Atmosphere model

The atmosphere model in use at Bure has been extended to compute the atmospheric excess pathlength by integrating the refractive index of wet air along the line of sight across the atmosphere.

The total precipitable water vapor content is computed, using an iterative procedure, to verify the condition:

$$\mbox{$T_{\rm EM}$}(observed) = (\mbox{$T_{\rm EM,S}$}(\mbox... ...ox{$T_{\rm EM,I}$}(\mbox{$w_{\rm\small\mbox{H$_2$O}}$}))/(1+G)$$

where G is the image side band gain ratio, and $T_{\rm EM,S}$ and $T_{\rm EM,I}$ are the radiation temperatures of the atmosphere computed from the atmosphere model in the signal and image side bands of the monitoring receiver. The model depends also on the ambient pressure, the ambient temperature, and the source elevation, which are known or directly measured.

The atmospheric excess pathlength $l$ at the observing frequency is then computed by integration when $w_{\rm\small\mbox{H$_2$O}}$ is known. One also computes the derivative $\frac{\partial l}{\partial \mbox{$T_{\rm EM}$}}$.