**4.1 Basic tasks of time services**

Basic tasks of exact time services are determination, storage and transmission of standard time signals, as well as determination and forecasting of the Earth's rotation parameters (ERP). There are primary and secondary time standards and frequencies, as well as reference standards, in some countries. A national primary time and frequency standard (NTFS) is designed for ensuring unity of measurements to a required degree of precision, carried out by time services of the country. This is achieved by a number of activities, and the main ones are as follows:


Dissemination of standard time and frequency is carried out by transmission of time and frequency signals over broadcast channels, by means of GNSS, as well as on the Internet. One of the main tasks of a time service is ensuring the possibility of signal reception anytime anywhere in the required area. Time precision in case of radio transmission is about 1 ms, transmission over television channels - 10 µS, over global positioning systems GPS/GLONASS - 10 ns, on the Internet – from 10 ms to 10 ns.

#### **4.2 Scheme of group time scale formation and storage**

Scheme of exact time units' formation with transmission to consumers over broadcast channels and the Internet is given in Figure 1 by the example of Novosibirsk time service.

Fig. 1. Block scheme of exact time scale formation equipment

A Standard's atomic time scale is realized by a group keeper which includes at least four hydrogen standards, one of which is selected as a key one. In order to calculate the scale, results of mutual comparison of the Standard keepers' frequency and time scales are used, this is internal comparison. The key keeper is compared to the national standard over communication channels. The results serve as the basis for forecast of frequency variations of the Standard's key keeper. Hereafter, the forecast data are used only in view of routine intercomparison of keepers' time scales. In practical work maintaining of the Standard's atomic scale comes down to determination of time corrections for a key hydrogen keeper.

Scheme of exact time units' formation with transmission to consumers over broadcast channels and the Internet is given in Figure 1 by the example of Novosibirsk time service.

**4.2 Scheme of group time scale formation and storage** 

Fig. 1. Block scheme of exact time scale formation equipment

A Standard's atomic time scale is realized by a group keeper which includes at least four hydrogen standards, one of which is selected as a key one. In order to calculate the scale, results of mutual comparison of the Standard keepers' frequency and time scales are used, this is internal comparison. The key keeper is compared to the national standard over communication channels. The results serve as the basis for forecast of frequency variations of the Standard's key keeper. Hereafter, the forecast data are used only in view of routine intercomparison of keepers' time scales. In practical work maintaining of the Standard's atomic scale comes down to determination of time corrections for a key hydrogen keeper.

Frequency and time group keeper is designed for generation of sinusoidal signal with standard frequency of 5 MHz.

Group keeper equipment includes:


Basic metrology characteristics:



Table 1. Alan variations of the group time keeper frequency

Transmission of exact time signals to consumers is carried out on the INTERNET by means of NTP protocol which ensures that error related to UTC (NOV) scale is not more than several milliseconds. At the same time, time and frequency signals transmitted by radio stations of short-wave, medium-wave and long-wave range and global positioning systems GLONASS and GPS are controlled.
