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The receiver is not purely single-sideband. Let us denote by Gl and Gu the
normalized gains in the receiver lower and upper sidebands,
Gl + Gu = 1. The
atmosphere opacity per km varies with altitude as does the air temperature.
Then, the above expressions of Tsky and Temi should be explicited for each
sideband (j= u or l):
Tskyj |
= |
![$\displaystyle (1-e^{-{\tau}^j}) T_{atm}^j$](img785.gif) |
(8.36) |
Temi |
= |
![$\displaystyle T_{sky}^l \eta_f G^l + T_{sky}^u \eta_f G^u+ (1-\eta_f) T_{gr}$](img786.gif) |
(8.37) |
The atmospheric transmission model ATM [Cernicharo 1988] allows to calculate
iteratively
from a load+ sky measurement. The values of
are calculated for the Standard US atmosphere (parameters are: Winter-, Spring-, or
Summer-temperature T, altitude, latitude, water vapor w) by summing up the
contributions of O2 (
), H2O (
) and O3 (
)
(including rare isotopes and vibrationally excited states):
![\begin{displaymath}\tau_\nu= A_\nu(T) + w B_\nu(T) + C_\nu(T)
\end{displaymath}](img792.gif) |
(8.38) |
Depending on
, the values of
are derived either
from a Table or estimated from empirical formulae. A first guess of the amount of
precipitable water is made from the ambient temperature, pressure and humidity by
using the relations of Sec.8.1.4. Then, the expected Tsky and
Temi are calculated from the two expressions above and Temi is compared to
its value measured from the the observation of the atmosphere and the load
(Eq.8.35). The value of w is changed and the calculation of
and Temi restarted. Normally, the process converges after 2 to 4
iterations. Once w and Temi are known, the calibration factor Tcal can
be derived
![\begin{displaymath}T_{cal}^j = ({T_{load} - T_{emi}})e^{\tau^j}
\end{displaymath}](img795.gif) |
(8.39) |
and the data
calibrated in the TA* scale using Eq.8.32.
Next: 9. Atmospheric Fluctuations
Up: 8.5 Correction for atmospheric TA*
Previous: 8.5.2 More realistic case
S.Guilloteau
2000-01-19