Space and time metrology is the science of making measurements that covers three main activities:
The definition of internationally accepted units of measurement; e.g., the metre and the second;
The realisation of units of measurement by scientific methods; e.g., the realisation of the second through the operation of atomic clocks;
The establishment of traceability chains by determining and documenting the value and accuracy of a measurement and disseminating that knowledge, e.g. the relationship between time scales realised in different scientific establishments.
Sciences are completely dependent on measurements. As an example Geologists measure shock waves when the gigantic forces behind earthquakes make themselves felt, and atomic physicists use local frequency references (lasers, microwave sources) to do spectroscopy on atoms, molecules and ions, just to give a couple of examples. Thus, accurate time and frequency measurements form the backbone of a variety of studies in the fields of geodesy, astronomy, space exploration, etc., either by measuring the time of arrival of propagation of a radio or light signal,or by measuring the change in frequency incurred by a propagating signal. This is also the case regarding the operation of a GNSS. On one hand, the successful operation of such systems relies on time and frequency metrology and, on the other hand, the availability of such systems supports several scientific activities.
The usage of the GNSS in metrology has promoted the development of techniques for time-frequency transfer. On the other hand, the strict timing requirements for spaceborne stable clocks is constantly pushing the industry for the development of new designs of atomic clocks.