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 will be provided for these new 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 the improved capabilities can be utilised.

Summary of capabilities

The new capabilities of the Australian VLBI National Facility are based around three new developments: 1) disk-based baseband recorders that support a maximum recorded data rate of 1 Gbps; 2) a software correlator that runs on the Beowulf supercomputer at the Swinburne University of Technology; 3) fast 1 Gbps network links to many of the VLBI telescopes. The development of a new disk-based recording system is significant in that it can support data rates in excess of the 128 Mbps data rate that was the maximum supported by the previous S2 tape-based recording VLBI system.

The disk-based recorders consist of server-class PC systems with VSIB data capture cards manufactured by the Metsahovi Radio Observatory (MRO) installed (the VSIB card is basically a direct memory access [DMA] card). The VSIB card interfaces to a VSIC board (also manufactured by the MRO) which in turn interfaces to either the S2 output port or the 64 MHz correlator output port of a VLBI data acquisition system (DAS) at each VLBI station in Australia. When the VSIC board is interfaced to the S2 output port of a DAS, the maximum data rate is 256 Mbps (4 x 16 MHz channels). When the VSIC board is interfaced to the correlator output of a DAS, the maximum data rate is 512 Mbps (2 x 64 MHz channels). The use of two DAS units at some antennas allows a maximum 512 OR 1024 Mbps recorded data rate (see table 1). The new disk-based recorders are known as the LBADR (Long Baseline Array Disk Recorder).

The second development, a VLBI software correlator running on the Swinburne University of Technology supercomputer, is significant in that it supports the high data rate recording provided by the new disk-based recorders. The older LBA S2 correlator in Marsfield could not interface to the new disk-based recorders and was limited to correlating data at a maximum rate of 128 Mbps. The software correlator has no such restriction on recorded data rate and the disk-based recording system has been designed to naturally interface to the software correlator. 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, and therefore has a number of advantages over the LBA hardware correlator in certain situations.

The combination of disk-based recorders and software correlator allows a number of VLBI observing modes that were not readily possible using the S2 and hardware correlator combination.

Antenna	# DAS units available	Max. data rate (LBADR)	Mark5 available?	K5 available?	Mark5/K5 Recording/correlation  supported?	Notes
Australia
ATCA	2	512/1024 Mbps	no	no	yes
ATCA split mode  - choice of two subarrays of ATCA antennas	2	2 x 256/512 Mbps (256/512 Mbps per subarray)	no	no	yes	Two tied arrays (i.e. antennas 1,2,3 and antennas 4,5,6) each at 256 Mbps. Instead of one tied array at 512 Mbps (i.e. antennas 1,2,3,4,5)
Parkes	2	512/1024 Mbps	yes	no	yes	Mk5b support only
Mopra	2	512/1024 Mbps	no	no	yes
Hobart	1	256/512 Mbps	yes	no	yes	Mk5a support
Ceduna	1	256/512 Mbps	no	no	yes
Tidbinbilla	1	256/512 Mbps	yes	no	yes	Mk5a support
International
Hartebeesthoek	1	--------	yes	no	yes	Mk5a support
VLBA	-	--------	yes	no	yes	Mk5a support
Kashima	-	--------	no	yes	yes	K5 support

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). At the remaining three antennas, only single DAS units are employed, giving maximum recorded data rates of 256 or 512 Mbps. 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. Table 1 also lists international antennas that utilise different disk-based recording systems (Mk5 and K5) that have been successfully used for VLBI observations in conjunction with the LBADR system and the Swinburne software correlator. The use of the Mark5 and K5 systems are supported by the software correlator. However, significant coordination is required to obtain the use of the international antennas and schedule/coordinate 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 Swinburne University of Technology 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-correlations 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.
		Only available at ATCA, Parkes, Mopra
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.
		Only available at ATCA, Parkes, Mopra
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.
		Only available at ATCA, Parkes, Mopra
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)
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)

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 bandwith observation would consist of 512 Mbps observations at the ATCA, Parkes, and Mopra (either in multiple 16 MHz or 32 MHz bands) and 256 Mbps observations at Ceduna, Hobart, and Tidbinbilla (depending upon the detailed polarisation setup required). The recorded data can be correlated on the Swinburne software correlator (all polarisation products, as required). Use of the 256/512 Mbps recording mode represents an approximate factor of 2 improved sensitivity over the standard 128 Mbps maximum data rate of the old S2 recording system.

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. The frequency channel limit for the LBA S2 correlator was 8196. For some spectral line observations, the LBADR and software correlator system is therefore more capable and more efficient than the S2 and hardware correlator combination.

Wide-bandwidth, high spectral resolution observations are also possible using this system, for example to cover multiple maser lines over wide bandwidths.

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 Swinburne software correlator running on the CPSR2 cluster at Parkes. 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 but the links across the Bass Strait are currently limited to 2 x 155 Mbps circuits. Test e-VLBI observations have been performed at 128 Mbps but connections at that rate are not reliable. Sustained e-VLBI observations that include Hobart (PAMHELA) can only be supported a rate 64 Mbps from Hobart.

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.

Professor Steven Tingay
Director, Science and Operations
Curtin Institute of Radio Astronomy
Department of Imaging and Applied Physics
Curtin University of Technology
Bentley, Western Australia
s.tingay@ivec.org OR s.tingay@curtin.edu.au

Dr Tasso Tzioumis
LBA Manager
Australia Telescope National Facility
CSIRO
Tasso.Tzioumis@csiro.au