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|Year of Publication||2013|
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.
- 1 Introduction
- 2 Time Transfer, Positioning and Navigation
- 3 Metrology Evolution
- 4 Credits
- 5 References
The comparison of distant clocks has always been an important part of time metrology and has for a long time been based on GPS time comparisons that will be complement by the emerging GNSS System Times, such as Galileo System Time. They allow the assessment of the properties of (primary) frequency standards; e.g., to judge whether they agree within their assigned uncertainties. They are needed also for providing the data which are subsequently used in the calculation of time scales on the basis of the readings of clocks located in different institutes spread world-wide. Looking from another perspective, function and performance of a GNSS are strongly dependent on the quality of clocks on the ground, onboard the satellites, and on the means of synchronization of the various elements. Therefore a strong interest exists in research aimed at improving clocks and time comparison techniques in support of GNSS evolution which consequently will influence GNSS-based applications using time metrology. Time metrology relies primarily on the Open Service of GNSS Systems, but a combination of the several available signals/frequencies can cancel simultaneously the first order ionospheric delays and the geometrical path delay. Such combination can be written either for the code modulated on the signals, or for the carrier frequencies. The first order ionospheric delays are cancelled, and the geometrical path can also be cancelled by the simple difference of two signals, provided the antenna phase centres of all signals are at the same point. The most severe drawback is that the noise of the combination results would be higher than when using just one single code/phase signal. It remains to be studied whether these techniques are competitive compared to one single signal. However GNSS already provide high accurate position determination using Differential GNSS (DGNSS) based techniques such as RTK or PPP that are being widely used in many scientific applications.
The processing of GNSS data enables estimation of several types of parameters. Those of interest in the present context are the position of the GNSS reference point (typically denoted as the antenna phase centre) and the time difference between the local reference clock and the GNSS time reference.
Progress in Clock Development
Europe has a long-established tradition in the making of atomic clocks:
- The first caesium clock was developed at the National Physical Laboratory (NPL) in the UK;
- Physikalisch-Technische Bundesanstalt (PTB) in Germany has designed and successfully operated thermal beam standards for over 30 years;
- The Paris Observatory (now Laboratoire National de Métrologie et d’Essais–Système de Références Temps-Espace or LNE SYRTE) is the forerunner of the use of cold atoms in primary clocks.
Only in Switzerland, however, exists commercialization of high level atomic-clock technologies been supported successfully for many years. The Observatoire de Neuchâtel produced active hydrogen masers for almost three decades, and later stepped into the research on miniaturized rubidium gas cell frequency standards. The passive hydrogen maser development has followed more recently. All initiatives led to space-qualified products: the active hydrogen maser will be part of ESA’s Atomic Clock Ensemble in Space (ACES) mission. The rubidium clock and the passive hydrogen maser are the current Galileo space clocks. The Observatoire de Neuchâtel’s clock research was recently transferred to the Laboratoire Temps–Fréquences (LTF) of the Université de Neuchâtel.
The Role of the GNSS System Time Service Provider
The additional use of GNSS as a time dissemination service requires that the relation between GNSS System Time and international references such as UTC and TAI, is well defined and broadcast in most of the GNSS navigation messages. As an example Galileo will provide a prediction of the time offset GST-UTC at least with an uncertainty of 28 ns (2 sigma). The required support for such ”metrological time-keeping” shall be guaranteed by the time facilities of each GNSS constellations providing so-called UTC correction so that each GNSS system time can be steered to UTC. UTC differs from TAI only by an integer number of seconds because of the introduction of leap seconds into UTC, but both scales have the same scale unit. The reference times of GNSS constellations also contribute to the overall UTC determination that is obtained from the contribution of many accurate clocks around the world.
Improving Time Transfer and Time References with GNSS
GPS Receivers of the first generation used for time comparison were single-channel, single frequency C/A-code (Coarse/Acquisition) receivers. Receiver manufacturers later developed multi-channel receivers, operating also at one frequency, but allowing simultaneous observations of several satellites at a time. The propagation of the signal is affected by atmospheric effects. The ionosphere provokes delays that introduce significant errors, particularly during periods of high solar activity. Dual frequency reception eliminates most of the ionospheric delays. Multi-channel, dual-frequency receivers have thus started to replace older equipment in a number of laboratories. Such receivers and the P3 evaluation method have helped to increase the accuracy of long-distance time transfer. The most advanced use of GNSS signals is based on the carrier phase. This allows frequency comparisons among remote standards with the lowest transfer noise of all current common methods. Such techniques are now common in the time and frequency community and, for example, they contribute to the computation of time links for TAI.
GNSS and the establishment of terrestrial reference frames
The positions of stations over the Earth’s surface can now be determined with a precision at the level of a few millimetres and their variation over time at the level of, or better than, 1 mm/year. This performance is only possible thanks to the tremendous progress achieved by space geodesy techniques and the high level scientific software packages developed by various analysis centres dealing with the accumulated geodetic observations over the last three decades. Continuous geodetic observations become more and more fundamental for many Earth science applications at the global and local levels:
- Large scale and local Earth crust deformation;
- Global tectonic motion;
- Redistribution of geophysical fluids on or near the Earth’s surface, including the ocean and atmosphere, cryosphere, and the terrestrial hydrosphere;
- Monitoring of mean sea level and its variability given its impact on global warming.
All of these important applications depend fundamentally on the availability and accuracy of a truly global Terrestrial Reference System (TRS) to be associated with the concept of ITRS (International Terrestrial Reference System) that only space geodesy can realize. In addition to these geoscience applications, a TRS, through its realization by a Terrestrial Reference Frame (TRF), is an indispensable reference needed to ensure the integrity of GNSS, such as GPS, GLONASS, Galileo, etc. It is believed that the science requirement, including the main stringent one, the mean sea level variability, is the availability of the reference frame that is reliable and accessible at the level of 1 mm and with a stability of 0.1 mm/year. Stability of the reference frame means here that no discontinuity or drift should occur in its time evolution, especially for its defining physical parameters, namely the origin and the scale. Unfortunately, the current level of reference frame accuracy (based on ITRF current achievement) is at least ten times worse than the science requirement. However the emerging GNSS Systems, such as Galileo and BeiDou as well the evolutions on the currently available systems (GPS and GLONASS) will contribute to get closer of such accuracy.
The presence of state of the art clocks in space opens a long-term prospect for the realisation in space of the international atomic time scale and its global dissemination. This will allow metrology to overcome the current limitations related to the fluctuations of the terrestrial environment.
The information provided in this article has been compiled by GMV. In some cases, figures, tables and paragraphs have been extracted from the indicated references, in particular from the Galileo Science Opportunity Document.
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- ^ Galileo Science Opportunity Document