## The Sentinel Constellation

The Sentinel satellite program was designed to replace the older Earth observation missions, which have reached retirement or are nearing the end of their operational life span.  The satellite array will ensure a continuity of data, so that there are no gaps in ongoing studies.

The European Space Agency $($ESA$)$ has been developing a series of next-generation Earth observation missions, on behalf of the joint ESA/European Commission initiative GMES $($Global Monitoring for Environment and Security$)$.  The collected data will be useful to many applications.

The Sentinel satellite family. Image: ESA

The Sentinel satellite family constitute the space segment of the European Union's Copernicus environmental monitoring programme.

Each mission is based on a constellation of two spacecrafts to fulfil revisit and coverage requirements, and provide robust data sets for the Copernicus Services.

These missions carry a range of technologies, such as radar and multi-spectral imaging instruments.

Each Sentinel constellation focuses on a different aspect of Earth observation:

• Atmospheric,
• Oceanic,
• Land monitoring.

Sentinel-1 launched in 2014.  Source: ESA

### Sentinel-1

With the objectives of Land and Ocean monitoring, Sentinel-1 will be composed of two polar-orbiting satellites operating day and night, and will perform Radar imaging, enabling them to acquire imagery regardless of the weather.  The first Sentinel-1 satellite was launched in April 2014.

The next frontier in agricultural science starts with the Sentinel-2 constellation.  Image: ESA

### Sentinel-2

The objective of Sentinel-2 is agricultural land monitoring, and the mission will be composed of two polar-orbiting satellites providing high-resolution optical imagery.  Vegetation, soil and coastal areas are among the monitoring objectives.

### Sentinel-3

The primary objective of Sentinel-3 is marine observation, and it will study sea-surface topography, sea and land surface temperature, ocean and land colour.  Composed of three satellites, the mission's primary instrument is a radar altimeter, but the polar-orbiting satellites will carry multiple instruments, including optical imagers.

Sentinel-4 Source: ESA

### Sentinel-4

Sentinel-4 is dedicated to air quality monitoring.  The Sentinel-4 UVN instrument is a spectrometer carried aboard Meteosat Third Generation satellites, operated by EUMETSAT.  The mission aims to provide continuous monitoring of the composition of the Earth's atmosphere at high temporal and spatial resolution and the data will be used to support monitoring and forecasting over Europe.

Sentinel-5P  Source: ESA

### Sentinel-5

Sentinel-5 is dedicated to air quality monitoring.  The Sentinel-5 UVNS instrument is a spectrometer carried aboard the MetOp Second Generation satellites.  The mission aims to provide continuous monitoring of the composition of the Earth's atmosphere.  It provides wide-swath, global coverage data to monitor air quality around the World.

### Sentinel-5P

A precursor satellite mission, Sentinel-5P aims to fill in the data gap and provide data continuity between the retirement of the Envisat satellite and NASA's Aura mission and the launch of Sentinel-5.  The mission will perform atmospheric monitoring.

Sentinel-6 Source: ESA

### Sentinel-6

Sentinel-6 carries a radar altimeter to provide high-precision and timely observations of the topography of the global ocean.  Such information is essential for the ongoing monitoring of sea-level changes, a key indicator of climate change.  It is also essential for operational oceanography.

Mapping up to 95% of Earth’s ice-free ocean every 10 days, Sentinel-6 will offer vital information on ocean currents, wind speed and wave height for maritime safety.  The data are important for the protection and management of the increasingly busy coastal zones.

Sentinel-6 builds upon the heritage from the Jason series of ocean topography satellites and from ESA’s CryoSat mission.  Crucially, this new mission is designed to complement the ocean information obtained from Sentinel-3.

The aftermath of the Nepal earthquake showing the devastation. Source: Aljazeera

## Sentinel-1 Seeing an Earthquake in Nepal

Europe's Sentinel-1A satellite surveyed the aftermath of Saturday 25th April 2015's big earthquake in Nepal.

The spacecraft carries a radar able to sense ground movement by comparing before and after imagery acquired from orbit.  Using this information, scientists can produce an interferogram - a colourful, highly technical, representation of the displacement that occurs on a fault.

The new data confirmed an area of 120 by 50 kilometres around Kathmandu lifted up, with a maximum rise of at least 1 metre.

A map of the aftermath of the Kathmandu earthquake. Source: Institut de Physique du Globe de Paris

Further to the northeast of the capital Kathmandu, the interferogram indicated that the ground subsided, which is exactly what would be expected following a shallow thrust.  Researchers were also able to see how the fault ruptured east from the epicentre, and did not break the surface.

This feature indicated that not all the strain built up in the rocks prior to the earthquake was released in the magnitude-7.8 event and subsequent aftershocks.

The interferogram was produced as part of a European Space Agency study called INSARAP.

One of the first images to be taken by Sentinel-2A showed the Po Valley in Italy. Source: ESA

## Sentinel-2 and Agriculture

Sentinels 2A and 2B are the centrepiece of the Copernicus Programme, because their free and open picture resource will almost certainly find the widest use.

The first Sentinel-2A satellite left Earth back in June 2015.  Sentinel-2B launched from Kourou in French Guiana on 7 March 2017.  It carries a large camera to image all land surfaces and coastal waters in visible and infrared light, and will join its identical twin satellite into orbit.

The two identical satellites will operate in the same orbit, 180° apart for optimal coverage and data delivery. Together they can cover all of Earth’s land surfaces, large islands, inland and coastal waters every five days at the equator.

Sentinel-2 constellation's 5-day revisit frequency, global coverage and compatibility to the Landsat missions offer new opportunities for regional to global agricultural management.

Both spacecrafts carry an innovative wide-swath multi-spectral imager sensitive to 13 spectral bands for new perspectives of our land and vegetation.  The 10-20 metre spatial resolution colour cameras will observe a range of properties in the things that they see, allowing for the distinction to be made between different types of crops and for their health to be assessed.

This combination of high-resolution cameras, along with their novel spectral capabilities, a swath width of 290 km and frequent revisit times will provide unprecedented views of Earth.

Together, the instruments will be producing something like 4 terabytes of data daily - a prodigious volume that will lean on the assistance of a relay link.  For each 10-minute spell in orbit, the satellite will beam their images not down to Earth but, rather higher up into the sky, to a geostationary satellite that will bounce them back to the ground.

Thousands of users have already registered to access the pictures, and downloaded them from the various distributed servers.  Applications range from urban planning and air quality monitoring to tracking deforestation and glacier retreat.

Information from the mission will be helping to improve agricultural practices, monitor the World's forests, detect pollution in lakes and coastal waters, and contribute to timely disaster mapping.

As night falls on 29 February 2016, a snowy Svalbard archipelago was the first to "pose" for Sentinel's Ocean and Land Colour Instrument $($OLCI$)$.

## Sentinel-3 Watching over the Earth

Sentinel-3 forms part of Europe's Copernicus environmental monitoring network, designed to "systematically measure Earth’s oceans, land, ice and atmosphere to monitor large-scale global dynamics and provide critical near real time information for ocean and weather forecasting".

The Sentinel-3A satellite delivered its first snaps from above our atmosphere, two weeks after it launched on 16th February 2016, on board a "Rockot" - a converted inter-continental ballistic missile.

The mission's main objective in support to the Marine Environment is to determine parameters such as sea-surface topography, sea-surface temperature and ocean-surface colour.

Sentinel-3 is packed with high-quality Earth monitoring instrumentation. Source: ESA

To achieve these objectives, the Sentinel-3A satellite packs four instruments that work in synergy:

• The Sea and Land Surface Temperature Radiometer $($SLSTR$)$, as its name suggests tasked with measuring sea and land temperatures to an accuracy of better than 0.3 K.  The SLSTR uses a dual viewing technique and operates across eight wavelength bands, providing even better coverage than its predecessor Envisat's Advanced Along Track Scanning Radiometer $($AATSR$)$, because of a wider swath width.
• The Ocean and Land Colour Instrument $($OLCI$)$, a spectrometer capturing a swath width of 1,270km to a resolution of 300 metres, operating across 21 spectral bands from ultraviolet to near-infrared, and which will allow ocean ecosystems to be monitored, support crop management and agriculture and provide estimates of atmospheric aerosol and clouds.
• Atmospheric pressure arises as a result of the weight of the air sitting above the observer in one atmospheric column.

A topography package system comprising a Synthetic Aperture Radar Altimeter $($SAR Altimeter or SRAL$)$, based on technologies previously used by ESA's Earth Explorer CryoSat mission, and working with
• a Microwave Radiometer $($MWR$)$ used to derive atmospheric correction and atmospheric column water vapour measurements.

• The Doppler Orbitography and Radiopositioning integrated by Satellite $($DORIS$)$ is a receiver for orbit positioning.
• Laser Retro-Reflector $($LRR$)$ will be used to locate the satellite in orbit accurately, using a laser ranging system.  In combination with SRAL, MWR and DORIS, it will gather detailed topographic measurements of the ocean and inland waters.
• Global Navigation Satellite System$($GNSS$)$, a GPS receiver that will provide precise orbit determination and track multiple satellites simultaneously.

Sentinel-3 has been delivering high-quality ocean and land temperature measurements since reaching its orbit.

The mission's main objective in support to the Marine Environment is to determine parameters such as sea-surface topography, sea-surface temperature and ocean-surface colour, over a lifetime of 7.5 years.

To achieve these objectives, the following set of observational requirements has been established:

• Sea surface topography $($SSH$)$ and significant wave height $($SWH$)$ over the global ocean to an accuracy and precision exceeding that of Envisat RA-2.
• Sea surface temperature $($SST$)$ determined globally to an equivalent accuracy and precision as that presently achieved by A/ATSR $($i.e. < 0.3 K$)$, at a spatial resolution of 1 km.
• Visible and thermal infrared radiances $($'Ocean Colour'$)$ for oceanic and coastal waters, determined to an equivalent level of accuracy and precision as MERIS data with complete Earth coverage in two to three days, and co-registered with SST measurements.

Given its extensive payload, Sentinel-3A is a real workhorse that is set to make a step change in the variety of data products provided to users.  The mission will be at the heart of a wide range of applications, from measuring marine biological activity to providing information about the health of vegetation.

After the nuclear test? A topographic profile of North Korea. While the yellow colour shows zones of uplift, the deep blue colour indicates subsidence. Source: ESA

## The Eye in the Sky

And it is not only earthquakes and natural events that the Sentinel constellations can keep an eye on...

The ground convulsion resulting from North Korea's underground nuclear bomb test on 6 January 2016 and described by North Korean media as a miniaturised hydrogen bomb detonation, were mapped by the Sentinel-1A radar satellite.

The EU spacecraft uses a technique called interferometry to sense surface movements.

Satellite interferometry works by finding the difference in "before" and "after" radar pictures of the Earth's surface.  It allows for even  subtle ground movements to be detected.

Sentinel-1A got its first view of the nuclear test site, following the explosion, on 13 January.  This result was compared with an observation acquired on 1 January.  The tell-tale effects of both subsidence and uplift are evident.  Its data shows rock above the blast zone going down by up to 7 centimetre in one area and rising by 2-3 centimetres in another.

The imagery was released by Germany's Institute for Geosciences and Natural Resources $($BGR$)$, which advises the federal government on matters related to the Comprehensive Nuclear-Test-Ban Treaty $($CTBT$)$.

All of North Korea's nuclear tests $($2006, 2009, 2013, 2016, 2017$)$ appear to have taken place at one site called Punggye-ri, also known as P'unggye-yok, in a remote region in the east of the country, near the town of Kilju.

Although the data picked up by international seismometers has given very good location information, the new Sentinel imagery refines these estimates further.

Whatever goes on there, you can be sure...

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