MMA Memo #221

Elevation Dependence in Fast Switching

M.A. Holdaway [1]

July 13, 1998

Keywords: phase stability, fast switching, stability

We investigate the dependence of fast switching phase calibration on elevation angle. We include elevation effects such as air mass, change in rms phase, change in the distance between the lines of sight, and the details of the antenna's proposed AZ-EL drive system which is singular at the zenith. We find that the AZ-EL drive system creates a noticeable, but small effect, making very high elevation observations (ie, 75-85 degrees elevation) slightly less sensitive than at the optimum elevation around 70 degrees. The overall elevation behavior is dominated by the increase in rms phase with decreasing elevation. A sensitivity analysis indicates that something like 20 degree residual phase errors will be optimal for high elevations, while 25 or 30 degree residual phase errors will be optimal for low elevations or the highest observing frequencies. We achieve sensitivities of about 80% of the "perfectly phase stable" atmospheric case (which would not require any phase calibration) when we including both the effects of time lost to fast switching calibration and the decorrelation losses due to the residual phase errors inherent in the fast switching phase calibration technique.

[1] NRAO

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Last modified: 09 December, 1999