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Time & Frequency and Fibre Optics
The NMISA currently maintains four Caesium atomic clocks, with one of the clocks being designated as the master clock for South Africa. Development work is in progress to design a steered frequency source so that the master clock for South Africa will become the average of all clocks in the ensemble.
 
Services offered:
The Time and Frequency Laboratory offers time distribution, calibration and consultation services. Time is distributed through the Network Time Protocol (time.nmisa.org) or satellite common view (GPS and GLONASS). More information is available on the SA Time page in the main menu. The information, together with more time related information, is also available in a monthly bulletin.
 
The Time and Frequency Laboratory is accredited for the work performed for customers on a regular basis. Below are descriptions of common calibration services
 
Frequency measurements
The frequency of the full range of clocks and oscillators can be calibrated, from crystal oscillators with drift rates of parts in 108 to Caesium devices with drift rates of parts in 1014. A variety of techniques are used and uncertainties quoted depend on the period of observation, the unit under test and the technique used. The accredited uncertainty for certain fixed frequencies are 1 part in 1013
 
The frequency output of a frequency source or generator can be measured from 1 mHz to 40 GHz where the best uncertainty is 2 parts in 1010.
 
Time Interval
Time interval measurements can be performed from 1 ns to 100 000 s with a best uncertainty of 2 parts in 1011.
Pulse characteristics
 
Calibration for the characterisation of pulses and the step response of oscilloscopes is provided by the use of a reference pulse generator traceable to an international laboratory. The best measurement uncertainty is 6 ps.
 
Phase angle measurements
The national standard for phase angle is generated by a phase standard capable of generating phase angle with a resolution of 0,001 degree at a frequency range from 1 Hz to 100 kHz for voltage output of 50 mV to 125 V (RMS). The best measurement uncertainty is a function of frequency and voltage and is available on our accreditation scope.
 
South African Standard Time (SAST)
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 the NMISA. This is currently the most accurate unit maintained, and its accuracy is disseminated to the public by the provision of time services.
If for example, a 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:
  • 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, a NTP server must be set up at the local site. Our stratum 1 server at time.nmisa.org is available to the public, but the settings will only allow another NTP server to access it. Other time servers in South Africa can be viewed on ZA Time Server pages.
 
The 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 satellite 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 against our own clocks. The data is reduced to a single point per day. A 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 better than parts in 1013.

Service areas have been identified to describe the work conducted in the Fibre Optics Laboratory, namely: optical time domain reflectometer (OTDR) parameters and fibre optic length, wavelength and fibre optic power responsivity.
 
Responsivity
A fibre optic power meter measures the intensity of light (optical power) exiting a fibre optic cable or being generated by an optical source (e.g. laser, laser diode). Optical power is a useful indicator of system performance because it yields the ‘efficiency’ of light transfer of the fibre optic link. Optical power measurements are therefore widely used during installation and maintenance of fibre networks to ensure that the maximum possible signal is transferred.
 
The absolute radiometer is gazetted as the National Measurement Standard for detecting optical radiant power, which is traceable to the SI units.
 
Wavelength
Wavelength measurements become progressively more important as the number of channels is increased. This is due to a concomitant reduction in the wavelength gap (channel spacing) maintained between adjacent channels. ‘Dense’ in DWDM refers to the fact that these channels are indeed very closely spaced. Accurate wavelength measurements are required to be able to differentiate between consecutive channels, so that their signals can be received independently. Currently, telecommunications applications utilise wavelengths around 850 nm (multimode fibres), as well as in the whole range from 1260 nm to 1675 nm (single-mode fibres). Owing to its importance for DWDM, the wavelength region in which the highest accuracies are required, is that between 1460 nm and 1625 nm (which includes the so-called S-, C, and L-bands). A Standard reference material, Acetylene cell coupled with fibre ends are used to provide traceable wavelength measurements.
 
Optical time domain reflectometers
Optical time domain reflectometers (OTDRs) are widely used in the ICT industry. These instruments are considered to provide the benchmark for measurements taken during installation and trouble-shooting of fibre optic networks. An OTDR creates a visual representation or ‘trace’ of the fibre under test (which may already form part of an installed network). From such a trace, two main parameters can be determined: the length of the fibre link; and the amount of light that is lost as the signal travels through the optical fibre.
 
OTDR users need to be sure that the OTDR is operating correctly and thus that the location and magnitude of each event is an accurate representation of the fibre link’s characteristics. OTDR length measurements are traceable to South Africa’s national time standard, the caesium clock.