Space-time Metrology - Revision history

Claudia.Prajanu at 21:03, 16 September 2018

2018-09-16T21:03:47Z

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	Space and time metrology is the science of making measurements that covers three main activities:		Space and time metrology is the science of making measurements that covers three main activities:
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	* Monitoring of mean sea level and its variability given its impact on global warming.		* 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 [[Reference Systems and Frames\|Terrestrial Reference System (TRS)]] to be associated with the concept of ~~[http://www.itrs.net/\|~~ ITRS (International Terrestrial Reference System)] that only space geodesy can realize<ref>Altamimi, Z., Angermann, D., Argus, D., Blewitt, G., Boucher, C., Chao, B., Drewes, H., Eanes, R., Feissel, M., Ferland, R., Herring, T., Holt, B., Johannson, J., Larson, K., Ma, C., Manning, J., Meertens, C., Nothnagel, A., Pavlis, E., Petit, G., Ray, J., Ries, J., Scherneck, H.-G., Sillard, P., and M. Watkins, “The terrestrial reference frame and the dynamic Earth”, Eos, Transactions, Am. Geophys. U., 82(25), p. 273-279.</ref><ref>Altamimi, Z., Sillard, P. and Boucher, C., “ITRF2000: A new release of the International Terrestrial Reference Frame for Earth science application”, J. Geophys. Res., 107(B10), 2214, doi:10.1029/2001JB000561.</ref><ref>Altamimi, Z., Collilieux, X., Legrand, J., Garayt, B. and Boucher, C., “ITRF2005: A New Release of the International Terrestrial Reference Frame based on time series of station positions and Earth orientation parameters”, J. Geophys. Res., 112, B09401, doi:10.1029/2007JB004949.</ref>.		All of these important applications depend fundamentally on the availability and accuracy of a truly global [[Reference Systems and Frames\|Terrestrial Reference System (TRS)]] to be associated with the concept of ITRS (International Terrestrial Reference System) that only space geodesy can realize<ref>Altamimi, Z., Angermann, D., Argus, D., Blewitt, G., Boucher, C., Chao, B., Drewes, H., Eanes, R., Feissel, M., Ferland, R., Herring, T., Holt, B., Johannson, J., Larson, K., Ma, C., Manning, J., Meertens, C., Nothnagel, A., Pavlis, E., Petit, G., Ray, J., Ries, J., Scherneck, H.-G., Sillard, P., and M. Watkins, “The terrestrial reference frame and the dynamic Earth”, Eos, Transactions, Am. Geophys. U., 82(25), p. 273-279.</ref><ref>Altamimi, Z., Sillard, P. and Boucher, C., “ITRF2000: A new release of the International Terrestrial Reference Frame for Earth science application”, J. Geophys. Res., 107(B10), 2214, doi:10.1029/2001JB000561.</ref><ref>Altamimi, Z., Collilieux, X., Legrand, J., Garayt, B. and Boucher, C., “ITRF2005: A New Release of the International Terrestrial Reference Frame based on time series of station positions and Earth orientation parameters”, J. Geophys. Res., 112, B09401, doi:10.1029/2007JB004949.</ref>.
	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.		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.		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.
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	=Credits=		=Credits=
	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.''<ref>Galileo Science Opportunity Document~~, http://egep.esa.int/egep_public/file/GSOD_v2_0.pdf~~</ref>		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.''<ref>Galileo Science Opportunity Document</ref>

Teresa.Ferreira at 12:45, 1 December 2013

2013-12-01T12:45:38Z

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	\|Editors=GMV		\|Editors=GMV
	\|Level=~~Basic~~		\|Level=Intermediate
	\|YearOfPublication=2013		\|YearOfPublication=2013
	\|Logo=GMV		\|Logo=GMV

Teresa.Ferreira: /* Improving Time Transfer and Time References with GNSS */

2013-12-01T12:45:00Z

Improving Time Transfer and Time References with GNSS

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	==Improving Time Transfer and Time References with GNSS==		==Improving Time Transfer and Time References with GNSS==
	GPS Receivers of the first generation used for time comparison were single-channel, single frequency [[GPS General Introduction# GPS Signal Structure\|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.		GPS Receivers of the first generation used for time comparison were single-channel, single frequency [[GPS General Introduction# GPS Signal Structure\|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 ~~(see 3.4)~~. 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<ref>Defraigne P., Petit, G., “Time transfer to TAI using geodetic receivers“, Metrologia 40 (2003) 184.</ref> have helped to increase the accuracy of long-distance time transfer. The most advanced use of GNSS signals is based on the carrier phase<ref>Schildknecht, T., Beutler, G., Gurtner, W., Rothacher, M., “Towards subnanosecond GPS time transfer using geodetic processing techniques”, Proc. 4th European Frequency and Time Forum 13 – 15 March 1990, 335.</ref><ref> Kouba, J., Héroux, P., “Precise Point Positioning using IGS Orbit and Clock Products”, GPS Solutions 5 (2001) 12.</ref><ref> Ray, J., Senior, K., “Geodetic techniques for time and frequency comparisons using GPS phase and code measurements“, Metrologia 42 (2005) 215.</ref>. This allows frequency comparisons among remote standards with the lowest transfer noise of all current common methods<ref>Bauch, A., Achkar, J., Bize, S., Calonico, D., Dach, R., Hlavac, R., Lorini, L., Parker, T., Petit, G., Piester, D., Szymaniec, K. and Uhrich, P., “Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level”, Metrologia, 43(1), pp. 109-120, doi: 0.1088/0026-1394/43/1/016.</ref>. Such techniques are now common in the time and frequency community<ref>Cerretto, G., Perucca, A., Tavella, P., Mozo, A., Píriz, R., Romay, M., “Time and Frequency Transfer through a network of GNSS receivers located in Timing Laboratories”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 1097.</ref> and, for example, they contribute to the computation of time links for TAI<ref>Petit, G., “The TAIPPP pilot experiment”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 116.</ref>.		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<ref>Defraigne P., Petit, G., “Time transfer to TAI using geodetic receivers“, Metrologia 40 (2003) 184.</ref> have helped to increase the accuracy of long-distance time transfer. The most advanced use of GNSS signals is based on the carrier phase<ref>Schildknecht, T., Beutler, G., Gurtner, W., Rothacher, M., “Towards subnanosecond GPS time transfer using geodetic processing techniques”, Proc. 4th European Frequency and Time Forum 13 – 15 March 1990, 335.</ref><ref> Kouba, J., Héroux, P., “Precise Point Positioning using IGS Orbit and Clock Products”, GPS Solutions 5 (2001) 12.</ref><ref> Ray, J., Senior, K., “Geodetic techniques for time and frequency comparisons using GPS phase and code measurements“, Metrologia 42 (2005) 215.</ref>. This allows frequency comparisons among remote standards with the lowest transfer noise of all current common methods<ref>Bauch, A., Achkar, J., Bize, S., Calonico, D., Dach, R., Hlavac, R., Lorini, L., Parker, T., Petit, G., Piester, D., Szymaniec, K. and Uhrich, P., “Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level”, Metrologia, 43(1), pp. 109-120, doi: 0.1088/0026-1394/43/1/016.</ref>. Such techniques are now common in the time and frequency community<ref>Cerretto, G., Perucca, A., Tavella, P., Mozo, A., Píriz, R., Romay, M., “Time and Frequency Transfer through a network of GNSS receivers located in Timing Laboratories”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 1097.</ref> and, for example, they contribute to the computation of time links for TAI<ref>Petit, G., “The TAIPPP pilot experiment”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 116.</ref>.

	==GNSS and the establishment of terrestrial reference frames==		==GNSS and the establishment of terrestrial reference frames==

Filipe.Pelica at 00:45, 1 December 2013

2013-12-01T00:45:04Z

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	=Metrology Evolution=		=Metrology Evolution=
	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 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.


	=Credits=		=Credits=

Filipe.Pelica: /* Metrology Evolution */

2013-12-01T00:44:40Z

Metrology Evolution

← Older revision		Revision as of 00:44, 1 December 2013
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	==Metrology Evolution==		=Metrology Evolution=
	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 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.


	=Credits=		=Credits=

Filipe.Pelica at 00:44, 1 December 2013

2013-12-01T00:44:20Z

← Older revision		Revision as of 00:44, 1 December 2013
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	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.		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.		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.


			==Metrology Evolution==
			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.

Filipe.Pelica at 17:50, 30 November 2013

2013-11-30T17:50:23Z

← Older revision		Revision as of 17:50, 30 November 2013
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	=Credits=		=Credits=
	The information provided in this article has been compiled by GMV. In some cases, tables and paragraphs have been extracted from the indicated references, in particular from the ''Galileo Science Opportunity Document.''<ref>Galileo Science Opportunity Document, http://egep.esa.int/egep_public/file/GSOD_v2_0.pdf</ref>		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.''<ref>Galileo Science Opportunity Document, http://egep.esa.int/egep_public/file/GSOD_v2_0.pdf</ref>

Filipe.Pelica at 17:44, 30 November 2013

2013-11-30T17:44:01Z

Filipe.Pelica at 17:41, 30 November 2013

2013-11-30T17:41:15Z

← Older revision		Revision as of 17:41, 30 November 2013
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	* The realisation of units of measurement by scientific methods; e.g., the realisation of the second through the operation of atomic clocks;		* 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.		* 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.		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.

Filipe.Pelica at 17:38, 30 November 2013

2013-11-30T17:38:59Z

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	{{Article Infobox2		{{Article Infobox2
	\|Category=Applications		\|Category=Applications
	~~\|Title={{PAGENAME}}~~
	\|Authors=ESA		\|Authors=ESA
			\|Editors=GMV
	\|Level=Basic		\|Level=Basic
	\|YearOfPublication=2013		\|YearOfPublication=2013
	\|Logo=GMV		\|Logo=GMV
			\|Title={{PAGENAME}}
	}}		}}
	Space and time metrology is the science of making measurements that covers three main activities:		Space and time metrology is the science of making measurements that covers three main activities:

← Older revision		Revision as of 17:44, 30 November 2013
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	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.		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.


	=Introduction=		=Introduction=
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	* 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.		* 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.		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 Role of the GNSS System Time Service Provider==
	The additional use of GNSS as a time dissemination service requires that the relation between [[Time References in GNSS\|GNSS System Time]] and international references such as [[Time References\|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 additional use of GNSS as a time dissemination service requires that the relation between [[Time References in GNSS\|GNSS System Time]] and international references such as [[Time References\|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 [[Time References #Description\|UTC determination]] that is obtained from the contribution of many accurate clocks around the world.		The reference times of GNSS constellations also contribute to the overall [[Time References #Description\|UTC determination]] that is obtained from the contribution of many accurate clocks around the world.


	==Improving Time Transfer and Time References with GNSS==		==Improving Time Transfer and Time References with GNSS==
	GPS Receivers of the first generation used for time comparison were single-channel, single frequency [[GPS General Introduction# GPS Signal Structure\|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.		GPS Receivers of the first generation used for time comparison were single-channel, single frequency [[GPS General Introduction# GPS Signal Structure\|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 (see 3.4). 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<ref>Defraigne P., Petit, G., “Time transfer to TAI using geodetic receivers“, Metrologia 40 (2003) 184.</ref> have helped to increase the accuracy of long-distance time transfer. The most advanced use of GNSS signals is based on the carrier phase<ref>Schildknecht, T., Beutler, G., Gurtner, W., Rothacher, M., “Towards subnanosecond GPS time transfer using geodetic processing techniques”, Proc. 4th European Frequency and Time Forum 13 – 15 March 1990, 335.</ref><ref> Kouba, J., Héroux, P., “Precise Point Positioning using IGS Orbit and Clock Products”, GPS Solutions 5 (2001) 12.</ref><ref> Ray, J., Senior, K., “Geodetic techniques for time and frequency comparisons using GPS phase and code measurements“, Metrologia 42 (2005) 215.</ref>. This allows frequency comparisons among remote standards with the lowest transfer noise of all current common methods<ref>Bauch, A., Achkar, J., Bize, S., Calonico, D., Dach, R., Hlavac, R., Lorini, L., Parker, T., Petit, G., Piester, D., Szymaniec, K. and Uhrich, P., “Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level”, Metrologia, 43(1), pp. 109-120, doi: 0.1088/0026-1394/43/1/016.</ref>. Such techniques are now common in the time and frequency community<ref>Cerretto, G., Perucca, A., Tavella, P., Mozo, A., Píriz, R., Romay, M., “Time and Frequency Transfer through a network of GNSS receivers located in Timing Laboratories”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 1097.</ref> and, for example, they contribute to the computation of time links for TAI<ref>Petit, G., “The TAIPPP pilot experiment”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 116.</ref>.		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 (see 3.4). 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<ref>Defraigne P., Petit, G., “Time transfer to TAI using geodetic receivers“, Metrologia 40 (2003) 184.</ref> have helped to increase the accuracy of long-distance time transfer. The most advanced use of GNSS signals is based on the carrier phase<ref>Schildknecht, T., Beutler, G., Gurtner, W., Rothacher, M., “Towards subnanosecond GPS time transfer using geodetic processing techniques”, Proc. 4th European Frequency and Time Forum 13 – 15 March 1990, 335.</ref><ref> Kouba, J., Héroux, P., “Precise Point Positioning using IGS Orbit and Clock Products”, GPS Solutions 5 (2001) 12.</ref><ref> Ray, J., Senior, K., “Geodetic techniques for time and frequency comparisons using GPS phase and code measurements“, Metrologia 42 (2005) 215.</ref>. This allows frequency comparisons among remote standards with the lowest transfer noise of all current common methods<ref>Bauch, A., Achkar, J., Bize, S., Calonico, D., Dach, R., Hlavac, R., Lorini, L., Parker, T., Petit, G., Piester, D., Szymaniec, K. and Uhrich, P., “Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level”, Metrologia, 43(1), pp. 109-120, doi: 0.1088/0026-1394/43/1/016.</ref>. Such techniques are now common in the time and frequency community<ref>Cerretto, G., Perucca, A., Tavella, P., Mozo, A., Píriz, R., Romay, M., “Time and Frequency Transfer through a network of GNSS receivers located in Timing Laboratories”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 1097.</ref> and, for example, they contribute to the computation of time links for TAI<ref>Petit, G., “The TAIPPP pilot experiment”, Proc. Joint 23rd European Frequency and Time Forum, and 2009 IEEE Frequency Control Symposium, Besançon, April 2009, 116.</ref>.


	==GNSS and the establishment of terrestrial reference frames==		==GNSS and the establishment of terrestrial reference frames==
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	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.		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.		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.


	=Credits=		=Credits=
	The information provided in this article has been compiled by GMV. In some cases, tables and paragraphs have been extracted from the indicated references, in particular from the ''Galileo Science Opportunity Document.''<ref>Galileo Science Opportunity Document, http://egep.esa.int/egep_public/file/GSOD_v2_0.pdf</ref>		The information provided in this article has been compiled by GMV. In some cases, tables and paragraphs have been extracted from the indicated references, in particular from the ''Galileo Science Opportunity Document.''<ref>Galileo Science Opportunity Document, http://egep.esa.int/egep_public/file/GSOD_v2_0.pdf</ref>


	=References=		=References=
	<references/>		<references/>


	[[Category:Applications]]		[[Category:Applications]]


	[[Category:Scientific Applications]]		[[Category:Scientific Applications]]