PLATO (PLAnetary Transits and Oscillation of stars) is a medium-class space mission part of the ESA Cosmic vision program. Its goal is to find and study extrasolar planetary systems, emphasizing on planets located in habitable zone around solar-like stars. PLATO is equipped with 26 cameras, operating between 500 and 1000nm. The alignment of the focal plane assembly (FPA) with the optical assembly is a time consuming process, to be performed for each of the 26 cameras. An automatized method has been developed to fasten this process. The principle of the alignment is to illuminate the camera with a collimated beam and to vary the position of the FPA to search for the position which minimizes the RMS spot diameter. To reduce the total number of measurements which is performed, the alignment method is done by iteratively searching for the best focus, decreasing at each step the error on the estimated best focus by a factor 2. Because the spot size at focus is similar to the pixel, it would not be possible with this process alone to reach an alignment accuracy of less than several tens of microns. Dithering, achieved by in-plane translation of the focal plane and image recombination, is thus used to increase the sampling of the spot and decrease the error on the merit function.
We present the first results of a study aimed at finding new and efficient ways to automatically process spacecraft telemetry for automatic health monitoring. The goal is to reduce the load on the flight control team while extending the "checkability" to the entire telemetry database, and provide efficient, robust and more accurate detection of anomalies in near real time. We present a set of effective methods to (a) detect outliers in the telemetry or in its statistical properties, (b) uncover and visualise special properties of the telemetry and (c) detect new behavior. Our results are structured around two main families of solutions. For parameters visiting a restricted set of signal values, i.e. all status parameters and about one third of all the others, we focus on a transition analysis, exploiting properties of Poincare plots. For parameters with an arbitrarily high number of possible signal values, we describe the statistical properties of the signal via its Kernel Density Estimate. We demonstrate that this allows for a generic and dynamic approach of the soft-limit definition. Thanks to a much more accurate description of the signal and of its time evolution, we are more sensitive and more responsive to outliers than the traditional checks against hard limits. Our methods were validated on two years of Venus Express telemetry. They are generic for assisting in health monitoring of any complex system with large amounts of diagnostic sensor data. Not only spacecraft systems but also present-day astronomical observatories can benefit from them.
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.
The ESA Herschel space observatory will be launched in 2008 into the Earth-Sun L2 orbit and the three instruments onboard
will be exposed to cosmic radiation during the 4 years lifetime of the satellite. To study the impact of ionizing
radiation on the Ge:Ga photoconductors of the PACS instrument (Photodetector Array Camera and Spectrometer), we
performed a series of irradiation measurements at the cyclotron of the University of Louvain la Neuve, Belgium
simulating the in-flight predicted proton fluxes including solar flare events. The PACS integral field spectrometer
contains two 25×16 pixel arrays of Ge:Ga crystals: a low stressed configuration is used in the wavelength range from 55
to 105 μm, and a high stressed device covers the range 105 to 210 μm. Calibration of the detector modules under
realistic IR background fluxes is done at MPE Garching and MPIA Heidelberg. 70 MeV protons were generated at the
cyclotron test site. They were attenuated on their way to the detectors by beam conditioning elements and the metal
shields of the cryostat before they reached the Ge:Ga crystals with a mean energy of 17 MeV and a standard deviation
of 1.5 MeV. According to predictions the expected proton fluxes were set to nominally 10 ps-1cm-2 and to 400 ps-1cm-2
simulating solar flares. We observed radiation-induced glitches in the detector signal, changes in responsivity, increase
in noise and transient behavior. The ongoing data evaluation indicates optimal operating parameters, the best curing
method and frequency, calibration procedures and data processing algorithms aiming for a high photometric accuracy.
The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments for ESA's far infrared and submillimetre observatory, Herschel. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 x 25 pixels, each, and two filled Si bolometer arrays with 16 x 32 and 32 x 64 pixels, respectively, to perform imaging line spectroscopy and imaging photometry in the 57-210 micron wavelength band. In photometry mode, it will simultaneously image two bands, 60-85 micron or 85-130 micron and 130-210 micron, over a field of view of ~ 1.75'x3.5', with full beam sampling in each band. In spectroscopy mode, it will image a field of ~ 50"x50", resolved into 5 x 5 pixels, with an instantaneous spectral coverage of ~ 1500 km/s and a spectral resolution of ~ 75 - 300 km/s. In both modes background-noise limited peformance is expected, with sensitivities (5σ in 1h) of ~3 mJy or 3-10x10-18 W/m2, respectively.