ALMA Memo 490: Effects of Atmospheric Emission Fluctuations and Gain Fluctuations on Continuum Total Power Observations with ALMA

Atmospheric water vapor emits strongly at millimeter wavelengths. Fast temporal and spatial fluctuations in the atmospheric water vapor results in problems in removing the atmospheric emission, which is done by very quick position switching or beam switching. Most phase fluctuations seen with the 11.2 GHz site testing interferometer are due to fluctuations in water vapor, so we can use the statistics of the phase fluctuation measurements to infer how well the cancellation of variable water vapor emission will be for a given observational strategy and a given atmosphere. In fact, we have found a (non-optimal but sufficient) way to match observations at all ALMA bands (1-10) to atmospheric opacity and stability conditions such that OTF total power continuum observations are essentially always thermal noise limited and never limited by atmospheric fluctuations. In addition to the problem of atmospheric fluctuations, the total power continuum observations also must contend with gain fluctuations, which in fact will limit these observations. Gain fluctuations of 1e-4 in one second will just barely limit total power continuum observations. However, gain fluctuations of 1e-3 will result in residuals which are an order of magnitude worse than the thermal noise limit. While other considerations might drive the decision for the receiver stability specification, it should be noted that a specification of 1e-3 in one second, or worse, will cause problems for ALMA continuum observations of large objects. All simulations were performed in AIPS++, and several reusable glish tools have been written.