PROBA3 is the first high precision formation flying (FF) mission under responsibility of the European Space Agency (ESA). It is a technology mission devoted to in-orbit demonstration of the FF techniques, with two satellites kept at an average inter-satellite distance of 144m. The guiding scientific rationale is to realize a diluted coronagraph with the telescope (ASPIICS) on one satellite and the external occulter on the other satellite to observe the inner Solar corona at high spatial and temporal resolution, down to 1.08R⊙. The two spacecraft will be orbiting in a high eccentricity geocentric trajectory with perigee at 600km and the apogee at 60000Km and with an orbital period of 19hrs. The FF acquisition and operations will last about 6 hrs around the apogee and different metrology systems will be used for realizing and controlling the FF. The alignment active most critical sub-system is the Shadow Positioning Sensors (SPS), a series of Si-PM (Silicon Photomultiplier) disposed around the ASPIICS telescope's entrance aperture and measuring the proper positioning of the penumbra generated by the occulter at the center of the coronagraph’s optical reference frame. The FF alignment measurement accuracies required to the SPS are: 500μm for lateral movements and 50mm for longitudinal movements. This paper gives an overview of the opto-mechanical and electronic design and of the software algorithm for the FF intersatellite positioning. The expected performance of the SPS metrology system are reported.
The Sentinel-4 mission (S4) is part of the Global Monitoring for Environment and Security (GMES) initiative and covers the needs for continuous monitoring of Earth atmospheric composition and air pollution .
PROBA3/ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is the first formation flight solar coronagraph, scheduled by ESA for a launch and currently in phase C/D. It is constituted by two spacecraft (one hosting the occulter, diameter 142 cm, and one with the telescope) separated by 144 m, kept in strict alignment by means of complex active and metrology custom systems. The stray light analysis, which is always one the most critical work packages for a solar coronagraph, has been only theoretically investigated so far due to the difficulty of replicating the actual size system in a clean laboratory environment. The light diffracted by the external occulter is the worst offender for the stray light level on the instrument focal plane, thus there is strong interest for scaling at least the occultation system of the coronagraph and test it in front of a solar simulator in order to experimentally validate the expected theoretical performance. The theory for scaling the occulter, the occulter-pupil distance and the source dimension has been developed and a scaled model is being manufactured. A test campaign is going to be conducted at the OPSys facility in Torino in front of a solar simulator (conveniently scaled). This work accounts for the description of the scaled model laboratory set-up and of the test plan.
The Photodetector Array Camera and Spectrometer (PACS), on board the Herschel Space Observatory, is designed for
imaging and low and medium resolution spectroscopy in the wavelength region between 57 and 210 μm. This paper
reports the design and the testing results of the grating cryogenic mechanism of the PACS spectrometer. The PACS
diffraction grating is made from an aluminium substrate, mechanically ruled with a periodicity of 8.5 grooves per mm
and gold coated for optimum reflectivity at PACS operating wavelengths. The grating mechanism is capable of accurate
positioning (4") of the flat diffraction grating within a large angular throw (44°) in cryogenic environment (4.2 K).
Technologies of actuators, position sensors, bearings, servo-control and cryogenic test set-up are presented. The grating
mechanism was thoroughly tested, alone and when integrated in the PACS Focal Plane Unit (FPU). The tests were
performed in cryogenic conditions, in a set-up fully representative of the flight conditions. Actual mechanical and
optical performance obtained with the Flight Model (FM) is presented in detail. Quality of the angular positioning of the
mechanism, spectral resolution and optical quality of the grating are analysed.