GNSS anti-JAM RF-to-RF on board unit for ERTMS train control

The GNSS technology has been selected as the key player for the modernization of the European Railways Train Management System (ERTMS). The main advantage of the satellite system is the possibility to realise cost-effective solutions able to increase the capacity and better allocate the railway resources. This cost reduction needs be performed without compromising the system safety that shall remain at the same level guarantee with the traditional systems. One of the problems that has to be accounted dealing with the GNSS is the Radio frequency interference like the intentional and not intentional. The presence of a jamming signal can degrade the performance of a GNSS location determination system and, if the ratio among the jammer is much higher the genuine signal, there could be a denial of service. One of the possible techniques able to mitigate such an issue is represented by the beamforming. In essence, having a CRPA (Controlled Reception Pattern Antenna) it is possible to identify the presence of the jamming signal and, eventually, combine the antennas' output to generate a signal that reduces the impact of such a treat. Particularly, in this work, we focused on a solution based on a four channel antenna processing chains constituted by a four element squared phased array and a 4 coherent channels front-end. The digital streams spilled after the ADCs (Analogue to Digital Converter) are the processed to identify the presence of a jamming signal. If the jamming signal is identified, then the processing unit will estimate the weights to implement a spatial filter able to minimize the impact of the jammer. The signal reconstructed after the beamformer is the put as input to a COTS/SDR receiver, making smoother the integration of this component in the navigation unit. The digital beamforming platform is designed to operate on scenarios typical of the railway environment. In this work we show the platform performance assessment of RF-2-RF GNSS anti-jam platform in terms of jamming detection and mitigation capabilities in presence of different attack scenarios potentially occurring in the ERTMS operative conditions. Particularly, we report the results of a test campaign by using Spirent tools, showing the capabilities of the anti-jam platform able to estimate the jammer direction of arrival, mitigate it cleaning the useful SIS (Signal in Space) from it and re-transmit it to a COTS/SDR receiver. The signal in output from these tools have been recorded by a four coherent channel frontend and then elaborated in post-processing with the algorithms running on the platform. This approach has been selected to guarantee the possibility to inject the front-end with signals that have the same phase shifts and attenuations that would have been experimented on the field with the given array geometry and element beam patterns.