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Aquest treball analitza els efectes de la radiacio a l'espai sobre components electronics. En particular, considerem els efectes de la radiacio sobre el Wikisat, un femto-satel?lit desenvolupat a l'EETAC que es llancara al 2012 i orbitara a 250 km sobre la superficie terrestre, amb una inclinacio de 28?. Al llarg del treball determinem els valors de les magnituds rellevants que descriuen la intensitat de la radiacio (fluencies i fluxos) a una orbita baixa (LEO), semblant a la que s'espera que descrigui el Wikisat i, per poder-ho comparar, tambe a una orbita d'altura mitjana (MEO). En tots dos casos es considera tant l'efecte dels protons i electrons atrapats com l'efecte de raigs cosmics i successos solars aillats. Per estudiar la interaccio del Wikisat amb la radiacio hem dissenyat un model de capes a escala amb els diferents components que el formen, amb i sense la bateria. A partir d'aquest model i de les condicions de radiacio a cada orbita hem pogut determinat els valors de les dosis (en rad) que cal esperar que s'acumulin a cada capa del model i, finalment, establir si aquestes dosis poden suposar un risc per la missio. Els nostres resultats confirmen que les probabilitats de supervivencia del Wikisat en orbita baixa son elevades, pero que un dispositiu com aquest a orbita mitjana requeriria l'us de blindatge per poder apantallar els efectes de la radiació

The present work analyses the requirements related to the orbit and the effects of the space radiation on Polosat, a toy-model satellite whose main goal is to monitor and image the Earth Poles. Our main objective has been to work on the so-called phase A of the design of a space mission, in which a preliminary study of the viability and characteristics of the mission is made. We present this project as a coordinated work due to the fact that its competition required the use of two relatively complex software tools, as well as the achievement of extra theoretical background on orbital mechanics, radiation and its effects. One of the software tools was STK (Satellite Tool Kit). It allows to compute with realism and high degree of accuracy the time evolution of the kinematics of an object in orbit around the Earth. The other tool was SPENVIS (Space Environment Information System). It also computes the time evolution of the orbital parameters of an object -although with less option for realism and accuracy than STK, and the characteristics of the incident radiation and its effects of the orbiting object. Both software tools are used in professional and academic environments and, therefore, becoming familiar with them and their use has been an important and useful part of our work. We compare the results that STK and SPENVIS yield for the computation of the orbital kinematics in the cases where both tools are appropriate (cases with negligible atmospheric drag and limited intervals of time). We check that in such cases STK and SPENVIS results yield a very good agreement and, most importantly, we have found the procedure in which results from STK can be transferred to SPENVIS, in order to model the situations we are interested in. As we show, by using both codes, we can take advantage of both the powerful numerical tools of STK in order to study orbital mechanics, and the capabilities of SPENVIS in order to model the incidence of radiation and its effects on the spacecraft. Finally, we have concluded that both STK and SPENVIS are convenient tools for developing the proposed project and have determined the way in which we can use them in order to accomplish our objectives.

Context: Attitude control of a satellite relies upon a precise determination of its inertia tensor. However, the determination of the inertia tensor of a complex system, as is the case of satellites, completely escapes analytic methods. Even though the inertia tensor can be obtained by means of CAD models, this method is subject to a degree of uncertainties due to required approximations in the materials definition and the geometry of some components. Furthermore, cabling is often difficult to integrate into CAD designs, which may introduce an additional source of error. Aims: In this Degree Thesis, we propose the design of a torsion pendulum which will allow us to determine all the components of the inertia tensor of a CubeSat-type object. We intend to achieve more efficiency and better accuracy than performing standard CAD simulations. Methodology: Our starting point is a general idea of a pendulum, Our sketch involves a set of fixed parts, mostly with structural purposes, and a set of mobile parts. The latter must be able to rotate about the axis of a torsion fiber. Besides, it must connect to a platform on top of which the CubeSat whose tensor of inertia we intend to determine will be placed. By measuring the period of rotation of the mobile parts (which include the platform and CubeSat), it is easy to derive the inertia moment of rotating set about the axis given by the torsion fiber. The detailed design of the pendulum was executed with the software SolidWorks. Foundations of Classical Mechanics and rotation matrix Algebra, as well as the software MatLab were required to develop the operation analysis of the proposed pendulum. Results: We designed, obtained detailed maps, and developed stress analysis calculations of the pendulum. Besides, we completed the mathematical analysis which allows to build the inertia tensor of a CubeSat, by using individual inertia moments, derived from a set of period measurements obtained with different orientations of the CubeSat. Finally, we used basic error analysis to show that the components of the inertia tensor can be obtained with the desire precision by using feasible period measurements. Contexto: El control de actitud de un satelite se basa en una determinaci ´ on precisa de ´ su tensor de inercia, sin embargo, la determinacion del tensor de inercia de un sistema ´ complejo, como es el caso de los satelites, escapa por completo a los m ´ etodos anal ´ ´ıticos. Aunque el tensor de inercia se puede obtener por medio de modelos CAD, este metodo ´ esta sujeto a un grado de incertidumbre debido a las aproximaciones requeridas en la ´ definicion de materiales y en la geometr ´ ´ıa de algunos de los dispositivos. Ademas, el ´ cableado a menudo es dif´ıcil de integrar en los disenos ˜ CAD, e introduce una fuente de error adicional. Objetivos: En este trabajo de final de grado, proponemos realizar el diseno y el an ˜ alisis ´ de las bases del funcionamiento de un pendulo de torsi ´ on que permite determinar todos ´ los componentes del tensor de inercia de un objeto tipo CubeSat, con mayor eficiencia y precision que la que nos permitir ´ ´ıa un analisis de ´ CAD. Metodolog´ıa: Partimos de una idea general de un pendulo, que incluye un conjunto de par- ´ tes fijas con funcion principalmente estructural, y de partes que pueden rotar alrededor de ´ una fibra de torsion, unida a una plataforma sobre la cual se puede situar el ´ CubeSat cuyo tensor de inercia deseamos conocer. A partir de las medidas del periodo de rotacion de ´ la plataforma podemos determinar de manera sencilla el momento de inercia del conjunto de partes giratorias alrededor del eje de la fibra de torsion. Para realizar el dise ´ no deta- ˜ llado del dispositivo utilizamos el software SOLIDWORKS. Para analizar las bases de su funcionamiento utilizamos conocimientos de Mecanica Cl ´ asica y ´ Algebra de matrices de ´ rotacion, as ´ ´ı como el software MATLAB. Resultados: Hemos disenado, presentado los planos, y realizado un an ˜ alisis num ´ erico de ´ estres sobre el p ´ endulo propuesto. Adem ´ as hemos completado el desarrollo matem ´ atico ´ que permite construir el tensor de inercia del CubeSat, a partir de valores individuales de momentos de inercia, derivados de las medidas del periodo de rotacion del sistema ´ con distintas orientaciones del CubeSat. Finalmente, nuestro calculo de errores b ´ asico ´ muestra que podemos obtener valores de las componentes del tensor de inercia con la precision deseada, a partir de medidas de periodos factibles.

This project aims to study the drag coefficient (CD) for satellites in a free molecular flow. The study is mainly based on Schaaf & Chambre and Schamberg models. These are old models (from 1958), but still are the most widely used nowadays. The two models are analyzed for flat and spherical geometries, considering both specular and diffuse reflection. In order to set the scenario of our study, we describe the thermosphere and justify the interest of studying this region of the terrestrial atmosphere. Then, we describe the specific type of flux considered in this work. The parameters which divide the behaviour of the gases into different regimes, as well as the basic characteristics of the free molecular flow and the interaction between it and the surface of the satellite, are explained. The first model studied is the one by Schaaf and Chambre, considering the two types of reflection for each geometry. We observe that, in the case of flat plate geometry, the angle of attack is the most important parameter for the determination of CD. For the spherical case, we see that CD adopts almost constant values, since the angle of attack is irrelevant due to the geometry, and the temperature does not cause great variations. With respect to Schamberg model we obtain large variations of CD due to variations in the angle of attack, as in the previous model. The case of the sphere also gives a constant CD in the specular case. There is also a clear dependency on the wall temperature for the diffuse and intermediate cases. The values obtained for CD under the diffuse reflection show large discrepancies between models. In fact, Schamberg's model gives values that are at odds with empirical values under certain conditions. For this reason, we also analyze a third model: Sentman's model for diffuse reflection. This model gives results similar to those of Schaaf & Chambre. Additionally, the bibliography points to serious shortcomings of Schamberg's model in the diffuse reflection case and low angles of attack. We have checked the practical necessity of combining these models for different reflection conditions. We have concluded that a first study of CD (and, eventually, of the thermospheric density) should make use of spherical satellites in order to minimize the number of relevant variables, and to better identify the most influential parameters. The problem of launching spherical satellites could be solved by holding them inside a suitable satellite dispenser, perhaps with a CubeSat-like configuration, and release them once in orbit. These satellites dispensers could release either one or several spherical satellites.