The objective of ESA’s Next Generation Gravity Mission (NGGM) is long-term monitoring of the temporal variations of Earth’s gravity field at high temporal (down to 3 days) and spatial (100 km) resolution. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere and land, and will complete our picture of Global Change with otherwise unavailable data. The observable is the variation of the distance between two satellites measured by a laser interferometer; ultra-precise accelerometers measure the nongravitational accelerations to correct the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (between 396 and 488 km) circular orbits, at 220 km separation, one pair quasi-polar and the other around 65°-70° inclination. The satellite-to-satellite tracking technique for detecting the temporal variations of gravity was established by GRACE (300-400 km spatial resolution at monthly intervals) using tracking in the microwave band. Today, GRACE is being continued by GRACE-Follow-On, with similar objectives, where the laser interferometry has improved the measurement resolution by a factor of 100 (upper MBW). At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, against 10% achieved with GRACE. High temporal resolution will reveal large-scale sub-weekly mass variations, with applications in water and emergency management. NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation in the framework of MAGIC. The paper focusses on the on-going Phase A system design and technology pre-development activities.
Significant progress has been made over the past years in the understanding of Earth’s climate; however, there are still uncertainties in the modelling of atmospheric radiative processes. The main radiative processes relevant for climatological models are related to atmospheric water vapour, clouds, and surface emissivity across the full thermal infrared spectrum. Whilst the mid-infrared outgoing radiation is currently measured from space (e.g. IASI), there is a lack of spectrally resolved measurements over a significant portion of Earth’s thermal emission in the Far-InfraRed (FIR) i.e. between 100 and 667 cm-1 (15-100 micron). FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) has been selected as the ESA’s ninth Earth Explorer mission in September 2019. FORUM aims at measuring the Earth’s Top-Of-Atmosphere emission spectrum from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). Two independent instrument concepts, based on a Fourier Transform Spectrometer, have been thoroughly studied in Phase A preparatory activities, including the development of a breadboard of the interferometer. In addition, critical technologies have been identified and subjected to comprehensive breadboard studies in order to mitigate development risks and to raise the Technology Readiness Level (TRL). This has led to the development and characterisation of several beamsplitter candidates, the measurement of the emissivity of the black coating for the on-board blackbody, the verification of noise and responsivity performance of pyroelectric detectors and the characterisation of the interferometer mechanism.
The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been selected in September 2019 as the 9th Earth Explorer mission of the European Space Agency. The mission aims to measure the Earth’s Top-Of- Atmosphere (TOA) emission spectrum in the spectral region from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). This will fill the current observational gap from space in the far-infrared region (FIR) from 100 to 667 cm-¹ (i.e. from 15 to 100 μm). FORUM measurements will improve the understanding of the climate system by providing, for the first time with high resolution, the spectral features of the far-infrared emission of the Earth with a focus on the contribution to the radiation budget of the continuum absorption of the water vapour rotational bands, on cirrus cloud properties, and on ice/snow surface emissivity. The FORUM mission requires a payload able to spectrally-resolve the Earth's outgoing longwave radiation across the Far-InfraRed (FIR) spectral range with high absolute radiometric accuracy. Characterisation of the surface, atmospheric and cloud/surface heterogeneity in the observed field-of-view is also required to help interpret the measured spectral radiance. These needs dictate the use of two instruments: a spectrometer and an infrared imager. The concepts of both instruments, thoroughly studied in phase A preparatory activities by two independent industrial consortia, are presented in this paper.
This paper presents an overview of an optical instrument for disaster and security monitoring from the geostationary orbit. The described instrument is the payload of the GEOBS (GEOstationary OBServatory) concept, which is the result of an ESA exploratory activity conducted at the ESA-ESTEC Concurrent Design Facility (CDF) in early 2020 in order to assess the technical and programmatic feasibility of optical mission concepts from GEO. Recent improvements in the maturity of critical technologies (e.g. adaptive optics, large CMOS detector arrays, ultra-stable materials,…), together with the development of adequate manufacturing facilities and the required manufacturing techniques in Europe and Japan, have provided the ground for further investigation of the current technical and programmatic feasibility of a geostationary observation system for disaster and security monitoring. The instrument on-board the GEOBS satellite provides medium spatial resolution imagery (10-20 m Ground Sampling Distance at equator) in three visible narrow bands (Red, Green and Blue - RGB), one Panchromatic band and one band in the Near Infrared (NIR). The instrument acquires images using a step-and-stare observation technique, enabling persistent monitoring of selected areas. The instrument is sized to provide a footprint >100 km at Equator and observe the area from 30° to 55° North in Latitude and from 10° West to 36° East in Longitude, with observations possible from 4:00 a.m. to 8 p.m. local solar time. The instrument architecture is based on a Korsch telescope, with an active optics correction loop (wavefront sensor, deformable mirror, 5 degrees-of-freedom mechanism on the secondary mirror), and a focal plane based on multiple CMOS detectors, assembled in either 1D or 2D large arrays to cover the full field of view. The instrument concept also includes ultra-stable structures and light-weight sun-shields to reduce straylight and thermos-elastic effect.
This paper examines the active optics of future large segmented telescopes from the point of view of dynamic simulation and control. The first part of the paper is devoted to the modelling of the mirror. The model has a moderate size and separates the quasi-static behavior of the mirror (primary response) from the dynamic response (secondary or residual response). The second part of the paper is devoted to control. The control strategy considers explicitly the primary response of the telescope through a singular value controller. The control-structure interaction is addressed with the general robustness theory of multivariable feedback systems, where the secondary response is considered as uncertainty. This approach is very fast and allows extensive parametric studies. The study is illustrated with an example involving 90 segments, 270 inputs, and 654 outputs.
This paper examines the possibility of constructing deformable mirrors for adaptive optics with a large number of degrees of freedom from silicon wafers with bimorph piezoelectric actuation. The mirror may be used on its own, or as a segment of a larger mirror. The typical size of one segment is 100 to 200 mm; the production process relies on silicon wafers and thick film piezoelectric material deposition technology; it is able to lead to an actuation pitch of the order of 5 mm, and the manufacturing costs appear to grow only slowly with the number of degrees of freedom in the adaptive optics.
This paper examines the possibility of constructing deformable mirrors for adaptive optics with a large number
of degrees of freedom, by assembling segmented silicon mirrors with bimorph piezoelectric actuation. The
production process relies on silicon wafers and thick film PZT deposition technology; it is able to lead to an
actuation pitch of the order of 5 mm, and the manufacturing costs appear to grow only slowly (linearly or less)
with the number of degrees of freedom in the adaptive optics.
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