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byAndrei FINKELSHTEIN, Dr. Sc. (Phys. & Math.), Director of the St. Petersburg Institute of Applied Astronomy, Russian Academy of Sciences

Our Institute of Applied Astronomy is through the first stage of a major research project, Quasar, which opens up new vistas in the exploration of the universe. Using the Quasar innovative technology, we shall be able to look into the galactical nucleus (core), the center of the quasar proper, and gain knowledge of the primordial state of the universe. Besides, we shall be able to peep deep into the earth's interior and develop a high-precision theory of the motion of the solar system's bodies. President Putin was among the first to note the significance of this undertaking. In his message of greetings to the Russian Academy of Sciences he stressed that "scientists have obtained a unique instrument for conducting research in the interests of basic and applied science".

ACCURACY OF MEASUREMENTS COMES FIRST

Taking a most general view of astronomy, we may divide it arbitrarily into two domains-astronomy of images and astronomy of positions.

The former includes astrophysics and cosmology (both are involved with the origin and evolution of celestial objects). Here we come to deal with a variety of most challenging problems, in theoretical terms too. For instance, with the energy sources of galactic and quasar nuclei; with the mechanisms implicated in the making of a large-scale structure of the universe and the nature of its primary state.* This field of research is always in flux, what with the ever changing theoretical concepts, and most sophisticated theories and costly techniques being employed.

The other domain, astronomy of positions, is concerned with the geometrical properties of the universe. This is a more traditional and even conservative sphere of scientific studies. Its aim is to construct high-precision radio-astronomical and optical reference systems (inertial ones, both on the earth and in outer space) and determine corresponding orientation parameters. And another aim is the dynamic modeling of the solar system and our home planet. These objectives may look quite commonplace and even dull. Yet once we come to grapple with them, we hit upon as many "blank spots" and "black holes" as in astrophysics and cosmology. Such puzzling problems as prediction of earthquakes,* circulation of air in the atmosphere and water in the World Ocean, vagaries of the plate tectonics and lithospheric phenomena, fluctuations in the ocean level and the like. Add to this a range of related problems-those of the gravitational field


* See a related article on the solar system and its stellar analogues in the present issue of our magazine.-?y.

See: V. Muravyov, G. Krasnopevtseva, "Violent Earthquakes: How Predictable", Science in Russia, No. 6, 2000.- Ed.

page 20


of the earth,* the dynamics of its core, solid-body and ocean tides. And, besides, a host of applied problems bearing on ballistic support of satellite navigation systems and high-precision synchronization of atomic time scales. And last, figuring prominently among basic problems is verification of some physical theories, above all, the general theory of relativity.

What we have just said sets stringent requirements to the accuracy of measurements which, in relative units, is equal to 5 ? lO -9 -5 ? 10 -2 . It is on this level that the most intriguing and enigmatical physical phenomena come into play.

The most effective in the panoply of present-day techniques are innovative methods of chronometry and coordinate measurements developing apace in the last twenty years. These include the laser detection and ranging of artificial satellites of the earth and the moon, the radiolocation of larger and smaller planets, radiotechnical observations carried out by satellites of the global navigation systems GLONASS and GPS, the satellite request system DORIS and, last, studies of extragalactical radiation sources, planets and space vehicles by means of radiointerferometry-very long baseline interferometers (VLBI). Of special importance here are optoastronomical satellites employed for high-precision measurements of relative angular distances between optical sources in outer space. VLBI techniques hold the best promise as the most universal technology ensuring comprehensive and top accuracy problem solving. VLBI are likewise all-important in studying the fine effects in the revolution of the earth which, in their turn, are connected with many significant physical phenomena, above all, with the internal structure of our planet. It is for this reason that the VLBI are used as a basic technology for tackling a wide range of coordinate- and time- related problems.

VERY LONG BASELINE RADIOINTERFEROMETRY

Radio astronomy, like astronomy in general, is actually geared to two problems-the high sensitivity and the high angular resolution of telescopes. The first parameter (high sensitivity) characterizes the ability of telescopes to register weak signals, i.e. those coming from ever more


*See an article on gravitation of celestial bodies and neutron fluxes in the present issue of our magazine.

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remote sources and indicative of the early stages of the evolution of the universe and its objects. The other parameter (high angular resolution) shows the possibilities of radio telescopes in looking into the structure of cosmic radiation sources and pinpointing their position.

Now any radio telescope is in fact an analog computer designed for performing a simple school operation that involves fields excited by cosmic radiation and brought into focus: the telescope adds field by field, raises this sum to the square and time- averages the result, as described by the equation:

The first term on the equation's right side describes the total power picked up by the telescope, a value that is proportional to its collecting surface, or the diameter's square (D 2 ). This term gives information on a flux emanating from the source and on its variability. The other term is a cross-correlation function which provides data on the image of the source and, consequently, on its internal structure and its position in the celestial sphere. This value is inversely proportional to the diameter of the radio telescope and directly proportional to the radiation wave-length- So in order to solve both problems one should increase the radio telescope's diameter.

Such was the pathway that scientists had been pursuing up until 1932, a year when an American engineer, Carl Jansky, detected radio emission from a cosmic source, the galaxy, and thus laid a groundwork for a new field of observational astronomy, radio astronomy (which has been instrumental in the discovery of radio galaxies, quasars, relict radiation and pulsars).

For years radio telescopes were manufactured with mirrors of ever larger diameter. Today the most powerful radio telescope is in Effelsberg, Germany, equipped with a 100 m mirror. This diameter is actually a finite one for rotary full-swing radar telescopes, for otherwise the metal-work of a telescope, deformed, deranges the entire focusing setup.

Time was when telescope experts suggested a way out by building alternate-shape aerial systems in which the collecting surface is composed of panels mutually oriented with high accuracy (for which purpose geodetic, radio-engineering or radio- astronomical methods are employed). This principle, proposed by the pioneers of Russian observational radio astronomy, S. Khaikin and N. Kaidanovsky, has been materialized in this country's largest radio telescope at the Pulkovo observatory and in the 600-meter radio telescope RATAN-600 (this one is the world's largest).

And yet telescopes of this kind have essentially limited possibilities in tracking objects of observation. This factor narrows a great deal their application range: first, because there is less time for accumulation of signals from weak cosmic sources and, second, these sources can hardly be watched at different hour angles.

The VLBI technology is quite opportune in this respect as the most effective way for the development of high-angular resolution radio astronomy. In a nutshell, here's what it's all about. Telescopes, installed far apart and not interconnected by cable lines, are synchronized in picking up high-frequency signals from cosmic radiation sources. Thereupon these signals are converted to video frequencies which are either registered in the digital form on magnetic tape or relayed via special communication lines. Then a computer determines a cross-correlation function.

The sensitivity of this system to a radiation flux is usually determined by the area of component radio telescopes, and the time of coherent accumulation. As to angular resolution, it is a function of the distance between the instruments and the interferometer's baseline. The latter, as an effective diameter of such a global system, is limited only by the diameter of the earth. Thereby the

page 22


detection and ranging accuracy is improved by nearly five orders-from several angular minutes to fractions of one millisecond-compared with a solitary radio telescope.

RADIOINTERFEROMETRIC NETWORK QUASAR

Worldwide today there are over a hundred radio telescopes operating-either on a now- and-then basis or permanently-in the radiointerferometry mode. Such telescopes have been installed on all the continents, including the Antarctic. This global, planetary system has become possible after, in the late 1970s and early 1980s, radio- astronomical research teams of the United States, the Soviet Union, Europe, Canada, Japan and China designed astrophysical and astronometric VLBI networks. In the Soviet Union this project was dubbed Quasar-KVO, stands in Russian for "coordinate-temporal support"; the very name reflects the mainstream of research in astrometry, geodynamics and ephemeris-time astronomy.

Originally the project provided for the construction of a radiointerferometer composed of five telescopes. Yet with the disintegration of the Soviet Union and because of broader international cooperation in this field, we decided to build only three radio telescopes, for this number was found to be sufficient for securing our independence in national problem solving and for effective integration of our network within the international radiointerferometric community.

New radio-astronomical observatories are to be set up at Svetloye (Leningrad administrative region), at Zeienchukskaya (Karachai-Circassian Republic in Northern Caucasia) and at Badary (Buryat Republic in Siberia). As a result, we obtain a VLBI triangular network, with its "sides" equal to 2015x4282x 4405 km; its control and processing center is in St. Petersburg, where a correlator has been installed. All these points have not been chosen at random. Not at all. Certain astronomical and geodynamic advantages are involved here, such as the presence of a considerable equatorial and polar projection of bases, and a large common zone of visibility for the network's radio telescopes tracking

page 23


Comparison of noise temperatures of VLBI stations.

Basic parameters of Quasar radio telescope

Main mirror:

diameter

32m

form

quasiparaboloid

Secondary mirror:

diameter

4m

form

quasihyperboloid

Focal distance

11.4m

Mounting

altazimuth

Rotation limit:

by azimuth

+/--240

by angle

-5-+95

Rate of movement (fast/slow):

by azimuth

1.5 о /s/l/5'/s

by angle of elevation

0.8 о /s/1.0'/s

Accuracy of surface

0.5 mm

Accuracy of guidance

10"

sources with a wide range of declinations. An adequate "radio climate", that is the lowest possible level of technogenic interferences (noises), was another important consideration. And last, these are the sites where other astronomical setups have long been in operation (for instance, a 6-meter optical and a 600-meter radio telescope at Zeienchukskaya and a solar radio telescope at Badary).

Two observatories have been commissioned within the Quasar network, one at Svetloye, and the other at Zeienchukskaya; both are integrated into an interferometer comprising two radio telescopes. A third observatory is about to go into service at Badary It is equipped with a full-swing rotary radio telescope of a new generation; its mirror measures 32 meters in diameter. It has been specially designed for work within the Quasar network, since astrometric and geodynamic tasks of this system make stringent demands on the structure and dynamic qualities of this high-precision astrometric instrument.

The radio telescope, modeled after a design suggested by N. Cassegrain*, is fitted out with a principal quasiparaboloid (focal length, 11.4 m) and a secondary mirror, which is a modified hyperboloid of revolution, 4 m in diameter. Besides, it is equipped with high-sensitivity radiometers of heterojunction transistors (NEMT-transistors) on wavelengths 1.35; 2.6; 3.5; 6.0; 13 and 18/21 cm, which make it possible to simultaneously receive two circular orthogonal polarization. To obtain noise temperatures of the radio telescope/radiometer system not above 50 K special microcryogenic systems of closed type cool low-noise devices of all bands as well as some of the input circuits down to 20 K (hydrogen level). Weak sources can thus be registered. This, in turn, makes it possible to draw up catalogs of radiation sources evenly distributed in the celestial sphere and use these sources effectively in differential measurements for suppressing the effects of a turbulent troposphere and of the unstable performance of instruments. Even the first measurements have shown: the noise characteristics of Quasar radio telescopes on different wavelengths are among the world's best.


* N. Cassegrain, 18th-century French physicist and telescope designer. -Ed.

page 24


An aligned antenna feed provides for simultaneous reception on wavelengths 3.5 and 13 cm (excluding the ionosphere's effects), which are the principal bands for astrometric and geodynamic investigation. A system of two-mirror feed ensures a rapid transition from one band to another and, consequently, enables multi-wave measurements in the mode of quasi-simultaneous reception (in this case secondary foci are aligned not on the principal mirror axis, but on a circumference with the center on this very axis). Such a transition is effected by turning the secondary mirror by a corresponding angle around its axis.

Another essential element of the Quasar observational network is materialized in a system of frequency/time synchronization made up of four hydrogen and two caesium standards with very high metrological parameters. This system is responsible for many important functions; for one, it generates heterodyne (local-oscillator) signals; it keeps perfect time for synchronization of recordings at different points; it induces reference voltage for such registering units. They also include facilities that allow preliminary time synchronization of atomic time scales thousands of kilometers apart with an error margin of no more than 0.1 ms. Thereby separate radio telescopes actually become an interferometer, one global radio telescope.

And last, the third major element of the Quasar observatories are satellite data transmission systems built on the basis of receiver/transmitter antennae 4 m in diameter and operating in the 14/11 GHz band. Using them we can transmit radio- astronomical and control signals at a rate of up to 4.5 megabits per second (Mbit/s) to ensure a real time regime in control and processing channels. The selfsame system allows for an error of 1 ns in adjusting the atomic time scales by transmission of signals through a satellite channel by a duplex method.

The development of VLBI systems operating in real time is a strategic technological task for many countries. Today only Japan is up to the mark in coping with it: that country has built a line for optical fiber communications with the aim of earthquake prediction. The VLBI community views the possibility of pooling the Europe-based radio telescopes by means of optofiber communication lines with a throughput of IT bit/s into a single grid of real time as a most promising trend. As a first step we are planning to hook the observatory at Svetloye to a high-rate optical communication line between Lappeenranta and St. Petersburg with a flow of 1 Gbit/s, which will make it possible to connect it directly with the correlator.

This one receives primary data from the observatories; to some extent it is a phase center of the Quasar network, for it is here that signals are added coherently, and autocorrelation and cross-correlation functions of video signals are computed. Besides, the correlator computes visibility function amplitudes that give data on the structure of a radiation source. Today the correlator is processing data supplied by the MkIII registration systems from three points simultaneously at a rate of 384 Mbit/s. Our I PA Institute is now working on a new-generation correlator for a new registration system, MkV, which we, with the aid of NASA (USA), are going to mount on radio telescopes of the Quasar

page 25


network. The new correlator will be operating on programmed logic integrated circuits capable of processing data of all standards used by the international VLBI community-namely, MkIII-MkIV(USA), SII-SV(Canada), KIII-KIV(Japan).

The hardware of the Quasar observatories-radio telescopes, radiometers, systems of frequency time synchronization and registration on magnetic tape-is controlled by a central computer supplied with the MkIV Filed System, which is an international standard for VLBI stations. This system has been adjusted to Russian hardware. Thus today radio telescopes of the Quasar network can respond to command files of international VLBI networks and, consequently, accommodate easily within international observation programs.

There is about a score of first-rate VLBI stations worldwide. Apart from purely radiointerferometric facilities, they are also equipped with radio-engineering and light-range finder systems GLONASS/GPS and ETALON/LAGEOS for observations carried out by artificial satellites of the earth, navigation and geodetic ones alike. This technology allows to intercalibrate various measurement facilities. Besides, it makes it possible to minimize the effect of systematic errors, expand the range of problems under study and investigate phenomena characterized by different time scales. These points were taken into consideration in designing the Quasar network. The observatories at Svetloye and Zeienchukskaya are equipped with transceivers of the GLONASS system and geodetic receivers of GPS. The Badary observatory is supported by the system DORIS. NASA has promised to help with installing laser range finders of the fifth generation at Zeienchukskaya and Badary.

To conclude, it should be stressed that the basic problems of astrometry and geodynamics are among those ones which are least affected by "inflationary" tendencies compared with many of the intriguing problems tackled by astrophysics and cosmology. This line of research relies on large and long-term regional and global programs in which the VLBI technology is playing a decisive role. At present these programs are coordinated by the international research body IVS (International VLBI Service for Geodesy and Astrometry) which pools the technical resources of 75 organizations from 15 advanced countries of America, Europe, Asia and Australia. The most significant one, covering a period of no less than two decades, is the global CORE program (Continuous Observations of the Rotation of the Earth) carried out under the scientific and technical guidance of NASA and involving dozens of countries. Intensive exchange of observation data and innovative technologies is proceeding within the framework of this very program. The Quasar network, with its first stage already onstream, enables Russia-who was no more than an onlooker up until recently-to turn to this job on her own merit.


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Andrei FINKELSHTEIN, RADIOINTERFEROMETRIK NETWORK QUASAR // London: Libmonster (LIBMONSTER.COM). Updated: 07.09.2018. URL: https://libmonster.com/m/articles/view/RADIOINTERFEROMETRIK-NETWORK-QUASAR (date of access: 02.12.2021).

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