The Heights for the time Measurement and the Time for the Heights Measurement
Alberto Cina, Politecnico di Torino, Italy; Davide Calonico, Istituto Nazionale di Ricerca Metrologica (INRIM), Italy; Paolo Dabove, Politecnico di Torino, Italy
Location: Big Sur
Measuring the time is something essential to humans: all our activities are marked by atomic clocks found in research laboratories, in companies and in many satellites above us. In this context, a common International Time scale has been necessary for several centuries. With the advent of new frequency standards, the accuracy obtainable has increased, reaching values of about 2 x 10-16.
To achieve this kind of accuracy, it is necessary to make some corrections, including one for the relativistic effect caused by the altitude at which the clocks are found.
Therefore, the importance of having correct and precise measurements of geodetic heights becomes fundamental in increasing the accuracy in time determination. Today, the precision of geodetic and metrological measurements are becoming comparable, making the measurement of time for the determination of height differences interesting: this involves the use of atomic optical clocks for the futuristic "relativistic geodesy".
It is sometimes the case that different disciplines find themselves closely interacting when high accuracy measurements are required – and so it is with the geodetic measurements of heights and the metrological ones of time and frequency. Such close interaction leads to a realistic scenario of relativistic geodesy (Muller et al., 2008).
This work is framed within the Italian national research project “Italian Link for Time and Frequency” (LIFT) funded by the Italian Ministry for University and Research and it is also part of the international research project entitled "International timescales with Optical clocks" (ITOC) funded by the European Union through the European Metrology Research Program. The primary objectives of these projects are: the improvement of frequency measurements with atomic clocks, generating an International Time scale based on new frequency standards, and the dissemination of standard time and frequency signals at national and international scale, with a level of accuracy and stability greater than those available today. Recent experiments (Calonico et al., 2014, Predel et al., 2012, Stefani et al., 2015, Calonico et al., 2015) have shown that atomic clocks can be synchronized with very high accuracies (even if they are at great distances) thanks to innovative techniques based on optical links, that enable great improvements compared to the traditional satellite techniques used up to now.
To achieve high accuracy in the metrological measurements of time and frequency, it is necessary to make some corrections, including one for the relativistic effect caused by the altitude at which the clocks are found.
The measure of the geodetic height becomes, therefore, an element whose importance increases as much as the desired accuracy in time determination. Today, the precision of geodetic and metrological measurements are becoming comparable, making the measuring of time for the determination of height differences interesting: this involves the use of atomic optical clocks for the futuristic "relativistic geodesy".
This work aims to demonstrate how it is possible to couple geodetic and atomic measurements, by addressing and solving problems due to heights determinations of some metrological sites and considering different geoid models.