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As described in sections 2.2.2, the actual calibration of data is
a two-steps process. In the first step,
is simply deduced from
Eq. 11 and the ATM model is then just used to associate to this
value, a value of the total opacity of the atmosphere at signal and
image frequencies. In the second step, the
value is computed with
Eq. 17 and then used on the data.
Hence, in Eq. 17, the value of the term
is
independent of the ATM model and/or of the coupling factors, while the
value of the term
is directly
dependent of the ATM modeling and/or of the coupling factors. Two different
situations arise: 1) the measured coupling parameters are at best known
only to a given accuracy and at worst biased and 2) the model is
inaccurate. To model both situations, we will assume that
,
,
and
are the actual values of those
different parameters while
,
,
, and
are the modeled or measured values. It is easy to show that the
relative error on
is
|
(18) |
Thus the accuracy of
directly depends on the accuracy of the
coupling factors and on the modeled opacity. It must be noted that 1) this
equation assumes that the load and data calibration are made at the same
airmass (i.e. elevation) 2)
is close to zero for the EMIR
receivers, implying that the
is one of the key parameter to get
high calibration accuracy and 3) it is better to underestimate the opacity
than to overestimate it.
Fig. 1 compares the values of opacities,
and
as a function of precipitable for both version of ATM. The
comparison is made at two different frequencies, which have a very
different opacity behavior:
- 115.3 GHz
- The opacity is dominated by the dry continuum due to the
transparency of water at this frequency and to the presence of a close
oxygen line. It is known that at 3mm under submillimeter weather
conditions, the measured sky emission is lower than the modeled one with
ATM 1985. This facts comes from the uncertainty on the knowledge of the
, an incorrect modeling of the dry continuum with ATM 1985, or a
combination of both. Anyway, this is why ATM 1985 authorizes negative
amount of water vapor. ATM 2009 makes the situation even worse because
the total opacity and thus emission are larger with ATM 2009 than with
ATM 1985. This comes mainly from the dry continuum opacity being larger
with ATM 2009 than with ATM 1985... On the other hand, the same amount of
atmospheric emission is reached with significantly less water vapor in
ATM 2009.
- 230.5 GHz
- The opacity is dominated by its wet component, the dry
continuum being almost negligeable. This is the reason why
sharply increases when the amount of water vapor goes to zero with ATM
2009. It is unclear why such an effect is not seen with ATM 1985. Here
also, the total opacity and thus emission are larger with ATM 2009 than
with ATM 1985, and the wet opacity increases slowlier with ATM 2009 than
with ATM 1985. But the effects are much less pronounced at 230.5 than at
115.3 GHz. The consequence is that the amount of precipitable vapor (and
thus of opacity) needed to reproduce the emission of the sky is similar
in both ATM versions.
Next: Recommendations
Up: Comparison of ATM 1985
Previous: Comparison at constant amount
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Gildas manager
2014-07-01