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MMA Memo #174

How Many Fast Switching Cycles Will the MMA Make in its Lifetime?

M.A. Holdaway [1]

June 16, 1997

Keywords: phase calibration, fast switching, switching cycles, self-calibration, fatigue

Estimating the MMA's frequency and configuration usage, we calculate the typical fast switching cycle times, calibration source parameters, and atmospheric characteristics for the MMA for an entire year. From these calculations, we are able to estimate during a 30 year life time of the MMA the antennas will perform about 30-50 million fast switching cycles. If radiometric phase correction works well, this may be reduced to about 3 million switching cycles. The uncertainties in these numbers are quite large, at least a factor of 2. The estimated number of switching cycles is required for a fatigue analysis of the MMA antenna members.

Several interesting results arise in conjunction with these calculations:

At 40 GHz, calibrators are typically 20 mJy and about 0.4 deg away from the target sources, while at 650 GHz, calibrators are typically 120 mJy and about 1.1 deg away. As the path length requirement becomes more severe at high frequencies, more time must be spent integrating on brighter calibrator sources which are typically farther away.

To achieve low residual phase errors, very short switching cycles are required, reducing the switching efficiency and increasing the noise. If large residual phase errors are permitted, the switching efficiency is high, but decorrelation losses are large, also increasing the noise. The optimal residual phase errors seems to be about 30 deg with an 80% efficiency with respect to decorrelation and time lost to calibration, but some future refinement can be done on this issue.

By matching the observing frequency to the atmospheric conditions, the D array is always phase stable at the observing frequency appropriate to the atmospheric conditions (as defined by 20 deg rms phase errors on the longest baselines) and the C array is often phase stable.


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Last modified: 09 December, 1999