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ISSN 0010 9525, Cosmic Research, 2014, Vol. 52, No. 5, pp. 319­325. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.V. Andreyanov, N.S. Kardashev, V.V. Khartov, 2014, published in Kosmicheskie Issledovaniya, 2014, Vol. 52, No. 5, pp. 353­359.

Space­Ground Radio Interferometer RadioAstron
V. V. Andreyanova, N. S. Kardashevb, and V. V. Khartov
a b b

Astrophysics Institute, Russian Academy of Sciences, Moscow, Russia e mail: andre@asc.rssi.ru Lavochkin NPO (Science and Production Corporation), Khimki, Russia
Received December 16, 2013

Abstract--The paper considers the design, features, and characteristics of the Russian space­ground very long baseline radio interferometer (VLBRI) RadioAstron. DOI: 10.1134/S0010952514050013

INTRODUCTION In the 20th century, the exploration of the Universe spread to all ranges of electromagnetic radiation. In addition to optical astronomy, gamma, X ray, infra red, and radio astronomy appeared. This led to the extraordinary expansion of knowledge of the Universe up to the development of the model of the Universe as a whole and understanding of the laws of the origin and evolution of most astronomical objects. However, many of the key questions in astronomy remain unsolved and new major challenges have arisen including the possible existence of the multicompo nent Universe. The progress in astronomy is closely connected both with the ability to conduct research from space and with the advent of new technologies for develop ing telescopes and techniques of radiation analysis in all ranges. The main parameters that limit research in radio astronomy are the sensitivity and angular resolution. Both of these parameters only have great prospects in space. Radio telescope sensitivity is mostly deter mined by the area of its antenna and receiver noise. Initially, the angular resolution was also limited by the antenna size; however, using the interferometric method, this relationship has been excluded. Thus, the resolution came to be determined by the distance between the antennas. With the advent of the possibil ity of signal detection in a digital form and their subse quent computer processing, as well as the use of very stable oscillators that make it possible to bind signals recorded at different telescopes by frequency and time, very long baseline interferometers (VLBI) appeared. By the end of the 20th century, bases of these systems have reached 10 000 km (about the Earth diameter). As a result of VLBI studies, it was revealed that many of the most interesting astronomical objects (active galactic nuclei, quasars; neutron stars, pulsars; star forming regions, cosmic masers) are very compact and cannot be efficiently studied by ground base inter

ferometers. In this regard, in the 1970s­1980s, the development of space­ground interferometers began [1]. These projects were widely discussed at interna tional conferences. In order to prepare for the imple mentation of the first space­ground interferometer aboard the manned space station, Salyut 6 astronauts V. A. Lyakhov and V. V. Ryumin deployed a prototype of the parabolic antenna of the radio telescope SRT 10 (with a diameter of 10 m) with a mesh reflective sur face [2]. The telescope operated in bands of 12 and 72 cm at a height of 400 km from July to August 1979. Based on the results of measurements by cosmic radio sources it was decided to design a telescope with greater surface accuracy since it was necessary to carry out research for the shortest waves in the centimeter range or at even shorter wavelengths (greater transpar ency of investigated sources themselves and higher res olution). In April 1979, the Institute of Space Research and NPO Lavochkin began to develop technical proposals on the creation of a space­ground interferometer with a base much larger than the diameter of the Earth for radio astronomy observations in the range of 1 cm to 1m [3, 4]. In May 1980 the Government of the Soviet Union issued a decree on the development of six unmanned space observatories by the NPO Lavochkin together with the USSR Academy of Sciences in the subse quent ten years. In 1983, the plan of launches was updated. In particular, Astron R (the shortest wave lengths in the centimeter range) was to be launched in 1987­1988 and Astron M (shorter wavelengths) was due to be launched in 1990. The first international meeting on the space­ ground interferometer RadioAstron was held in Mos cow on December 17­18, 1985. Major bands (1.35, 6.2, 18, and 92 cm) of the space radio telescope were determined. Other parameters of the interferometer were discussed, in particular those related to the choice of its orbit. By the time of radical changes in the

319


320 Table 1 f
m

ANDREYANOV et al.

22.232 GHz 8 bands within 7 GHz 32 MHz â 2

4.832 MHz 100 MHz 32 MHz â 2

1664 MHz 100 MHz 32 MHz â 2

324 MHz 16 MHz 16 MHz â 2

f F

c

country several, these meetings had been held. On October 21, 1991, in Pushchino near Moscow (Radio Observatory of the Astro Space Center of FIAN) the 13th international meeting took place. In 2003­2004, in Pushchino, the already assembled space radiotele scope was tested on all bands, and the prototype was presented for joint tests with the Spektr R spacecraft [5­7]. On July 18, 2011, the space radio telescope was successfully launched by the rocket Zenit 3F with the upper stage Fregat­SB from the Baikonur Cosmo drome. The deployment of a parabolic antenna and first tests including the single radio telescope and interferometer mode were also successfully carried out in 2011. Then, there were successful tests on all bands of the space radio telescope, which confirmed that its efficiency as the largest fixed space antenna with a diameter of 10 m and the largest interferometer with a base of up to 350 000 km. For systematic radio astron omy studies, international cooperation has been orga nized, which includes more than 30 ground based tele scopes and two data collection stations (in Pushchino, Russia, and in Green Bank, United States) [8], http://www.asc.rssi.ru/radioastron/index.html. The International Program Committee selects applications for research. The on orbit operation of this radio telescope is provided by the multifunction space platform Naviga tor, on which it is installed. The preparation and conduction of research with space­ground interferometer RadioAstron described in the later papers are of interest for further develop ment in this direction (creation of the Earth­Space interferometer at shorter wavelengths and with larger bases, the Millimetron project of the Federal Space Pro gram of Russia) [9], http://asc lebedev.ru/index.php. The prospects of creating fixed parabolic antennas of higher accuracy in space for operation at shorter wave lengths down to the infrared range is also quite impor tant. The creation of very large antennas and multiple element interferometers with space only bases is also a prospect [10, 11]. 1. FUNDAMENTAL DIFFERENCES BETWEEN VLBRI AND VLBI For a single telescope, the angular resolution = /D, where D is the telescope effective diameter. For VLBRI, = /B1, where B1 is the projection of the base B of a pair of radio telescopes (a space radio tele scope, SRT, and a ground radio telescope, GRT) on

the plane perpendicular to the direction to the source S (Fig. 1). The configuration and composition of the VLBRI RadioAstron is shown in Fig. 1. It can be seen that its distinctive feature is the VLBRI arm from the space radio telescope to outputs of the tracking station (TS). The SRT, as well as the GRT, receives the source radiation in radioastronomical bands from the direc tion of S, converts it into a desired form, and transmits it in real time over a radio link in the direction of R to the Earth, where it is reconstructed and stored in the data logger on the TS (Fig. 2). Therefore, all of the factors that change the input phase of the source on the way to the TS output (by time delays and frequency shifts) should be considered in the correlation with data from the GRT. Table 1 shows medium frequen cies fm of the SRT receiver tuning, bands of receiving frequencies by input f and used for the correlation processing of the video Fc. Factor 2 in the third row means that the radio emission can be received simul taneously in two circular polarizations (left and right). As can be seen, VLBRI is a super system of synchro nously operating telescopes (SRT and GRT), means of data transmission from SRT to Earth (to the TS) and as fast as possible data delivery for the correlation processing. The task of the interferometer as the phase system is to achieve the best resolution = /B1 in the study of celestial objects at the selected wavelength . The measured difference between radiation source phases reaching telescopes at the same time can be dif ferent from the purely geometrical (in vacuum) by the presence of bad constituents i. In terrestrial VLBIs, the base B between GRTs does not change during observation, and the projection of the base B1 changes slowly because of the rotation of the Earth and according to an established law. The clock on the GRT (usually a hydrogen clock) is periodically com pared using GPS or UT systems; therefore, the syn chronization error of these clocks does not usually exceed 10­100 ns. Data received by telescopes taken together with the current time tGRT are recorded to the loggers for delivery to the correlator and, in the VLBA system, data are transmitted over communication lines [12]. Thus, the measured phase difference can be different from a geometrical difference only because of the clock synchronization error and the possible dif ference of the effect on the phase of the Earth's atmo sphere at GRT locations. In the VLBRI, the base between SRT and GRT telescopes and its projection B1 is continuously and rapidly changing because of the SRT orbital motion and the Earth's rotation. SRT data are specially converted to noise resistant digital data streams and are transmitted in real time to the TS
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SPACE­GROUND RADIO INTERFEROMETER RADIOASTRON Antenna equivalent by the angular resolution m S Source B1 R Synchronization lines GRT 1 Tracking station (TS) c GRT n Delivery of data from the GRT and the TS Correlation and spectral processing K (, F)max max series Construction of radio images Scientific data processing and interpretation o n t r o l 90° B r/a data transmission line SRT + SC

321

Fig. 1. Configuration and basic segments of the VLBRI. m is the difference of geometric (in vacuum) delays from the source to the SRT and GRT, (, F)max max are maximum correlation values for delay and frequency interference.

via radio link f0 over the carrier of 15GHz. There, they are decoded, reconstructed, and recorded together with the current time tTS by the clock of the TS (Fig. 2). The TS clock is synchronized with the GRT clock, but the SRT time tSRT = tTS ­ tSRT TS remains undefined because of the changing delay tSRT TS in the radio channel SRT TS. A two way coherent synchroniza tion line (the TS SRT TS phase loop) makes it possi ble to determine the current values of tSRT TS more accurately and significantly more often than by the ballistic prediction [13]. All communication frequen cies of the SRT with the TS are selected according to the Regulation of the International Radio Consulta tive Committee. The data delivery from the TS and GRT for the correlation processing is conducted on carriers of their Registrars or over the Internet. For the correlation data, the SRT and all GRTs should also be identical functionally and by a number of parameters, i.e., to operate in the same frequency bands and polar
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izations and to have the same recorded frequency bands and positions of the local oscillator frequencies, as well as known instrumental delays. The sum of phase errors added to the geometric phase difference geom is determined by bad time delays and frequency shifts on the way to the correlator. 2. SPACE RADIO TELESCOPE A key new segment of the radiointerferometer is the space radio telescope (SRT), which is a radio receiving electronic complex with a 10 m parabolic antenna with 27 rigid carbon fiber petals that unfurl in space. A space telescope is the payload of the far orbit satel lite Spektr R. It was launched into a high elliptical sat ellite orbit with an apogee of about 350 000 km for observations of celestial sources synchronously with a network of terrestrial telescopes. The SRT is con nected to the tracking station via airborne means of


322

ANDREYANOV et al. Radio astronomical sources of radiation Radiation perceived by the SRT in four ranges allocated for the radio astronomy near frequencies fs 22; 4.8; 1.6 and 0.32 GHz Synchrocode generation. TM data input rom the SRT and satellites. Data formatting (formation of the serial data stream). Differential encoding.

fs

Onboard scientific equipment complex (SRT + VIRK)

Synchronization lines in ranges 7.2 GHz up and 8.4 GHz down.

Scientific data transmission line in a range of 15 GHz Radio reception. DRPM demodulation. Diff. decoding. Synchrocode detection, isolation and decoding of the entire header.

To the correlator

TS

Fig. 2. Space arm of VLBRI RadioAstron. In the box in the right part of the diagram actions are listed that are performed on the SRT and the TS for data transmission from the SRT via radio.

the high capacity radio complex (VIRK). With such bases the interferometer provides information with a record angular resolution up to 10 µs of arc (for the shortest wavelength = 1.35 cm). For comparison, the most powerful ground based optical systems have a resolution of 0.1­0.01 arcsec. SRT systems perform the following functions (Fig. 3): the reception and amplification of radio emission of both polarizations for the investigated galactic or extragalactic source (an antenna and devices of the cold plate), the transfer of the frequency spectrum of received signals in the video area without losing the phase of the received signals (instruments of focal and instrument containers), the conversion of these signals together with the SRT intrinsic noise in binary form, the formation of digital data streams (instrument container), and their transfer to a ground tracking station over the air with the carrier f0 = 15 GHz, as well as the formation of a phase loop on tone signals 7.2075/8.4 GHz (the VIRK with the tracer for the TS antenna). As can be seen from the SRT diagram in Fig. 3, the main interferometric mode involves all subsystems and devices, excluding reserve systems, and the two or three receivers not included in the range of current observations.

Heterodyne and clock signals are required for the operation of the receivers in the FC and the formatter in the IC. Their formation in frequency synthesizers 1 and 2 (VHFFB and HCFFB) can occur either from the reference signal from the output of the loop phase transponder (15 MHz) received from the TS or from the onboard autonomous rubidic oscillator, or from an autonomous airborne H maser (15 MHz). Syn thesizer 2 forms clock frequencies of 64 and 72 MHz and 4 heterodyne frequencies of 250, 254, 258, and 262 MHz for the formatter, as well as frequencies of 64 and 160 MHz for further formation of frequencies for heterodyne receivers in the synthesizer 1 (200, 1152, and 4320 MHz). Thus, the SRT signal path is ready to receive signals from the onboard antenna feed (OAF) elements. Signals from each of the variously polarized outputs of the OAF are amplified by low noise ampli fiers (LNA) and fed through FC pressure seal connec tors to the inputs of two channel receivers; in the range of 324 MHz the LNA is not cooled and is placed directly in the receiver of this FC range. Spectra of the received signals transferred to an intermediate fre quency (about 512 MHz) together with the SRT intrinsic noise are fed from a pair of outputs of each receiver to the selector (IF selector on the scheme). Depending on the chosen observation mode the selec
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SPACE­GROUND RADIO INTERFEROMETER RADIOASTRON

323

Deployment mechanism

10 m antenna reflector coax. two polarization antenna feeds

Radiator

22 GHz 4.8 GHz 1.6 GHz LNA MHz
two channel Frequency 8 4320 RAR synth. 1152 1 two channel two channel

0.327 GHz

Cold plate (FN) Focal container (FC)

LNA

LNA

RAR

RAR

two channel Phase LNA and RAR calibration Commands Control unit

200

160

72 Power supplies

IF Selector

~6 m
Frequency synth. 2

Formatter 2 MHz 72 250­262 Rb osc 1 Rb osc 2
antenna depl.

Formatter 2 (redundant) Control unit 2 4m Instrument container (IC)

control

maser

Tr dr 7.2/8.4 GHz Tr dr 7.2/8.4 GHz VIRK

DRPM 15 GHz ADT 1

DRPM 15 GHz ADT 2

Tracking antenna
Fig. 3. SRT and VIRK block diagram. Key: LNA is the low noise amplifier, FN is the focal node containing a block of antenna feeds and LNA, RAR is the radio astronomy receiver, IF is the intermediate frequency, TM is the telemetry, Rb osc 1, 2 are rubidic oscillators, H maser is the hydrogen oscillator (two copies), VIRK is the high capacty radio system, Tr dr 7.2/8.4 is the transponder of the input frequency of 7.2 GHz in to the output frequency of 8.4 GHz, ADT 1, 2 are transmitters of astronomical data of the carrier frequency of 15 GHz.

tor chooses any two channels by commands (two from any radioastronomic receiver (RAR) or by one from any two RAR, or any one RAR output is fed to the two Selector outputs) and feeds them to the FC output connectors. Then, these signals are inputted to the IC by rigid coaxial cables extending along rods and are fed to the formatter (the second one is on cold standby). There, they are converted into a video range and a dig ital form. Next, they are compacted in two consecutive binary data streams where antijamming synchrocode and other data that form a data format of the SRT are also added [13]. Both streams (I and Q) are derived from the IC by the paraphase circuit and fed to inputs of the modulator of astronomic data transmitters
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(ADT) of 15 GHz of the VIRK by cables of equal length. Next, modulated double relative phase modu lation (DRPM) signals are transmitted to Earth via the VIRK antenna tracking the TS. The described modu lation makes it possible to transmit two data streams simultaneously in a single frequency band at a speed of 72 Mbit/s each. The antenna tracking for the selected TS and its refocusing on another TS should be initi ated by software commands from the SC onboard computer. The first two years of the mission only used one tracking station located in Pushchino, Russia. At the end of 2013, the US TS (Green Bank) was put into operation.


324 Table 2

ANDREYANOV et al.

1 Pointing to the source (provided by the SC) 2 SRT antenna

Triaxial stabilization, pointing accuracy: ±2 arcmin, stabilization error: ±32 arcsec, max reorientation rate 0.1 deg/s 10 m diameter, unfurlable, parabolic. Coaxial feeders, in the primary focus, with outputs of the right and left circular polarization. Max deviation from the parabolic shape is ~2 mm 324 ± 8, 1664 ± 32, 4832 ± 32, 22 232 ± 32 MHz (18­25 GHzfor the multifrequency synthesis mode) <200, <45, <130, <80 K
2

3 Input frequency range and used bands 4 Noise temperatures of the SRT system in the specified ranges 6 Frequencies of heterodynes for both receiver channels in the specified ranges

5 Effective antenna areas in the specified ranges >30 m2, >41 m2, >35 m2, >7.5 m

200, 1152, 4320, 21 720 (4320) MHz

7 Heterodyne frequencies of video converters of 500 (250 â 2); 508 (254 â 2); 516 (258 â 2); 524 (262 â 2) MHz the Formatter 8 Sources of reference frequencies for all fre quency transformations and their stability Rubidic oscillator 5 MHz: 10­12 per 100 s; Hydrogen oscillator of 5 and 15 MHz: 5 â 10­15 per 1000 s; Reference signal from TS (on the output of the VIRK transponder): 15 MHz with the Doppler re sidual error Frequencies of TS synchronization lines: 7.2075 GHz up, 8.4 GHz down, 1­4 W (carrier). Scientific data transmitter: DRPM modulation of 15 GHz, 40 (4) W; transfer rate of 2 â 72 Mbaud, or 2 â 18 Mbaud, the tracking antenna, diameter of 1.5 m 300 124 <1150 W (depending on mode) <2500 kg Length of 7460 mm, diameter of 3550 mm Apogee of about 350 000 km, the inclination of 79.70 (April 2012), period of 8.5 days, and initial perigee of 500 km 8 arcsec (in the range of 1.35 cm)

9 VIRK parameters

10 Number of control commands CCW functional 11 Power consumption is 27 V from the onboard network 12 SRT weight (with VIRK) 13 Folded dimensions (without VIRK) 14 Orbit 15 Best angular resolution of the interferometer

16 Best sensitivity of the SRT GRT interferome 10 mJy ter (VLA)

Table 2 gives the values of the parameters in the draft of SRT and VLBRI that make it possible to achieve scientific goals of the mission, i.e., to obtain images, coordinates, and the evolution of the angular structure of celestial radio sources in the Universe with ultra high angular resolution, as well as to conduct gravity experiments. 3. RESULTS (1) An ultra high angular resolution of radio astro nomicalal sources is achieved based on fundamental differences between VLBRI and ground VLBI, i.e., the placement of the SRT in the satellite orbit, the establishment of the radio communication complex with the Earth (with TS), and a special conversion of

signals from the SRT for further transmission to TS and restoration for correlation with data from the GRT. (2) A four band SRT (from 1.35 to 92 cm) is cre ated on the basis of the 27 petal antenna unfurling in space and radiation cooled LNA. (3) The energy potential of the communication with the Earth is capable of transmitting data from the SRT at a speed of up to (72 â 2) Mbit/s in two elliptical polarizations. (4) The SRT (and airborne transmitters) synchro nization is possible through the phase loop of hydro gen clocks of the TS, which are in turn synchronized with the GRT clock. Synchronization with an auton omous onboard H maser or rubidic oscillator is also provided, which potentially makes it possible to con duct gravity experiments.
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ACKNOWLEDGMENTS The RadioAstron project is carried out by the Astro Physics Institute of the Russian Academy of Sciences and NPO Lavochkin under contract with the Russian Space Agency and many technical and scien tific organizations in Russia and other countries. REFERENCES
1. Kardashev, N.S., Pariiskii, Yu.N., and Sokolov, A.G., Space radio astronomy, Usp. Fiz. Nauk, 1971, vol. 104, pp. 328­331. 2. Arsent'ev, V.M., Berzhatyi, V.I., et al., The CRT 10 space radio telescope, Dokl. Akad. Nauk SSSR, 1982, vol. 264, pp. 588­591. 3. Andreyanov, V.V., Kardashev, N.S., et al., RadioAstron: A radio interferometer with the Earth­space base, Astron. Zh., 1979, vol. 63, pp. 850­855. 4. Andreyanov, V.V., Biryukov, A.V., et al., Multi fre quency receiving for image synthesis in the Radio Astron project, Trudy Fiz. Inst. im. P.N. Lebedeva, Akad. Nauk SSSR, 2000, vol. 228, pp. 13­22. 5. Aleksandrov, Yu.A., Andreyanov, V.V., Kardashev, N.S., et al., RadioAstron (the Spektr R project): a radio tele scope much larger than the Earth. Basic parameters and tests, Vestnik NPO im. S.A. Lavochkina, 2011, no. 3, pp. 11­18. 6. Aleksandrov, Yu.A., Babakin, N.G., Babyshkin, V.E., et al., RadioAstron (the Spektr R project): a radio tele

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scope much larger than the Earth. Ground based seg ment and basic lines of research, Vestnik NPO im. S.A. Lavochkina, 2011, no. 3, pp. 19­30. Avdeev, V.Yu., Zelenyi, L.M., Kardashev, N.S., Khar tov, V.V., et al., The RadioAstron space mission. First results, Vestnik NPO im. S.A. Lavochkina, 2012, no. 3, pp. 4­21. Kardashev, N.S., Khartov, V.V., et al., RadioAstron A telescope with a size of 300000 km: Basic parameters and first results of observations, Astron. Zh., 2013, vol. 90, pp. 179­222. Kardashev, N.S., Andreyanov, V.V., et al., The Millimet ron project, Trudy Fiz. Inst. im. P.N. Lebedeva, Akad. Nauk SSSR, 2000, vol. 228, pp. 112­128. Buyakas, V.I. Gvamichava, A.S., et al., An unlimitedly enlarged space radio telescope, Kosm. Issled., 1978, vol. 16, p. 767. [Cosmic Research, p. 621]. Gvamichava, A.S., Buyakas, V.I., et al., Design prob lems of large space mirror radiotelescopes, Acta Astro naut., 1981, vol. 8, pp. 337­348. Andreyanov, V.V., Compatibility problems of Radio Astron, VSOP, VLBI, and VLBA, in Frontiers of VLBI, Hirobayashi, H., Inoue, M., Kobayashi, H., Eds., Tokyo: Universal Academy Press, 1991, pp. 163­167. Andreyanov, V.V., Radio telescope in the sky, Science in Russia, 1993, nos. 3­4, pp. 8­13.

Translated by O. Pismenov

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