Horizontal Protection Level

Description

The Horizontal Protection Level (HPL) is a statistical measure of the integrity or trustworthiness of a calculated horizontal position fix. It is not the position error itself, but rather an upper bound on the horizontal position error, computed in real-time by the positioning system. Specifically, the HPL defines the radius of a circle, centered on the reported position estimate, within which the true horizontal position of the device is guaranteed to lie with a very high probability (e.g., 99.999%). If the HPL exceeds a predefined alert limit for a given application, the system must issue an integrity alert, indicating that the position fix is not sufficiently reliable for safe use.

In 3GPP standards, introduced in Release 17 for enhanced positioning services, the HPL is part of a broader framework for high-integrity positioning. The calculation of HPL typically involves analyzing the geometry of the satellites or base stations used (Geometric Dilution of Precision - GDOP), the estimated ranging errors (pseudorange residuals), and known error models for each signal source. For GNSS-based positioning, factors like satellite clock/ephemeris errors, ionospheric delay, and receiver noise are accounted for. For terrestrial positioning (e.g., using LTE or NR signals), errors from timing advance, angle-of-arrival measurements, and multipath are considered. The positioning engine, which could be in the device (UE-based) or the network (UE-assisted), runs these computations to output both a position estimate and its associated HPL.

The role of HPL within the 3GPP architecture is tied to location services (LCS) and the Location Management Function (LMF). When an application (like a vehicle's autonomous system) requests a high-integrity position, the LMF or the UE's positioning engine must provide the HPL alongside the coordinates. The application then compares the HPL to its own Horizontal Alert Limit (HAL). If HPL < HAL, the position fix is deemed usable. This integrity monitoring is continuous. The specifications (e.g., TS 37.355 for LTE positioning protocol and TS 38.857 for NR positioning) define how HPL-related data (like error models and confidence levels) are communicated between the network and the UE to support these calculations for various positioning methods including A-GNSS, OTDOA, and multi-RTT.

Purpose & Motivation

The drive to standardize Horizontal Protection Level within 3GPP, starting in Release 17, was motivated by the stringent requirements of new vertical applications that depend on highly reliable and accurate positioning. While traditional cellular positioning (e.g., for emergency services or basic location-based services) focused primarily on accuracy, applications such as autonomous vehicles, advanced drone operations, and industrial automation demand both high accuracy and guaranteed integrity. Integrity answers the critical question: "Can I trust this position fix enough to make a safety-critical decision?"

Before its formal inclusion, 3GPP positioning standards provided accuracy estimates but lacked a standardized framework for integrity metrics like HPL. This was a barrier for safety-of-life applications that require compliance with standards like those from aviation (RAIM) or automotive (ISO 26262). The introduction of HPL addresses this gap. It allows cellular networks—either standalone or as a complement/supplement to GNSS—to provide positioning services with known, bounded error levels. This is particularly important in urban canyons or other GNSS-challenged environments where cellular signals can improve availability. By defining HPL, 3GPP enables a unified integrity framework across hybrid positioning sources (GNSS + terrestrial), facilitating the development of reliable positioning solutions for V2X, UAVs, and other mission-critical IoT use cases that form a core part of 5G-Advanced evolution.