In the visible spectral range of solar radiation (~ 400 ... 700 nm) light can penetrate water bodies and its color can change due to scattering and absorption processes in the body of water or at its bed. This makes it possible to derive from optical remote sensing data information about the characteristics of the water body, the type and concentration of its various components, and the nature of the underlying ground. In order to do so, subtle “color” differences must be detected and quantified. This is accomplished by measuring the radiation in an appropriately large number of narrow spectral bands with the help of spectrometers. Whereas nonimaging sensors are employed on board ships or in the water itself, so-called imaging spectrometers are used on aircraft and satellites. The latter can also provide a spatial image for each spectral band. Remote sensing is the only technology which can be employed to monitor the high spatial and temporal dynamics of water bodies, because with buoys or ships it is in practice impossible to achieve adequately dense and at the same time wide-area coverage.
Bio-optical and radiation transport models
The measured spectrum, that is, the amount of radiation in the various bands, is determined by the scatter and absorption processes associated with the water molecules and the various substances in the water. The main group of water components which can be detected with remote sensing are phytoplankton (because of their various pigments), organic and anorganic suspended matter, and dissolved organic substances. The relationship between the concentrations of substances in the water and the color of the water can be determined with the help of bio-optical models, which can be tailored for different species, the season of the year, and the particular water body. Since the satellite sensor is located above the earth’s atmosphere, the influence of the atmosphere also has to be taken into account in these simulations, something which is accomplished by using radiation transport models. With these models, the measurement data expected for each situation or viewing geometry can be simulated and used to develop and optimize remote sensing algorithms.
The team works with various radiation transport models which can be customized for different sensors or the topics under investigation. The bio-optical models which are incorporated are either provided by project partners or derived from DLR bio-optical measurements recorded during special on-site measurement campaigns.
The signal received by the satellite sensor is influenced by the optical characteristics of the substances in the water and by the atmospheric parameters, which evolve simultaneously and independently of one another. Various approaches are taken to quantify the individual variables, with physical, model-based multivariate inversion having proven to be the most useful since it permits optimization of the inversion process for various criteria and relatively unproblematic adaption to different sensors and recording conditions. Various algorithms can be used for the mathematical implementation of the inversion process; neuronal nets or iterative spectral matching techniques are especially common, for example. The Principal Component Inversion method was developed by the team and is being used in modified form for various applications.
Besides “primary” parameters such as chlorophyll concentration or absorption and scatter coefficients, “secondary” characteristics like water transparency can also be derived from remote sensing data.
The developed algorithms are implemented in prototype processors and, depending on the specifications, incorporated in operational chains with the help of external partners or the German Remote Sensing Data Center DFD to provide information products in near-real-time.
Projects and applications
All these methodologies are being further developed together with oceanographic research institutions or public bodies and are in routine use. The MOS-IRS imaging spectrometer developed by DLR (and flown on the IRS-P3 satellite from 1996 to 2005) was, as the first instrument of its kind, primarily used for basic research and to demonstrate the feasibility of quantitative satellite remote sensing. It was followed by the imaging spectrometer MERIS on board the ESA environmental satellite ENVISAT, which marked the transition to routine provision of data for environmental monitoring by government authorities and to confirming adherence to European guidelines as part of the GMES programs (see the Web page on GMES services).
For some time now, the development of algorithms to detect and quantify harmful algal blooms, particularly cyanobacteria in the Baltic Sea, has been a concern of the team. The relevant investigations began as part of the EU HABILE project and are now being continued in the context of the EOS program and the EU AquaMAR project.
Besides radiation transport models and inversion methodologies, the team also focuses on developing and verifying bio-optical models and validating satellite measurements, and operates several spectrometers for the purpose. These can be used to measure the radiation field of the atmosphere, the spectrum returned by the water body, and the scalar and vector light field in the water body (down to a depth of 50 m). These instruments are operated on board research ships in cooperation with oceanographic institutes in Germany and abroad.