EXTERNAL versus INTERNAL phase calibration

For the 1.3mm receivers, there are two ways of calibrating the phase: either INTERNAL or EXTERNAL. INTERNAL phase calibration is just what is done for the 3mm receiver: the measured phased if fit by a continuous curve. However, because of higher phase noise at 1.3mm (both thermal and atmospheric), 2$\pi$ ambiguities are more frequent and more difficult to resolved. One can use instead the EXTERNAL phase calibration, in which phase derived from the 3mm receiver is first removed (with correction for the frequency difference), and then a residual phase offset between receivers is fit.

There are still two ways to compute the EXTERNAL phase calibration. In the first mode, the ``standard'' 3mm calibration curve is stored as external phase reference curve. The fit on the residual 1.3mm phase thus incorporates atmospheric fluctuations: the derived phase noise is representative of the atmosphere, but the phase difference between both receivers is not measured in an optimum way. In the second mode, a ``self-calibration'' derived from the 3mm data is stored as external phase reference curve. The fit on the residual 1.3mm phase no longer includes any atmospheric noise. The phase difference between both receivers is thus measured in an optimum way.

Switching between EXTERNAL and INTERNAL calibrations is done using command

SET PHASE INTERNAL     or  SET PHASE EXTERNAL

Both solutions can be stored independently in the .IPB file.