Non-Geostationary Satellites

Description

Non-Geostationary Satellites (NGSO) are satellites that orbit the Earth at altitudes lower than geostationary orbit (35,786 km), typically in Low Earth Orbit (LEO, 500–2,000 km) or Medium Earth Orbit (MEO, 2,000–35,786 km). Their orbital periods range from approximately 1.5 hours for LEO to about 10 hours for MEO, meaning they do not remain fixed relative to a point on the Earth's surface. This mobility necessitates a constellation of multiple satellites to ensure continuous coverage, along with sophisticated handover mechanisms to maintain service as satellites move in and out of view.

In the context of 3GPP, NGSO satellites are a key component of Non-Terrestrial Networks (NTN), which aim to extend 5G coverage to remote, underserved, or disaster-affected areas. 3GPP Release 19 introduced specifications for NGSO-based NTN, particularly in TS 38.742 (Study on New Radio (NR) to support non-terrestrial networks) and TS 38.863 (Study on enhanced NR support for non-terrestrial networks). These documents define the architecture, radio interface, and procedures for integrating NGSO satellites into the 5G system.

The operation of NGSO satellites in 5G involves several unique challenges. First, the high relative velocity between the satellite and the ground user (up to 7.5 km/s for LEO) causes significant Doppler shift, which must be compensated by the UE and the network. Second, the propagation delay is much larger than in terrestrial networks, varying from a few milliseconds to tens of milliseconds, requiring adjustments to timing advance and HARQ processes. Third, the continuous movement of satellites means that the serving satellite changes frequently, necessitating seamless handover between satellites and between satellite and terrestrial cells.

To address these challenges, 3GPP has defined enhancements to the NR air interface, including extended timing advance, frequency pre-compensation, and mobility procedures tailored for NTN. The satellite constellation is typically managed by a gateway that connects the satellite network to the 5G core network. The gateway tracks satellite positions and schedules handovers, ensuring that the UE maintains connectivity as satellites pass overhead.

NGSO satellites can operate in both transparent and regenerative modes. In transparent mode, the satellite acts as a repeater, amplifying and forwarding signals between the UE and the gateway without demodulation. In regenerative mode, the satellite carries a gNB (next-generation NodeB) and performs baseband processing, effectively acting as a base station in the sky. Regenerative mode reduces latency and backhaul requirements but increases satellite complexity.

The integration of NGSO satellites into 5G is a major step toward ubiquitous connectivity, enabling services in areas where terrestrial infrastructure is impractical or unavailable. It also supports new use cases such as maritime and aeronautical communication, disaster recovery, and remote industrial IoT. As the technology matures, NGSO constellations are expected to play a vital role in the global 5G ecosystem.

Purpose & Motivation

Non-Geostationary Satellites exist to provide 5G coverage in areas where terrestrial networks are not economically or technically feasible, such as oceans, deserts, mountains, and remote rural regions. Unlike geostationary satellites, which are fixed relative to the Earth but suffer from high latency (approximately 250 ms round-trip), NGSO satellites operate at lower altitudes, reducing latency to levels compatible with 5G requirements (e.g., 10–50 ms for LEO). This makes them suitable for interactive services like voice and video.

The primary motivation for using NGSO satellites in 5G is to extend the reach of mobile networks beyond the limits of fiber and microwave backhaul. Terrestrial networks cover only about 20% of the Earth's landmass and a small fraction of the oceans, leaving vast areas unserved. NGSO constellations can blanket the entire globe, providing connectivity to ships, aircraft, and remote communities. They also offer resilience against natural disasters, as they are not dependent on ground-based infrastructure that can be damaged.

Historically, satellite communication was dominated by geostationary satellites, which are simple to manage because they appear stationary from the ground. However, their high latency and high cost of launch made them unsuitable for many 5G use cases. The advent of low-cost launch vehicles and miniaturized satellite technology has made NGSO constellations economically viable, leading 3GPP to standardize their integration into 5G. The Release 19 work on NTN represents a significant milestone, as it brings satellite and terrestrial networks under a unified 5G framework, enabling seamless roaming and service continuity.