Ingo Walter

Mercury radiometer and thermal infrared spectrometer--a novel thermal imaging spectrometer for the exploration of Mercury

The MERTIS instrument is a state of the art imaging spectrometer in the TIR range onboard ESAs Bepi Colombo mission to the planet Mercury. MERTIS has four science goals: the study of Mercurys surface composition, identification of rock-forming minerals, mapping of the surface mineralogy, and the study of surface temperature variations and of the thermal inertia. The instrument is designed to achieve a signal-to-noise-ratio above 100 in the 7-14 µm range with a spectral channel width of 90 nm and a nominal spatial ground resolution of 500 m within the complex thermal and radiation environment of Mercury.

MERTIS: reflective baffle design and manufacturing

Optical instruments for remote sensing applications frequently require measures for reducing the amount of external, unwanted stray light in the optical instrument path. The reflective planet baffle design and manufacturing process for the thermal infrared imaging spectrometer MERTIS onboard of ESAs cornerstone mission BepiColombo to Mercury is presented. The baffle has to reflect the unwanted solar flux and scattered IR radiation, and minimize the heat load on the instrument. Based on optical stray light simulations and analyses of different baffle concepts the Stavroudis principle showed the best performance and the smallest number of internal reflections. The setup makes use of the optical properties of specific conic sections of revolution. These are the oblate spheroid, generated by rotating an ellipse about its minor axis, and the hyperboloid of one sheet, obtained by the rotation of a hyperbola around its conjugate axis. Due to the demanding requirements regarding surface quality, low mass and high mechanical stability, electroforming fabrication was selected for the baffle. During manufacturing, a layer of high strength nickel alloy is electrodeposited onto a diamond turned aluminum mandrel. The mandrel is subsequently chemically dissolved. Not only the baffle, but also the baffle support structure and other mating components are electroformed. Finally, the baffle and support structure are assembled and joined by an inert gas soldering process. After the optimum baffle geometry and surface roughness has been realized, the remaining total heat flux on the baffle is only dependent on the selection of the appropriate, high reflective coating.

Deep space instrument design for thermal infrared imaging with MERTIS

MERTIS is a miniaturized thermal infrared imaging spectrometer onboard of ESAs cornerstone mission BepiColombo to Mercury. It shall provide measurements in the spectral range from 7-14 μm with a spatial resolution of maximal 300 m and 80 spectral channels in combination with radiometric measurements in the spectral range from 7-40 μm. The instrument concept therefore integrates two detector systems sharing a common optical path consisting of mirror entrance optics and reflective Offner spectrometer. Uncooled micro-bolometer and thermopile radiometer technology are implemented for lowest power consumption. Subsequent viewing of different targets including on-board calibration sources will provide the desired performance. Special attention is spent on the fully passive thermal design in the harsh environment around Mercury. The article will provide an overview of the 3 kg - instrument design and highlight the concept of the subsystems and technologies used. The status of the development process will be reported.

MERTIS-thermal infrared imaging of Mercury: advances in mid-IR remote sensing technology for planetary exploration

MERTIS (MErcury Radiometer and Thermal infrared Imaging Spectrometer) is part of ESAs BepiColombo Mercury Planetary Orbiter mission to the innermost planet of the Solar system. MERTIS is designed to identify rock-forming minerals, to map the surface composition, and to study the surface temperature variations with an uncooled microbolometer detector in the hot environment of Mercury. MERTIS is an advanced IR instrument combining a pushbroom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-fight calibration components for a wavelength range of 7-14 and 7-40 μm, respectively. First results of the ongoing MESSENGER project at Mercury have shown a more complex geology and higher variability of features than previously thought. The MESSENGER studies have demonstrated the need to gain global high-resolution mid-IR spectral and temperature data to achieve a better understanding of the planetary genesis. The MERTIS measurements will acquire this currently missing data set. This article gives a summary of the instrument requirements and its design. We are reporting on the actual instrument development progress, and the status of system and subsystem qualification efforts.

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: 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.

The Venus Emissivity Mapper (VEM) Concept

Based on experience gained from using the VIRTIS instrument on Venus Express to observe the surface of Venus and the new high temperature laboratory experiments, we have developed the multispectral Venus Emissivity Mapper (VEM) to study the surface of Venus. VEM imposes minimal requirements on the spacecraft and mission design and can therefore be added to any future Venus mission. Ideally, the VEM instrument will be combined with a high-resolution radar mapper to provide accurate topographic information, as it will be the case for the NASA Discovery VERITAS mission or the ESA EnVision M5 proposal.

A system on a chip concept for the Mercury Thermal Imaging Spectrometer MERTIS

A very advanced Mercury Thermal Infrared Spectrometer (MERTIS) was proposed by the German Aerospace Center (DLR) fulfilling the extreme resource restrictions of the ESA deep space mission BepiColombo to the inner planet Mercury. The design drivers for the MERTIS instruments are the limited mass and the harsh radiation environment. Derived from these main constrains are power-, volume- and thermal restrictions. This paper presents some key technologies applied to keep the BepiColombo mission limits. It will touch the extreme light weight and compact optic design but focuses on the system on a chip application of the MERTIS instrument digital electronics and its influence to the system structure and reliability concept. It will further show some techniques making the operation of the MERTIS instrument more robust in this radiation exposed environment. One example is the integration of a digital thermal controller for a thermo electrical cooler with less than 10 mK accuracy. It is synthesized into the MERTIS system on a chip instrument controller based on a field programmable gate array (FPGA)

MERTIS - a highly integrated IR imaging spectrometer

With a background of several instrument developments in the past the German Aerospace Center in Berlin proposed for ESAs deep space mission BepiColombo an imaging spectrometer which meets the challenges of limited technical resources and a very special operational environment. An 80-channel push broom-type spectrometer has been drafted and it s development has been started under the name MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer). The instrument is dedicated to the mineralogy surface science and thermal characteristics studies of the innermost planet. It is based on modern un-cooled micro-bolometer technology and all-reflective optics design. The operation concept principle is characterised by intermediate scanning of the planet, deep space and black bodies as calibration targets. A miniaturised radiometer is included for low level temperature measurements. Altogether the system shall fit into a CD-package sized cube and weigh less than 3 kg. The paper will present the instrument architecture of MERTIS, its design status and will show the results of first components being built.

Sensor system for fire detection on-board the small satellite BIRD

With the successful launch of BIRD satellite in October 2001, new possibilities of the observation of hot events like forest fires, volcanic eruptions a.o. from space are opened. The BIRD (Bi-spectral Infrared Detection) is the first satellite which is equipped with space instrumentation dedicated to recognize high temperature events. Current remote sensing systems have the disadvantage that they were not designed for the observation of hot events. Starting with the FIRES Phase A Study, the principle requirements and ideas for a fire recognition system were defined. With the German BIRD demonstrator mission, a feasible approach of these ideas has been realized and work now in space. This mission shall answer technological and scientific questions related to the operation of a compact bi-spectral infrared push-broom sensor and related to the detection and investigation of fires from space. The payload of BIRD is a multi-sensor system designed to fulfil the scientific requirements under the constraints of a micro satellite. The paper describes the basic ideas for fire detection and the estimation of fire temperature, fire size, and energy release in the sub-pixel domain and describes the technical solution for the infrared sensor system on board of BIRD.