R. Doerffer

ROSIS imaging spectrometer and its potential for ocean parameter measurements (airborne and space-borne)

Abstract The reflective optics system imaging spectrometer (ROSIS) is an advanced sensor particularly tailored to measure ocean parameters, including chlorophyll fluorescence. Further applications are described. Emphasis is placed on the wide range of applications for both airborne and space-borne instruments, including water biomass detection and water quality measurements, atmospheric applications as well as those over ice and land surfaces. It provides ≤5 nm spectral resolution at 128 selectable spectral bands, dispersed by a grating onto a matrix detector array and 32° field of view, which may be more than doubled when matching more than one optical module, especially for space-borne missions where a high repetition rate is required. A ROSIS airborne prototype is currently under development in a joint project of DLR, GKSS and MBB. This instrument will become operational in mid-1991. The ROSIS concept and design is outlined, as well as the planning status for extensions.

ROSIS (Reflective Optics System Imaging Spectrometer) - A Candidate Instrument For Polar Platform Missions

The ROSIS imaging spectrometer concept is based on all-reflective optics and matrix CCD detector arrays. The instrument concept was defined and detailed within a national study for chlorophyll measurements from space platforms, which was the design determining mission objective in terms of high spectral and radiometric and moderate spatial resolution. A great variety of further mission applications in the field of ocean, land and atmospheric remote sensing is foreseen. Meanwhile, a first airborne prototype version is under development for delivery in late 1988, funded commonly by DFVLR, GKSS and MBB, which will be used to verify the technical sensor concept of ROSIS and its full application spectrum. ROSIS is designed to cover a spectral range from 430 to 960 nm in resolution steps of 5 nm per channel; a set of up to 28 spectral channels can be read out simultaneously at full spatial resolution, the full spectrum at reduced resolution. The selection of channels can be pre-programmed per orbit or aircraft flight out of the available 106 channels. The radiometric resolution is defined as 0.05% of the apparent albedo. The total FOV covers ± 16° per optics module. In view of the ESA (and NASA) planning for Polar Platform Missions, ROSIS could represent a promising candidate of multi-purpose remote sensing instruments.

Remote sensing of water parameters in Madura Bay

Abstract During the Madura Bay Remote Sensing Experiment, which was conducted as part of the Snellius-II Expedition in August, 1984, space-borne, air-borne and ship-borne radiometric data were collected. They have been analysed in terms of chlorophyll, suspended matter, yellow substance and sea-surface temperature to map the distribution patterns and temporal variability of different water masses. A general correlation analysis between ship-borne radiance measurements and biochemical data (sea truth) indicates that the dominant factors which determine the reflectance spectra are the total amount of scattering and absorbing substances in the water, sun and sky glitter, and the chlorophyll and phaeophytin concentration. For a coastal area strongly influenced by river run-off, the chlorophyll concentration is comparatively low. As a result, the chlorophyll fluorescence is weak, but can still be detected. The blue/green colour ration and the fluorescence line height along the flight tracks derived from the Ocean Color Radiometer (OCR) over three consecutive days indicate at least three different types of water, viz. clear ocean water in the eastern part of the Bay, mixed water with moderate contents of chlorophyll and suspended matter in the middle and western parts of the Bay and finally estuarine and river water containing large amounts of inorganic and dissolved organic matter near the mouths of Solo and Brantas rivers and in the Strait itself. Distribution patterns change from one day to the next, possibly as a result of tidal effects. Vertical radiation profiles derived from aircraft flights at different altitudes and from modelling the radiative transfer through the atmosphere give an indication of how well ocean colour/chlorophyll fluorescence can be monitored through a typical tropical atmosphere from satellite altitude. Model calculations and comparative measurements show that even the small fluorescence signals (corresponding to a low pigment concentration) can still be detected above the atmosphere, while colour ratios, especially in the blue part of the spectrum, are heavily masked by aerosol and Raleigh scattering and have to be corrected carefully prior to any interpretation. At all wavelengths, upwelling spectra contain a considerable amount of light specularly reflected at the sea surface.

Future systems for global monitoring of ocean colour

Future earth observation satellites will carry advanced sensors incorporating multispectral scanners and imaging spectrometers. Due to their flexibility especially with regard to spectral channel selection, the latter types permit a variety of applications in regional or global monitoring of the marine and terrestrial biosphere. Planned and approved missions are discussed, such as e.g. NASA’s Earth Observation System, ESA’s Earth Observation Programme and other missions like FY-1 (People’s Rep. of China), PRIRODA (USSR), ADEOS (Japan), ATMOS (FRG) and GLOBESAT (France). A brief survey of relevant sensors like MOS, OCTS, POLDER, SeaWiFS, ROSIS, MODIS, MERIS and their performance is presented.