NMISA Time

 
September 2014
2
Tuesday
 

 

Daily time is derived from the second, which is one of the seven SI units. Time is maintained in the Time and Frequency Laboratory of NMISA. This is currently the most accurate unit maintained, and its accuracy is disseminated to the public by the provision of time services.

If your browser supports Java, a clock should be shown to the right. The applet synchronises with our time servers after loading the applet. It does not check the status of the time server and does not correct for network delays. Normally, the time displayed here will be accurate to within a second, but may be affected by unusual network delays.

The following time services are currently offered by the laboratory.

Telephone Time Service

The Telephone Time Service (TTS) provides a traceable time synchronisation service to users with a computer and a modem using the local telephone network. A time code is provided that is suitable for the automatic setting of your computer directly traceable to the national standard for time.

Note: This service is being phased out due to the age of the hardware. Please first try and use one of the other services. Should you have a need that can only be solved using the TTS system, then please contact us with the details.

Internet Time Service

A number of protocols are used to disseminate time using the Internet. At NMISA, the Network Time Protocol (NTP) is used. In order to make use of the service, NTP or SNTP client software is required, which can be found on many websites on the Internet. The use of the NMISA Stratum 2 server at time.nmisa.org is available to the public. Other time servers in South Africa can be viewed on ZA Time Server pages.

The TTS and NTP services will provide single shot accuracy that is dependent on the network, but should give users millisecond accuracy. This is typically good enough for most users. For more accurate measurements, a common view approach should be used.

Note: Not all computer operating systems support millisecond timing resolution. This means that the accuracy of your computer time may be limited to tens of milliseconds (approximately 50ms being typical).

Satellite time transfer

Global positioning systems, like GPS, GLONASS, or GALILEO, have high accuracy atomic clocks on board that are steered to a local equivalent of UTC. This allows users to use special timing versions of the satelite receivers to extract the time from the satellite. Using the time directly from such a receiver should give users microsecond accuracy. NMISA publishes a monthly bulletin where we track the performance of GPS and GLONASS against our own clocks. The data is reduced to a single point per day. An user in South Africa can proof traceability to the South African time scale to sub-microsecond accuracies (provided the local clock is good enough) if the laboratory performs similar measurements. Over the long term, with continuous observations, the frequency of the local clock can be determined to parts in 1013.