SAMURAI Implementation

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 for SAMURAI.

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.

Hosted by NRAO on behalf of the US VSOP-2 Science Team.