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To work optimally, interferometers like the Australia telescope Compact Array
need to combine the signals from all antennas in phase.
The dominant source of phase errors in interferometric radio observations
at frequencies above 5 GHz are fluctuations in the tropospheric water vapour
content along the line of sight of the individual interferometer elements.
The effect of the fluctuations scales linearly with frequency, making observations
at wavelengths less than ~1cm particularly prone to atmospheric phase changes.
When the ATCA was upgraded to operate at mm wavelengths in the early 2000s,
it was recognised that a means of characterising the phase stability was
required, and a dedicated two-element interferometer, observing a tone from
a geostationary satellite on a fixed 230m baseline was installed.
The initial system
was described by Middelberg, Sault and Kesteven and has been upgraded
and refurbished several times since then. One element of the seeing monitor
system is shown in the foreground.
Indermuehle and Burton
examined nearly 9 years of seeing monitor data, and confirmed the earlier
finding that the phase stability is best between 03:00 and 06:00 local time
in winter, and worst between 12 noon and 15:00 local time in summer.
However, typical fluctuations during summer nights were similar to those
occurring during winter days, indicating millimetre-wave interferometry
was not as restricted as might have been thought.
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