Highly-Eccentric Orbiting satellites

Description

Highly-Eccentric Orbiting (HEO) satellites are a class of spacecraft operating in elliptical orbits with a high eccentricity, typically characterized by a low perigee (closest point to Earth) and a very high apogee (farthest point). This orbital geometry results in the satellite spending a prolonged period near its apogee, where its velocity is lowest, effectively creating a 'quasi-geostationary' appearance over a specific region, particularly beneficial for high-latitude areas. Within the 3GPP framework, HEO systems are analyzed as a component of Non-Terrestrial Networks (NTN) to extend 5G New Radio (NR) and LTE services. The architecture involves the satellite acting as a relay node or a base station (gNB in NR), connecting User Equipment (UE) on the ground to a Gateway Earth Station, which then interfaces with the 5G Core Network. Key technical considerations include managing the long propagation delays (ranging from tens to over a hundred milliseconds), significant Doppler shifts due to the satellite's high relative velocity during parts of the orbit, and the need for large beam footprints or steerable spot beams to maintain coverage as the satellite moves. The radio interface must be adapted to handle these challenges, involving enhancements to synchronization, timing advance, and handover procedures. HEO constellations are envisioned to operate in various frequency bands, including L, S, C, and Ka bands, as defined in 3GPP studies for satellite radio interface and deployment scenarios. Their role is to provide seamless service continuity, backhaul connectivity, and direct-to-device services in areas where terrestrial infrastructure is economically unviable or physically impossible to deploy, such as polar regions, oceans, and air routes. Integration with the 5G system requires modifications to core network functions for mobility management, session management, and QoS handling to account for the unique satellite link characteristics.

Purpose & Motivation

The primary purpose of standardizing HEO satellite support in 3GPP is to enable global and ubiquitous 5G coverage, a key objective of IMT-2020. Traditional terrestrial networks have limited reach, leaving vast geographical areas, including maritime, aerial, and polar regions, without connectivity. HEO satellites address this coverage gap by leveraging their unique orbital mechanics to provide persistent, high-latitude coverage that Geostationary Earth Orbit (GEO) satellites often struggle with due to their equatorial alignment and low elevation angles at high latitudes. Historically, satellite communication systems operated in isolation from cellular standards, leading to fragmented services and complex user equipment. The integration into 3GPP, starting from Release 14 study items, aims to unify terrestrial and non-terrestrial networks under a common framework, allowing for economies of scale, reduced device complexity, and seamless service experience. This addresses limitations of previous proprietary satellite systems which were not natively integrated with cellular core networks, hindering features like mobility, network slicing, and consistent QoS. The motivation is driven by growing demand for connectivity everywhere, support for Internet of Things (IoT) in remote areas, and governmental mandates for emergency and public safety communications. HEO satellites, with their favorable high-latitude dwell time, present a complementary solution to Low Earth Orbit (LEO) mega-constellations and GEO systems, forming a heterogeneous NTN ecosystem.