Two Line Elements

Description

Two Line Elements (TLE) is a data format originally developed by NORAD and adopted by 3GPP for Non-Terrestrial Network (NTN) operations. A TLE set consists of two lines of text containing a set of orbital parameters, known as Keplerian elements, for a specific satellite or space vehicle at a given epoch (reference time). These parameters include the satellite's inclination, right ascension of the ascending node, eccentricity, argument of perigee, mean anomaly, and mean motion. The format also includes identification and classification data. In the 3GPP context, the network can broadcast TLE information for relevant NTN platforms (e.g., Low Earth Orbit satellites) via system information blocks (SIBs). User Equipment (UE) and network nodes can use this information, along with standard orbital propagation models like SGP4 (Simplified General Perturbations 4), to compute the satellite's current and future ephemeris (position and velocity). This computed ephemeris is fundamental for several NTN-specific procedures. For example, it enables the UE to determine the correct timing advance for uplink transmission to compensate for the large and variable propagation delay. It also assists in predicting cell coverage areas for cell (beam) reselection and handover, as the satellite's movement causes its service area to move rapidly over the Earth's surface. The TLE data must be periodically updated by the network as the satellite's orbit is perturbed by factors like atmospheric drag and gravitational anomalies. The integration of TLE into 3GPP specifications provides a lightweight, standardized method for ephemeris dissemination without requiring constant, high-bandwidth transmission of raw positional data.

Purpose & Motivation

The purpose of integrating the TLE format into 3GPP standards is to enable efficient and predictable communication with mobile platforms in Non-Terrestrial Networks. Traditional terrestrial networks rely on fixed base station locations, but NTN introduces moving network nodes (satellites) with high velocities and large, time-varying propagation delays. Direct, continuous transmission of precise GPS-like coordinates for each satellite would consume significant broadcast channel resources. The TLE format solves this by providing a compact, parametric description of the orbit. From this small dataset (typically less than 200 characters), any receiver can independently generate accurate ephemeris predictions for a significant time window (often several days) using publicly available algorithms. This addresses the key challenge of letting UEs and network elements autonomously calculate essential parameters like propagation delay and satellite visibility, which are prerequisites for synchronization, random access, and mobility management in a dynamic NTN environment. Its adoption leverages a long-standing, proven, and computationally efficient standard from the aerospace community, avoiding the need to invent a new, complex ephemeris dissemination protocol for 5G and beyond.

Key Features

• Compact orbital parameterization using Keplerian elements
• Enables autonomous ephemeris prediction using models like SGP4
• Broadcast via 3GPP system information for NTN cell acquisition
• Supports calculation of time-varying propagation delay for timing advance
• Facilitates prediction of satellite visibility and cell coverage for mobility
• Standardized format allows use of existing, proven aerospace libraries

Evolution Across Releases

Defining Specifications