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The water vapor radiometer operates around the water line at 22
GHz. By monitoring the amount of water vapor along the line of sight
of each antenna, one can derive a corresponding atmospheric phase at
the frequency of interest and correct the individual dumps from the
correlator by this phase before averaging, hence mitigating
atmospheric decorrelation (see also section 3.3.2).
This requires calibration of the data coming from the radiometer. A
number of modes are available to derive the receiver gain,
,
and noise,
, either from tabulated laboratory measurements or
observations of loads of known brightness. The classical way of doing
this for NOEMA is to use
derived from skydip measurements
which are carried out regularly. The use of a hot load then allows us
to derive the gain
. With
and
, the sky
antenna temperature,
, for the different channels can be
computed.
The first generation of WVR used at NOEMA had 3 channels (centered at
frequencies
,
and
), that were combined in a way
that removed any
contribution (hence cloud emission):
|
(59) |
As for the astronomical receiver calibration, the water vapor content
of an atmospheric model is varied until a good match with the measured
triple temperature is found. The derivative of the optical pathlength
to the channel temperatures is then computed, in order that a scaling
factor can be applied to the radiometer counts, so that their
variation track atmospheric phases.
The offline version will do the same, but the spectral averaging was
done online. So it is useful only to check calibration or do use an
updated calibration for continuum records.
Next: Monitor
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Previous: Command ATMOSPHERE
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Gildas manager
2024-03-29