**1. Introduction**

This chapter covers some issues of measuring, storage and reproduction of a precise time scale and reference frequencies, as well as techniques and methods for providing consumers with this scale and Earth rotation parameters (ERP) data.

The first paragraph contains historical information on development of time measurement and storage instruments. It reviews development stages of national and world time services, their duties and goals within the scope of an assurance of time and frequencies measurement and reproduction uniformity.

The second paragraph represents a brief classification of time measurement systems basing on different natural periodic process, which are used as a standard time unit. Possibilities of use a pulsar time scale for long-time storage of high-stable time intervals and synchronization of group time and frequencies references are also reviewed in this paragraph.

The third paragraph states the main objectives of national time and frequencies services, describes a drawing scheme for a group time scale, its generation algorithms and a structure of a measurement instruments complex for time and frequency storage and reproduction.

The fourth paragraph studies methods for synchronization of reference clocks, which are distant from each other making a direct connection between them impossible. Different types of radio and wire communications systems used as solutions for providing various consumers with standard time signal are also described briefly. For example, there are following such consumers: communications and telecommunications, information technologies computer systems and computer servers, space and ground-based navigation, space geodesy, geodynamics, transport management systems, etc. Some synchronization methods based on the use of GPS/GLONASS satellite groups are presented.

The fifth paragraph covers algorithmic and software tools, which allow to evaluate the accuracy of solving of time and coordinates support problems by means of a simulation modelling. In particular, a brief description of the simulation model of the quantum clock instability developed by the authors and used in a software simulator of the measurement data, received from a network of non-query measuring stations through the GLONASS and GPS navigation satellites is given in this paragraph.

A comparative analysis based on the main metrological characteristic of the modern methods of ERP determination and prediction is conducted in the sixth paragraph. The ERP prediction accuracy requirements for the time intervals of the coordinate and ephemeris support of GNSS operation are given within the scope of metrological support issues of coordinate and navigational determinations for GPS and GLONASS GNSS. The basic principles and approaches used by authors for the development of new high-precision ERP prediction method are set forth.
