Ephemeris (NTN)

Description

In the context of 3GPP Non-Terrestrial Networks (NTN), an ephemeris is a set of parameters that describe the movement of an NTN node (e.g., a satellite or an airborne platform) over time. This information is crucial for the network to predict the node's position and velocity, enabling accurate timing advance, Doppler compensation, and handover decisions. The ephemeris data is typically provided by the NTN node itself or by a ground-based control center and is used by both the User Equipment (UE) and the network (e.g., gNB or eNB) to maintain synchronization and optimize resource allocation. The concept was introduced in Release 17 as part of the initial NTN support in 5G NR and LTE.

Technically, an ephemeris consists of parameters such as orbital elements (for satellites) or flight path data (for airborne platforms). For satellites, common formats include Two-Line Elements (TLE) or equivalent representations like position and velocity vectors at a given epoch, along with perturbation models. These parameters allow the calculation of the node's position and velocity at any future time. In 3GPP, the ephemeris is transmitted to the UE via System Information Block (SIB) messages, specifically SIB19 for NR NTN (as defined in TS 38.331 and TS 36.331 for LTE). The UE uses this data to pre-compensate for the large propagation delay and Doppler shift inherent in satellite communications, ensuring that uplink transmissions arrive at the satellite within the correct timing window. The network also uses ephemeris to schedule handovers between satellites and to manage beam pointing.

The key components of an ephemeris for NTN include the reference frame (e.g., Earth-Centered Inertial or Earth-Centered Earth-Fixed), the epoch time, and the set of parameters that define the trajectory. For satellites in Low Earth Orbit (LEO), the ephemeris might include semi-major axis, eccentricity, inclination, right ascension of the ascending node, argument of perigee, and mean anomaly at epoch. For simpler cases, a position and velocity vector at a given time may suffice. The accuracy of the ephemeris directly impacts the performance of the NTN link; hence, updates are provided periodically to account for orbital perturbations and maneuvers.

The use of ephemeris in NTN enables several critical functions: (1) Timing Advance (TA) pre-compensation: UEs calculate the round-trip time to the satellite based on ephemeris and adjust their uplink timing to avoid interference; (2) Doppler shift pre-compensation: UEs adjust their frequency to counteract the Doppler effect caused by the satellite's motion; (3) Handover management: The network can predict when a UE should switch to another satellite or beam; (4) Resource scheduling: The network can allocate resources efficiently by knowing the satellite's coverage area over time. These functions are essential for providing reliable broadband connectivity via satellite, especially in remote areas.

In Release 18 and 19, the definition of ephemeris remains unchanged, but ongoing work enhances the accuracy and flexibility of ephemeris provisioning. For instance, Release 18 studies on NTN mobility and coverage enhancements, while Release 19 may refine the signaling and formats. The concept is also applicable to other NTN platforms like High Altitude Platform Stations (HAPS). Overall, ephemeris is a foundational enabler for integrating satellite and airborne networks into the 5G ecosystem, ensuring that the unique challenges of non-terrestrial propagation are effectively mitigated.

Purpose & Motivation

The concept of ephemeris in NTN exists to address the fundamental challenge of communicating with non-geostationary nodes, where the large distances and high relative velocities introduce significant propagation delays and Doppler shifts. Traditional terrestrial networks assume static or slowly moving base stations, but NTN nodes like LEO satellites move at speeds of approximately 7.5 km/s, causing rapid changes in link characteristics. Without accurate knowledge of the node's position and velocity, it would be impossible for UEs to synchronize their transmissions, leading to failed connections and inefficient use of spectrum. Ephemeris provides the necessary data to predict these variations and compensate for them proactively, enabling reliable and high-capacity NTN services.

Historically, satellite communication systems have used ephemeris data for tracking and pointing antennas, but 3GPP's integration of NTN into 5G NR and LTE required standardizing the format and signaling of ephemeris to UEs. Before Release 17, there was no standard way for a UE to obtain and utilize ephemeris for pre-compensation; each system was proprietary. 3GPP introduced ephemeris in Release 17 to enable a unified approach, allowing any compliant UE to connect to any NTN network. This standardization is crucial for the success of mega-constellations like Starlink and OneWeb, which rely on seamless handovers and precise timing. By defining ephemeris in specifications like TS 36.300 and TS 36.331, 3GPP ensures interoperability and paves the way for ubiquitous global connectivity, including in unserved and underserved areas.