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IAA Transactions, No. 8, ``Celestial Mechanics'', 2002
High orbit for the RadioAstron project
N. S. Kardashev, B. B. Kreisman, Yu. N. Ponomarev
Astro Space Center of the Lebedev Physical Institute, Moscow, Russia
RadioAstron project is an international collaborative mission to launch a free
flying satellite carrying a 10­meter radio telescope in elliptic orbit around the
Earth [1,2]. The RadioAstron mission uses the satellite SPECTR (astrophysical
module), which is under development by Lavochkin Association of Russian Avi­
ation and Space Agency. According to the currently accepted resolutions, Space
Radio Telescope (SRT) will be launched (in 2005­2006) from the Baikonur Space
Center by the Proton carrier rocket (CR) and 11C824F upper­stage rocket (USR).
The SRT antenna consists of a deployable parabolic reflector (10­m diameter)
which is made of 27 carbon fiber petals and central solid portion (3 m in diame­
ter). The radio telescope has focus to diameter ratio F/D=0.43 and overall RMS
surface accuracy 0.7 mm. Observing frequencies are 0.324, 1.66, 4.83 and 22.2
GHz. A concentric feed arrangement at prime focus will provide the possibility
of observing at two frequencies or two circular polarizations simultaneously. For
RadioAstron mission it is proposed to use one K­band circular polarized chan­
nel at fixed frequency 22.232 GHz. The second simultaneously operating channel
with opposite circular polarization can be switched in the band from 18.392 GHz
to 25.112 GHz (the number switching frequencies N=8). It will be provided for
a wide band synthesize in the Earth­Space interferometer for the astrometrical
tasks. The maximum data rate of downlink is 128 Mbit/s (32 MHz wide band
for one circular polarization). Data transmission is being done by VIRK system
through the high gain antenna at frequency 15 GHz. VIRK provides also the
two--side coherent link (phase transfer) at 7.21 up/8.47 down GHz.
The aim of the mission is to use the space telescope to conduct VLBI obser­
vations in conjunction with the global ground VLBI network in order to obtain
images, coordinates, and evolution of angular structure of different radio emitting
objects in the Universe with the extraordinary high angular resolution. Since the
observations with the base up to ¸ 10 000 km at a wave of ¸ 3 mm with a
resolution of 50 ¯s of the arc are regular on the Earth [3], a base of ¸ 450 000 km
is necessary for the shortest wave of 1.35 cm in the RadioAstron mission and the
10--fold gain in resolution up to 5 ¯s. The orbit of the RadioAstron provides three
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types of study, i.e. 1) rough images of radio sources with ultra--high angular reso­
lution, using all baselines up to the radius of the apogee, that is much larger than
Earth diameter; 2) high accuracy measurements of coordinates, proper motion
and changes in source structure, with ultra--high angular resolution determined
by the largest baseline; 3) high quality imaging of radio sources, with moderate
resolution, by observing either with small projected base lines Earth--spacecraft,
i.e. close to the orbital plane, or near the perigee. In both cases the effective
baselines are only several times larger than the Earth's diameter. For this aim an
orbit was chosen with high apogee and with period of satellite rotation around
the Earth equal to 9.5 days, which evolves in result of weak gravitational pertur­
bations from the Moon and the Sun [4]. The perigee radius varies from 10 000
to 70 000 kilometers, the apogee radius --- from 310 000 to 390 000 kilometers.
The main orbit evolution is in the rotation of its plane around a weakly evolving
apsides axis. The normal to the orbital plane traces an oval on the celestial sphere
with the major axis about 150 degrees, and minor axis about 40 degrees, in about
3 years. The linear parameters of the orbit vary with a period of about 1.5 years,
and angular parameters vary with a period about 3 years.
The Dorman--Prince method of the 8(7)th order was used for numerical in­
tegration. In the calculations of the SRT orbit, we used the model of the GEM--
T2 geopotential truncated to the 17th order [5]. The model of ephemerides
DE403/LE403 developed at the JPL NASA, USA, was used to take into account
perturbations from the Moon and Sun [6]. The effects of the solar radiation and
the Earth atmosphere were ignored in the calculations.
References
1. Andreyanov V. V., Kardashev N. S. Kosmich. Issled., 1981, 19, 763--772 (in
Russian).
2. Kardashev N. S. Experimental Astronomy, 1997, 7, 329--343.
3. Rantakyro F. I. et al. A&AS, 1998, 131, 451--467.
4. Kardashev N. S., Kreisman B. B., Ponomarev Yu. N. In: Radioastron. engi­
neering and methods. M.: LPhI, 2000, 228, 3--12 (in Russian).
5. Marsh J. G. et al., Geophys. Res., 1990, 95, No. B13, 22043--22071.
6. Standish E. M. et al., JPL Planetary and Lunar Ephemerides,
DE403/LE403, JPL IOM, 1995, 314, 10--124.
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