## 2 Methodology
### Network Time Protocol (NTP)  
Let there be a layered system using independent time sources. Then, each system can be organized hierarchically, which is termed as a stratum. Each stratum consists of \(n\) synchronized stratum servers, and the number \(n\) depicts the distance from the reference clock system, which is stratum 0. Note that the number of the stratum may not always indicate the quality of the inner clock's precision or accuracy. Then, the whole structure of the layered system is as shown in Figure 1.  
In detail, a regular network time protocol (NTP) can be established using the above-mentioned layered system, and the client calculates a communication time offset and a round-trip delay as follows [34]:  
\[\delta=(t_{3}-t_{0})-(t_{2}-t_{1}), \tag{1}\]  
where \(\delta\) is the round-trip delay, \(t_{0}\) is the time when a poll is requested from the client, \(t_{1}\) is the time when the poll request is received from the server, \(t_{2}\) is the time of the response from the server, and, finally, \(t_{3}\) is the time the response is received by the client as follows:  
As shown in Figure 2, clock synchronization can be performed by computing the offset between the server and the client. \[\theta=\frac{(t_{1}-t_{0})+(t_{2}-t_{3})}{2}, \tag{2}\]  
where \(\theta\) is the clock offset. It should be noted that the time offset can be a positive value or a negative value depending on the instance timeline information of the client and the server. Next, in order to derive the amount of the offset in real time, a request packet can be utilized, such as  
\[t_{0}=t_{1}-\theta-\delta/2, \tag{3}\]  
for the case of the response packet, and  
\[t_{3}=t_{2}-\theta+\delta/2, \tag{4}\]  
and, finally, the \(\theta\) can be solved. However, it is important to remove the outliers of the estimated \(\theta\) and \(\delta\) to synchronize the inner clocks for the layered system. The above-mentioned NTP can be utilized based on a global positioning system (GPS) or a global navigation satellite system (GNSS) when applied to individual vehicles. After decoding, NMEA (National Marines Electronics Association)-0183 sentences, which are encoded packets received from multiple satellites at the time when the packets are sent, can be known [35]. However, a fundamental problem of using NMEA sentences is that there is no firm timing protocol for when to initiate packet sending after a new second starts. Therefore, it is important to know that, if the client receives an NMEA packet, it does not have any evidence of whether it has been sent at the start of the new second or the end of the new second. For this issue, 1 pulse per second (PPS) can be useful for enhancing the NMEA to have better accuracy. The 1-PPS signal is a type of pulse signal that indicates the start of the new seconds and is considered to have less than 40 ns average precision. Figure 1: Schematic diagram showing the layer of the network time protocol where distributed layered systems using independent time sources are synchronized hierarchically (\(n\)th stratum is connected to \(n+1\)th stratum). Therefore, by combining the NMEA sentence timestamps and the 1-PPS signal, the system can have better accuracy and better precision [36]. Figure 3 shows the one-way communication delay measurement using the NTP server shown in the above. After both the teleoperated vehicle and the control center have synchronized clock systems, by sending the sent time with the control command from the control center, a vehicle controller inside of the teleoperated vehicle is able to compute the one-way communication delay. In this scenario, numerous approaches to the communication method can be used such as (but not limited to) 5G/LTE and DSRC [37] with appropriate protocols, e.g., SAE J2735 [38]. As for the common failure scenarios for using such wireless communication methods, the performance of the connectivity can be degraded due to systematic failures, low sensitivity, and even weather conditions [39].