# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 1 Introduction  
Historically, the commercial satellite communications (SatCom) sector has been dominated by geosynchronous orbits (GSOs), especially geostationary orbits (GEOs) [1]. However, in recent years the use of non-geosynchronous orbits (NGSOs) has been a steadily growing feature of the industry [2]. GSO and NGSO systems have been typically designed in isolation, serving different markets and customers. Both systems have their own strengths. GSO systems can offer broad contiguous service regions, fixed ground antenna orientations, longer satellite lifetimes, a simpler frequency coordination environment, lower financial costs to achieve regional coverage, and lower maintenance/operational costs. On the other hand, NGSO systems, such as low Earth orbit (LEO) constellations, can offer improved latency, significantly reduced path losses enabling miniaturization of ground terminals, have the potential to provide truly global coverage also including the polar areas, and support the maximum total system capacity within a given spectrum allocation. Future SatCom system topologies should benefit a range of diverse space elements, integrating together GEO, LEO, or medium Earth orbit (MEO) satellites [3]. This leads to the multi-layered SatCom system (MLSS) architecture studied previously, e.g., between LEO and MEO systems in [4, 5]. The MLSS definition we adopt is that of a satellite communication network that aggregates multiple satellite layers, where different layers refer todifferent orbiting altitudes of satellite constellations and that has been thoroughly designed to operate as a multi-layer system, rather than a loose aggregation of different layers. Consequently, we make an assumption that an MLSS is expected to have at least two satellite orbital layers in addition to potentially any other terrestrial layers. Such a system should be capable of supporting the full range of both current and future satellite service types, but with increased overall spectral efficiency and interoperability with aerial platforms, such as high-altitude platforms (HAPSs) and unmanned aerial vehicles (UAVs), and terrestrial networks. There have been numerous papers published in recent years proposing three-dimensional (3D) satellite-terrestrial network architectures [6; 7; 8]. Those papers have covered topics such as human-centric application areas and high-security requirements [6], the role of 5G non-terrestrial networking (NTN) to support 6G requirements [7; 8], and the role of satellite communications to support remote areas and sustainable development [8]. The findings reported in these papers have pointed out the importance of LEO satellite systems but also that the role of satellites in higher orbits remains important for resilience [9; 10]. Integration of UAV networks and satellites has been studied in [11; 12; 13], which covered local capacity enhancements, interference management, and trajectory optimizations. However, a detailed analysis of modern MLSS systems covering technical enablers, economic factors, and the current regulatory landscape is missing in the literature. While several review papers consider conceptually the full space-aerial-terrestrial integrated network, the authors pay less attention to the important MLSS part of the overall network. Therefore, in this paper, we will first conduct a market analysis to present the most potential application areas. Through technology research, we then aim to answer the following research questions:  
* How does the introduction of multiple satellite layers benefit and challenge future SatCom system architecture? * What are the main enablers for more optimized coordination of different layers in future MLSS systems? To answer those questions, one must consider the whole network topology and space segment design, as well as access protocols, routing, network operations, inter-satellite link (ISL) technologies, and ground antenna technologies. The novelty of this paper is four-fold:  
1.