Space missions to visit small bodies of the Solar System are important steps to improve our knowledge of the Solar System. Usually those bodies do not have well known characteristics, as their gravity field, which make the mission planning a difficult task. The present paper has the goal of studying orbits around the triple asteroid 2001SN 263 , a Near-Earth Asteroid (NEA). A mission to this system allows the exploration of three bodies in the same trip. The distances reached by the spacecraft from those three bodies have fundamental importance in the quality of their observations. Therefore, the present research has two main goals: (i) to develop a semi-analytical mathematical model, which is simple, but able to represent the main characteristics of that system; (ii) to use this model to find orbits for a spacecraft with the goal of remaining the maximum possible time near the three bodies of the system, without the need of space maneuvers. This model is called ''Precessing Inclined Bi-Elliptical Problem with Radiation Pressure" (PIBEPRP). The use of this model allow us to find important natural orbits for the exploration of one, two or even the three bodies of the system. These trajectories can be used individually or combined in two or more parts using orbital maneuvers.

Abstract    This work is concerned with orbit determination using GPS signals. An especial case of truncated arcs assuming that GPS receiver data, which is available only when the satellite carrying the receiver flies over a ground tracking station, is the main issue. To analyse the behaviour of an Extended Kalman filter (EKF) in real time satellite orbit determination using short arcs of data, the algorithm developed by Chiaradia et.al. (2013) is used. This algorithm has been formerly qualified using the raw single frequency pseudorange GPS measurements of the Topex/Poseidon (T/P) satellite as observations by the Kalman Filter, and for this reason, is used as reference in this work. However, these real data are truncated as if they had been collected by the tracking and control station of INPE in Cuiabá, Brazil. That is, the data are obtained only when the satellite T/P is in the viewing area of the Cuiaba Brazilian station. The behaviour of the Kalman filter is analysed under such premises. The results of research are presented showing the degradation of performance when compared to the full arc orbit determination. Keywords    Kalman filter, short arcs, GPS, orbital dynamics. Resumo    Este trabalho trata-se de determinação de órbita usando sinais GPS. Um caso especial de arcos truncados é a questão principal, isto é, será suposto que os dados do receptor GPS somente estarão disponíveis quando o satélite com um receptor GPS abordo estiver sobrevoando uma estação terrestre de rastreamento. Para analisar o comportamento de um filtro de Kalman estendido (EKF) na determinação de órbita de satélites em tempo real usando arcos curtos de dados é usado o algoritmo desenvolvido por Chiaradia et. al. (2013). Este algoritmo foi anteriormente qualificado usando as medidas GPS de pseudodistância real em uma única frequência do satélite Topex/Poseidon (T/P) como dados de observações do filtro Kalman e, por esta razão, é usado como referência neste trabalho. No entanto, estes dados reais são truncados, como se eles tinham sido coletados pela estação do Centro de Controle e Rastreamento do INPE em Cuiabá, Brasil. Ou seja, os dados são obtidos somente quando o satélite T/P está na área de visibilidade da estação brasileira de Cuiabá. O comportamento do filtro de Kalman é analisado. Os resultados da pesquisa são apresentados mostrando a degradação do desempenho quando comparado com a determinação da órbita de arco completo. Palavras-chave Filtro de Kalman, arcos curtos, GPS, dinâmica Orbital.

This work is concerned with short arcs orbit determination using GPS signals. A special case of truncated arcs assuming that GPS data is only available when the satellite carrying the GPS receiver passes over a ground tracking station is presented. The behaviour of an Extended Kalman filter (EKF) in real time satellite orbit determination using short arcs of data is analysed. The algorithm is a simplified and compact model with low computational cost, and uses the EKF to estimate the state vector, composed of position and velocity components, and GPS receiver clock parameters. The algorithm may use different step-sizes between the GPS signal measurements. Its force model in the motion equations considered the perturbations as being due to the geopotential up to the 10thorder and degree of the spherical harmonics. The algorithm has been formerly qualified using raw single frequency pseudorange GPS measurements of the Topex/Poseidon (T/P) satellite, and used as reference in this work. However, the GPS data are truncated as if they had been collected by a single ground tracking station. In other words, the data are obtained only when the satellite T/P is within the viewing area of the station. The research results are presented showing the degradation of performance when compared to a full arc orbit determination.