Very Long Baseline Interferometry
Very Long Baseline Interferometry ( VLBI ) or Langbasisinterferometrie is a method of radio astronomy for measurements with high spatial resolution and position accuracy. It is used both for astronomical observations and for geodetic studies in the field of geodesy.
The spatial resolution of an interferometer is determined by the wavelength and the greatest distance between the participating antennas. In normal IRAM the signals from the individual antennas are generally combined using waveguides and brought to interference. Instead, the signals from the individual antennas are stored together with very precise time references and later correlated mathematically in the VLBI. This makes it possible to obtain interference over intercontinental distances, or even with an antenna in space ( space - VLBI ).
The exact position measurements VLBI are important for defining the coordinate system astronomical. The achievable spatial resolution with VLBI is currently the superior in other regions of the electromagnetic spectrum, however, limited to objects with bright radio emission. With VLBI, the effluent from the surroundings of black holes in active galactic nuclei "Jets " of energetic particles are investigated. Other objectives include masers in star-forming regions, in the atmosphere of stars and turn around active galactic nuclei.
In May 2012, VLBI was first used for a SETI project. Here, the star Gliese 581 with the instruments of the Australian Long Baseline Array was explored.
Geodesy is the science of surveying and mapping the earth's surface. Not only gauges and satellites, but also VLBI measurements are used for orientation on the surface. Distant celestial bodies that appear point-like for us because of their high distance and also apparently have no proper motion, are observed and used as a basis to determine locations on the earth's surface. That is, the distances between the radio telescopes and to each other are measured by their movements and movement directions within a few millimeters determined accurately. So it is possible to identify potential deviations by comparisons with previous measurements.
Principle of measurement
By precisely measuring the signals with two or more radio telescopes and their time-stamped storage of a kind travel time measurement is possible. The data is moved by means of a correlator for so long on the timeline, until almost complete coincidence of the peaks reached. After this, the correlating shift corresponds to the transit time or path difference? T1, 2 from the quasar to the two (or more) telescopes. By measuring towards several quasars (5-20 in one hour ) is set up a kind of survey network. Because change the individual? T by the Earth's rotation continuously, except for the coordinates of the instantaneous pole of rotation and also the astronomical time can be determined.
The accuracy is about 0.1 ns ( billionths of a second ), on route converted at a few centimeters. Due to the large number of measurements (mostly automatic) networks can be calculated to within ± 1 cm.
Data reduction and results
The measurements must be corrected due to various influences:
The results are easily combined with other methods - eg using GPS and its method to determine the second correction.
Through long-term determination of coordinates of radio telescopes, the movements of the continents by plate tectonics can be determined. For several years, this is possible with accuracies in the millimeter to centimeter range. The ten large plates move against each other with 2 to 20 cm per year.
The most important currently used VLBI networks are:
- VLBA: Very Long Baseline Array (USA)
- EVN: European VLBI Network
- LBA: Long Baseline Array (Australia)
- VERA: VLBI Exploration of Radio Astrometry (Japan)
- IVS: International VLBI Service for Geodesy and Astrometry