  The emergence of low Earth orbit (LEO) satellite mega-constellations is
dynamically transforming the space sector. While free-space optical (FSO) links
efficiently facilitate intersatellite data forwarding, they suffer from
atmospheric/weather conditions in the space-to-ground link. This study delves
into utilizing high-altitude platform stations (HAPS) as elevated relay
stations strategically positioned above terrestrial ground stations. We
introduce the concept of high-altitude ground stations (HAGS), an innovative
approach to enabling the development of all optical LEO satellite
constellations. The first contribution is an analysis of the HAGS-based network
architecture where the LEO spacecraft only hosts FSO transceivers. Secondly, we
execute an extensive simulation campaign to determine the gain of HAGS,
including a new equivalency model with the traditional ground station approach.
Finally, we examine the research challenges of implementing HAGS-based, all
optical LEO mega-constellations.
