Astrometry VLBI in Space (AVS)

V.Altunin (JPL,USA), E.Akim (IAM, Russia), V.Alekseev (NIRPhI, Russia),
M.Eubanks (USNO,USA),K.Kingham (USNO,USA),K.Sukhanov(Lavochkin
Assoc.,Russia),R.Treuhaft (JPL,USA)

    This article describes a proposal for a new space radio astronomy mission
for astrometry, using Very Long Baseline Interferometry (VLBI), called
Astrometry VLBI in Space (AVS). The ultimate goals of AVS are improving the
accuracy of radio astrometry measurements to the microarcsecond level in one
epoch of measurements and improving the accuracy of the transformation between
the inertial radio and optical coordinate reference frames.

    The primary scientific goals for the ORAM mission are:
1) A two order of magnitude improvement in solar system tests of General
Relativity,
2) A rigorous determination of extragalactic source stability and estimation of

proper motions of extragalactic radio sources of possible cosmological origin,
3) Construction of the first global astrometric surveys in several frequencies,
and a one order of magnitude or better improvement in the tie between the radio
and optical astrometric reference frames.
    The secondary mission goals include:
1) Tracking of interplanetary spacecraft on a target of opportunity basis,
2) Several order of magnitude improvements in terrestrial time transfer,
3) Test of Frame Dragging at the 20% level using the Sagnac effect,
4) Monitoring of changes in extragalactic radio sources at very high angular
resolution,
5) Improvements in geophysical determinations of geocenter motions and changes
in Earth's rotation.
    A basic element of the proposed mission is a space-based radiointerferometer
composed of two free-flying antennas. The antennas will be located in orbit such
that  they are visible to each other most of the time. A microwave (or laser)
link will be established between the Space Radio Telescopes (SRTs) to provide a
direct measurement of the radio interferometer baseline length, and
synchronization of the L.O's and SRT's clocks. Along with radio interferometry
equipment, each spacecraft will carry an optical beacon and optical (CCD)
astrometry camera. The camera will determine the position of the optical beacon

(the spacecraft with the radio telescope) relative to the optical reference
stars.
    The nominal mission includes two identical spacecraft , each equipped with
one 4 meter with 4 receivers, nominally for 8-9 GHz, 22-24 GHz, 32 GHz, and 44-
48 GHz and 70m ground antenna. Each spacecraft opticalpackage should contain a
very low power continuously operating laser and a small (~ 10 cm diameter)
telescope aimed at the other spacecraft. The optical system telescope can use
CCD detectors capable of simultaneously observing the quasi-stationary laser
beacon on the other spacecraft and stars passing through the field of view. The
nominal magnitude limit for this system is 14th magnitude. This subsystem should
provide for measurements of a space radio telescope baseline orientation in an
optical reference frame with an accuracy better than 0.001 arc sec.
    The proposed mission will be capable of a wide range of measurements which
are not  prime goals or capabilities of the Space Very Long Baseline
Interferometry (SVLBI) missions currently under development. The proposed VLBI
astrometric measurements can be implemented with much smaller orbital radio
telescopes than those of the currently proposed SVLBI imaging missions with a
corresponding savings in cost.
    The proposed radio astrometry mission, if implemented in the time frame
considered under this NRA, can also be a valuable complement to the new
generation of optical astrometry missions that  are to be launched in the same
period of time.