Skydip

The goal of ``atmospheric calibration'' is to estimate the receiver gain, the atmospheric transmission and the antenna efficiency in order to rescale the observed intensities onto the scale that would be obtained using a perfect antenna outside the atmosphere (Brightness Temperature scale). With the cold load, the skydip procedure (the OBS $\backslash$SKYDIP command) allows the estimation of 3 parameters, namely the water vapor content (wH₂O in mm), the antenna forward efficiency Feff, and the receiver temperature Trec. In principle the antenna efficiency Feff is not a function of time, but it does depend on the surface state of the antenna, especially if there is any ice or condensed water. Under normal conditions Feff = 0.93 at 86 GHz.

Skydips are necessary if antenna efficiency variations are suspected (icing or condensing conditions). They can be done using OBS $\backslash$SKYDIP command. The continuum detectors measure the total power with an integration time of 8 seconds on the cold load, then on the warm load (table), then on the sky. The measurement is repeated at 6 different elevations. The correlator sampling levels are adjusted (Tweaking) at each subscan. Data are stored in the Date.IPB file to maintain completeness of the calibration parameters in this file.

Reduction is done in CLIC by command CLIC $\backslash$SOLVE SKYDIP