Global Navigation Satellite System

Description

Global Navigation Satellite System (GNSS) is a generic term encompassing all satellite-based positioning systems, including GPS (USA), Galileo (EU), GLONASS (Russia), and BeiDou (China). Within 3GPP specifications, GNSS is integrated as a key method for determining the geographical location of User Equipment (UE). The architecture involves the UE containing a GNSS receiver capable of processing signals from one or multiple satellite constellations. The network can assist this process through control plane or user plane location protocols, such as LTE Positioning Protocol (LPP) or Secure User Plane Location (SUPL), by providing aiding data like ephemeris, almanac, and approximate location to reduce the UE's Time To First Fix (TTFF) and improve accuracy.

The fundamental operation relies on the UE measuring the time of arrival of signals from multiple visible satellites. By calculating the signal travel time and knowing the satellite positions (from decoded or network-provided navigation messages), the UE can compute its own three-dimensional position and precise time through trilateration. Key components include the satellite constellations themselves, the UE's GNSS receiver and antenna, and network elements like the Location Management Function (LMF) in 5GC or the Enhanced Serving Mobile Location Centre (E-SMLC) in EPS, which manage the positioning session and calculations.

GNSS positioning in 3GPP supports multiple modes: standalone (UE operates independently using only satellite signals), UE-assisted (UE sends measurements to the network for position calculation), and UE-based (UE calculates its own position, potentially using network-provided assistance data). Its role is critical for emergency services (e.g., E911/E112), location-based services, lawful interception, network optimization (e.g., for mobility management), and various IoT applications requiring asset tracking. The specifications detail requirements for performance (accuracy, sensitivity), testing, and integration with radio access technologies from UMTS to 5G NR.

Purpose & Motivation

The primary purpose of integrating GNSS into 3GPP standards is to provide a reliable, globally available method for determining UE location. This addresses regulatory mandates for emergency caller location (e.g., FCC E911 in the USA, E112 in the EU), which require mobile networks to provide accurate position information for emergency calls. It also enables a vast ecosystem of commercial location-based services (LBS), such as navigation, fleet management, and location-aware applications, creating new revenue streams for operators.

Historically, cellular-only positioning methods like Cell-ID, Enhanced Cell-ID, and Observed Time Difference of Arrival (OTDOA) had limitations in accuracy, especially in rural or sparse cell deployment areas. GNSS provides superior outdoor accuracy, often down to a few meters, filling this gap. Its inclusion starting in 3GPP Release 7 formalized the support for assisted-GNSS (A-GNSS), which leverages the network to deliver assistance data to the UE, significantly improving startup performance (TTFF), sensitivity (allowing fixes in weaker signal conditions), and UE battery life compared to standalone GNSS operation.

The motivation extended beyond compliance and services to network optimization and new use cases. Accurate location aids in radio resource management, handover decisions, and network planning. For later technologies like V2X and IoT, precise GNSS-based timing and positioning are foundational. The evolution to support multiple constellations (multi-GNSS) and hybrid positioning (combining GNSS with terrestrial signals) in subsequent releases was driven by the need for improved availability, robustness in urban canyons, and even higher accuracy for advanced applications like autonomous driving and industrial automation.