Carsten Paproth

Thermal infrared imaging of Mercury - MERTIS - a new remote sensing technology

MERTIS (MERcury Thermal infrared Imaging Spectrometer) is an advanced infrared remote sensing instrument that is part of the ESA mission BepiColombo to the planet Mercury. The scientific goals of MERTIS science are surface composition analyses, identification of rock-forming minerals, mapping of the surface mineralogy, and studies of surface temperature variations. MERTIS combines a push-broom IR grating spectrometer (TIS) with a radiometer (TIR), which operate in the wavelength region of 7-14 μm (TIS) and 7-40 μm (TIR), respectively. The instrument represents a modular concept of the sensor head, electronic units and power/calibration systems. The integrated instrument approach allows the subsystems TIS and TIR to share the same optics, instrument electronics and in-fight calibration components. The instrument is designed to achieve a signal-to-noise ratio above 100 in the 7-14 μm wavelength range with a spectral channel width of 90 nm. The TIS optical design combines a three mirror anastigmat (TMA) with a modified Offner spectrometer. The spatial resolution will be about 500 m globally and better than 500 m for 5-10% of the Mercurys surface. With an uncooled microbolometer detector, the instrument can be operated in the hot environment of Mercury without the need for a cryogenic cooling system. We are reporting on the measurement requirements, the status of the instrument development, and ongoing qualification efforts.

MERTIS: system theory and simulation

The deep-space ESA mission BepiColombo to planet Mercury will contain the advanced infrared remote sensing instrument MERTIS (MErcury Radiometer and Thermal infrared Imaging Spectrometer). The mission has the goal to explore the planets inner and surface structure and its environment. With MERTIS investigations of Mercurys surface layer within a spectral range of 7-14μm shall be conducted to specify and map Mercurys mineralogical composition with a spatial resolution of 500m. Due to the limited mass and power budget the used micro-bolometer detector array will only have a temperature-stabilization and will not be cooled. The theoretical description of the instrument is necessary to estimate the performance of the instrument especially the signal to noise ratio. For that purpose theoretical models are derived from system theory. For a better evaluation and understanding of the instrument performance simulations are performed to compute the passage of the radiation of a hypothetical mineralogical surface composition through the optical system, the influence of the inner instrument radiation and the conversion of the overall radiation into a detector voltage and digital output signal. The results of the simulation can support the optimization process of the instrument parameters and could also assist the analysis of gathered scientific data. The simulation tool can be used as well for performance estimations of MERTIS-like systems for future projects.

MERTIS: shutterless background signal removal

MERTIS (MERcury Thermal infrared Imaging Spectrometer) is an advanced infrared remote sensing instrument that is part of the ESA mission BepiColombo to planet Mercury. The enabling technology that allows sending the first spectrometer for the thermal infrared spectral range to Mercury is an uncooled microbolometer. One of the challenges is the calibration of the instrument. Radiometric and spectroscopic breadboard models of MERTIS were used to develop proper calibration methods. In the context of the calibration we are reporting on the ongoing efforts to separate non-scene and scene signal portions from each other. The non-scene signal portion is contained in the raw image data sets and is usually the dominating signal contribution. The conventional method to measure the non-scene signal contributions using a shutter or spaceview and perform a time-interpolation is compared to an approach using linear pixel-to-pixel relations in which information from the outer regions of the image matrix is used for the estimation of the non-scene signal components of the inner regions where additional scene signal components exist. The results of both methods are discussed in terms of noise or errors of the extracted scene information. The proposed method could be used without further instrument modifications offering a functional redundancy which is important to keep alive the MERTIS operation in the case of a breakdown of the mechanically stressed high-speed shutter device.

MERTIS: from laboratory to Mercury

MERTIS (MERcury Thermal infrared Imaging Spectrometer) is an advanced infrared remote sensing instrument that is part of the ESA mission BepiColombo to planet Mercury. The enabling technology that allows sending the first spectrometer for the thermal infrared spectral range to Mercury is an uncooled microbolometer. With this detector the instrument can be operated in the hot environment of Mercury without the need for a cryogenic cooling system. The challenge is the characterization and calibration of the instrument. We are reporting on the ongoing calibration efforts including laboratory measurements of analogue materials, end-to-end simulations and a detailed characterization of all components and discuss each of the three elements. The measurement of planetary analogue materials in grain sizes <25 μm and at temperatures up to 500°C relevant for Mercurys surface provide a realistic input signal for the end-to-end simulation. A radiometric and a spectroscopic breadboard model of MERTIS are used to derive all necessary parameters of the instrument, for example the spectral resolution or the wavelength registration on the detector. This parameters support setting up the end-to-end simulation which can then process the spectra of the planetary analog materials as input signal to create a realistic representation of the MERTIS output signal.

MERTIS - Identifiability of spectral mineralogical features in dependence of the signal to noise ratio

The ESA deep-space mission BepiColombo to planet Mercury will contain the advanced infrared remote sensing instrument MERTIS (MErcury Radiometer and Thermal infrared Imaging Spectrometer). The mission has the goal to explore the planets inner and surface structure and its environment. With MERTIS, investigations of Mercurys surface layer within a spectral range of 7 μm to 14μm shall be conducted to specify and map Mercurys mineralogical composition with a spatial resolution of 500 m. Due to the limited mass and power budget, the used micro-bolometer detector array will only have a temperature-stabilization and will not be cooled. The performance of the instrument is estimated by the theoretical description of the signal to noise ratio and the optics including the Offner spectrometer. The expected signal to noise ratio will be in the order of 100 and is mainly dependent on the surface temperature and the wavelength. The derived theoretical models are used to execute simulations to compute the passage of the infrared radiation of a hypothetical mineralogical surface composition and surface temperature through the optical system of MERTIS. The resulting noisy spectra are used to determine spectral features of the minerals. So it is possible to evaluate the conditions which are necessary to achieve the scientific goals of MERTIS. The intent is to estimate the spectral positions of mineralogical features like the Christiansen feature. This will be difficult because of the low signal to noise ratio and the low contrast of real mineral spectra.

MERTIS: background signal removal and signal simulation

MERTIS (MERcury Thermal infrared Imaging Spectrometer) is an advanced infrared remote sensing instrument that is part of the ESA mission BepiColombo to planet Mercury. The enabling technology that allows sending the first spectrometer for the thermal infrared spectral range to Mercury is an uncooled microbolometer. With this detector the instrument can be operated in the hot environment of Mercury without the need of a cryogenic cooling system. The challenge is the calibration of the instrument. A radiometric and a spectroscopic breadboard model of MERTIS were used to develop proper calibration methods and to derive system parameters that support the setup of an end-to-end simulation which can process spectra of planetary analog materials from the DLR Planetary Emissivity Laboratory (PEL) as input signal in order to create a realistic representation of the MERTIS output signal. In the context of the calibration we are reporting on the ongoing efforts to remove the background signal which is contained in the raw image data sets and actually being the dominating signal portion. A background measuring method with using a shutter together with a noise reduction method based on a pixel-by-pixel correlation approach - are discussed and related to the remaining errors of the emissivity spectra which were calculated from raw images of laboratory experiments using onground calibration data sets. The results of the error evaluation and new emissivity spectra from the PEL for high temperatures of planetary analog materials are input parameters for the end-to-end simulation of MERTIS. Regarding the instrument´s SNR a comparison of the simulation results and the experimental data is given and the effect of the noise reduction method.

High Temperature Fire Experiment for TET-1 and Landsat 8 in Test Site DEMMIN (Germany)

In 2012, the German Aerospace Center (DLR) launched the small satellite TET-1 (Experimental Technology Carrier) as a test platform for new satellite technologies and as a carrier for the Multi-Spectral Camera System (MSC) with five spectral bands (Green, Red, Near Infrared, Middle Infrared, and Thermal Infrared). The MSC has been designed to provide quantitative parameters (e.g. fire radiative power, burned area) observing high-temperature events. The detection of such events provides information for operational support to fire brigades, to change detection of hotspots, to assess CO2 emissions of burning vegetation, and, finally, contributes to the monitoring programs that support climate models. In order to investigate the sensitivity and accuracy of the MSC system, a calibration and validation fire campaign was developed and executed, to derive characteristic signal changes of corresponding pixels in the MWIR and LWIR bands. The planning and execution of the validation campaign and the results are presented.

System theoretical aspects for designing opto-electronic sensors for remote sensing

The design of passive optical sensor systems for remote sensing requires more or less complex theoretical investigations. Based on the user requirements which are characterized by data products all relevant physical effects influencing the generation of an image have to be considered and modeled. The goal is to build the whole chain of signal generation and transformation: from the source of light, through the atmosphere to the object to be observed, through the atmosphere again to the sensor system. These models can be applied for simulations which can be used for the optimization of sensor parameters and observation conditions and for the estimation of the potential performance of such a system. For this task, specific retrieval algorithms have to be considered closing the loop of developing camera systems. The paper contains classical approaches for a system design as a standard tool used in the Institute for Robotics and Mechatronics including methods for modeling the geometrical and radiometric relations and the description of camera hardware. The results of a complex investigation of the point spread function of camera systems is core topic of this paper.

MERTIS: configuration of measurement sequences for a maximized image SNR

MERTIS (MERcury Thermal infrared Imaging Spectrometer) is an advanced thermal infrared remote sensing instrument which is a part of the ESA mission BepiColombo to planet Mercury. Since the instrument is designed to work as in the thermal infrared range detecting radiation using an uncooled micro-bolometer matrix it is necessary to pay special attention to the development of proper scene signal extraction methods for the elimination of undesired signal portions from the measurement data - typically being achieved by subtraction of shutter images from scene images. It is shown here how the noise of the resulting difference images for different periodic measurement modes can be predicted and minimized using a theoretical model considering measurement sequences the MERTIS instrument can be driven with. The model introduced is reflecting the noise characteristics of the instruments analog image data channel statistically so that the analog channel itself is not modeled explicitly. Nevertheless a precise noise strength prediction can be achieved. The prediction results depend both on the specific shutter open/close sequence used and a system specific spatial-temporal autocovariance function which can be easily estimated from simple image data cubes. The predictions become very precise if a proper preprocessing removing the most strong disturbing signal portions from the image datasets is done before. Being able to predict the noise strength for arbitrary measurement sequences and giving respect to the system´s physical constraints - e.g. maximum shutter speed - an optimal measurement sequence can be found giving a maximized SNR of the images of MERTIS.