ATNF Long Baseline Array Correlator
W.E.Wilson, P.P.Roberts, E.R.Davis.
Presentation made to the Asia Pacific Telescope Workshop
Sydney, December 1995
The Australian Long Baseline Array
The Long Baseline Array (LBA) was an important component of the original
specification of the Australia Telescope, construction of which commenced
in 1983 following funding by the Australian Federal Government.
The LBA was originally conceived as an array of three telescopes connected by
microwave links. The telescopes were the Parkes 64M antenna, the Mopra 22M
antenna and the ATNF Compact Array at Narrabri. It was also envisaged that a
fourth telescope, from the NASA Deep Space Tracking Station at Tidbinbilla,
would be able to join the array when available. As design of this
system progressed, and as more telescopes became available for possible
inclusion in the array, in particular the 26M antenna at Hobart, and as it
became apparent that a good imaging capability required at least six stations,
the inflexibility of the connected element interferometer became more and
more apparent. This lead to the decision to modify the LBA design to a
tape based VLBI array.
Three VLBI tape recording systems were evaluated for use in the LBA. These
were the reel based VLBA recorder and the cassette based K4 system from
Japan and S2 system from Canada. The S2 system, a system capable of
recording at a maximum data rate of 128MBits/sec for 5 hours on standard
S-VHS cassette tapes, was finally chosen as the
only option which satisfied the technical specifications and allowed the
outfitting of a sufficient number of stations within the funding available.
The funding from the original AT construction budget had, by this time (1992),
been augmented by an Australian Research Council grant to a consortium of
universities led by the University of Tasmania. This allowed the purchase
of a total of five record terminals, six playback terminals and six data
acquisition systems. The three element ATNF LBA had become a six element
Development of the LBA Correlator
The first two S2 record terminals were delivered to ATNF in July 1992.
In August of that year they were installed at Narrabri and Mopra and used
in an experiment which attempted to detect, and perhaps resolve, the supernova
remnant SN1987A. On-line fringe rotation was performed on the Compact Array
antennas at Narrabri, with the Mopra antenna defined as the reference antenna.
Two polarisations at 4MHz bandwidth with 2 bit quantisation were recorded
at Mopra and from four of the six antennas at Narrabri. This made use of the
full 128MBits/sec recording capability at Narrabri, whereas at Mopra the
recorded data rate was 32MBits/sec. The data was correlated in Sydney on
the ATNF test correlator using the same two record terminals. As the record
terminals have only a limited playback capability, with a maximum playback
data rate of 16MBits/sec, multiple passes were required. It was, however, the
availability of this playback function which allowed the experiment to be
undertaken. The experiment failed to detect SN1987A, but did detect a number
of weak sources contained within the primary beam. It was judged a resounding
success in its demonstration of the flexibility of the S2 systems in their
first use in astronomy.
In May 1993 three S2 playback terminals were delivered to ATNF and a record
terminal was delivered to Hobart. In October 1993 a test VLBI experiment
was carried out between Hobart and Parkes. This data was correlated on a
single baseline correlator which had been developed at ATNF as a prototype
for the LBA correlator. In November 1993 the first S2 intercontinental VLBI
experiment was carried out between Parkes, Hobart and the 70M antenna at
Ussuriysk, involving a collaboration of ....russian...???, the Institute
of Space and Terrestrial Science, Canada, the University on Tasmania and the
ATNF. Initial correlation of this data was carried out on the single baseline
prototype correlator, but by May 1994 this system had been extended to a
three baseline system.
In October 1994 regular scheduled observations with the LBA commenced,
initially with three stations. The first six station experiment was carried
out in July 1995. The final six station correlator is now in place
and will be officially opened at this meeting.
The LBA Correlator System
A block diagram of the system is shown below. Although the current system
is configured for six stations ( i.e. six S2 playback terminals ) it can be
expanded to a maximum of twelve stations. The bulk station delay is taken
out in the S2 terminals whilst the delay unit handles the incremental delay
during a scan by means of sample insertion. The output of the delay units
is connected via a
bus structure to a large crossbar switch which allows a highly fexible
routing of staion signals to the baseline components.
The baseline component contains the fringe rotators, the data validity
processors, the vernier delay components, a sideband inversion facility
and the recirculation memories. The fringe rotators use
four level sine, cosine functions and produce real and imaginary rotated
outputs for the complex correlator which follows.
Recirculation is only used at bandwidths less than 4MHz. An expansion path
is available by way of adding an additional crossbar switch/baseline
component/correlator, as shown in the block diagram.
- Up to 4 input data channels per station.
- Maximum bandwidth 16MHz on any one channel.
- 1 or 2 bit samples.
- Maximum total data rate of 128MBits/sec per station.
- Normal operating modes:
- 1,2 or 4 frequencies x 1 polarisation.
- 1 or 2 frequencies x 2 polarisations.
|Input data channels ||Maximum baselines
| || ||
|1 ||64 ||10
|2 ||32 ||8
|4 ||16 ||6
The correlator is made up from 8 standard ATNF correlator blocks
containing 64 correlator module boards. It has the following specifications:
- Total frequency channels
| ||Real ||Complex
| || ||
|At 16MHz BW
|At 4MHz BW
- Continuum: Channels per product at maximum number of staions
| ||2 Stokes ||Full Stokes
| || ||
|At 16MHz BW
|At 8MHz BW
- Line: Channels per product at 6 stations - 2 products per baseline
|At 8MHz BW
|At 4MHz BW
|At 2MHz BW
|At 1MHz BW
|At 0.5MHz BW
Last updated: Warwick Wilson 24 January 1996