GLObal'naya NAvigatsionnaya Sputnikovaya Sistema (Global Navigation Satellite System)

Description

GLONASS is a space-based satellite navigation system that constitutes one of the major Global Navigation Satellite Systems (GNSS) alongside the US GPS, European Galileo, and Chinese BeiDou. Within the 3GPP ecosystem, specifications define how User Equipment (UE) can utilize signals from GLONASS satellites (as well as other GNSS constellations) to compute their own position. This capability is a key enabler for 3GPP Location Services (LCS). The system operates by transmitting precise timing signals from a constellation of satellites in Medium Earth Orbit. A GNSS receiver in the UE measures the time delay of signals received from multiple visible satellites; with knowledge of the satellites' orbits (ephemeris data) and the signal transmission time, the receiver can calculate its distance to each satellite and trilaterate its own position.

The integration of GLONASS into 3GPP standards involves specifications across multiple layers. Radio access network specs (e.g., 25.331 for UTRAN, 36.331 for E-UTRAN, 38.331 for NR) define signaling protocols for Assisted GNSS (A-GNSS), where the network can provide assistance data (like satellite almanac and ephemeris) to the UE to significantly speed up the time-to-first-fix and improve sensitivity. Core network specifications cover the positioning protocols and architecture. Furthermore, performance requirements (e.g., in 25.171, 36.171, 38.171) define minimum performance levels for UE receivers supporting GLONASS, including sensitivity, accuracy, and time-to-first-fix metrics under various conditions.

From a network architecture perspective, the UE's GNSS capability, including GLONASS support, is reported to the network. The Location Management Function (LMF) in 5GC or the Enhanced Serving Mobile Location Centre (E-SMLC) in EPS can then use this capability when deciding on a positioning method for a location request. Using multiple GNSS constellations, a technique known as multi-GNSS, improves performance significantly. It increases the number of visible satellites, which is crucial in urban canyons or other challenging environments, leading to better accuracy, availability, and integrity of the position fix. 3GPP's support for GLONASS ensures that UEs can leverage this additional constellation for robust positioning services globally.

Purpose & Motivation

3GPP's incorporation of GLONASS support was driven by the growing demand for accurate and reliable location-based services and regulatory requirements like Enhanced 911 (E911). Relying solely on the US GPS system presented limitations, including potential single points of failure, regional signal vulnerabilities, and sometimes insufficient satellite visibility for a fast, accurate fix. The inclusion of GLONASS as an alternative and complementary GNSS constellation directly addresses these limitations by increasing the number of available navigation satellites.

The primary motivation was to enhance the performance of mobile positioning. By supporting multiple constellations, User Equipment can achieve a position fix more quickly (reduced Time To First Fix - TTFF), with higher accuracy, and with greater reliability, especially in dense urban areas where buildings can block signals from some satellites. This multi-GNSS approach future-proofed 3GPP standards, allowing for the integration of other systems like Galileo and BeiDou as they became operational. It also catered to global market needs, ensuring that mobile devices could provide effective location services worldwide, regardless of regional preferences or the operational status of any single satellite system.