public:vlbi_fundamantals:introduction

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public:vlbi_fundamantals:introduction [2014/07/15 11:46] admin [History and technological developments] |
public:vlbi_fundamantals:introduction [2014/07/16 21:37] (current) admin [Geometric principle] |
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$$ \tau=-\frac{b \cdot s_0}{c}=t_2-t_1 $$ | $$ \tau=-\frac{b \cdot s_0}{c}=t_2-t_1 $$ | ||

- | The delay $\tau$ is time-dependent, and the largest contribution to its variation is due to the fact that the interferometer is fixed to the Earth's surface and thus follows its diurnal rotation with respect to the celestial reference system that is realized by positions of radio sources. The geodetic VLBI concept uses two or more radio telescopes to observe numerous extragalactic radio sources distributed across the skies, mostly quasars or radio galaxies. In geodetic VLBI since the end of the 1970's the observations are done within S-band (2.3 GHz) and X-band (8.4 GHz)\footnote{A change of the frequency setup, e.g. observing on a frequency band between 2 and 14 GHz, is envisaged for the next VLBI generation, VLBI2010 ([[public:bibliography#Petrachenko, 2009|Petrachenko, 2009]]), and the data are recorded and time-tagged using very stable and precise time signals obtained from hydrogen masers. These data are then sent to particular correlation centers for cross-correlation to generate so-called fringes and to obtain the group delay observable $\tau$ which is relevant for geodetic and astrometric applications. From these delays, the baseline lengths $b$ and other geodetic parameters can be derived nowadays with sub-centimeter accuracy. The VLBI technique measures very accurately the angle between the Earth-fixed baseline vector $b$ and the space-fixed radio sources $s_0$ which have to be transformed into a common system for the evaluation of the Eq. from above by parameter estimation techniques. Thus, even the most subtle changes in the baseline lengths and in the angles between the reference systems can be detected, and the main geodynamic phenomena such as Earth orientation parameters can be monitored with unprecedented accuracy ([[public:bibliography#Schuh, 2000|Schuh, 2000]]). However, //'... if we leave the Euclidean geometry in empty space and return to the real world with curved space, flickering quasars, billowing atmospheres, wobbling axes, and drifting continents, we have to delve into layers of complexity, fortunately not only as a chore but also as an opportunity to gain a wealth of new knowledge about our system Earth.' ([[public:bibliography#Campbell, 2000|Campbell, 2000]])// More details about the complexity of VLBI are provided in the next sections. | + | The delay $\tau$ is time-dependent, and the largest contribution to its variation is due to the fact that the interferometer is fixed to the Earth's surface and thus follows its diurnal rotation with respect to the celestial reference system that is realized by positions of radio sources. The geodetic VLBI concept uses two or more radio telescopes to observe numerous extragalactic radio sources distributed across the skies, mostly quasars or radio galaxies. In geodetic VLBI since the end of the 1970's the observations are done within S-band (2.3 GHz) and X-band (8.4 GHz) (A change of the frequency setup, e.g. observing on a frequency band between 2 and 14 GHz, is envisaged for the next VLBI generation, VLBI2010 ([[public:bibliography#Petrachenko, 2009|Petrachenko, 2009]])) and the data are recorded and time-tagged using very stable and precise time signals obtained from hydrogen masers. These data are then sent to particular correlation centers for cross-correlation to generate so-called fringes and to obtain the group delay observable $\tau$ which is relevant for geodetic and astrometric applications. From these delays, the baseline lengths $b$ and other geodetic parameters can be derived nowadays with sub-centimeter accuracy. The VLBI technique measures very accurately the angle between the Earth-fixed baseline vector $b$ and the space-fixed radio sources $s_0$ which have to be transformed into a common system for the evaluation of the Eq. from above by parameter estimation techniques. Thus, even the most subtle changes in the baseline lengths and in the angles between the reference systems can be detected, and the main geodynamic phenomena such as Earth orientation parameters can be monitored with unprecedented accuracy ([[public:bibliography#Schuh, 2000|Schuh, 2000]]). However, //'... if we leave the Euclidean geometry in empty space and return to the real world with curved space, flickering quasars, billowing atmospheres, wobbling axes, and drifting continents, we have to delve into layers of complexity, fortunately not only as a chore but also as an opportunity to gain a wealth of new knowledge about our system Earth.' ([[public:bibliography#Campbell, 2000|Campbell, 2000]])// More details about the complexity of VLBI are provided in the next sections. |

===== History and technological developments ===== | ===== History and technological developments ===== | ||

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