Abstract
The proliferation of low Earth orbit (LEO) constellations for observation, communications, and navigation has intensified the demand for space-based relay systems that provide continuous inter-satellite links. Existing geostationary Earth orbit (GEO) relay systems suffer from high-latitude blind zones and cannot achieve seamless global coverage. This study proposes a heterogeneous joint-orbit relay constellation integrating GEO, IGSO, and MEO satellites to deliver continuous coverage for LEO users at 200 km–2000 km altitudes. A multi-altitude coverage evaluation model is developed, incorporating inter-satellite visibility geometry, beamwidth, and minimum elevation angle constraints. Five candidate configurations are optimized using the NSGA-III multi-objective algorithm with total satellite count and weighted coverage multiplicity as dual objectives. The optimal GEO+IGSO+Walker configuration, comprising 14 satellites, achieves global minimum twofold coverage throughout the constellation period and offers the best balance of relay performance, robustness, and engineering feasibility. This ensures that every LEO user within the target altitude range is simultaneously served by at least two relay satellites at any epoch, guaranteeing reliable, uninterrupted data relay communication links between LEO spacecraft and ground stations.