Sentinel-3 Sets Sail

Artist's view of Sentinel-3. Image courtesy of ESA–Pierre Carril.

Artist’s view of Sentinel-3. Image courtesy of ESA–Pierre Carril.

At 17.57 GMT yesterday (16th February 2016) Sentinel-3 set sail from the Plesetsk Space Centre in Russia, heading for its 814 km sun-synchronous low Earth orbit. Like all the other Sentinel launches, we were at home watching the live feed!

This is the third Sentinel launch of the European Commission’s Copernicus Programme, following Sentinel-1 and 2. Sentinel-3, like its predecessors, will be part of a twin satellite constellation with Sentinel-3B’s launch expected to be in 2017.

Sentinel-3 carries four scientific instruments:

  • Sea and Land Surface Temperature Radiometer (SLSTR) will measure temperatures of both the sea and land, to an accuracy of better than 0.3 K. This instrument has 9 spectral bands with a spatial resolution of 500 m for visible/near-infrared wavelengths and 1 km for the thermal wavelengths; and has swath widths of 1420 km at nadir and 750 km looking backwards. It’s worth noting that two thermal infrared spectral wavebands are optimised for fire detection, providing the fire radiative power measurement.
  • Ocean and Land Colour Instrument (OLCI) has 21 spectral bands (400–1020 nm) focussed on ocean colour and vegetation measurements. All bands have a spatial resolution of 300 m with a swath width of 1270 km.
  • Synthetic Aperture Radar Altimeter (SRAL) which has dual frequency Ku and C bands. It offers 300 m spatial resolution after SAR processing, and is based on the instruments from the CryoSat and Jason missions. This will be first satellite altimeter to provide 100% coverage of the Earth’s surfaces in SAR mode.
  • Microwave Radiometer (MWR) dual frequency at 23.8 & 36.5 GHz, it is used to derive atmospheric column water vapour measurements for correcting the SRAL instrument.

The scientific instruments are supported by four positioning/navigation instruments to ensure the satellite maintains its precise orbit.

Sentinel-3 will mainly be focussing on ocean measurements and will include the measurement of sea-surface height (similar to the recently launched Jason-3); however it will also measure sea surface temperature, ocean colour, surface wind speed, sea ice thickness and ice sheets. Whereas over land the satellite will provide indices of vegetation, measuring the height of rivers and lakes and help monitor wildfires.

Sentinel-3 is a very exciting satellite for us, as the data and products it will produce are very much within the wheelhouse of the services that Pixalytics offers. Sam’s background is in ocean colour, she’s world renown for atmospheric correction research and we offer a variety of agritech services including vegetation indices. You can probably now see why we’re so excited!

The satellite is currently in its commissioning phases where ESA tests the data produced by the sensors. This is undertaken in conjunction with a group of users, and Pixalytics is one of them! This phase is expected to last five months, after which the satellite will be transferred to Eumetsat and the data should be released.

Like all the data from the Copernicus programme, it will be offered free of charge to users. This will challenge organisations, like us, to see what innovative services we can offer with this new data stream. Exciting times ahead!

Sentinel-2A dips its toe into the water

Detailed image of algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

Detailed image of algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

With spectacular images of an algal bloom in the Baltic Sea, ESA’s Sentinel-2A has announced its arrival to the ocean colour community. As we highlighted an earlier blog, Sentinel-2A was launched in June predominately as a land monitoring mission. However, given it offers higher resolution data than other current marine focussed missions; it was always expected to dip it’s toe into ocean colour. And what a toe it has dipped!

The images show a huge bloom of cyanobacteria in the Baltic Sea, with the blue-green swirls of eddies and currents. The image at the top of the blog shows the detail of the surface floating bloom caught in the currents, and there is a ship making its way through the bloom with its wake producing a straight black line as deeper waters are brought to the surface.

Algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

Algal bloom in the Baltic Sea acquired by Sentinel-2A on 7 August 2015. Data courtesy of Copernicus Sentinel data (2015)/ESA.

To the right is a wider view of the bloom within the Baltic Sea. The images were acquired on the 7th August using the Multispectral Imager, which has 13 spectral bands and the visible, which were used here, have a spatial resolution of 10 m.

The Baltic Sea has long suffered from poor water quality and in 1974 it became the first entire sea to be subject to measures to prevent pollution, with the signing of the Helsinki Convention on the Protection of the Marine Environment of the Baltic Sea Area. Originally signed by the Baltic coastal countries, a revised version was signed by the majority of European countries in 1992. This convention came into force into force on the 17th January 2000 and is overseen by the Helsinki Commission – Baltic Marine Environment Protection Commission – also known as HELCOM. The convention aims to protect the Baltic Sea area from harmful substances from land based sources, ships, incineration, dumping and from the exploitation of the seabed.

Despite the international agreements, the ecosystems of the Baltic Sea are still threatened by overfishing, marine and chemical pollution. However, the twin threats that cause the area to suffer from algal blooms are warm temperatures and excessive levels of nutrients, such as phosphorus and nitrogen. This not only contributes towards the algal blooms, but the Baltic Sea is also home to seven of the world’s ten largest marine dead zones due to the low levels of oxygen in the water, which prevent marine life from thriving.

These images certainly whet the appetite of marine remote sensors, who also have Sentinel-3 to look forward to later this year. That mission will focus on sea-surface topography, sea surface temperature and ocean colour, and is due to the launched in the last few months of 2015. It’s an exciting time to be monitoring and researching the world’s oceans!

Goodbye HICO, Hello PACE – Ocean Colour’s Satellite Symmetry

HICO™ Data, image of Hong Kong from the Oregon State University HICO Sample Image Gallery, provided by the Naval Research Laboratory

HICO™ Data, image of Hong Kong from the Oregon State University HICO Sample Image Gallery, provided by the Naval Research Laboratory

Ocean colour is the acorn from which Pixalytics eventually grew, and so we were delighted to see last week’s NASA announcement that one of their next generation ocean colour satellites is now more secure with a scheduled launched for 2022.

Unsurprisingly the term ocean colour refers to the study of the colour of the ocean, although in reality it’s a name that includes a suite of different products, with the central one for the open oceans being the concentration of phytoplankton. Ocean colour is determined by the how much of the sun’s energy the ocean scatters and absorbs, which in turn is dependent on the water itself alongside substances within the water that include phytoplankton and suspended sediments together with dissolves substances and chemicals. Phytoplankton can be used a barometer of the health of the oceans; in that phytoplankton are found where nutrient levels are high and oceans with low nutrients have little phytoplankton. Sam’s PhD involved the measurement of suspended sediment coming out of the Humber estuary back in 1995, and it’s remained an active field of her research for the last 20 years.

Satellite ocean colour remote sensing began with the launch of NASA’s Coastal Zone Colour Scanner (CZCS) on the 24th October 1978. It had six spectral bands, four of which were devoted to ocean colour, and a spatial resolution of around 800m. Despite only having an anticipated lifespan of one year, it operated until the 22nd June 1986 and has been used as a key dataset ever since. Sadly, CZCS’s demise marked the start of a decade gap in NASA’s ocean colour data archive.

Although there were some intermediate ocean colour missions, it was the launch of the Sea-viewing Wide Field-of-view (SeaWiFS) satellite that brought the next significant archive of ocean colour data. SeaWiFS had 8 spectral bands optimized for ocean colour and operated at a 1 km spatial resolution. One of Sam’s first jobs was developing a SeaWiFS data processor, and the satellite collected data until the end of its mission in December 2010.

Currently, global ocean colour data primarily comes from either NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the twin Aqua and Terra satellites, or the Visible Infrared Imaging Radiometer Suite (VIIRS) which is on a joint NOAA / NASA satellite called Suomi NPP. MODIS has 36 spectral bands and spatial resolution ranging from 250 to 1000 m; whilst VIIRS has twenty two spectral bands and a resolution of 375 to 750 m.

Until recently, there was also the ONR / NRL / NASA Hyperspectral Imager for the Coastal Ocean (HICO) mission on-board the International Space Station. It collected selected coastal region data with a spectral resolution range of 380 to 960nm and 90m spatial resolution. It was designed to collect only one scene per orbit and has acquired over 10,000 such scenes since its launch. However, unfortunately it suffered during a solar storm in September 2014. Its retirement was officially announced a few days ago with the confirmation that it wasn’t possible to repair the damage.

In the same week we wave goodbye to HICO, NASA announced the 2022 launch of the Pre-Aerosol and ocean Ecosystem (PACE) mission in a form of ocean colour symmetry. PACE is part of the next generation of ocean colour satellites, and it’s intended to have an ocean ecosystem spectrometer/radiometer called built by NASA’s Goddard Space Flight Centre and will measure spectral wavebands from ultraviolet to near infrared. It will also have an aerosol/cloud polarimeter to help improve our understanding of the flow, and role, of aerosols in the environment.

PACE will be preceded by several other missions with an ocean colour focus including the European Sentinel-3 mission within the next year; it will have an Ocean and Land Colour Instrument with 21 spectral bands and 300 m spatial resolution, and will be building on Envisat’s Medium Resolution Imaging Spectrometer (MERIS) instrument. Sentinel-3 will also carry a Sea and Land Surface Temperature Radiometer and a polarimeter for mapping aerosols and clouds. It should help to significantly improve the quality of the ocean colour data by supporting the improvement of atmospheric correction.

Knowledge the global phytoplankton biomass is critical to understanding the health of the oceans, which in turn impacts on the planet’s carbon cycle and in turn affects the evolution of our planet’s climate. A continuous ocean colour time series data is critical to this, and so we are already looking forward to the data from Sentinel-3 and PACE.