Above Ground Level

Description

Above Ground Level (AGL) is a vertical reference convention that expresses altitude as the height of an object above the terrain or ground surface directly beneath it, as opposed to Above Mean Sea Level (AMSL) or a fixed ellipsoidal datum such as WGS-84. In 3GPP Rel-20, AGL appears in the context of aerial user equipment (UAV/drone) and non-terrestrial network (NTN) studies documented in TR 22.870, where accurate vertical positioning and altitude reporting are prerequisites for services such as flight-path management, geofencing, and interference coordination between aerial and terrestrial users.

The distinction between AGL and other altitude references is operationally significant. Terrestrial cellular networks were historically designed with the assumption that user equipment sits at or near ground level, so radio propagation models, antenna downtilt, and interference budgets implicitly assumed a small vertical offset. When a UE is airborne, its height above the local terrain determines line-of-sight conditions, the number of cells it can see, and the interference it causes to distant cells. Because terrain elevation varies, an aircraft at a constant AMSL altitude has a varying AGL altitude, and it is the AGL value that most directly correlates with radio propagation and with regulatory airspace rules such as maximum permitted drone heights.

From an architectural standpoint, AGL is not a network function or protocol entity; it is a reference quantity that must be derived and reported. A UE equipped with barometric sensors, GNSS, or both can estimate its altitude, but converting a geodetic or barometric altitude into AGL requires knowledge of local terrain elevation, typically obtained from a digital elevation model (DEM) or terrain database. In 3GPP studies, this raises questions about where the terrain data resides, whether the UE or the network performs the conversion, how accurate the resulting AGL estimate is, and how that accuracy should be signalled or assured. These questions feed into requirements on positioning accuracy, altitude reporting granularity, and location services for aerial UEs.

AGL also interacts with regulatory and safety frameworks. Aviation authorities commonly regulate unmanned aircraft operations in terms of AGL limits (for example, maximum heights for visual line-of-sight operations), and geofencing around airports or sensitive areas is often defined in three dimensions. For a mobile network to support such services, it must be able to determine, or assist in determining, the AGL altitude of a connected aerial UE and to expose that information to authorized applications, for example through location services APIs. This links AGL to the broader 3GPP work on enhanced positioning, UAV identification and tracking, and network exposure functions.

In summary, AGL is a foundational vertical-reference concept for 3GPP's aerial and NTN work. It matters because altitude-dependent behaviour — radio propagation, interference, mobility management, and regulatory compliance — cannot be modelled or enforced correctly without a well-defined and reasonably accurate notion of height above ground. Its treatment in TR 22.870 reflects the study-phase nature of the concept in Rel-20, where use cases, service requirements, and feasibility are being scoped before normative specification work.

Purpose & Motivation

AGL exists as a concept because cellular systems increasingly serve airborne terminals, and the traditional assumption that devices operate at or near ground level no longer holds. Altitude references such as AMSL or ellipsoidal height are geodetic quantities that do not directly describe the relationship between a device and the terrain beneath it, yet that relationship is what governs radio propagation, interference coupling between aerial and terrestrial links, and compliance with aviation regulations that are themselves written in terms of height above ground.

Previous generations of 3GPP specifications largely avoided the problem by scoping terrestrial mobility to ground-level users. As UAV and NTN use cases matured, it became clear that altitude reporting, geofencing, and flight authorisation would require a consistent vertical reference. AGL provides that reference, but it also exposes a dependency: deriving AGL requires terrain elevation data, which is not natively part of the mobile network. The Rel-20 study in TR 22.870 therefore examines how AGL can be estimated, with what accuracy, and how it can be used by services without overburdening the UE or the network.

The motivation is thus twofold: to enable new aerial services with well-defined vertical semantics, and to ensure that network planning and interference management for mixed terrestrial-aerial deployments rest on a physically meaningful altitude measure rather than an arbitrary datum.