7.3.3 Phase referencing using another band or another frequency

It is generally easier to measure the path lengths fluctuations at a lower frequency (even though the phase scales like frequency), due to both better receiver sensitivity and larger flux of the referencing source. Moreover, in marginal weather conditions, if the rms phase fluctuations at 100 GHz is $\sim 40^\circ$, then at 230 GHz they are of $\sim 100^\circ$, and the phase becomes impossible to track directly due to $2 \pi$ ambiguities.

If two receivers are available simultaneously, one may subtract to the high frequency phase the phase measured at the low frequency. The atmospheric fluctuations are cancelled and only a slow instrumental drift remains. The gain curve at the high frequency is then determined as the sum of two terms: the low frequency gain curve (including the slow atmospheric terms) plus that slow instrumental drift (which represents any phase fluctuation affecting one of the signal paths of the two receivers).

This method is currently used at Plateau de Bure.