22 NTP server controlled by the national time etalon

22.1 Introduction

Main goal of the project is the development and operation of a time server (called TimeCZ) controlled by the Czech national time etalon. The project has started with cooperation of the Institute of Radio Engineering and Electronics of the Academy of Science of the Czech Republic (IREE), which operates the time etalon.

The main feature of this time server, compared with other time servers, is its independence from any third party time signal (e.g., navigation systems or public time services). The national time etalon is a trustable source with a metrologically defined relation to the UTC time.

22.2 Structure of the time server

The server consists of three main functional blocks:

computer running ntpd daemon,
control system KPC,
microprocessor system FK.

Figure 22.1: Functional blocks of the server

Detailed description of components, processes and relations between them was presented in the 2002 report. After one year of experimental operation we see no need to change the design and we are thus confident the server can be used in a production environment.

22.3 Progress in 2003

In 2003 we focused on experimental verification and improvement of server features.

22.3.1 New version of the FK system

We designed and manufactured a new microprocessor system FK in order to replace the former development version. The new version has the following features:

more stable and reliable main board,
generation of leap second flag,
more accurate measurement of the difference between server time and internal time.

22.3.2 New version of software

We wrote new software for the KPC control system. Our operational experiences showed the system was not sufficiently tolerant to packet loss and so we decided to improve the communication algorithm between KPC and other components.

22.3.3 WWW pages

As the time server is intended for public usage, it is also important to prepare appropriate documentation for both casual and expert users. We started to collect all necessary information on WWW pages, which now contain an explanation of server principles, user instructions, server characteristics and links to recommended NTP clients. Unfortunately, we did not succeed to finish the WWW pages yet and so the task continues.

22.4 Characteristics of server

The server checks continually the difference between its internal time and the national etalon. In addition to that, the internal time is compared to other independent time sources - a GPS receiver and an external NTP server. The control systems can block server output and switch it effectively off if the difference indicates a system malfunction.

Apart from checking against the second label, the internal time is also checked with a resolution of 100 ns. It is necessary to do both checks, as the etalon output also consists of two signals: the second label and the PPS (Pulse per Second) signal. We have to eliminate the theoretical situation when the time provided by the server differs from the exact time by an integer multiple of a second.

The complete list of checks is as follows:

matching server time against the second label of the FK system,
matching server time against the GPS time,
matching server time against an independent external NTP server,
measuring the difference (with 100 ns accuracy) of PPS signals between the server and the etalon.

The output of the time server is blocked no later than 3 seconds after a conflict is discovered between second labels of the server and the system FK or as soon as the measured difference of PPS signals exceeds 30 us. When a conflict is found between the server time and GPS or external NTP time, a warning is generated without blocking the server output. In this case the error may actually be in the external system.

22.4.1 Accuracy of the time server

Figure 22.2: Absolute error of the time server

The Figure shows the measured absolute difference between the server and etalon times. The 90-minute interval shown in the figure represents a typical behaviour of the measured value, with the absolute error lying in the range of 500 ns around the mean value of 1 microsecond. As the mean value was stable throughout the observed period (i.e., several months), it is possible to compensate for it and thus reach the absolute accuracy about 500 ns.

The accuracy is better than expected. It is mainly due to the usage of a temperature compensated oscillator (TCXO) and a special card for processing the PPS input without any latency. Placing the server in a well air-conditioned room probably helped as well.

22.5 Plans for future

We have to finish the WWW pages of the server in a near future. We intend to continue the observation and measurement of the server characteristics. We consider to develop a real-time hardware module having the synchronization algorithm implemented independently of the main processor.

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