In all the calibration modes, is derived from by correcting for the forward efficiency. Hence it is important to give correctly this forward efficiency. This efficiency can be measured using a skydip (command OBSSKYDIP).
In a skydip, the total power on the sky is measured at various elevation. Since the sky brightness varies with the elevation, while the rearward losses are essentially constant, it is possible in a skydip to derive simultaneously the atmospheric transmission (i.e. the water vapor content, ) and the forward efficiency . Provided is known (at all elevations), a skydip can be done. can be calculated from the gains on the Chopper and on the Cold Load when Nitrogen is available, or given by user input in TREC mode. In the first case, elevation dependent can be accounted for, while the second case require constant .