Technological Research Lines

Time and Frequency Lab

A Laboratory for TIME MEASUREMENTS at the Astronomical Observatory of Cagliari

An international convention, has declared that BIPM (Bureau International des Poids er Measures) is the authority for the realization and conservation of the unit measurement samples  for all physical quantities, including time units, which are expressed in seconds. This is universally accepted and it represents the common ground for the construction of the Universal Time Coordinate (UTC).

The second, like any other physical unit is defined by international conventions which are determined by the experts of that given field of research, e.g., astronomers, metrologists etc. The definition of the second has changed during the years accordingly with society's needs, scientific requirements and new technologies which have enabled higher instrumental precision.

Up to 1967 the second was defined as: "1 sec = 1/86400 of the solar mean day referred to 1900". The controlling authority was one of IAU's commissions which through various astronomical observatories studied the Earth's rotation with optical instruments.

Since the beginning of 1900 the Astronomical Observatory of Cagliari has contributed to the monitoring of the time unit through the use of its telescopes and other sophisticated mechanical time measuring instruments. A pendulum clock was used to time the duration of the events. This clock was substituted in the 60s by more stable and precise quartz laboratory clocks which remained until the 70s.

In 1967 the 13th General Conference for weights and measurements defined the second as: " the duration of 9192631770 periods of the radiation produced by hyperfine level transitions for caesium -133 in the ground state". This definition was not connected to the Earth's rotation, therefore a new atomic time scale was defined (TA).

The Earth's rotation is not uniform, therefore in order to maintain alignment between the two scales when they diverge by more than half a second, a second is added to the UTC scale (civil time).

Currently the two scales differ by 34 seconds.

The atomic clock era had started, the Observatory of Cagliari purchased its first caesium atomic clock in 1974, an EBAUCHES B-5000, a very complex laboratory instrument which needed a lot of attention compared with current 5017 clocks.

These clocks are more precise and stable (1 sec every 10 to the 14) than the previous ones, therefore to control its beat sophisticated instruments with high precision interval counters were needed (TIC Time Interval Counter) which are able to measure time intervals of the order of one thousandth of a billionth of a second).

High precision clocks are needed in astronomical observatories in order to perform the timing of an event with great accuracy.

Every time a clock is installed the first thing that has to be done is to synchronize it with the Universal Time scale. This scale is built with the BIPM and is called UTC (Universal Time Coordinate), in the following paragraphs we will see how it is represented.

During the years different methods were used in to synchronize a clock with the time scales and as technology evolved these methods where more and more sophisticated.
During the 70s some metrology laboratories transmitted signals (bip) which could enable, through the use of other instruments (oscilloscope), a synchronization that had a precision of the order of a few thousandths of seconds. In the following few years new systems such as OMEGA ans Loran-C which received signals from more than one station and had precisions up to on millionth of a second.  

The Astronomical Observatory of Cagliari followed this trend by purchasing adequate instruments.

 Faster and faster TIC receivers

The current systems are based on the great performance given by artificial satellite constellations such as the GPS, GLONASS and the latest one GALILEO. The synchronization precision now obtainable is of the order of one billionth of a second. The laboratory has got receivers which can perform measurements with respect to the three constellations.

This precision has enabled us to perform laser telemetry, (the measurement of the time taken by light to reach a given object) and will enable the SRT to perform interferometry measurements.

In the last couple of years,  the possibility of constructing time scales based on frequencies that correspond to pulsar signals, has been hypothesised.  For this task it is necessary to combine instruments from the time laboratory to the radiotelescope. Our time laboratory will be transferred to the the SRT site and will be therefore in optimal conditions for the aforementioned tasks.

UTC, this is how it works

There are about 50 laboratories distributed in various continents which account for about 50 clocks.

In italy there are two time laboratories: ours (CAO) and the INRIM “G. Ferraris”.

These clocks are constantly compared to atomic clocks on board of satellites which are moving above the correspondent region. Every week the data is sent to BIPM.

The time data corresponds to different laboratories and is averaged  accordingly with algorithms that have been adopted by the scientific community of time metrologists and which calculate a UTC point every 5 days and the offset of each clock in this calculation. The results are then sent to each laboratory in order for them to eliminate the offset. The data is handed out by BIPM monthly through the newsletter T a published on the web site

Each clock has been characterized by a weight which is an index of how reliable the laboratory is, the linearity the beat is and continuous functioning.

An atomic clock must not be turned off ever. Every time a clock is turned off and on again there will be a loss of continuity of the scale, and its weight will be tuned to zero for several months.

In the recent months one of our clocks has been characterized with the maximum weight possible.

The Time and Frequency laboratory of the Astronomical Observatory of Cagliari enables the synchronization of the network computer clocks by connecting to the TimeServer

Laboratory resources

Currently the laboratory has:

  • 2 Cs atomic clocks, 1 Rb, and 1 hydrogen maser clock
  • 1 GPS receiver monofrequency and monochannel.
  • 1 GPS receiver monofrequency and multichannel
  • 1 receiver. Combined GPS, GLONASS, GALILEO
Future activities

The laboratory and all its resources will soon be transferred to the Sardinia Radio Telescope site (SRT), where it will assist the telescope through it time and frequency signals.  These will be useful for the telescope movements, its receivers and to all the control rooms and laboratories.