Satellite Constellation

Description

A satellite constellation in 3GPP terminology is defined as a set of satellites working together as a system or network. These satellites may be placed in the same orbit type or in different orbits, including Geostationary Orbit (GSO) and Non-Geostationary Orbit (NGSO), with varying characteristics such as altitude, inclination, and coverage footprint. The concept is introduced in Rel-20 and specified in TR 22.870, which studies satellite access in 5G-Advanced and beyond. The constellation is the space segment of a Non-Terrestrial Network (NTN) and determines the overall coverage, capacity, and service continuity characteristics.

Constellations are designed to provide continuous or near-continuous coverage over a target area. GSO satellites appear stationary relative to the Earth, offering stable coverage but with high latency (approximately 250 ms one-way) and limited coverage at high latitudes. NGSO constellations, particularly LEO, offer lower latency (5–20 ms) but require many satellites and complex handover management because each satellite is visible only for a few minutes. A constellation may combine GSO and NGSO satellites to balance coverage, capacity, and latency, or use multiple orbital planes to ensure global coverage.

From a 3GPP perspective, the constellation is abstracted as a network of access nodes. Each satellite can serve as a gNB or relay, and the constellation as a whole must support mobility procedures, timing advance, and Doppler compensation. The network must track satellite ephemeris data to predict coverage and manage handovers between satellites and between satellite and terrestrial cells. Inter-satellite links (ISL) may be used for routing and synchronization, forming a mesh network in space.

Key design parameters include orbital altitude, number of satellites, number of orbital planes, inclination, and inter-satellite link availability. These parameters affect the link budget, the number of handovers per unit time, and the overall system capacity. The constellation also influences regulatory and spectrum sharing considerations, as NGSO systems must avoid interference with GSO systems and terrestrial networks.

In 3GPP, the satellite constellation is not specified as a physical entity but as a functional concept that impacts network architecture, mobility, and service continuity. The study in TR 22.870 identifies use cases such as global IoT coverage, disaster recovery, and mobile broadband in unserved areas, all of which rely on the constellation's ability to provide ubiquitous connectivity. The constellation concept also enables new business models, such as satellite operators leasing capacity to mobile network operators.

Purpose & Motivation

The satellite constellation concept exists to provide continuous, wide-area coverage that a single satellite cannot achieve. A single satellite, whether GSO or NGSO, has a limited coverage footprint and, for NGSO, is visible only for a short period. By coordinating multiple satellites, a constellation ensures that at least one satellite is always in view of a given user or area, enabling seamless service.

Historically, satellite systems like Iridium and Globalstar demonstrated the feasibility of LEO constellations for voice and data, but they used proprietary protocols. The integration of constellations into 3GPP standards allows mobile operators to leverage these space assets as part of a unified 5G network, extending coverage without deploying terrestrial infrastructure. The motivation is to serve remote areas, maritime and aeronautical routes, and emergency scenarios where terrestrial networks are unavailable or destroyed.

The limitations of previous approaches included high cost, limited capacity, and lack of interoperability with cellular networks. 3GPP's study of satellite constellations in Rel-20 addresses these by defining how constellations interact with the 5G core, how mobility is managed across satellites, and how quality of service can be maintained despite varying latency and bandwidth.