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Radio astronomy is the study of celestial phenomena through measurement of the characteristics of radio waves emitted by physical processes occurring in space. Radio waves are much longer than light waves. In order to receive good signals, radio astronomy requires large antennas.

Radio astronomy is a relatively new field of astronomical research. The earliest investigations into extraterrestrial sources of radio waves were by Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, in the early 1930s. Following World War II, substantial improvements in radio astronomy technology were made by astronomers in Europe and the United States, and the field of radio astronomy began to blossom.

Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including pulsars, quasars and radio galaxies. Such objects represent some of the most extreme and energetic physical processes in the universe. Radio astronomy is also partly responsible for the idea that dark matter is an important component of our universe; radio measurements of the rotation of galaxies suggest that there is much more mass in galaxies than has been directly observed. And the cosmic microwave background radiation was first detected using radio telescopes. However, radio telescopes have also been used to investigate objects much closer to home, including observations of the Sun and solar activity, and radar mapping of the planets.

### VLBI

Multiple antenna radio astronomy is also known as interferometry. When the antennas are too far apart to allow the antennas to be connected by conventional cables, the data are recorded on magnetic tape (or recently hard disks) and shipped to a central processing location (correlator). This technique is known as Very Long Baseline Interferometry (VLBI). VLBI involves using a number of antenna linked together to create a giant antenna which can resolve features with smaller angles. The band of radio waves used depends on what they want to achieve for the particular experiment.

Some of the scientific results derived from VLBI include:

• Motion of the Earth's tectonic plates
• Regional deformation and local uplift or subsidence.
• Definition of the celestial reference frame
• Variations in the Earth's orientation and length of day.
• Maintenance of the terrestrial reference frame
• Measurement of gravitational forces of the Sun and Moon on the Earth and the deep structure of the Earth
• Improvement of atmospheric models.
• Astronomical benefits too

VLBI is used mostly for mapping and timing. It is essential for accurate spacecraft tracking that the positions of the antennas is known to the milimetre. This technique measures the time differences between the arrival of radio waves from distant sources (such as quasars) at two separate antennas. Using large numbers of time difference measurements from many quasars observed with a global network of antennas over a period of time, it is possible to map movements of tectonic plates to within milimetres.

### Space VLBI

The latest development in radio astronomy observations is the Space Very Long Baseline Interferometry (SVLBI) program. This is used to perform radio astronomy with an extended baseline VLBI, of which one element is a space-based antenna.

The JPL SVLBI[?] project, funded by NASA, supports the VSOP[?] (VLBI Space Observatory Program) mission developed by the Institute of Space and Astronautical Science[?] (ISAS) in Japan. The VSOP spacecraft consists of an eight meters radio telescope. It was launched in February 1997 and is orbiting the Earth in an elliptical orbit to enable VLBI observations on baselines between space and ground telescopes. The primary targets are active galactic nuclei, water masers[?], OH masers[?], radio stars[?], and pulsars will also be observed.

The baselines between space and ground telescopes will provide 3 to 10 times the resolution available for ground VLBI at the same observing frequencies. Four ground tracking stations are involved with the SVLBI project.

The whole system was supposed to operate automatically, needing only the observing schedule, Doppler predicts, and spacecraft state vectors to perform all the acquisition and tracking functions, with no operator inputs. This however has not yet been achieved and an operator is required for all supports on this system.

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