Geostationary-Satellite Orbit

Description

Geostationary-Satellite Orbit (GSO) refers to a specific high-altitude orbit used for telecommunications satellites. A satellite in GSO is positioned at an altitude of approximately 35,786 kilometers directly above the Earth's equator. At this altitude, the satellite's orbital period is exactly 24 hours, synchronizing with the Earth's rotational period. Consequently, when observed from the ground, the satellite appears stationary in the sky. This characteristic is crucial for establishing fixed ground antenna pointing, simplifying the ground station and user terminal design, as they do not need to track satellite movement.

Within the 3GPP framework, starting from Release 15, GSO satellites are defined as a component of Non-Terrestrial Networks (NTN). The 3GPP specifications define the technical parameters for integrating GSO satellites into the 5G NR radio access network. This includes defining the specific radio characteristics, such as the very large propagation delay (approximately 250 ms one-way) and Doppler shift characteristics, which are negligible for GSO compared to Low Earth Orbit (LEO) satellites due to the fixed relative position. The radio interface must be adapted to handle these unique channel conditions.

The system architecture for GSO-based NTN involves the satellite acting as a radio relay node, or in some scenarios, a base station (gNB). The satellite communicates with User Equipments (UEs) on the service link and with a ground-based gateway station on the feeder link. The gateway then connects to the 5G core network. Key challenges addressed in the specifications include timing advance management for the enormous delay, handling of discontinuous coverage (for regenerative payloads), and mobility procedures adapted for a virtually fixed cell from the user's perspective. The radio specifications (e.g., 38.101, 38.306) define frequency bands, UE requirements, and performance aspects for operation with GSO satellites.

Purpose & Motivation

The integration of GSO satellites into 3GPP standards was motivated by the need to provide seamless global coverage, including in remote, maritime, and aerial areas where terrestrial networks are economically or physically impractical to deploy. Traditional terrestrial cellular networks have coverage gaps that satellites are uniquely positioned to fill. GSO satellites, with their fixed footprint covering roughly a third of the Earth's surface, offer a proven and reliable method for broadcast and wide-area communications.

3GPP's work on NTN, including GSO, aims to unify terrestrial and non-terrestrial networks under a single 5G system architecture. This creates a true global network, enabling service continuity for users moving between terrestrial and satellite coverage. It also allows for new use cases like massive IoT sensor networks in remote areas, backhaul for terrestrial networks, and communications for transportation sectors (aviation, shipping). GSO was included alongside LEO and MEO orbits to provide a range of solutions balancing coverage area, latency, and infrastructure cost, with GSO offering the advantage of continuous coverage over a vast region with a small number of satellites.