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!

New Horizons for Remote Sensing

This image of Pluto from New Horizons’ Long Range Reconnaissance Imager (LORRI) was received on July 13, and has been combined with lower-resolution colour information from the Ralph instrument. Credits: NASA-JHUAPL-SWRI.

This image of Pluto from New Horizons’ Long Range Reconnaissance Imager (LORRI) was received on July 13, and has been combined with lower-resolution colour information from the Ralph instrument. Credits: NASA-JHUAPL-SWRI.

You can’t have failed to have seen the amazing images of Pluto taken by the New Horizon spacecraft over the last week. What you may not have thought about, is that these images are taken using remote sensing technology.

Remote sensing, particularly when referred to as Earth observation, is thought of as a scientific field focussed on looking at our planet – we often use this in our own marketing! However, the simplest definition of remote sensing is being able to know what an object is without being in physical contact with it (inspired by Sabins 1978). Although the Earth is the most obvious example, European Space Agency’s Rosetta mission to comet 67P highlighted, remote sensing can go extra-terrestrial, or in this case interplanetary!

New Horizons has seven scientific instruments: three optical, two plasma, a dust sensor and a radio science receiver/radiometer. The three optical instruments are:

  • Long Range Reconnaissance Imager (LORRI) that’s a panchromatic high magnification imager, which at its closest approach will have a pixel size of approximately 50 m.
  • Ralph is a visible and infrared imager and spectrometer that has three panchromatic and four colour imagers within its Multispectral Visible Imaging Camera, which takes images twice a day; it has a pixel size of around 250 m. In addition, Ralph has a Linear Ealon Imaging Spectral Array (LEISA) that’s an infrared spectrometer with 1.25 – 2.50 micron wavelengths, which will produce thermal maps of Pluto.
  • Alice is an ultraviolet imaging spectrometer with 1 024 spectral channels at 32 spatial locations along its rectangular field of view. It analyses the composition and structure of Pluto’s atmosphere, by measuring either ultraviolet emissions or absorption of sunlight by the atmosphere. A basic version of Alice is also onboard Rosetta.

The image at the top of the blog, which was first shown around the world last week, was taken in black and white by LORRI, but has been combined with lower resolution colour information from the Ralph instrument – a technique call image fusion.

The remaining scientific instruments are:

  • REX (Radio Science EXperiment): measures atmospheric temperature and pressure.
  • SWAP (Solar Wind Around Pluto): studying Pluto’s interaction with solar winds.
  • PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation): a directional energetic particle spectrometer, measuring the density, composition, and nature of particles escaping Pluto’s atmosphere.
  • SDC (Student Dust Counter): built and operated by students of the University of Colorado, it measures the concentration of space dust in the solar system.

The pictures produced by the instruments on New Horizons are fantastic. However, with extra-terrestrial remote sensing the travel time involved is significant; New Horizons was launched on the 19 January 2006 and Rosetta on the 2 March 2004. This means the technology onboard is a decade old, although they were cutting edge instruments at launch and so the lag is probably not ten years, but it is behind what we can do now. For example, QuickBird-2 has a pixel size of 0.61 – 0.72 m. However, improved spatial resolution is not only dependent on the technology. The high speed flyby of Pluto meant sensors only had a short amount of time to take the images, and so the focus needed to be the whole planet, or specific areas, in high resolution detail. There is also the problem of bandwidth, and the difficulty in transferring large amounts of data back to Earth.

Remote sensing is a fast moving field of science, technological advances and innovative ideas for data that mean exciting discoveries happen regularly. Last week’s UK Space Conference gave an insight into what’s happening next. Make sure you’re onboard!

 

Pixalytics blog with contributions from Davydh Tretheway.

What do colours mean in satellite imagery?

False colour image of phytoplankton blooming off the coast of Patagonia. Acquired 2nd Dec 2014. Image Courtesy of NASA/NASA's Earth Observatory

Phytoplankton blooming off the coast of Patagonia on 2nd Dec 2014.
Image Courtesy of NASA/NASA’s Earth Observatory

Satellite images are a kaleidoscope of colours, all vying for attention. It’s important to be clear what the colours are showing, and more importantly, what they may not be showing, to interpret the image correctly. For example, a patch of white on an image might indicate snow or ice, sunglint off the ocean, fog or it could just mean it was cloudy.

On the earth’s surface different colours represent different land types:

  • Vegetation appears as shades of green from pale for grasslands to dark for forests – although some forests will progress from green to orange to brown in autumn.
  • Ocean colour is significantly influenced by phytoplankton, which can produce a range of blue and green colours. A fantastic example of this can be seen in the image at the top of the blog showing phytoplankton blooming off the cost of Patagonia.
  • Snow and ice can appear white, grey, or slightly blue.

As noted in the opening, colours can also mislead with cloud cover being the natural nemesis of optical remote sensing. However, you also have to be careful with effects such as:

  • Smoke: ranges from brown to grey to black.
  • Haze: a pale grey or a dirty white.
  • Dust: can be brown, like bare ground, but also white, red and black.
  • Shadow: Clouds or mountain shadows can look like dark surface features.

There is a good article here from NASA’s Earth Observatory giving more details on the different colours of surface land types. So far, we’ve focussed on natural colour signatures; but man-made structures also appear on imagery. Generally, urban areas tend to be silver or grey in colour; although larger objects also show up in their own right such as the bright red roof of Ferrari World in the middle of the Abu Dhabi Grand Prix Circuit – as discussed in a previous blog.

Composite Google Earth image of the entrance to the Panama Canal: Data courtesy of DigitalGlobe

Composite Google Earth image of the entrance to the Panama Canal: Data courtesy of DigitalGlobe

We tried to repeat the identification of man-made objects for this blog using the coloured roofs of the Biomuseo building, located on the Amador Causeway – at the entrance to the Panama Canal in the Pacific Ocean. Sadly, Landsat 8 pixels are too coarse; and Google Earth has fallen prey to cloud cover preventing visibility, as shown in the image on the right. What you can see though is the buildings in Panama City and the yachts in the marinas and clustered around the four islands (Naos, Perico, Culebra and Flamenco) at the end of the Amador Causeway.

The final thing to remember when considering colours, is the format of the image itself. Some images use true-colours from the red, green and blue wavelengths, which produce colours as if you were looking at the scene directly, so trees are green, sea is blue, etc. However, other images incorporate infrared light to enhance the detection of features not easily distinguished on a true-colour image; this means colours aren’t what you would expect, for example, the ocean may appear red.

Colour is central to use of satellite imagery, but you need to know the properties of the rainbow you are looking at or you may never find the pot of satellite gold.