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The ATNF Long Baseline Array Correlator
W.E.Wilson, P.P.Roberts and E.R.Davis
Australia Telescope National Facility
December 1995
1 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 Tidbin­
billa, 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 was realised that
a good imaging capability required at least six stations, the inflexibility of the
connected element interferometer became more and more apparent. This led to
the decision to modify the LBA 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
S­VHS 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 Australian LBA.
2 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
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remnant SN1987A. On­line 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 experi­
ment 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 the Russian Astro Space Centre, the In­
stitute 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.
3 The LBA Correlator System
A block diagram of the system is shown in Figure 1. 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 station signals to the baseline components.
The baseline component contains the fringe rotators, the data validity pro­
cessors, the vernier delay components, a sideband inversion facility and the recir­
culation 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 expan­
sion path is available by way of adding an additional crossbar switch/baseline
component/correlator, as shown in the block diagram.
2

1
7
2
3
4
5
6
12
DELAY
UNIT
BASELINE
FRINGE
ROTATION
RECIRCULATION
SWITCH
CORRELATOR
10
12
CROSS­BAR
STATION COMPONENTS
COMPONENTS
S2 PT
S2 PT
S2 PT
S2 PT
S2 PT
S2 PT
ATNF LBA CORRELATOR
Figure 1: Block Diagram
4 Specifications
ffl Up to 4 input data channels per station.
ffl Maximum bandwidth 16MHz on any one channel.
ffl 1 or 2 bit samples.
ffl Maximum total data rate of 128MBits/sec per station.
ffl Normal operating modes:
-- 1,2 or 4 frequencies x 1 polarisation.
-- 1 or 2 frequencies x 2 polarisations.
ffl
Input data channels Maximum baselines Maximum stations
1 64 10
2 32 8
4 16 6
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5 Correlator
The correlator is made up from 8 standard ATNF correlator blocks containing
64 correlator module boards. It has the following specifications:
ffl Total frequency channels
Bandwidth Real Complex
16MHz 8192 4096
4MHz 32768 16384
ffl Continuum: Channels per product at maximum number of stations
Bandwidth 2 Stokes Full Stokes
16MHz 64 32
8MHz 128 64
ffl Line: Channels per product at 6 stations---2 products per baseline
Bandwidth
8MHz 256
4MHz 512
2MHz 1024
1MHz 2048
0.5MHz 4096
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