Space VLBI Co-Observing at the DSN

Introduction

The Space Very Long Baseline Interferometer VLBI (SVLBI) co-observing program, using the Deep Space Network (DSN), was initiated in 1993 under the auspices of the SVLBI Project. because of their superb sensitivity and important strategic location, the DSN 70-m radio telescopes play an important role in the Project's ground-based observing network, which consists of a few tens of radio telescopes around the world. As a partner in this venture, the DSN Science Office provides R&D hardware and software, which enables DSN participation.

The DSN played a key role in enabling Space VLBI. In 1986, the 70-m antenna at the Canberra Deep Space Communications Complex (CDSCC) was the first ground-based radio telescope to participate in the highly successful ground-space VLBI experiments that used the communication satellite TDRSS as the space-borne element, thus demonstrating the viability of the Space VLBI concept. It was clear that the first generation of space VLBI missions - VLBI Space Observatory Programme (VSOP) and RadioAstron - with small, space-based antennas with diameters of 8 and 10 m respectively (not much larger than the TDRSS 4.5-m antenna), would require co-observing support of large, ground-based radio telescopes. The 70-m DSN radio telescopes are among the largest such telescopes in the world.

The Space VLBI mission VSOP was launched on February 12, 1997. After successful deployment, the VSOP mission was renamed Highly Advanced Laboratory for Communications and Astronomy (HALCA). During the first half year, performance of the Earth-to-space interferometer and its elements was extensively tested. The first interferometric fringes at L- and C-band were detected, and the first images of radio sources observed at baselines up to two Earth diameters were published. The HALCA mission is now in its operational stage.

Since the Space VLBI mission forms an interferometer between the Earth-based and space-based radio telescopes, reliable performance of the former is crucial to the mission. To make the DSN 70-m telescopes work reliably and efficiently with the HALCA space radio telescope, the DSN's receiving and VLBI equipment and operations procedures had to be significantly improved.

SVLBI Co-Observations Upgrade Task

The SVLBI upgrade task began in the fall of 1995. It included an upgrade of the DSN's MKIII VLBI recorders to MKIV, improvements of the L-band and K-band receivers, and development of automtion to allow SVLBI co-observing sessions with minimal involvement by DSN personnel.

The DSN VLBI equipment upgrade included changes to the recording hardware (data acquisition terminals - (DATs) - and tape recorders), the VLBI controller and software (PC Field System - PCFS), and the observing script handler (DSN VLBI Scheduling Processor - DSVP). The MKIII DSN DATs and recorders had to be upgraded to provide compatibility with the VSOP VLBI recording modes. The DSN decided to upgrade its MKIII VLBI recorders to the newly developed MKIV recorders, since they had a wider recording band, were better suited to the other DSN VLBI tasks like VLBI geodesy, and were cheaper than the comparable VLBA recorders. Also, the Goddard Space Flight Center (GSFC) and United States Naval Observatory geodetic networks and the European VLBI Network, a major VLBI radio astronomy network, had decided to adopt the MKIV recorders. Use of the MKIV recording systems also allowed the GSFC-developed PCFS to be used as the VLBI controller, instead of having to modify the DSN VLBI controller. To reduce the labor involved in processing files with generic observing instructions into files with specific instructions for the DSN antennas and equipment, software was developed for automatic processing of the VLBI schedule files.

HALCA has three observing frequency bands: L (1612-1720 MHz), C (4.7-5.0 GHz) and K (22.2-22.3 GHz). Two of these bands, L and K, were covered by existing radio astronomy receivers on the 70-m DSN antennas. The SVLBI upgrade task included replacement of the K-band masers by cooled High Electron Mobility Transistor Low Noise Amplifiers (HEMT LNAs), since the HEMT LNAs are simpler to operate and maintain, and refinement and automation of the noise and tone calibrations at both L and K bands Installation of C-band receivers on the DSN 70-m antennas was considered too expensive.

A key new element for automating SVLBI co-observing sessions was the Equipment Activities Controller (EAC). Originally developed for automating radio astronomy observations with DSN radio telescopes, the EAC functions as the interface between the PCFS, which controls the station's VLBI recorder, and DAT, the Radio Astronomy Controller (RAC), which controls the radio astronomy receiver configuration and calibration, and DSN standard subsystem controllers such as the Antenna Pointing Assembly (APA). The EAC provides the main interface for the DSN operators, through which they direct the operations, which include observing session initiation, station configuration, configuration and performance tests, and antenna pointing during the observations.

The RACs also had to be standardized across the Network, and new software was written to provide an R&D equipment control service to such clients as the EAC.

Significant efforts went into streamlining the SVLBI co-observing logistics, such as calibration procedures, tapes shipment, log and clock correction files delivery to the correlator, etc. The Goldstone Complex reported readiness to support SVLBI co-observing in November 1997. In January of this year, the Madrid Complex also reported itself ready. Problems unique to the Canberra Complex have delayed readiness, but these are expected to be solved soon. A formal release and transfer of the software to DSN Operations is scheduled for the Ides of March.

In addition to the standard configuration, a Canadian-built S-2 recorder has been installed at the Canberra Complex by the CSIRO Australia Telescope national facility (ATNF), an Australian organization that participates in radio astronomy observations at CDSCC.

First Space VLBI Results

The first fringes of the interferometer between the HALCA space radio telescope and the ground-based 64-m radio telescope in Usuda, Japan, were detected in May 1997. The DSN's 70-m Tidbinbilla radio telescope was the first non-Japanese radio telescope with which fringes were detected in an experiment held a few weeks later. The first observations involving Tidbinbilla were done at L-band, recording in S-2 format. The data were processed at the S-2 correlator in Canada. Figure 1 shows these first fringes between Tidbinbilla and HALCA (courtesy of Dominion Radio Observatory, Canada).

During June-July 1997, a series of observations with the VLA and VLBA obtained fringes at L-band on multiple Earth-space baselines. A few images of radio sources were produced. These images revealed unresolved structure in radio sources observed with baselines of more than two Earth diameters. An example of such an image from the VSOP observation with VLBA is given in Figure 2 (courtesy of NRAO). The image of this distant quasar, 1156+295, made from ground telescopes only, is shown on the left. On the right is another image of the quasar, but with the radio signals from VSOP satellite added. The material blown out from the quasar can be seen more clearly in this second picture. This unprecedented, angular resolution at the observing wavelength 18 cm (L-band) allows radio astronomers to probe closer to the center of the quasar, in order to understand the nature of these objects.

After the first successful observation with S-2 recording in Tidbinbilla, the Goldstone and Tidbinbilla radio telescopes have participated in several observing sessions using MKIV recording. The Goldstone 70-m radio telescope was the first MKIV radio telescope with which the Earth-to-space fringes were detected.

Conclusions

Space VLBI co-observing, one of the most important space radio astronomy projects in the DSN in recent years, is entering its operation phase. The first dedicated Space VLBI mission -- HALCA -- has begun to produce important radio astronomical results, and the DSN 70-m telescopes are playing a significant role in this exciting undertaking.

The DSN 70-m telescopes will play an important role in future Space VLBI missions. The next scheduled space telescope is the Russian Radio Astron. It will have 50 percent more collecting area than HALCA and will be equipped with dual polarization receivers. The DSN K-band receivers are dual polarizations, and a redundant, down-conversion chain will allow two polarizations to be recorded, simultaneously. Converting the DSN 70-m L-band systems to dual polarization is being considered. A Japanese follow-on mission tentatively being called VSOP2, with a 15-20 m telescope operating at 22 GHz and possibly higher frequencies, is also being discussed.

An advanced U.S. space VLBI mission called ARISE is being planned. This will also involve a larger antenna, perhaps 25-m in diameter, and operate at higher frequencies, including 43 GHz and possibly 86 GHz. DSS-13 has been operating as a radio telescope in the 4050 GHz band for several years, and a receiver capable of 86 GHz observations is nearing completion. There are few ground-based radio telescopes capable of observation at 86 GHz. The DSN 34-m beam waveguide (BWG) subnet, suitably equipped, could be a key asset to such a project.

• Top of Page
• Return to Homepage

• TMOD Reorganization: Changes for the Better
• Improvement in the Water Vapor Emission Model from GPS/WVR Comparisons Development
• Raster-Scan for Calibration of DSN Antennas
• ARTSN: An Automated Real-Time Spacecraft Navigation System