ALMA Memo 590: Atmospheric dispersion and the implications for phase calibration

This memo was posted in December.


The success of any ALMA phase-calibration strategy, which incorporates
phase transfer, depends on a good understanding of how the atmospheric
path delay changes with frequency (e.g. Holdaway & Pardo 2001). We
explore how the wet dispersive path delay varies for realistic
atmospheric conditions at the ALMA site using the ATM transmission
code. We find the wet dispersive path delay becomes a significant
fraction (5 per cent) of the non-dispersive delay for the high-frequency
ALMA bands (160 GHz, Bands 5 to 10). Additionally, the variation in
dispersive path delay across ALMA's 4-GHz contiguous band- width is not
significant except in Bands 9 and 10. The ratio of dispersive path delay
to total column of water vapour does not vary significantly for typical
amounts of water vapour, water vapour scale heights and ground pressures
above Chajnantor. However, the temperature profile and particularly the
ground-level temperature are more important. Given the likely
constraints from ALMA's ancillary calibration devices, the uncertainty
on the dispersive-path scaling will be around 2 per cent in the worst
case and should contribute about 1 per cent overall to the wet path
fluctuations at the highest frequencies.

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