WVR

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, $T_{cal}$, and noise, $T_{rec}$, either from tabulated laboratory measurements or observations of loads of known brightness. The classical way of doing this for NOEMA is to use $T_{rec}$ derived from skydip measurements which are carried out regularly. The use of a hot load then allows us to derive the gain $T_{cal}$. With $T_{rec}$ and $T_{cal}$, the sky antenna temperature, $T_i$, for the different channels can be computed.

The first generation of WVR used at NOEMA had 3 channels (centered at frequencies $\nu_1$, $\nu_2$ and $\nu_3$), that were combined in a way that removed any $\nu^2$ contribution (hence cloud emission):

$$T_{triple} = \frac{1-\left(\frac{\nu_1}{\nu_2}\right)^2}{1-\left(... ...}{\nu_3}\right)^2\right)-\left(T_1-T_2\left(\frac{\nu_1}{\nu_2}\right)^2\right)$$ (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.