Memo 590: Atmospheric dispersion and
the implications for phase calibration
Emily Curtis, Bojan Nikolic, John Richer and Juan Pardo
ABSTRACT
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 (&ge 5 per cent) of
the non-dispersive delay for the high-frequency ALMA bands (&ge 160 GHz, Bands 5 to 10).
Additionally, the variation in dispersive path delay across ALMA's 4-GHz contiguous bandwidth
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.
FULL TEXT
The full text of the memo is available
here.
SOFTWARE
The following Python scripts interface with the
ATM software
to produce the results presented in this memo.
- dispersivephase.py
finds the ratio of dispersive path delay to total column
of water for various atmospheres, in particular reproducing
Figs. 4-10.
Prerequisites