The facility Optical Information Systems (OS) at the Robotics and Mechatronics Center of the German Aerospace Center (DLR) has more than 30 years of experience with high-resolution imaging technology. This paper shows the scientific results of the institute of leading edge instruments and focal plane designs for EnMAP VIS/NIR spectrograph. EnMAP (Environmental Mapping and Analysis Program) is one of the selected proposals for the national German Space Program. The EnMAP project includes the technological design of the hyper spectral space borne instrument and the algorithms development of the classification. The EnMAP project is a joint response of German Earth observation research institutions, value-added resellers and the German space industry like Kayser-Threde GmbH (KT) and others to the increasing demand on information about the status of our environment. The Geo Forschungs Zentrum (GFZ) Potsdam is the Principal Investigator of EnMAP. DLR OS and KT were driving the technology of new detectors and the FPA design for this project, new manufacturing accuracy and on-chip processing capability in order to keep pace with the ambitious scientific and user requirements. In combination with the engineering research, the current generations of space borne sensor systems are focusing on VIS/NIR high spectral resolution to meet the requirements on earth and planetary observation systems. The combination of large swath and high spectral resolution with intelligent synchronization control, fast-readout ADC chains and new focal-plane concepts open the door to new remote-sensing and smart deep space instruments. The paper gives an overview over the detector verification program at DLR on FPA level, new control possibilities for sCMOS detectors in global shutter mode and key parameters like PRNU, DSNU, MTF, SNR, Linearity, Spectral Response, Quantum Efficiency, Flatness and Radiation Tolerance will be discussed in detail.
At the German Aerospace Center (DLR), within the department Optical Information Systems, investigations are
currently being performed on time delay and integration charge coupled devices, with respect to their applicability on
satellites for earth observing missions. This paper contains first results of dynamic measurements of point spread
function and modulation transfer function of a sensor with 9000 pixels and 64 time delay integration steps. The influence
of a mismatch between the line synchronisation frequency and satellite ground speed, as well as the effect of angle
misalignment between satellite flight direction and the orientation of the sensor itself onto point spread function, and
modulation transfer function was investigated. The performance of the test equipment will also be presented.
At the German Aerospace Center an airborne multi-wavelength differential absorption LIDAR for the measurement of
atmospheric water vapour is currently under development. This instrument will enable the retrieval of the complete
humidity profile from the surface up to the lowermost stratosphere with high vertical and horizontal resolution at a
systematic error below 5%. The LIDAR will work in the wavelength region around 935 nm at three different water
vapour absorption lines and one reference wavelength. A major sub-system of this instrument is a highly frequency
stabilized seed laser system for the optical parametrical oscillators which generate the narrowband high energy light
pulses. The development of the seed laser system includes the control software, the electronic control unit and the opto-mechanical
layout. The seed lasers are Peltier-cooled distributed feedback laser diodes with bandwidths of about
30 MHz, each one operating for 200 μs before switching to the next one. The required frequency stability is
± 30 MHz ≅ ± 10-4 nm under the rough environmental conditions aboard an aircraft. It is achieved by locking the laser
wavelength to a water vapour absorption line. The paper describes the opto-mechanical layout of the seed laser system,
the stabilization procedure and the results obtained with this equipment.
The paper presents the current status of the operational calibration facility that can be used for radiometric, spectral and geometric on-ground characterisation and calibration of imaging spectrometers. The European Space Agency (ESA) co-funded this establishment at DLR Oberpfaffenhofen within the framework of the hyper-spectral imaging spectrometer Airborne Prism Experiment (APEX). It was designed to fulfil the requirements for calibration of APEX, but can also be used for other imaging spectrometers. A description of the hardware set-up of the optical bench will be given. Signals from two sides can alternatively be sent to the hyper-spectral sensor under investigation. Frome one side the spatial calibration will be done by using an off-axis collimator and six slits of different width and orientation to measure the line spread function (LSF) in flight direction as well as across flight direction. From the other side the spectral calibration will be performed. A monochromator provides radiation in a range from 380 nm to 13 μm with a bandwidth between 0.1 nm in the visible and 5 nm in the thermal infrared. For the relative radiometric calibration a large integrating sphere of 1.65 m diameter and exit port size of 55 cm × 40 cm is used. The absolute radiometric calibration will be done using a small integrating sphere with 50 cm diameter that is regularly calibrated according to national standards. This paper describes the hardware components and their accuracy, and it presents the software interface for automation of the measurements.
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.
The success of the Modular Optoelectronic Scanner MOS on the Indian Remote Sensing Satellite IRS-P3 during the 6 years mission time has been based on its sophisticated in-orbit calibration concept to a large extent. When the internal lamp and the sun calibration failed in September 2000 we tested the possibility of ground target based (or vicarious) calibration of the MOS instruments to continue the high data quality. This is essential for future watching of global changes of the ocean coastal zones (phytoplancton, sediments, pollution, etc.) using spectral measurements of the VIS/NIR MOS spectral channels.
The investigations have shown the suitability of a part of the Great Eastern Erg in the Sahara desert for this purpose. The satellite crosses this very homogeneous area every 24 days. Because of the good cloudfree conditions we can use 6 - 8 overflys a year for calibration. The seasonal variability of the surface reflectance is very small so that we obtain relative calibration data of sufficient accuracy even without ground truth measurements for most of the channels.
The trend of this "vicarious" calibration corresponds very well with the previous trend of the failed lamp and sun calibration. Dfferences between the three methods will be discussed.
In the paper we will also present the results of a comparison between SeaWiFS and MOS data of comparable spectral channels from the Great Eastern Erg area. They confirm the suitability of this area for calibration purposes too.
DLR's 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.
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.
Long term orbit missions have the general problem of checking the instrument parameters in order to provide data with a high and equal reliability during the whole mission. Different methods of end-to-end or only partly recalibrations have been used in the past. The behavior of the first pushbroom imaging spectrometer in orbit, the Modular Optoelectronic Scanner MOS, has been observed during 3 years operation in orbit. Two procedures are used to check periodically some parameters: internal calibration with 2 minilamps in each module and external calibration with the sun irradiance onto a white diffuser in front of the entrance optic. The results show different developments of the parameters in the modules: increase of the dark current has not stopped up to now; the sensitivity of MOS-A channels remains constant in the frame of plus or minus 0.5%, while the MOS-B channels reach a constant level after a long period of increasing now. No wavelength shift could be observed. The PRNU of the CCD lines remains inside a plus or minus 1% interval. Spectral variations in the channel sensitivity of MOS-B can be observed and corrected with an uncertainty of plus or minus 0.5%. In general, the in- flight calibration methods allow to characterize the instrument parameters during the mission with an uncertainty of plus or minus 0.5%.
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.
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.
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.
Bernd Harnisch, Marino Fabbricotti, Roland Meynart, Bernd Kunkel, Winfried Posselt, Elke Schmidt, Robert Davancens, Olivier Donnadieu, Olivier Saint-Pe, Dave Charlton, Liz Sankus, Giuseppe Basile, L. Calamei, Juergen Schweizer, Hans Juranek, Rolf Sand, Horst Schwarzer, Karl-Heinz Suemnich, Philip Slater
The recent developments within the ESA funded HRIS (high resolution imaging spectrometer) technology program -- aiming at an airborne demonstrator model -- yielded rather successful subsystem developments. HRIS is designed as a true pushbroom hyperspectral imager with comparatively high spatial and spectral resolution, covering the spectral range from 450 to 2350 nm. The main breadboard units, with a space-near design, are essentially: a TMA (three mirro anastigmat, Carl Zeiss) front optics, a dual path spectrometer optics (Officine Galileo) with a novel in-field spectral separation unit, a 2-D SWIR CMT detector array with a dedicated CMOS readout multiplexer (GEC Marconi IR, MATRA MSF for testing), the signal processing electronics (DSS), some calibration elements (DLR + DSS), and the extensive testing of all units. The paper presents the essential results per unit, with possible exception of the front optics (which may not be completed at the conference paper presentation yet), including derived further development efforts. Also, the remaining steps towards an airborne test mission are outlined, together with a brief description of the envisaged high-altitude aircraft. We hope that this paper may also stir some potential users of later airborne HRIS test missions over dedicated target areas. Positive responses would support ESA to pursue the program. The technology units development under the HRIS contract have turned out useful for follow-on instrument developments such as the ESA Explorer mission candidate PRISM (processes research by an imaging space mission). This leads to the conclusion that the achieved development results are a sound basis for future airborne and spaceborne hyperspectral imager developments in Europe. A brief survey of the current PRISM baseline concept is added to the paper.
The MOS instruments on the Indian satellite IRS-P3 are now working for one year in orbit. Beside the calibration of the instrument during laboratory experiments the methods and tools of the in-orbit calibration gives a reliable basis for the interpretation of the remotely sounded data from the earth atmosphere-surface system. As a part of the in-orbit quality assurance the relative internal calibration together with the absolute recalibration with the extraterrestrial sun irradiance provides data for checking the instrument parameters. The principles of the system and the results of the one year operations are discussed.
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.
The requirements concerning the radiometric accuracy of optical remote sensing systems for earth and environmental observations especially to high resolution imaging spectro- radiometers are increasing more and more. Accurate and conscientious on-ground and in-flight calibration of the sensors is one of the baselines to meet this requirement. From this point of view the polarization sensitivity of the sensors plays an important role because it is present more or less every time. Polarization sensitivity and its changes affect directly the radiometric accuracy of the estimated radiances of the polarized radiation coming from the scenes under investigation. In this paper an equipment for in-flight monitoring the polarization sensitivity of the sensor as part of the calibration procedure is presented. It can be used for measuring the plarization state of the incoming radiation too.
The space borne remote sensing imaging spectrometer MOS will be launched to the Russian MIR-station in autumn 1995. It was developed for monitoring large scaled effects of the oceans, the atmosphere and land such as chlorophyll content, yellow substances, sediments, aerosol parameters, vegetation stress, and vegetation index. Information about such state parameters will be derived from the spectral function of the backscattered sun radiation of the objects under investigation. This spectral function will be measured in 13 wavelength channels from 408 nm to 1010 nm with 10 nm halfwidth for ocean, land, and atmospheric purposes and in 4 channels at the O2-A-absorption band from 757 nm to 767 nm with 1.4 nm half width for atmospheric purposes only. The reliability of the thematic interpretation depends strongly on the accuracy of the measured data, and it depends on the on-ground and the in-flight calibration methods and procedures. The determination of the spectral sensitivity function of each sensor element, their shape as well as the absolute values on ground and the check of the stability of the scanner properties during the mission time are described.
The imaging pushbroom scanner MOS measures the spectral radiance of the backscattered radiation of the earth surface in the VIS/NIR spectral region from orbit altitudes. The two main problems, the investigation of ocean/land and atmospheric properties, require a modular optomechanical and -electronical spectrometric device: MOS-A especially for data acquisition in the absorption band of the atmospheric oxygen in 4 spectral channels with 1,4 nm bandwidth and MOS-B with 13 channels of 10 nm bandwidth between 0,4 micrometers and 1,0 micrometers . The spectral and radiometric properties of MOS are chosen with respect to the spectral characteristic of the upwelling radiation, which is influenced by the object spectral reflection function. They are a compromise between the dominant measuring quantities like radiometric, geometric and time resolution requirements. These problems in connection with the design and calibration concepts will be discussed.
HRIS is proposed as a spaceborne, high-resolution imaging spectrometer designed to image a variable (+/- 30 degree(s)) 30 km swath with 40 m SSP pixel size in the spectral range from 450 to 2340 nm with an average 10 nm spectral bandwidth. HRIS is conceived as a push-broom imager with two-dimensional detector arrays for spectral and spatial coverage. The challenging requirements for this instrument will be discussed as well as the concept derived against these requirements. Emphasis is on the optical definition, particularly the spectrometer optics, the focal plane assembly--here mostly the hybrid SWIR CMT detector array--and the calibration concept which includes two external references, ratioing radiometers and an internal reference. The other subunits will be described briefly only. The presentation will conclude with a preliminary development plan.
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.
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