“True colour images based on satellite data show astonishing details, ranging from algal rafts and ships to waves and foam on waves breaking in storms, as well as the structure of ice and passages formed therein by vessels traveling in the sea,” says SYKE researcher Sakari Väkevä.
Satellite remote sensing is based on the measurement of sunlight reflected from land or water, or the thermal radiation emitted by the Earth. The orbits of satellites, 500 – 1000 kilometres above the surface of the Earth, have been synchronised with the movements of the sun in such a way that they all pass over the areas that they are imaging in all places at the same time of the day, usually about noon.
The most important satellites used by SYKE are the Sentinels (Sentinel-2A/B, Sentinel-3A/B) that are part of the Copernicus programme of the European Union and the European Space Agency (ESA), and the Landsat-8 of the United States Geological Survey (USGS). In addition, NASA's Modis Terra satellite is used for monitoring the ice cover on lakes and the fractional snow cover on land.
Daily images from the Baltic Sea area
“The satellites with the best terrain resolution are Sentinel-2A and 2B, whose pixel (the smallest image unit) in the terrain is ten metres wide in the visible light spectrum. These satellites overfly the same geographic location in Finland every 2–3 days and with each pass, they can observe a strip of the Earth's surface about 300 kilometres wide. Operating together, satellites of the Sentinel-2 series cover about 30–60 % of the Baltic Sea area each day. Observations by Sentinel-2 on water quality are published in a terrain resolution of 60 metres,” explains SYKE IT Specialist Mikko Kervinen.
Sentinel-3A and 3B are known as medium resolution satellites, as their pixel width in the terrain is 300 metres. On one overpass, they can cover a strip of the Earth's surface more than 1,200 kilometres wide, and as the swaths partly overlap, they can get pictures of the entire Baltic Sea area every day. In addition to an optical camera, the Sentinel-3 satellites carry a device that measures thermal radiation emitted by the Earth and is used by SYKE to calculate the surface temperature of lakes and the Baltic Sea.
Satellite images are processed in Sodankylä
The satellite images are processed at the Sodankylä National Satellite Data Centre in northern Finland, whose main Finnish users are SYKE and the Finnish Meteorological Institute. The Centre is part of the Copernicus Sentinel Collaborative Ground Segment, which makes the first observed data available usually a few hours after the overpass of a satellite. The centre uses the Calvalus computer cluster for the processing of large volumes of satellite material. In the early hours of the morning the system uses observational data from satellites to derive observables describing water quality, such as turbidity, Secchi disk transparency, chlorophyll concentration, and humus absorption.
Operators alternate weekly. They are responsible for the handling of the newest pictures each morning. Observations that are of interest from the point of view of the algae situation are passed on to officials responsible for algae issues at SYKE and the ELY Centres. Clouds are removed from sea surface temperature (SST) observations, and efforts are made to publish the SST information each morning before 10:00 o’clock. After the exclusion of observations made under cloudy conditions, the most representative observations of turbidity, humus, and surface algae are published.
The material is usually delivered through the SYKE TARKKA service on the workday after the observation was made and it is also available through SYKE open data interfaces from where they can be inserted as a layer to GIS softwares, for example. The processing chain is long, and both planned and unplanned interruptions or delays can occur. In such situations, the latest satellite observations do not always reach TARKKA as scheduled, and there might be a delay of a day or two.
The TARKKA service also shows true colour imagery taken by all satellites, with colours that mimic those that can be seen by the human eye. True colour images are freely available to the public and the media on the condition that SYKE and the original supplier of the data are mentioned as sources. SYKE's remote sensing services help in the interpretation of true colour images.
Blue-green algae monitoring to continue into the autumn
SYKE issues weekly reports on blue-green algae from June to August, and if necessary, separate bulletins are issued outside the summer period. As it is not possible to make direct interpretations of the blue-green algae content of water based on true colour images shown in the TARKKA service, SYKE has developed a method for interpreting surface algal blooms based on the strength of the reflection in some of the satellites’ wavelength ranges. The reflection in areas with surface algal blooms is stronger than in areas with no algae.
Maps on the blue-green algae situation in the open sea and in coastal waters are compiled daily. The presence of algae is quantified on a four-level ordinal scale (No algae, Potential algae, Likely algae, Evident algae). The interpretation map represents the likelihood that surface algae will appear in a certain area. At the end of the blue-green algae season, a seasonal aggregate of the maps will be released to facilitate comparisons of the appearance of blue-green algae between different years.
A summary of algae rafts on the surface is made weekly, combining satellite pictures, observations by Finnish border officials’ patrol flights, and observations by members of the public and the ELY Centres. This information is delivered every Thursday to the Finnish Meteorological Institute, which makes a forecast on the movements of surface rafts and their possible drifts to the Finnish coast. The forecast is made using HELCOM's Seatrack Web drift calculation system. Even if no algae have risen to the surface, drift forecasts are updated constantly, because the readiness to make forecasts must be preserved in case of oil spills, for example. Drift forecasts can be made daily during exceptional blooms of blue-green algae.
In addition to information in the form of images, satellite observations are also used for compiling statistical information, for example regional time series, mainly used by experts. Some of the time series observations are also available at itämeri.fi, the Baltic Sea portal.
Further information
- TARKKA service
- SYKE remote sensing services on Twitter
- Information on remote sensing
- Researcher Sakari Väkevä, Finnish Environment Institute SYKE tel. +358 295 252 088, firstname.lastname@ymparisto.fi
- IT Specialist Mikko Kervinen, SYKE tel. +358 295 251 275, firsname.lastname@ymparisto.fi
- SYKE remote sensing services eotuki.syke@ymparisto.fi