# Trusted GNSS-Based Time Synchronization for Industry 4.0 Applications
## 2 GNSS RF Spectrum Attack Vector  
GNSS technologies [14; 15] have shown a remarkable growth in the last years, being nowadays adopted in the most different fields of applications such as: consumer devices (e.g., smartphones, cameras, wearable devices, etc.), vehicular applications (e.g., road user charging, bike sharing, connected and automated driving, etc.), manned and unmanned aviation (e.g., drones), industrial applications and even in critical infrastructures. Every system that makes use of GNSS-derived data must trust them before taking decisions, especially in case of safety-critical or liability-critical applications.  
The widespread adoption of GNSS technologies creates incentives for the attackers that want to impair or fool any system that rely on GNSS to estimate the position, the velocity and, especially, the time information [26]. GNSS receivers and connected devices integrating these receivers are all vulnerable to intentional attacks, aiming to affect the availability and the reliability of the GNSS signals and data. In general, GNSS RF attacks are classified in three main categories [24; 26; 27]:  
1. _Jamming_, that is the blocking of the reception of GNSS signals by intentionally emitting RF interferences to disrupt the functionalities of the receivers, in order to reduce the signal-to-noise power level;
2. _Meaconing_, that corresponds to the rebroadcasting of delayed GNSS signals, without any distinction between signals received from different satellites;
3. _Spoofing_, that refers to the transmission of counterfeit GNSS-like signals, with the intent to produce false position and/or time data at the victim receiver.  
Each of these three categories can be put in place in different ways, implying a different complexity and cost at the attacker side. Among the possible attacks, only few can be considered likely to be deployed in real applications [12; 16; 24].  
Different countermeasures against these attacks are already available and widely discussed by the GNSS research community [26]. Please note that a comprehensive review of the state of the art related to GNSS RF attacks and proposed solutions is out of the scope for this paper. Nonetheless, interested readers can refer to [17; 26; 27] and the references therein.  
Among the most recent solutions, it is worth to point out the on-going effort of the European Galileo program to gradually add authentication services to its first and second generation of satellites signals, in order to enable authentication functionalities for future civil receivers [27]. The Galileo Open Service Navigation Message Authentication (OSNMA) consists in a mechanism that allows the receivers to verify the authenticity of GNSS information, making sure that the data they receive are indeed from Galileo satellites and have not been modified in any way [32]. GNSS receivers can take advantage of such capability for implementing simple but effective detection methods against several spoofing attacks [16; 24; 33]. The first-ever signal-in-space transmission of Galileo OSNMA started on November 2020, and tests are currently underway in order to consolidate the service [34].