudes, the revisiting time is even shorter. The data which the satellites record provide information about the composition of the Earth’s surface and vegetation, which are primarily used for agriculture and forestry.
The Sentinel-3 mission is the most complex project in the Copernicus program to date, as the two identical satellites carry four different instruments which, working together, provide a comprehensive view of the Earth:
The Sea and Land Surface Temperature Radiometer (SLSTR) measures the temperatures of land surfaces and oceans on a daily basis with high accuracy using the emitted infrared radiation. In addition, the instrument includes two channels for thermal infrared radiation, which are used to actively search for fires. The SLSTR has a spatial resolution of 1,000 meters on these channels and 500 meters on the other channels. The width of the recorded strips is 1,420 kilometers.
The Ocean and Land Color Instrument (OLCI) is an imaging spectrometer with 21 discrete spectral channels spanning a wavelength range from 400 to 1,200 nanometers. This area of the electromagnetic spectrum provides relevant information on marine ecosystems, permits the management of agriculture and forestry by tracking the state of soils and vegetation, and also maps atmospheric aerosols and clouds. The OLCI has a spatial resolution of 300 meters for all measurements and records strips with a width of 1,270 kilometers.
The two aforementioned instruments are supplemented by a SAR altimeter and a microwave radiometer. In addition to topography measurements over land, the altimeter also records the exact heights of sea levels, sea ice, rivers and lakes. In addition, wave heights and wind speeds over the sea can also be determined. The microwave radiometer determines the atmospheric correction and thus enhances the accuracy of the topographic measurements.
Under development: Sentinel-4 to Sentinel-6
The next Copernicus instruments will be launched with the third-generation geostationary sounder satellites of the Meteosat weather satellites (MTG) currently being built at OHB. Like Sentinel-5, Sentinel-4 is designed to monitor the earth’s atmosphere and provide information on air quality, ozone levels and solar radiation. OHB is making a significant contribution to the development of the ultraviolet-visible-near-infrared spectrometer (UVN spectrometer) for the Sentinel-4 mission.
The Sentinel-5 instruments will also be placed on European weather organization EUMETSAT’s weather satellites. The second-generation meteorological operational satellites (MetOp-SG) are near-earth polar orbiting satellites designed to complement the MTG satellites. OHB Italia is developing the microwave imager (MWI), which is the main instrument for these satellites, a microwave radiometer for detecting aerosols in the atmosphere. The MWI is complemented by the Sentinel-5 instrument, a UVN shortwave spectrometer. Pending the launch of Sentinel-5, the gap in data caused by the failure of Envisat failure in 2012 will be closed at least partially by the single Sentinel-5P satellite (Sentinel-5 precursor) launched in 2017.
Sentinel-6 is a mission for collecting data on the topography of the oceans. The satellites will be carrying high-precision radar altimeters and providing information on changes in sea level as an indicator of climate change. Every 10 days, 95% of the ice-free oceans are to be mapped for this purpose.
Addition to the Copernicus program: Sentinel Expansion
Encouraged by the great success of the first Sentinel satellites, the EU is already preparing the expansion of the Sentinel family. After analyzing the user requirements that are currently not being addressed, six potential missions to complement Copernicus have been identified:
CHIME – Copernicus Hyperspectral Imaging Mission
The satellites of the CHIME mission will carry hyperspectral sensors, perform soil composition analyses in a continuous spectrum from visible light to near infrared and complement the multispectral data collected by Sentinel-2.
CIMR – Copernicus Imaging Microwave Radiometer
The CIMR mission is a response to the special requirements of satellite monitoring in polar regions. The satellites are to be equipped with multi-frequency microwave radiometers, which will detect wide strips of the polar regions by means of conical scanning. Among other things, the collected data will provide information about the temperature and salinity of the sea surface as well as the concentration of sea ice.
CO2M – Copernicus Anthropogenic Carbon Dioxide Monitoring
CO2M is to measure the carbon dioxide produced by humans in the atmosphere and thus make it possible to identify regions which have particularly high emissions. Observance of agreed climate targets can also be verified. The main instruments required for this are a near-infrared and a short-wave infrared spectrometer.
CRISTAL – Copernicus Polar Ice and Snow Topography Altimeter
The CRISTAL mission satellites will determine the thickness of the sea ice and the snow lying on top of it as a basis for planning activities in the polar regions and for monitoring climate change. For this purpose, they are to carry multi-frequency radar altimeters and microwave radiometers.
LSTM – Copernicus Land Surface Temperature Monitoring
LSTM is responsible for the thermal monitoring of land areas. To this end, satellites fitted with sensors for thermal infrared radiation with high spatial and temporal resolution are to be placed in orbit. Measurements of the surface temperature of land masses are important indicators of climate change and facilitate the prediction of droughts and the management of water resources.
ROSE-L – L-band Synthetic Aperture Radar
The ROSE-L mission will complement the data gained from the Sentinel-1 mission. Wherever the shorter wavelength C-band SAR of Sentinel-1 does not reach the ground due to vegetation, snow or ice, the longer wavelength L-band SAR of ROSE-L will step in to fill the gap. It will be used in agriculture and forestry and for monitoring climate changes through the observation of the polar ice caps, sea ice and seasonal snow cover.
The missions have been undergoing more detailed planning since mid-2018. OHB System AG has been selected as the main contractor for the studies on the CHIME, CO2M and LSTM missions. OHB Sweden has been awarded the contract for the CRISTAL mission and OHB Italia is the main payload contractor for CIMR. Which missions will ultimately be implemented will be decided at the ESA Ministerial Council Conference Space19+ in November 2019.