Estimate problems for satellite clusters

Accurate determination of the kinematic state of a spacecraft is quite important in order to correctly exploit its mission and to address payload operation. Such a determination can be a challenging task, especially if close spacecraft are involved or if certain mission phases (separation, rendezvous etc.) are considered. Traditional estimation is based on Kalman Filtering, in both the linear and the non linear (extended) versions. Progress in computation power allows to consider alternative approaches as the unscented transformation or the particle filters, possibly able to cope with critical” situation. All these techniques can be applied to the orbit determination of a single spacecraft, or repeated for all spacecraft belonging to a formation or cluster. Recent developments suggest that the estimation can take into account a sort of global motion of the spacecraft cluster, leading to the approach known as dense target tracking. In such a way, a number of bulk variables can be considered as acting on all involved satellite, or, alternatively, some variables related to a single satellite can be strictly related to the corresponding ones of the other bodies, therefore reducing the number of degrees of freedom for the system. These estimate techniques do not simply translate to iterating the single platform approach, and should apply quite well when an external observer, unaware of some system characteristics, is considered. The paper is aimed to consider these different techniques and their application to the orbital determination in formation flying. Algorithms implemented in extensive numerical simulations are reported, and relevant performances, both in accuracy and in computation time, are discussed. The possibility to include a “global motion” concept, leading to dense target tracking, is investigated