Optimal Interferometric Maneuvers for Distributed Telescopes

The scienti c community has proposed several missions to expand our knowledge about the universe, its formation and search for distant Earth-like planets. Most of the present space-based observation missions have reached angular resolution limits, therefore the potential bene ts concerning angular resolution and intensity that can be reaped from the realization of interferometry within a distributed satellite telescope have led to the proposal of several multi-spacecraft systems. Among these missions synthetic imaging space based interferometers, consisting of multiple telescope apertures ying in controlled formation in order to combine received information from each of the otilla members are nowadays the subject of interesting research. The objective of synthesizing images with high angular resolution, low ambiguity and high intensity is always a tradeo with the whole fuel consumption of the mission. As a consequence, this research focuses on the design of interferometric maneuvers and optimal interferometric controllers balancing image performance and energy consumption. The rest part of the thesis presents the optimization and design process of coordinated spiral maneuvers due to their interferometric interest when lling the frequency plane of the observed image. On the other hand, the second part of this work focuses in the resolution of an optimal control problem within the LQ framework, to determine the optimal imaging recon gurations of a formation ying system. Its objective is to balance the quality of the celestial observation and the usage of fuel, which are the key aspects of any space-based observation mission. This study concerning implementability and performance of interferometric maneuvers will lead towards the enlargement of mission lifetime and exibility of the system conserving acceptable quality observations.Preprin

Magnetic diagnostics algorithms for LISA Pathfinder: system identification and data analysis

LISA (Laser Interferometer Space Antenna) is a joint mission of ESA and NASA, which aims to be the first space-borne gravitational wave observatory. LISA will consist in a constellation of three spacecraft at the vertexes of an equilateral triangle of side 5 million kilometers. The constellation will orbit around the Sun trailing the Earth by some 20 degrees. Each of the spacecraft harbors two proof masses, carefully protected against external disturbances such as solar radiation pressure and charged particles, which ensures they are in nominal free-fall in the interplanetary gravitational field. Gravitational waves will show as differential accelerations between pairs of proof masses, and the main aim of LISA is to measure such acceleration using laser interferometry. The technologies required for the LISA mission are many and challenging. This, coupled with the fact that some flight hardware cannot be tested on ground, led ESA to define a technology demonstrator to test in flight the required critical technologies. This precursor mission is called LISA Pathfinder (LPF). The payload of LISA Pathfinder is the LISA Technology Package (LTP), and will be the highest sensitivity geodesic explorer flown to date. The LISA Technology Package is designed to measure relative accelerations between two test masses in nominal free fall placed in a single spacecraft, since one LISA arm is squeezed from 5 million kilometer to 35 cm. Its success will prove the maturity of the necessary technologies for LISA such as the Optical Metrology System and the Drag Free concept. The differential acceleration reading will be perturbed by identified disturbances, such as thermal fluctuations or magnetic effects. These disturbances are monitored by the Diagnostics Subsystem. The Magnetic Diagnostics System is one of its modules and is a critical subsystem, since magnetic noise is apportioned to 40% of the total noise budget. In this respect, to estimate the magnetic noise contribution, the Magnetic Diagnostics Subsystem will have two main tasks: (1) estimate the magnetic properties of the test masses, i.e., their remanent magnetic moment and susceptibility, and (2) infer the magnetic field and its gradient at the location of the test masses. To this end, the Magnetic Diagnostics Subsystem includes two coils which generate controlled magnetic fields at the locations of the test masses. These magnetic fields will excite the dynamical response of both test masses. Thus, by adequate processing of the kinematic excursions delivered by the interferometer, the magnetic characteristics of the test masses can be estimated within 1% accuracy level. Additionally, the Magnetic Diagnostic Subsystem includes a set of four tri-axial fluxgate magnetometers. However, the magnetic field and its gradient need to be measured at the positions of the test masses and the readouts of the magnetometers do not provide a direct measurement of the magnetic field at these positions. Thus, an interpolation method must be implemented to calculate them. This is a difficult problem, mostly because the magnetometers are too distant from the locations of the test masses (more than 20 cm away) and because there are not sufficient magnetic channels to go beyond a classical linear interpolation method, which yields extremely poor interpolation results. Consequently, in this thesis we present and validate an alternative interpolation method based on neural networks. We put forward its robustness and accuracy in several mission scenarios and we stress the importance of an extensive magnetic testing campaign. Under these assumptions, we deliver magnetic field and gradient estimates with 10% accuracy. Finally, the estimate of the magnetic noise contribution to the total acceleration between the two LPF’s test masses is determined with an accuracy of 15%. This result represents an enhancement of the estimation quality in one order of magnitude with respect to former studies.LISA (Laser Interferometer Space Antenna) és un missió espacial conjunta de l’ESA i la NASA, que serà el primer detector d’ones gravitacionals a l’espai. LISA consisteix en una constel·lació de tres satèl·lits situats als vèrtexs d’un triangle equilàter de 5 milions de quilòmetres de costat. La constel·lació orbitarà al voltant del Sol seguint la Terra a uns 20 graus. Cada un dels satèl·lits contindrà dues masses de prova, curosament protegides de pertorbacions externes com la pressió de la radiació solar, assegurant que estiguin en una caiguda lliure nominal en el camp gravitacional interplanetari. Les ones gravitacionals creen acceleracions diferencials entre el parell de masses de prova. Així doncs el principal objectiu de LISA és mesurar l’esmentada acceleració utilitzant interferometria làser. Les tecnologies necessàries per LISA són molt exigents. A més, la majoria d’elles no poden ser testejades a la Terra. Per tant, l’ESA va determinar la necessitat de llançar una missió precursora que actués com a demostrador tecnològic, aquesta missió és LISA Pathfinder (LPF). La seva càrrega útil és el LISA Technology Package (LTP) i serà el sensor geodèsic de més alta sensitivitat a l’espai. El LISA Technology Package està dissenyat per mesurar acceleracions diferencials entre dues masses de prova en caiguda lliure situades en un sol satèl·lit, reduint un dels braços de LISA des de 5 milions de quilòmetres fins a 35 cm. L’èxit de la missió suposaria la demostració de la maduresa de les tecnologies necessàries per LISA, com són el Optical Metrology System i el concepte Drag Free. La mesura de l’acceleració diferencial estarà afectada per certes pertorbacions com podrien ser les fluctuacions tèrmiques o els efectes magnètics a l’interior del satèl·lit. Aquestes pertorbacions són monitoritzades pel Subsistema de Diagnòstic. El Subsistema de Diagnòstic Magnètic és un dels seus mòduls i és un sistema crític, perquè el soroll magnètic representa un 40% del soroll total. Amb la finalitat d’estimar la contribució del soroll magnètic, el Subsistema de Diagnostic Magnètic ha de (1) estimar les propietats magnètiques de les masses de prova, i.e., el seu moment magnètic remanent i la seva susceptibilitat, i (2) estimar el camp magnètic i el seu gradient a la posició de les masses de prova. Així doncs, aquest subsistema integra dues bobines per generar camps magnètics a la posició de les masses. Aquests camps magnètics exciten la resposta dinàmica de les dues masses. Finalment, amb el processament de les excursions cinemàtiques proporcionades per l’interferòmetre podem estimar les característiques magnètiques amb una precisió de l’1%. D’altra banda, el Subsistema de Diagnòstic Magnètic també integra 4 magnetòmetres triaxials. No obstant, el camp magnètic i el seu gradient ha de ser mesurat a la posició de les masses de prova i les lectures dels magnetòmetres no estan situades en aquestes posicions. Per tant, cal implementar un sistema d’interpolació. Aquest problema presenta una dificultat especial perquè els magnetòmetres estan situats lluny de les masses de prova (més de 20 cm) i perquè només hi ha mesures magnètiques per realitzar una interpolació de primer ordre. Aquest mètode dóna resultats inacceptables, per tant en aquesta tesi presentem i validem un mètode d’interpolació alternatiu basat en xarxes neuronals. En demostrem la seva robustesa i exactitud en diferents casos i remarquem la importància de disposar d’una extensa campanya de tests magnètics. Sota aquests supòsits, estimem el camp magnètic i el seu gradient amb un error inferior al 10%. Finalment, l’estimat de la contribució del soroll magnètic en la mesura de l’acceleració diferencial de les dues masses de prova es pot determinar amb una exactitud del 15%. Aquest resultat suposa una millora de la qualitat d’estimació en un ordre de magnitud en comparació a estudis previs

Algorithm specification for the Magnetic Diagnostics onboard LPF

Document tècnic per la missió espacial LISA Pathfinder. Missió espacial de l'Agència Espacial Europea (ESA).Preprin

Neural Network algorithms for magnetic diagnostics in the LTP

Document tècnic per la missió espacial LISA Pathfinder. Missió espacial de l'Agència Espacial Europea (ESA).Preprin

LTP Magnetic Field Interpolation

Document tècnic per la missió espacial LISA Pathfinder. Missió espacial de l'Agència Espacial Europea (ESA).Preprin

Magnetic experiments on board the LTP

Document tècnic per la missió espacial LISA Pathfinder. Missió espacial de l'Agència Espacial Europea (ESA).Preprin

Interpolation of the magnetic field at the test masses in eLISA

A feasible design for a magnetic diagnostics subsystem for eLISA will be based on that of its precursor mission, LISA Pathfinder. Previous experience indicates that magnetic field estimation at the positions of the test masses has certain complications. This is due to two reasons. The first is that magnetometers usually back-act due to their measurement principles (i.e., they also create their own magnetic fields), while the second is that the sensors selected for LISA Pathfinder have a large size, which conflicts with space resolution and with the possibility of having a sufficient number of them to properly map the magnetic field around the test masses. However, high-sensitivity and small-sized sensors that significantly mitigate the two aforementioned limitations exist, and have been proposed to overcome these problems. Thus, these sensors will be likely selected for the magnetic diagnostics subsystem of eLISA. Here we perform a quantitative analysis of the new magnetic subsystem, as it is currently conceived, and assess the feasibility of selecting these sensors in the final configuration of the magnetic diagnostic subsystem.Peer ReviewedPostprint (author's final draft

Design of the magnetic diagnostics unit onboard LISA Pathfinder

LISA (Laser Interferometer Space Antenna) is a joint mission of ESA and NASA which aims to be the first space-borne gravitational wave observatory. Due to the high complexity and technological challenges that LISA will face, ESA decided to launch a technological demonstrator, LISA Pathfinder. The payload of LISA Pathfinder is the so-called LISA Technology Package, and will be the highest sensitivity geodesic explorer flown to date. The LISA Technology Package is designed to measure relative accelerations between two test masses in nominal free fall (geodesic motion). The magnetic, thermal and radiation disturbances affecting the payload are monitored and dealt by the diagnostics subsystem. The diagnostics subsystem consists of several modules, and one of these is the magnetic diagnostics unit. Its main function is the assessment of differential acceleration noise between test masses due to the magnetic effects. To do so, it has to determine the magnetic characteristics of the test masses, namely their magnetic remanences and susceptibilities. In this paper we show how this can be achieved to the desired accuracy.Preprin