Gerhard Zimmermann

Imaging Spaceborne and Airborne Sensor Systems in the Beginning of the Next Century.

The beginning of the next millennium promises an explosion in the quantity and quality of global data available from imaging remote sensing systems. The scientific and commercial communities become aware of unique hyperspectral imaging data acquisition opportunities. A brief profile of over 80 high resolution spaceborne and airborne earth observation sensor systems (H less than 800 km) planned to be operating in the year 2000 and beyond are presented in this paper. This overview covers multi- and hyperspectral civil, land and ocean nadir viewing observation sensors in the spectral range from the ultraviolet to the thermic infrared. A summary of the performance of each system, from image parameters (spectral and ground resolution) to the image generating procedure (spectral selection mode, image acquisition mode) is presented. At this point some caution is due since not all these concepts and plans will come to pass. The cuts in the government budget and the containment of commercial plans for new sensor systems will affect the realization of the present plans. However, the year 2000 will see at least four large area vegetation and ocean mappers, three landsat-like systems and two commercial high resolution systems in polar orbit simultaneously. A fleet of over 40 airborne sensor systems gives the final polished form of the future data acquisition opportunities.

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.

MOS/PRIRODA - An Imaging VIS/NIR-Spectrometer for Ocean Remote Sensing

The Modular Optoelectronic Scanner MOS is a spaceborne imaging spectrometer in the VIS/NIR range of optical spectrum. It was especially designed for remote sensing of the atmosphere-ocean system providing 17 channels at high radiometric resolution and high absolute calibration accuracy. It will be launched to the Russian space station MIR on board of the PRIRODA remote sensing module in the mid of 1994. The paper presents the sensor concept of an atmospheric and a biospheric spectrometer blocks and the scientific goals of the German participation within PRIRODA as well as main aspects of the entire PRIRODA mission.

Spaceborne spectrometer calibration with LEDs

Until now incandescent lamp, sun and moon calibrations have been successfully applied for in-flight calibration of spaceborne Earth observation imaging sensors. The performance development of LEDs in the past decade guided to higher luminous efficiencies, broader spectral coverage, lower degradation of light output over time and lower power consumption. These advantages make LEDs to a candidate for radiometric and spectral calibration of spaceborne spectrometers. For analysing LEDs for space in-flight calibration a set of LEDs has been characterised and a simulation of space radiation quantities (i.e. proton and electron radiation for a polar low-Earth orbit) has been carried out. Additional vacuum tests (outgassing behaviour) demonstrated a possible application of LEDs with epoxy housing for the future space environment. Further on, a concept for long-term temperature stabilisation has been developed for solving the main problem of LED in-flight calibration, i.e. the temperature dependency of the irradiance. Consequently, this study demonstrates that (1) a degradation of LEDs due to space environment is not expected, that (2) long-term temperature stability of LEDs can be ensured, and that (3) the higher blue part of ‘white’ LEDs would best suit ocean-colour scientists needs.

Capabilities for the retrieval of coastal water constituents (case II) using multispectral satellite data

The difficulty of the remote sensing of coastal water is the presence of more than one constituent with high variability ranges, different correlation and spectral behavior. They are superimposing in their influence on the resulting total spectrum. Simple ratio algorithms applied to remote sensing data fail on the quantitative determination of the single constituents. However, coastal regions are of great interest for remote sensing since most of the consequences of urbanization are manifested here. For the improvement of remote sensing of coastal zones it is not only necessary to build a new generation of sensors that offer spectrally higher resolved data, but one has to develop a new methodology that allows the separation and determination of the water constituents based on the entire spectral signature of the different components of the water body. The imaging spectrometer MOS flying on the Indian remote sensing satellite IRS-P3 provides since March 1996 remote sensing data in 13 spectral channels for the scientific community. We implemented a new methodological approach to derive different case II water constituents as well as atmospheric turbidity for the application of MOS-data in costal regions. A new point of the method is the uniform consideration of atmospheric and water constituent influences on the remote sensing signal. The paper will present a short overview on the algorithms essentials and examples for the large variability of coastal waters around Europe basing on the results of the retrieved water constituents using the MOS algorithm. It will demonstrate the promising potential of this new algorithm for discrimination of single constituents under case II conditions. Derived maps of chlorophyll like pigments, sediments and aerosol optical thickness are shown and will be discussed.

Calibration of the Modular Optoelectronic Scanner (MOS) flight models

Since March 1996 the Modular Optoelectronic Scanner (MOS) provides remote data from a 820 km sun synchroneous polar orbit. It measures the spectral radiance of the atmosphere- surface system in 18 spectral channels and up to 420 pixels in a 200 km swath. MOS consists of two imaging spectrometers A and B with gratings and a camera C with an interference filter. MOS-AA has 4 channels with a spectral halfwidth (Delta) (lambda) approximately equals 1.4 nm in the absorption band of atmospheric oxygen near 760 nm, MOS-B has 13 channels between 400 and 1010 nm with (Delta) (lambda) approximately equals 10 nm and the MOS-C channel is at 1.6 micrometers with (Delta) (lambda) approximately equals 100 nm. Beside the on ground laboratory calibration as the basis of calculating the spectral radiance of the earth objects, the long time mission requires a periodic recalibration or at least a stability check of instrument properties in orbit to support the reliability of the remote data. Internal lamps and the extraterrestric sun radiation provide actual data sets to derive corrections on remote data if any changes in the performance data arises.

Four years of ocean colour remote sensing with MOS-IRS

The imaging spectrometer MOS on IRS-P3 was launched in March 1996 as the first example of a new generation of ocean colour sensors. It consists of three different spectrometers in the visible/near-infrared spectral region with 18 channels. The IRS-P3 mission is focused on the remote sensing of case 2 water, particularly the derivation of different water constituents in coastal waters. Due to the more complex spectral behaviour of case 2 water, a new methodological approach was developed which works directly with satellite measured top-of-atmosphere radiance and accounts for the correlation of the different water constituents as well as for the spectral shape. This paper gives an overview of the mission, the scientific goals and the development and improvement of the retrieval algorithms. The potential of the algorithm is demonstrated and examples of selected European coasts are shown. Derived maps of water constituents are presented.

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.