An accurate and trustable positioning is mandatory for safety-critical vehicular applications, such as the ERTMS (European Railways Train Management System) and the CCAM (Connected, Cooperative, and Automated Mobility) driving market growth. Vehicle localization relies on onboard sensors with GNSS playing a major role thanks to multiple satellite constellations, augmentation techniques and Real-Time Kinematic (RTK) and Precise Point Positioning (PPP).
GNSS-based positioning is challenged by signal obstructions in urban canyons, along tunnels, vulnerability to jamming and spoofing highlighting for more robust and non-exclusive dependency. Radiolabs trustworthy positioning platforms for autonomous driving, based on sensor fusion to combine data from other sensors as IMU, cameras, LiDAR, can benefit by 5G positioning being standardised by the 3GPP. Advanced positioning capabilities into 5G have been introduced from 3GPP Release 16 up to 19, including wide bandwidths, advanced waveforms, and dedicated ranging signals positioning 5G as a significant complement to the GNSS technology.
GNSS and 5G positioning are based on different system architectures, but both rely on RF signal measurements and share several common principles in signal processing and position estimation.
This convergence has the potential to attract the interest from railways industry to realize the ASTP– Advanced Safe Train Positioning, a self-standing device specified for the ERTMS to enhance train positioning capabilities, leveraging multi-sensor fusion including GNSS and IMUs.
Radiolabs – a pioneer of GNSS and satcom for train positioning leveraging on a cross-domain rail-automotive synergic development has presented at the ION’s Pacific PNT Conference 2026 a paper on A resilient high integrity train positioning based on GNSS and FRMCS/5G.
The novelty is to combine the consolidated GNSS technologies with the 5G being 5G the underlining technology of the FRMCS, the new standard to meet the railway requirements for replacing the obsolete GSM-R telecom network of the ERTMS.
5G should be regarded not merely as a “signal of opportunity,” but rather as an integral component of a multi-sensor train localization system according to Valeria Ioannucci of Radiolabs presenting the paper.
This new positioning platform can be implemented either when the FRMCS network is owned and managed by the railway infrastructure operator (that is the case of the GSM-R) or when the FRMCS service is provided by an external Mobile Network Operator. Yet, it is compatible with future alternative to GNSS based on a LEO-PNT constellations contributing toward a system of systems paradigm, where diverse space and non-space technologies are seamlessly integrated to enhance resilience autonomy while expanding overall coverage.
The widespread adoption of 5G within the FRMCS is an incentive to enhance the robustness of the actual GNSS-IMU architecture of the ASTP otherwise depending on the availability of the GNSS signals. The architecture developed by Radiolabs, added Ioannucci, is able to guarantee a Protection level of 10m (SIL 4) even in absence of GNSS signals that represents the most critical requirement for the ASTP.
Yet, she added that following a trade-off among possible alternative solutions based on 5G, the Sidelink Round-Trip Time (SL-RTTS) solution is preferred in terms of robustness and lower synchronisation requirements. Sidelink positioning relies on direct device-to-device signal exchanges, where the user equipment (UE) perform positioning measurements without necessarily involving the network infrastructure in the radio transmission phase. In this paradigm, selected UEs act as anchors by transmitting or receiving SL positioning reference signals, while a target UE measures timing, angle, or power-related observables from these signals. With the introduction of Release 18 positioning capabilities have been extended to sidelink communications through the definition of a new reference signal, namely the Sidelink Positioning Reference Signal (SL PRS). This signal enables UEs to perform sidelink-based positioning measurements, supporting multiple localization techniques.
The train positioning architecture presented at ION Pacific ensures high integrity performance, including the GNSS-denied environments, by adopting a redundant positioning architecture based on two independent sensor sets and associated processing chains.
The performance of the FRMCS positioning chain has been evaluated in a standalone configuration, representing a performance-bound operational scenario.
The Sidelink communication system is represented by a mobile user device, denoted as UET, installed on board the train, and a set of K fixed Position Reference Units (PRUs) distributed along the track, denoted as PRUk, k = 1, …, K.
Under these assumptions, a configuration with PRUs spaced at 5 km with a nominal RTT measurement error standard deviation σRTT1000 = 0.5m at 1 km, allows the position estimation error to be kept within values compatible with the 10m protection level under SIL-4 constraints of the ASTP. Conversely, larger PRU spacings can be implemented in the railways sections where a 60m Protection level is required or a closer PRU spacing in the railways sections where there is a need to discriminate parallel tracks.
Looking forward, Radiolabs will continue this research with the aim to strengthen the positioning function while reducing its dependency from a single technology.
