Explanatory note on the operation of "HeureLegaleFrancaise.fr"

The operating principle of this web page is illustrated in Figure 1. It is inspired by the NTP protocol, but works in a simpler way. The HTML page uses a program coded in Javascript running on the remote equipment (computer, tablet or smartphone). The remote equipment sends an HTML timing query on date t1 to one of the NTP servers of Observatoire de Paris. Our server returns its timing information t2, which is compared to the time of the remote equipment t3 at the reception of the information from the server. From these data, the application calculates an estimate of the desynchronization between the remote equipment and the server, as well as the transmission delay of the query, assuming the round trip delay is symmetric in both directions. The estimation of the desynchronization is then corrected for this transmission delay. This estimate of the desynchronization is used to display legal time and UTC(OP) time, constructed respectively from the UTC time and the local time of the remote equipment, corrected for the desynchronization. To improve resolution and to detect possible errors, the process is repeated 4 times in a period of about two seconds. The result is refreshed every 180 s. Between two refresh events, the displayed time is driven by the remote equipment clock frequency. Errors in displayed time depend on the remote equipment clock frequency, on the asymmetries of back-and-forth paths, as well as on the internal processes of our NTP server and in the application running on the remote equipment. The estimate of the transmission delay can however be considered an upper bound of the systematic uncertainty on the desynchronization.

Figure 1: operating principle of the "heurelegalefrancaise.fr" web page

In practice, our reference NTP server is directly connected to the UTC(OP) time scale, which is the basis of legal time in France (decree no. 2017-292 of March 6, 2017). The UTC(OP) time scale is kept within a few ns with respect to UTC. The desynchronization of our NTP server is typically in the range of milliseconds. We can therefore consider that the application provides timing information with an uncertainty at the sub-second level that can be used to perform some kinds of calibrations. For the calibration of an absolute timing, the uncertainty budget must take into account the uncertainty related to the method implemented, the desynchronization, the round trip delay, as well as the uncertainties related to our NTP server, typically less than 10 milliseconds. The application does not correct the remote equipment clock frequency. It can be used for the calibration of time intervals (or in other words to estimate a frequency), by analyzing the evolution over time of the estimated values ​​of the desynchronization. It is therefore recommended to carry out and analyze the measurements over several periods of 180 s, in order to have enough statistical data. Time zones The application provides the time generated at Observatoire de Paris given in UTC time and in legal time (UTC+1 or UTC+2 in France, depending whether the Daylight Saving Time applies or not ). It is up to the user to check or take into account the time zone / Daylight Saving Time configuration of his equipment. Precautions for use It should be noted that this application cannot perform a synchronization of a remote equipment, as it only estimates its desynchronization: the user should set up an NTP or other suitable client if a synchronization is needed. The application may generate wrong values ​​if time settings has just been adjusted on the remote equipment (manual, Daylight Saving Time, leap second). In these cases, it is better to wait for the end of the 180 s refresh period or to force a refresh of the web page. The values ​​displayed may be affected by the traffic disruptions on the network between the remote equipment and Observatoire de Paris, on which the Observatoire de Paris has no responsibility. Furthermore, the timing information displayed by the application on the remote equipment may be affected by the load of the processor or the graphics board of the remote equipment. It is users’ responsibility to ensure that their equipment is not overloaded. Service availability The application is made available free of charge in "best effort" mode. It may happen that it is not available for short periods because of maintenance of our services or in the event of an incident on the network. Under these conditions, the timing and desynchronization information is not provided by the application in order to avoid wrong results. Application display clearly indicates such a condition. Other synchronization sources For more demanding users, other sources of synchronization or calibration are available to them: - Synchronize to one of our NTP servers and perform an analysis of the log files; - Use the signal broadcasted by the ALS162 long-wave transmitter, using a suitable receiver. The ALS162 signal includes a time code generated from atomic clocks located at the transmitter, connected to UTC(OP) via satellite link; - Use the SCPTime service broadcasting time via the internet. The primary servers are hosted at Observatoire de Paris and directly referenced to UTC(OP); - Use a COFRAC accredited laboratory in time and frequency metrology; - For users of GNSS signals, we provide them our monitoring data of GNSS timescales relative to UTC(OP). We also carry out GNSS station relative calibrations for laboratories and companies specialized in time-frequency metrology.