# Trusted GNSS-Based Time Synchronization for Industry 4.0 Applications
## 1 Introduction  
The cornerstone of Industry 4.0 [1] are cyber-physical systems (CPS), which are closely connected with computer systems and which can interact and collaborate with other CPS systems [2]. These concepts represent the basis of decentralization and cooperation between connected systems: they are closely linked to the Industry 4.0 paradigm and fully enabled by information and communication technologies [3; 4; 5]. Networked environments and Industrial Internet of Things (IIoT) [4] need accurate, reliable and trusted time synchronization and time distribution [6], with different requirements in terms of synchronization accuracy and precision. For instance, the scheduling and synchronization of multiple factories represents an interesting high level application of the Industry 4.0 paradigm [5]. Due to rapidly changing market requirements, factories have shifted from a centralized to a more decentralized structure in many areas of decision making, including scheduling. Since limited resources make scheduling an important decision in the production, accurate time synchronization and efficient scheduling solutions are vital for improving the productivity in a multi-factory production network [7]. Another relevant example is the case of Digital Twins (DTs) in Industry 4.0 [8]: the underlying idea is that a real product and its virtual counterpart are twins that travel a parallel journey from design and development to production and service life. A DT must be always in sync with the corresponding physical asset. Such synchronization is needed to correctly update and to keep the operational data (i.e., failure and erroneous data) consistent within the production system. Nowadays, several Industry 4.0 applications already imply stringent synchronization requirements (e.g., sub-microsecond accuracy). Among the others, we consider the following ones especially relevant:* _Automation and control systems_[9], that need an accurate common notion of time as well as a reliable shared communication medium for timely data exchange, for instance to synchronize multi axis drive systems and subsystems with cyclic operation;
* _Measurement and automatic test systems_[10], which usually take advantage of accurate time stamping of logged data, for example to correlate acquired values in decentralized locations;
* _Power generation, transmission and distribution systems_[11; 12], requiring a precise time synchronization of all the critical points within the power grid to accurately measure the delivered/consumed power and to predict critical load situations. In this context, satellite-derived timing information plays a key role in the provisioning of an absolute time reference to a significant number of current and future connected systems. Global Navigation Satellite System (GNSS) receivers combined with specific transport protocols or with distributed synchronization approaches can indeed satisfy the stringent requirements foreseen in several industrial applications [13]. Figure 1 provides a common scheme of a synchronization network, representative of a typical time distribution network and highlighting the role of GNSS technologies. In this scheme, accurate time information is generated by each GNSS satellite and then transmitted by means of properly formatted radio-frequency (RF) signals [14; 15]. A Master Clock node of the network accurately synchronizes its local clock using a GNSS receiver, taking advantage of the signals received from the GNSS satellites visible at its location [16]. As illustrated in Figure 1, a distribution network allows us to distribute the timing information to all the nodes, achieving an accurate synchronization between the Master Clock and multiple Slave Clocks. The distribution network can take advantage of different protocols and technologies, depending on the application requirements and constraints.