Impact of eccentricity build-up and graveyard disposal strategies on MEO navigation constellations

With currently two constellations being in or close to the build-up phase, in a few years the Medium Earth Orbit (MEO) region will be populated with four complete navigation systems in relatively close orbital altitudes: The American GPS, Russian GLONASS, European Galileo, and Chinese BeiDou. To guarantee an appropriate visibility of constellation satellites from Earth, these constellations rely on certain defined orbits. For this, both the repeat pattern, which is basically defined by the semimajor axis and inclination, as well as the orbital planes, which are defined by the right ascension of ascending node, are determining values. To avoid an overcrowding of the region of interest, the disposal of satellites after their end-of-life is recommended. However, for the MEO region, no internationally agreed mitigation guidelines exist. Because of their distances to Earth, ordinary disposal manoeuvres leading to a direct or delayed re-entry due to atmospheric drag are not feasible: The needed fuel masses for such manoeuvres are by far above the reasonable limits and available fuel budgets. Thus, additional approaches have to be applied. For this, in general two options exist: disposal to graveyard orbits or the disposal to eccentricity build-up orbits. In the study performed, the key criterion for the graveyard strategy is that the disposed spacecraft must keep a safe minimum distance to the altitude of the active constellation on a long-term time scale of up to 200 years. This constraint imposes stringent requirements on the stability of the graveyard orbit. Similar disposals are also performed for high LEO satellites and disposed GEO payloads. The eccentricity build-up strategy on the other hand uses resonant effects between the Earth's geopotential, the Sun and the Moon. Depending on the initial conditions, these can cause a large eccentricity build-up, which finally can lead to a re-entry of the satellite. In this paper, the effects of applying either the first or the second disposal strategy on all payloads from all MEO navigation constellations are investigated. Long-term simulations over 200 years of the overall space debris environment using the tool LUCA (Long-Term Utility for Collision Analysis) are performed. Furthermore, a reference scenario was implemented. This reference scenario mimics the disposal strategies used up to the current date by active constellations (GPS and GLONASS), and proposes similar approaches for the future constellations (Galileo and BeiDou). The results of these simulations are analysed with regard to flux and collision probabilities. To this end, intra-constellation impacts between disposed and active satellites are shown, as well as inter-constellation impacts between disposed satellites from other constellations and active payloads. Additionally, the impact of eccentricity build-up disposal on other orbital regimes such as LEO and GEO is analysed. For all performed analyses, a special focus is put on the Galileo constellation.