Thomas Säuberlich

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: understanding Mercury's surface composition from mid-infrared spectroscopy

The Mercury Radiometer and Thermal Infrared Imaging Spectrometer MERTIS on the joint ESA-JAXA mission BepiColombo to Mercury is combining a spectrometer using an uncooled microbolometer in a pushbroom mode with a highly miniaturized radiometer. A full development model of MERTIS is now available. So, after three flybys of Mercury by the MESSENGER mission and with the Planetary Emissivity Laboratory at DLR in Berlin that can routinely obtain infrared emission spectra at high temperatures it is a good time to review the MERTIS science requirements and the performance in perspective of our new knowledge of Mercury.

Image Quality of Optical Remote Sensing Data

Photogrammetry and remote sensing (RS) provide procedures for deriving geometric, radiometric and thematic information from image data. A variety of aircraft and space-borne sensors are available to capture image data. Different standards and specifications of quality assessment for optical remote sensing data are available. Due to the possibilities of absolute geometric and radiometric calibration digital sensors provide new promising opportunities to create value added products like digital elevation models, land-use maps etc. Such cameras combine the high geometric quality with the radiometric standards of earth observation systems. The determination of image quality of remote sensing data can be distinguished in (spectral) radiometric and geometric aspects. Standards contains different metrics for accuracy issues (spectral, radiometric and geometric accuracy) and for performance parameters like SNR, MTF. Image artefacts (caused e.g. by compression) are an additional important topic. The paper gives an overview of the current debate and the possibility of standardization.

Developing of MERTIS as an advanced process from the study up to the flight model

ESA’s mission BepiColombo will be launched in 2016. MERTIS (Mercury Radiometer and Thermal imaging Spectrometer) is one of the key instruments. MERTIS is an imaging infrared spectrometer and radiometer using an uncooled detector technology with very small resources in terms of mass and power. The incentive of the MERTIS development is scientific requirements to study the surface composition and temperatures of Mercury under the extreme environmental condition at Mercury. Therefore, the state-of-the-art optical performance of MERTIS is unique. Components based on innovative technologies have been developed and qualified to realize the project. This approach required an advanced model philosophy and development process from the study up to the flight model completed in 2013. This paper describes the development process as well as challenges from the management and system engineering point of view up to a lessons learnt that lead to important conclusions.

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: geometrical calibration of thermal infrared optical system by applying diffractive optical elements

Geometrical sensor calibration is essential for space applications based on high accuracy optical measurements, in this case for the thermal infrared push-broom imaging spectrometer MERTIS. The goal is the determination of the interior sensor orientation. A conventional method is to measure the line of sight for a subset of pixels by single pixel illumination with collimated light. To adjust angles, which define the line of sight of a pixel, a manipulator construction is used. A new method for geometrical sensor calibration is using Diffractive Optical Elements (DOE) in connection with laser beam equipment. Diffractive optical elements (DOE) are optical microstructures, which are used to split an incoming laser beam with a dedicated wavelength into a number of beams with well-known propagation directions. As the virtual sources of the diffracted beams are points at infinity, the resulting image is invariant against translation. This particular characteristic allows a complete geometrical sensor calibration with only one taken image avoiding complex adjustment procedures, resulting in a significant reduction of calibration effort. We present a new method for geometrical calibration of a thermal infrared optical system, including an thermal infrared test optics and the MERTIS spectrometer bolometer detector. The fundamentals of this new approach for geometrical infrared optical systems calibration by applying diffractive optical elements and the test equipment are shown.

Pointing and spectral assignemnt design and control for MERTIS

The development of MERTIS, a miniaturized thermal infrared imaging spectrometer onboard of ESAs cornerstone mission BepiColombo to Mercury has been completed. Qualification of the design is followed by the calibration of the instrument showing up first results of the technology used. Based on subsequent viewing of different targets including on-board calibration sources the push-broom instrument will use a 2-dimensional bolometer detector to provide spatial and spectral information. Here repetition accuracy of pointing and spectral assignment is supported by the design of instrument components under the restriction of limited resources. Additionally a concept of verification after launch and cruise phase of the mission was developed. The article describes how this has been implemented and what the results under environment testing are.

MERTIS on BepiColombo: seeing Mercury in a new light

The MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) is part of the payload of the Mercury Planetary Orbiter spacecraft of the ESA-JAXA BepiColombo mission. MERTIS’s scientific goals are to infer rockforming minerals, to map surface composition, and to study surface temperature variations on Mercury. To achieve these science goals MERTIS combines a imaging spectrometer covering the wavelength range from 7-14 microns with a radiometer covering the wavelength range from 7-40 microns. MERTIS will map the whole surface of Mercury with a spatial resolution of 500m for the spectrometer channel and 2km for the radiometer channel. The MERTIS instrument had been proposed long before the NASA MESSENGER mission provided us with new insights into the innermost of the terrestrial planets. The discoveries of the MESSENGER fundamentally changed our view of Mercury. It revealed a surface that has been reshaped by volcanism over large parts of geological history. Volatile elements like sulfur have been detected with unexpectedly high abundances of up to 4%. MESSENGER imagined structures that are most likely formed by pyroclastic eruptions in recent geologic history. Among the most exciting discoveries of MESSENGER are hollows – bright irregularly shaped depressions that show sign of ongoing loss of material. Despite all this new results the MERTIS dataset remains unique and is now more important than ever. None of the instruments on the NASA MESSENGER mission covers the same spectral range or provides a measurement of the surface temperature. The MERTIS will complement the results of MESSENGER. MERTIS will for example be able to provide spatially resolved compositional information on the hollows and pyroclastic deposits – both among the most exciting discoveries by the MESSENGER mission for which the NASA mission can not provide compositional information.