VLBI National Facility upgrade
Introduction
The capabilities of the Australian VLBI National Facility have been upgraded for wide bandwidth disk-based or real-time e-VLBI operation, via a collaboration between the Swinburne University of Technology (funded through the Federal Government's Major National Research Facilities [MNRF] Programme), the Australia Telescope National Facility, and the University of Tasmania. These capabilities are now available to users of the VLBI National Facility. Extensive user support is provided for these facilities, including assistance with proposal preparation, scheduling, observing and data reduction.
This document outlines the new capabilities of the VLBI National Facility, describes the opportunities available to users, and the conditions under which they can be utilised.
Summary of capabilities
The capabilities of the Australian VLBI National Facility are based around three features: 1) disk-based baseband recorders that support a maximum recorded data rate of 1 Gbps; 2) a software correlator (DiFX) developed at the Swinburne University of Technology; 3) fast 1 Gbps network links to many of the VLBI telescopes.
The disk-based recorders in use at the ATNF telescopes are known as the LBADR (Long Baseline Array Disk Recorder). The UTAS, Warkworth and Hartebeesthoek telescopes now use variants of the DBBC developed by the EVN with a Mark5 or Flexbuf disk recorder.
The second development, a VLBI software correlator (DiFX) developed at the Swinburne University of Technology, is significant in that it supports the high data rate recording provided by the new disk-based recorders. In addition, the software correlator has no significant restriction on the spectral resolution or number of products that can be correlated in a single pass. The DiFX software correlator has been adopted at many institutes internationally (VLBA, Bonn,...). For LBA recorded observations correlation is performed at the Pawsey Centre for SKA Supercomputing in Perth.
The combination of disk-based recorders and software correlator allows a number of VLBI observing modes.
| Antenna | # DAS units available | Max. data rate | DBBC available? |
Notes |
| Australia | ||||
| ATCA | 2 | 1024 Mbps | no | |
| ATCA split mode - choice of two subarrays of ATCA antennas | 2 | 2 x 512 Mbps (512 Mbps per subarray) | no | Two tied arrays (i.e. antennas 1,2,3 and antennas 4,5,6) each at 512 Mbps. Instead of one tied array at 1024 Mbps (i.e. antennas 1,2,3,4,5) |
| Parkes | 2 | 1024 Mbps | no | MEDUSA DAS and recorder now also available allowing higher data rates |
| Mopra | 2 | 512/1024 Mbps | no | 512 Mbps maximum at frequencies below 3 GHz due to an IF limitation |
| Hobart | - | 1024 Mbps | yes | Using DBBC/Flexbuf |
| Ceduna | - | 1024 Mbps | yes | Using DBBC/Flexbuf |
| Tidbinbilla | - | 1024 Mbps | no | Using VRA (NASA data acquisition system) |
| ASKAP 12m antenna | Not available | -------- | no | Not currently available. |
| AuScope Yarragadee | - | 1024 | yes | Using DBBC/mark5 |
| AuScope Katherine | - | 1024 | yes | Using DBBC/mark5 |
| International (LBA) | ||||
| Warkworth | - | 1024 | yes | DBBC with Flexbuf |
| Hartebeesthoek | - | 1024 | yes | DBBC with Flexbuf |
As can be seen in Table 1, the three ATNF antennas in Australia utilise 2 DAS units each, allowing maximum recorded data rates of 512 Mbps ( 8 x 16 MHz channels) or 1024 Mbps (4 x 64 MHz channels).
Lower data rates are possible, for narrow-band spectral line observations. Table 2 summarises the observing modes and bandwidths that have been successfully tested using the LBADR system. These data rates can be matched (or exceeded) by the DBBC+Mark5/Flexbuf systems at the other telescopes.
Warkworth and Hartebeesthoek are offered as part of the standard LBA and can be requested in OPAL. Other VLBI arrays such as the EAVN, EVN, KVN and VLBA are known to also be compatible and are supported by the software correlator. However, significant coordination is required to obtain the use of international antennas other than Hartebeesthoek and Warkworth and to schedule the observations. It is suggested that users consult with the VLBI National Facility (contact information below) well in advance of preparing a proposal, in order to determine the feasibility of using the international antennas in conjunction with the LBA.
The VLBI National Facility will provide full support for users who wish to use this system. Support will include:
- Assistance with proposal preparation and evaluation of feasibility. This will be limited to the technical requirements of the observation. The scientific objective of the proposal is the user's responsibility.
- Scheduling support. The VLBI National Facility will produce schedules for the requested observations, in consultation with the user. It is the user's responsibility to secure time on the required antennas through the peer reviewed proposal process of the Australia Telescope National Facility and the Time Assignment bodies of other required facilities e.g. the NRAO if VLBA antennas are required.
- Observing support. The VLBI National Facility will assist users with the observations as per the normal VLBI National Facility policies (see http://www.atnf.csiro.au/vlbi/observing/vlbi_guide/node5.html)
- Correlation support. The VLBI National Facility will provide full support for software correlation of these observations.
- Data reduction support. The VLBI National Facility will provide assistance to users in terms of post-correlation data reduction.
| Mode | Total data rate | comments |
| 4 x 64 MHz | 1024 Mbps | DAS1: 2 x 64 MHz bands (1 x 64 MHz RCP, 1 x 64 MHz LCP) |
| DAS2: 2 x 64 MHz bands (1 x 64 MHz RCP, 1 x 64 MHz LCP) | ||
| 2 bit sampling | ||
| DAS1 and DAS2 run at different centre frequencies. | ||
| DBBC stations will observe either 8x32 MHz or 16x16 MHz. Mixed mode correlation is supported by the DiFX correlator. | ||
| 8 x 16 MHz | 512 Mbps | DAS1: 4 x 16 MHz bands (2 x 16 MHz RCP, 2 x 16 MHz LCP) |
| DAS2: 4 x 16 MHz bands (2 x 16 MHz RCP, 2 x 16 MHz LCP) | ||
| 2 bit sampling | ||
| DAS1 and DAS2 run at different centre frequencies. | ||
| Also available at DBBC stations. | ||
| 4 x 32 MHz | 512 Mbps | DAS1: 2 x 32 MHz bands (1 x 32 MHz RCP, 1 x 32 MHz LCP) |
| DAS2: 2 x 32 MHz bands (1 x 32 MHz RCP, 1 x 32 MHz LCP) | ||
| 2 bit sampling | ||
| DAS1 and DAS2 run at different centre frequencies. | ||
| Also available at DBBC stations, through mixed mode correlation. | ||
| 2 x 64 MHz | 512 Mbps | DAS1 or DAS2: 2 x 64 MHz (1 x 64 MHz RCP, 1 x 64 MHz LCP) |
| 2 bit sampling | ||
| Available at all antennas (refer to frequency and polarisation information for each telescope on LBA website http://www.atnf.csiro.au/vlbi) | ||
| Also available at DBBC stations, through mixed mode correlation. | ||
| 4 x 16 MHz | 256 Mbps | DAS1 or DAS2: 4 x 16 MHz (2 x 16 MHz RCP, 2 x 16 MHz LCP) |
| 2 bit sampling | ||
| Available at all antennas (refer to frequency and polarisation information for each telescope.) | ||
| 2 x 32 MHz | 256 Mbps | DAS1 or DAS2: 2 x 16 MHz (1 x 32 MHz RCP, 1 x 32 MHz LCP) |
| 2 bit sampling | ||
| Available at all antennas (refer to frequency and polarisation information for each telescope) | ||
| 4 x 8 MHz | 128 Mbps | DAS1 or DAS2: 4 x 8 MHz (2 x 8 MHz RCP, 2 x 8 MHz LCP) |
| 2 bit sampling | ||
| Available at all antennas (refer to frequency and polarisation information for each telescope) | ||
| 4 x 4 MHz | 64 Mbps | DAS1 or DAS2: 4 x 4 MHz (2 x 4 MHz RCP, 2 x 4 MHz LCP) |
| 2 bit sampling | ||
| Available at all antennas (refer to frequency and polarisation information for each telescope) |
Mixed mode correlations - i.e. different subbands observed by different telescopes, are supported as standard by the correlator.
Example Applications
The upgraded capabilities of the Australian VLBI National Facility are aimed at two broad primary goals: 1) wide bandwidth, maximum sensitivity observations; 2) high spectral resolution observations.
An example of a wide bandwidth observation would consist of 1024 Mbps observations at all stations. Higher bandwidths will be available in the near future when the BIGCAT project at ATCA is completed.
An example of a high spectral resolution observation could consist of 4 MHz bands with 16,392 or more frequency channels correlated using the software correlator.
Wide-bandwidth, high spectral resolution observations are also possible using this system, for example to cover multiple maser lines over wide bandwidths. In addition, if larger bandwidths are observed, it is possible to correlate just a small part of the band (centred on the spectral line) at high resolution in a so-called zoom band correlation.
Examples of science possible with such a system are:
- Wide field of view imaging of weak radio sources;
- Surveys of weak AGN populations;
- Detection and imaging of galactic X-ray binary systems;
- Proper motion studies of weak pulsars;
- Wide bandwidth, high spectral resolution observations of extragalactic water masers;
- Sensitive 3mm VLBI between the ATCA and Mopra antennas.
In addition to interferometric capabilities, the LBADR system can be used for single dish recording of pulsars or for single dish autocorrelation spectroscopy. For example, at the ATCA a 32 processor PC cluster is available as an autocorrelation spectrometer that can be used for spectroscopy or radio frequency interference (RFI) monitoring. The system at the ATCA is also capable of recording data from two separate tied arrays, allowing (for example) data to be software correlated on the ATCA baselines.
Examples of science possible with such a system are:
- RFI monitoring and characterisation for RFI excision in ATCA datasets;
- Pulsar timing;
- Ultra-high spectral resolution single dish spectroscopy;
e-VLBI Capabilities
The third major development of the VLBI National Facility is the provision of real-time transport of 1 Gbps data from some antennas (ATCA, Mopra, Parkes) to a software correlator via 1 Gbps optical fibre. This has allowed the development of the Parkes-ATCA-Mopra-Electronic-Longbaseline-Array (PAMELA), which utilizes a version of the DiFX software correlator running on the CPSR2 cluster at Parkes and/or the CAVE cluster at Narrabri. This supports data rates of up to 512 Mbps from the three ATNF antennas in real time and is available for general usage. 1 Gbps single baseline rates are available between ATCA and Parkes (single baseline Mopra and Parkes may also be available if requested).
The Hobart antenna is also connected via a 1 Gbps link to the University of Tasmania and the links across the Bass Strait were upgraded to 10 Gbps in 2011. Test e-VLBI observations have been performed at 512 Mbps and Hobart can be included in scheduled e-VLBI observations.
The Warkworth antenna in NZ joined the LBA in 2011 and is also connected to the network via a 1 Gbps link. e-VLBI observations with Warkworth were successfully demonstrated in February 2011, and this antenna may be requested for scheduled e-VLBI observations up to 512 Mbps.
Contact information
Users should contact the VLBI National Facility (contact details below) for the latest information on these developments, and the opportunities to utilise them for scientific observations.
Dr Phil Edwards
LBA Lead Scientist
Australia Telescope National Facility
CSIRO
(phil.edwards [at] csiro.au)