Thomas Walzel

Investigation of interpretation possibilities of spectral high-dimensional measurements by means of principal component analysis: a concept for physical interpretation of those measurements

Subject of the paper is the investigation of the information content of high dimensional multispectral remote-sensing measurements in the VIS-NIR region for ocean-atmosphere problems. The final goal of such measurements is the separation of atmospheric influence and the retrieval of detailed information of water constituents. Primary questions appearing during interpretation process are: how many independent parameters can be found from the measurements? what is the physical sense of these parameters? what is the accuracy of the parameters? One has to take into account that the properties of the measuring device, like channel position, bandwidth, number of channels and measurement accuracy have a great influence on the interpretation. One possible method to get answers to the above questions is the Principle Component Analysis (PCA). A problem in PCA is the physical interpretation of the mathematically obtained results - Eigenvalue, Eigenvector and Principle Components. Because the results of PCA interpretation depend on the statistical properties of the measurement data, they must be mapped back to the absolute measurement quantities (radiances). To get a physical interpretation of the PCA results a detailed investigation with a simulated data set using a simplified (but nonlinear) model was realized (atmosphere after Gordon, Sturm, water reflectances after Sathyendranath, Morel, Prieur). It will be presented a concept, how in-situ measurements can be involved into interpretation model with PCA.

The processing chain and Cal/Val operations of the future hyperspectral satellite mission EnMAP

The German Aerospace Center DLR - namely the Applied Remote Sensing Cluster CAF and the German Space Operations Center GSOC - is responsible for the establishment of the ground segment of the future German hyperspectral satellite mission EnMAP (Environmental Mapping and Analysis Program). The Applied Remote Sensing Cluster has long lasting experiences with air- and spaceborne acquisition, processing, and analysis of hyperspectral image data. This paper mainly addresses the concept of the operational and automatic processing chain and the calibration/data quality to generate high quality data products.

Regional products for the Baltic Sea using MERIS data

For many questions of the coastal zone management the knowledge on the biological and ecological state of coastal waters is of high importance. Due to the complexity of this water type, characterized by different classes of water constituents, a sophisticated methodology needs to be applied for quantitative remote sensing. Within the MAPP‐Project (MERIS Applications and Regional Products Project) a specific remote sensing interpretation algorithm was developed for the regional assessment of water constituents in the Baltic Sea. This is a part of the ESA Cat‐1 proposal ID 1413 GEMEL‐3 (“Generation of MERIS Level‐3 products for European multidisciplinary regional applications”). The operational implementation in a MERIS‐value‐adding system allows a near real‐time estimation of chlorophyll sediment and gelbstoff concentrations on a regular basis. The algorithm, based on a principal component inversion (PCI) technique and the model will be introduced. The applicability of the model will be demonstrated on a couple of comparisons of MERIS reflectances and in‐situ measurements. The potential of the model shall be demonstrated on available MERIS data.

Application of a multispectral interpretation algorithm to remote sensing data over the Baltic Sea

In the Institute for Space Sensor Technology a new generation of remote sensing imaging spectrometers was developed, measuring the reflected from the ocean atmosphere system radiance in the visible to near-infrared spectral range. This Modular Optical Scanner was successfully launched on 21 March 1996 with an Indian satellite to a polar sunsynchronous orbit, and on 23 April 1996 with the Russian Priroda Module on the MIR station. For the purpose of interpretation of these measurements over oceans and coastal zones has been developed a special algorithm based on Principal Component Analysis, using a special inversion technique for a given ocean-atmosphere physical mode. An important question in the description of such models are the inherent optical properties of the water. In the paper will be given a description of the derivation of the interpretation algorithm for different water constituents, with an inherent atmospheric correction. It will be shown how specific optical properties are influencing the interpretation results. This work was performed in cooperation with the Baltic Sea Research Institute Warnemuende.

Resume of seven-year MOS in-orbit calibration: events, effects, and explanations

The in-orbit calibration of the Modular Optoelectronic Scanner MOS on the Indian Remote Sensing Satellite IRS-P3 has delivered the actual radiometric recalibration coefficients with sufficient accuracy for most of the 18 spectral channels in the VIS/NIR spectral range during the 7 years mission time. This has been the basis for the thematic interpretation of the MOS data. The three different and independent in-orbit calibration methods: lamp calibration, sun calibration and ground target based (vicarious) calibration as well as different possibilities of dark signal determination and the extensive knowledge of instrument performance data and instrument characteristics from the lab measurements have enabled us to overcome all failures and difficulties of the instrument which occurred in orbit. The failure of the lamp and sun calibration equipment in September 2000 has been overcome by using the vicarious calibration and dark signal measurements at the earth night side at new moon. The failure of the thermo-electric cooling of the detectors in November 2002 could be overcome only by the knowledge of the temperature dependence of the spectral responsivity of the different spectral channels and its dark signals. Thus we are able to continue the determination of the time trend of the recalibration coefficients in spite of these problems. In the paper we will give a resume of the most important events concerning the in-orbit calibration during the mission time, try to find explanations for some effects and present the results of determining the recalibration coefficients and the accuracy reached under the concrete environmental and instrumental conditions in orbit.

Potentials of combined in-orbit calibration methods demonstrated by the MOS-IRS mission

In-orbit calibration is an absolutely necessary and accepted tool to update the pre-flight calibration sets of remote sensing instruments on satellites. Only such a periodical recalibration guarantees the long term quality and accuracy of the data and the reliability of the thematic interpretation. Especially for watching global changes of the ocean coastal zones (phytoplancton, sediments, pollution etc.) using spectroradiometric measurements in the VIS/NIR spectral range we need high radiometric accuracy because of small and often only slightly different signals. The Modular Optoelectronic Scanner (MOS) on the Indian Remote Sensing Satellite IRS-P3 has been showing its capacity in this field for more than 4 years. One reason for this success is the sophisticated in-flight calibration using different methods, first an internal parameter check with lamps and second the absolute recalibration with the sun via spectralon diffusers. These two methods together allow the radiometric recalibration with an uncertainty of ± 0.5% with respect to the initial state and enables us in many cases to recognize which opto-electronical component is responsible for which kind of change in different spectral channels and spatial pixels. Radiation stress, satellite and orbit environment, degradation and surface cleaning effects in vacuum are some items which affect the opto-electronical components in different ways. Comparative investigations of some MOS optical components by experimental simulation of the radiation environment in the 820 km IRS sun synchronous orbit for 1, 2, 3 and 10 years radiation load confirm the in-orbit calibration results. The BRDF of the spectralon sun diffuser, the reflectance of anodized aluminum surfaces and the transmission of the front end quartz window did not change by the radiation stress but the transmittance of the front end optics decreases in the blue spectral region up to 10%. These results will be presented together with the in-orbit calibration results of MOS.

Comparison of Vicarious and In-Orbit Calibrations of the Imaging Spectrometer MOS

DLRs Modular Optoelectronic Scanner MOS on the Indian Remote Sensing Satellite IRS-P3 has been working now for almost 5 years in orbit. In September 2000 the power supply for driving the internal lamps and the sun calibration equipment failed so that we no longer have actual in-orbit calculation values. However the spectrometers themselves are still working and nadir remote data collection is running. To remedy this situation we have tried to use vicarious calibrations over the Sahara desert. The Great Eastern Erg near the border between Tunisia and Algeria has been selected for this purpose. Because we do not have any ground truth measurements from this area, we have investigated the correlation between the in-orbit sun and internal lamp calibration data and the upwelling radiance data of this area in the VIS/NIR-channels of MOS from May 1996 to August 2000. The vicarious calibration data were corrected with respect to actual sun irradiance only, but not to atmospheric conditions. Nevertheless there is a remarkably high correlation between the in-orbit calculations and these vicarious calculations. This enables us to continue to generate calibration data sets for MOS only by using actual vicarious calibration data in place of the in-orbit calibration data. Additionally the vicarious calibrations can be compared with the extrapolated results of the time trend of the radiometric sensitivity of all spectral channels which we found from the previous in-orbit calibrations. The results of all these investigations are presented in this paper.

Three years of successful in-orbit calibration of the modular optoelectronic scanner (MOS) on the Indian IRS-P3 mission

The Modular Optoelectronic Scanner MOS of the German Aerospace Center (DLR) has now been working about three years on board the INdian Remote Sensing Satellite IRS-P3 very successfully. It consists of three instruments: the spectrometer MOS-A, the spectrometer MOS-B and the camera MOS-C. To meet the sophisticated radiometric and spectral requirements especially for ocean purposes, a 16 bit dynamic range and a qualified in-flight calibration concept including sun calibration and internal lamp calibration have been established. All three instruments have shown a remarkably high data stability and quality during the three years mission time. The highest changes of radiometric sensitivity of +7 percent were found in the SWIR- channels of MOS-B and the lowest changes of -1 percent in the MOS-A channels. Spectral shifts of center wavelengths could not be found. Small differences between the result of the two calibration methods are due to the fact that they do not cover the same optimal components exactly. But this enables us to allocate the sensitivity changes to those components of the instruments which cause them. The measured in-orbit calibration values were used to update the calibration coefficients.

Experience and results of long-term in-orbit sun calibration of the Modular Optoelectronic Scanner (MOS) on the Indian IRS-P3 mission

The Modular Optoelectronic Scanner MOS was developed at the Institute of Space Sensor Technology/Berlin of the German Aerospace Center (DLR) and specially designed for observations of medium scale effects of the system surface-atmosphere. MOS consists of the two VIS/NIR imaging spectrometers MOS-A and MOS-B and the SWIR camera MOS-C. It was launched on March 21, 1996 on board the Indian Remote Sensing Satellite IRS-P3 together with the Indian Wide Field Scanner WIFS and an X-ray instrument. Two different in-orbit calibration devices are integrated into the MOS equipment: (1) the internal calibration system based on two minilamps and (2) the sun calibration based on spectralon diffusers for absolute radiometric recalibration and long-term stability check of the sensitivity. Thus it is possible to determine the actual relative calibration data with an accuracy of about 0.5%. The interpretation of the calibration data of the MOS-IRS mission in orbit for two years shows that all detector elements really are working normally. The behavior of the sensitivity of all elements of a CCD-line is nearly identical. Altogether, the sensitivity of the MOS-A channels remains constant in an interval of plus or minus 0.7%, increases by different amounts for the MOS-B channels up to 6% and decreases for MOS-C about 1%. The results of the in-orbit calibrations are the basis for a consistent interpretation of the remote sensing measurements of the environment.

Algorithm development for the retrieval of coastal water constituents from satellite Modular Optoelectronic Scanner images

DLRs imaging spectrometer the Modular Optoelectronic Scanner (MOS) on the Indian remote sensing satellite IRS-P3 has been orbiting since March 1996. MOS consists of two spectrometers, one narrow band spectrometer around 760 nm for retrieval of atmospheric parameters and a second one in the IVS/NIR region with an additional line camera at 1,6 micrometers . The instrument was especially designed for the remote sensing of coastal zone water and the determination and distinction of its constituents. MOS was developed and manufactured at the Institute of Space Sensor Technology (ISST) and launched in a joint effort with the Indian Space Research Organization (ISRO). The high spectral resolution of MOS offers the possibility of using the differences in spectral signatures of remote sensing objects for quantitative determination of geophysical parameters. In ISST a linear estimator to derive water constituents and aerosol optical thickness has been developed, exploiting Principal Component Inversion (PCI) of modeled top-of- atmosphere and experimental radiance data sets. The estimator results in sets of weighting coefficients for each measurement band, depending on the geophysical situations. Because of systematic misinterpretation due to non- adequateness of model and real situation the further development implies the parallel improvement of used water models and recalibration with in-situ data. The paper will present for selected test sites of the European coasts results of algorithm application. It will show the improvement of the estimated water constituents by using regional specific model parameter. Derived maps of chlorophyll like pigments, sediments and aerosol optical thickness ar presented.