Total Electron Content

Description

Total Electron Content (TEC) is a geophysical parameter representing the columnar density of free electrons in the ionosphere. It is defined as the integral of the electron density along a straight-line path from a Global Navigation Satellite System (GNSS) satellite to a receiver on or near the Earth's surface. The standard unit is the TEC Unit (TECU), where 1 TECU = 10^16 electrons per square meter. The ionosphere is a dispersive medium for radio signals in the frequency bands used by GNSS (like GPS, Galileo), meaning it causes a frequency-dependent delay in signal propagation. This ionospheric delay is a primary source of error in standalone GNSS positioning.

In the 3GPP context, TEC data is utilized within location services (LCS), specifically for Assisted-GNSS (A-GNSS) and Advanced A-GNSS. The network (e.g., a Location Server like the Enhanced Serving Mobile Location Centre - E-SMLC or Location Management Function - LMF) can provide assistance data to User Equipment (UE) to improve positioning speed and accuracy. This assistance data can include ionospheric models or corrections, which are derived from TEC measurements. The network estimates the TEC for the region relevant to the UE, often using data from reference networks of GNSS receivers. It then calculates and transmits correction parameters, such as coefficients for the Klobuchar model or more advanced NeQuick model, which the UE applies to its GNSS measurements to compensate for the ionospheric delay.

The specification of TEC-related assistance data is covered in 3GPP TS 36.305 (E-UTRAN Stage 2 - UE positioning), TS 37.355 (LTE Positioning Protocol - LPP), and TS 38.305 (NG-RAN Stage 2 - UE positioning). These documents define how TEC models are formatted, transmitted, and applied. The process involves the UE requesting or the network providing an ionospheric model valid for a specific geographical area and time. The UE uses this model to correct the pseudo-range measurements from multiple satellites, significantly reducing the positioning error, which can be tens of meters without correction, to a few meters or less.

Purpose & Motivation

The incorporation of TEC modeling into 3GPP standards addresses the critical need for highly accurate and reliable positioning services, which are mandated for emergency calls (e.g., E-911, E-112) and are essential for numerous commercial applications like navigation, asset tracking, and location-based advertising. Standalone GNSS receivers, especially in consumer devices with small antennas, suffer from significant errors due to ionospheric variability, which changes with time of day, solar activity, and geographical location.

Previous approaches in early A-GNSS primarily provided satellite ephemeris and almanac data to reduce Time To First Fix (TTFF). However, they offered limited correction for atmospheric errors. The introduction of standardized TEC-based ionospheric correction in 3GPP, particularly from Release 16 onwards, was motivated by the demand for meter-level accuracy. It solves the problem of the UE needing a clear, sustained view of multiple satellites to compute its own ionospheric model, which is impractical in urban canyons or indoors. By having the network compute and deliver region-specific TEC models, the UE can achieve faster and more accurate fixes even under challenging conditions. This leverages the network's ability to aggregate data from multiple sources to create a more accurate and timely ionospheric picture than any single device could.