The VSOP-2 mission includes the ASTRO-G spacecraft and its control
station; several science telemetry stations (STSs); ground radio telescopes
(GRTs--notably the VLBA); correlators; an orbit determination center; and a mission
operations center. (The combination of the ASTRO-G spacecraft and several
STSs is canonically referred to as the "Space Radio Telescope," or SRT.)
The SAMURAI program would use a subset of the VSOP-2
mission elements to accomplish its specific scientific goals. In the
data-flow description in the following paragraph, mission elements to
be supplied as part of SAMURAI are indicated in bold-face type.
During an astronomical observation, signals from the SRT are
recorded at a STS, and signals from each VLBA antenna are recorded
locally. After the observation, the recorded signal streams ("VLBI data")
are shipped on disk packs from the STSs and VLBA to the VLBA correlator,
along with a time-correction file derived from the round-trip timing
signal and a log of events that occurred during the observation. The
orbit determination center provides a precise orbit to the
correlator. This information enables the correlator to find the desired
(weak) astronomical signals in cross-correlations of the recorded signals.
The correlator output, representing the interferometric complex
visibilities, is then analyzed by the science team to produce
images and related astrophysical measurements.
VSOP-2 extends the highly successful VSOP-1 mission. It builds on the
earlier experience by using observing frequencies up to eight times
higher for finer angular resolution; deploying a 25% larger area orbiting
antenna and processing eight times larger bandwidth for improved
sensitivity; including an ability to scan rapidly between a target source
and a nearby calibrator for visibility phase determination; and improving
orbit determination accuratcy using an on-board GPS receiver. Although
these enhancements require new hardware, the mission retains extensive
heritage from VSOP-1 in both the space and ground segments.
Techniques and specific methods have high heritage from VSOP-1 to
VSOP-2. Interfaces among mission elements, including file formats for
data exchange, are nearly identical. Design principles for space and
ground elements are the same. Ground software for mission operations will
be re-used. These features translate into a particularly low risk
The technology and methodology of ground-only VLBI radio astronomy are
well established, providing an additional body of knowledge and experience.
Space VLBI uses the same techniques and hardware for signal recording,
correlating, and image formation. Account must be taken of the fact that
one telescope is in orbit, so it is rapidly moving and it is connected to
its timing reference by a variable-length link. The STS architecture and
correlator interfaces needed for this were successfully developed for
VSOP-1 and will be re-used in the SAMURAI activity.
Use of the existing VLBA is essential to this mission, since it is
the only dedicated VLBI telescope that operates routinely at 43 GHz
and provides a range of baselines that enable a good imaging aperture
when combined with the ASTRO-G spacecraft. This makes it
necessary to implement the SRT so that it is
compatible with the current state-of-the-art in ground VLBI. In
particular, the polarizations, bandwidths, and signal digitizations
used on the ASTRO-G satellite will agree with those available at the VLBA;
and the recording medium and format used at the STSs will be
compatible with the playback systems at the correlator.
Leading members of the
SAMURAI team played similar roles in the implementation, operations
and science teams of VSOP-1.
Last modified on
Wednesday, 14-Jan-2009 10:22:07 MST
Hosted by NRAO on behalf of the US VSOP-2 Science Team.