MEO dynamics and GNSS disposal strategies

In the last years significant understandings in the knowledge of the dynamics of the MEO satellites were achieved. Much work was done in the analysis of viable disposal strategies and technologies for the spacecraft belonging to the Global Navigation Satellite Systems (GNSS), with particular emphasis on the European Galileo system. In the framework of an ESA-ESOC Contract an extensive numerical simulations of different long term evolution scenarios, implementing different disposal strategies were performed. A detailed analysis of the collision risk and manoeuvres need, related to the different scenarios, was performed. In terms of the long term evolution, the scenarios where the orbital instabilities are exploited to remove the objects from the operational regions seems favourite. That is, if the focus is on the long term sustainability of the space environment, the possibility to dilute the collision risk and to aim at the re-entry in the atmosphere of a subset of the disposed GNSS spacecraft is the most attractive. The most "problematic" constellations are Glonass and Beidou. This conclusion is driven by the future launch traffic hypothesized for these constellations and by the past practices that left already a significant number of large uncontrolled spacecraft in the constellation orbital zone, in the case of Glonass. On the other hand, the Galileo constellation is well detached from the others and faces the lowest collision risks. The Stable scenarios seems to minimize the interactions (crossings) with the operational constellations and, therefore, might be preferred for operational reasons. In particular, in the Stable scenarios the inter-constellations interaction is negligible. Particular care should be devoted to the efficiency and reliability of the disposal manoeuvers. A significant share of the collision risk faced by the operational satellites in every simulated scenario can be traced back to the ``failed'' satellites (the success rate of the disposal manoeuvers was assumed to be 90 % for all the constellations).