Twin satellites in orbit

The launch of Sentinel-3A (S3A) was in early 2016, and the launch of its twin satellite - Sentinel-3B - is now planned for 25 April 2018. Both have the same optical and electronic design, and are built based on the experience from ESA’s MERIS sensor that orbited space for a decade (2002-2012). The new satellite S3B will orbit around the Earth in tandem with its twin satellite S3A with a delay of 30 seconds. The instrument was shipped this week to Plesetsk, Russia, where it will be prepared to be launched into Space.

The satellite will be launched the 25th of April. Photo: From the video "The Science of Ocean Colour" a film by Roland Doerffer.

Algal blooms strive in sun light

The satellites’ instruments will provide information about sea surface temperature and water quality and are especially designed for monitoring coastal zones.

-  The satellite will also provide long-term data to look at changes in algal bloom development and distribution, says Susanne Kratzer.

Susanne Kratzer.

It is important to keep track of algal blooms since they can have toxins that can lead to fish kills.  Algal blooms consist mostly of phytoplankton and cyanobacteria in the Baltic Sea. They are stimulated by nutrients from land.  The spring blooms is triggered by increase in sun light, and in summer cyanobacteria blooms are stimulated by warm water temperatures. Filamentous cyanobacteria can fix nitrogen and are therefore quite dominant  in summer.  Chlorophyll is usually used as an indicator for phytoplankton biomass and it can be measured from space.

Correcting for gasses in the atmosphere

The method that ocean colour satellites use is called “optical remote sensing” and it can be used to derive chlorophyll and other biogeochemical variables - such a coloured dissolved organic matter and suspended material - from space.

One challenge in optical remote sensing of the sea is that gasses and aerosols in the atmosphere affect the signal that the satellite sensor measures. Therefore, algorithms are used to process the satellite data, correcting for different atmospheric affects. Only after that the water quality parameters can be retrieved correctly.

Algal blooms on the coast of Gotland, seen from space. Algal blooms contain chorophyll, a green pigment crucial for photosynthesis, allowing plants to absorb energy from light. Photo Credit: USGS/NASA/Landsat 7

- Our group is mostly funded by the Swedish National Space Board and we deliver our validation results to The European Space Agency (ESA). We do not only work with validation of the data, but also with algorithm development. We develop Baltic Sea-specific algorithms in order to improve the monitoring of both coastal and open sea areas, says Susanne Kratzer.

If you want to read more about the satellite Sentinel-3B and what it can achieve, discover it here. If you want to know more about the optical remote sensing, watch the video: "The Science of Ocean Colour" - a film by Roland Doerffer.