The Fine Guidance System of the PLATO Mission

PLATO - PLAnetary Transits and Oscillation of stars - is a medium-class mission in the European Space Agency (ESA) Cosmic Vision programme, whose launch is foreseen by 2026. The objective is the detection and characterization of terrestrial exoplanets up to the habitable zone of solar-type stars by means of their transit signature in front of a very large sample of bright stars. The seismic oscillations of the parent stars orbited by these planets is measured in order to understand the properties of the exoplanatory systems. The PLATO payload consists of an instrument with 26 cameras for star observation; 24 normal cameras grouped in four subsets with six cameras each and two fast cameras. Besides providing scientific data for very bright stars, the fast cameras also serve as two redundant Fine Guidance System (FGS) and will be an integral part of the Attitude and Orbit Control System (AOCS). This ensures a very high pointing precision which is needed to achieve a high photometric precision. Working as a star-tracker, the attitude calculation is based on guide star positions on the focal plane and their reference directions given by a star catalogue. Compared to predecessor missions like CoRoT, Kepler, or TESS, the precision of the fine guidance algorithm needs to be increased significantly. This is especially challenging as the optical design is identical for all cameras and optimized to meet the science objectives rather than to serve as a star-tracker. Therefore, a novel approach based on a Gaussian fit is proposed. The shown algorithm provides a noise optimal estimation of the guide star positions which propagates to an optimal attitude estimation. Although, computational more expensive than conventional methods, its suitability for a real-time on-board application is proven with an implementation on the target hardware. Furthermore, its robustness and precision is assessed theoretically and with simulated star images sequences

CO2 Image: The design of an imaging spectrometer for CO2 point source quantification

CO2Image is a satellite demonstration mission, now in Phase B, to be launched in 2026 by the German Aerospace Center (DLR). The satellite will carry a next generation imaging spectrometer for measuring atmospheric column concentrations of Carbon Dioxide (CO2). The instrument concept reconciles compact design with fine ground resolution (50-100 m) with decent spectral resolution (1.0-1.3 nm) in the shortwave infrared spectral range (2000 nm). Thus, CO2Image will enable quantification of point source CO2 emission rates of less than 1 MtCO2/a. This will complement global monitoring missions such as CO2M, which are less sensitive to point sources due to their coarser ground resolution and hyperspectral imagers, which suffer from spectroscopic interference errors that limit the quantification

In situ science on Phobos with the Raman spectrometer for MMX (RAX): preliminary design and feasibility of Raman meausrements

Mineralogy is the key to understanding the origin of Phobos and its position in the evolution of the Solar System. In situ Raman spectroscopy on Phobos is an important tool to achieve the scientifc objectives of the Martian Moons eXploration (MMX) mission, and maximize the scientifc merit of the sample return by characterizing the mineral composition and heterogeneity of the surface of Phobos. Conducting in situ Raman spectroscopy in the harsh environment of Phobos requires a very sensitive, compact, lightweight, and robust instrument that can be carried by the compact MMX rover. In this context, the Raman spectrometer for MMX (i.e., RAX) is currently under development via international collaboration between teams from Japan, Germany, and Spain. To demonstrate the capability of a compact Raman system such as RAX, we built an instrument that reproduces the optical performance of the fight model using commercial of-the-shelf parts. Using this performance model, we measured mineral samples relevant to Phobos and Mars, such as anhydrous silicates, carbonates, and hydrous minerals. Our measurements indicate that such minerals can be accurately identifed using a RAX-like Raman spectrometer. We demonstrated a spectral resolution of approximately 10 cm−1, high enough to resolve the strongest olivine Raman bands at ~820 and ~850 cm−1, with highly sensitive Raman peak measurements (e.g., signal-to-noise ratios up to 100). These results strongly suggest that the RAX instrument will be capable of determining the minerals expected on the surface of Phobos, adding valuable information to address the question of the moon’s origin, heterogeneity, and circum-Mars material transport

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 space-view 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

SENSOR++: Simulation of remote sensing systems from visible to thermal infrared

During the development process of a remote sensing system, the optimization and the verification of the sensor system are important tasks. To support these tasks, the simulation of the sensor and its output is valuable. This enables the developers to test algorithms, estimate errors, and evaluate the capabilities of the whole sensor system before the final remote sensing system is available and produces real data. The presented simulation concept, SENSOR++, consists of three parts. The first part is the geometric simulation which calculates where the sensor looks at by using a ray tracing algorithm. This also determines whether the observed part of the scene is shadowed or not. The second part describes the radiometry and results in the spectral at-sensor radiance from the visible spectrum to the thermal infrared according to the simulated sensor type. In the case of earth remote sensing, it also includes a model of the radiative transfer through the atmosphere. The final part uses the at-sensor radiance to generate digital images by using an optical and an electronic sensor model. Using SENSOR++ for an optimization requires the additional application of task-specific data processing algorithms. The principle of the simulation approach is explained, all relevant concepts of SENSOR++ are discussed, and first examples of its use are given, for example a camera simulation for a moon lander. Finally, the verification of SENSOR++ is demonstrated

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 Mercury’s surface layer within a spectral range of 7 − 14μm shall be conducted to specify and map Mercury’s 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