Horst Schwarzer

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.

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.

sCMOS detector for imaging VNIR spectrometry

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.

Dynamic PSF and MTF measurements on a 9k TDI CCD

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.

APEX calibration facility: status and first commissioning results

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.

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.

Modular Optoelectronic Scanner (MOS): an imaging spectrometer for remote sensing of the environment

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.

High-resolution imaging spectrometer (HRIS): optics, focal plane, and calibration

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.

Development of a wavelength stabilized seed laser system for an airborne water vapour lidar experiment

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.

HRIS technology development results and their implementation in future hyperspectral imagers

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.