WVR and Phase Metrics

ALMA uses WVR (water vapour radiometers) to measure the water vapour content in the atmosphere. This water vapour is the primary cause of refraction at sub-millimetre wavelengths which acts to de-correlate signals between antennas in a baseline, reducing the source flux and image quality.
At Allegro the WVR signals and phase data are examined from testing data. We are parameterising the effects of water vapour in various weather conditions and also optimising the application of the WVR solutions.

Our studies have found that the WVR solutions can be scaled by an empirically derived
factor using calibrator data. This scaling can in some cases result in an improved
phase solution, which can then act to improve images in terms of signal-to-noise and
potentially fidelity (more details here).

ALMA adopts a fast-switching technique and water vapor radiometer (WVR) data to correct phase. However it becomes increasingly difficult to control phase stability with increasing observing frequency. In particular, for high frequency observation of ALMA Band 8,9 and 10, signal-to-noise ratio of calibrators drops dramatically due to the strong water and oxygen absorption lines in the Earth atmosphere and thus the gain solutions from the calibrators are much more noisy compared with those from the low frequency observation. This invokes Band-to-Band (B2B) phase transfer methods transferring reliable phase solution from the low frequency observation to the high frequency data.