  Global Navigation Satellite Systems (GNSS) provide ubiquitous, continuous and
reliable positioning, navigation and timing information around the world.
However, GNSS space segment design decisions have been based on the precursor
Global Positioning System (GPS), which was designed in the 70s. This paper
revisits those early design decisions in view of major technological
advancements and new GNSS environmental threats. The rich tradespace between
User Navigation Error (UNE) performance and constellation deployment costs is
explored and conclusions are complemented by sensitivity analysis and
association rule mining. This study finds that constellations at an orbit
altitude of ~2 Earth radii can outperform existing GNSS in terms of cost,
robustness and UNE. These insights should be taken into account when designing
future generations of GNSS.
