Memo Review Reply Memo: 403 - Fast Switching Phase Correction Revisited for 64 12 m Antennas Holdaway, 2001Dec17 Reviewer: Dave Woody Date Received: 2002Aug08 Reply from: Mark Holdaway Date Received: 2002Aug23 Reply: 1) The glish scripts I used for the calculations are available from http://www.tuc.nrao.edu/~mholdawa/fastswitching_scripts/index.html 2) OK, I apologize for leaving out some of the relevant suporting information. 3) Fast switching does not depend critically on ANYTHING My experience with these simulations over the past 8 years has lead me to an understanding of fast switching as the optimization of a very mushy multi-dimentional system. For example, lets say that our 90 GHz (ie, cal frequency) sensitivity turns out to be a factor of 2 worse. So, we spend 4 times as long omtegrating on the same calibrator to achieve our required sensitivity. Woop-de-doo, the calibrator integration time was only a small fraction of the entire cycle time, so the cycle time goes up by 10%. The phase structure function goes less than time to the unity power, so the effect is small in the increased residual phase error. OR, we could realize that this small increase in residual phase error indicates that we don't need to measure the phase on the calibrator so accurately, so we only integrate 3.4 times longer (as oppose to 4 times longer). OR, we realize that there is a slightly brighter calibrator source slightly further away which results in a minimal efficiency penalty because we have a 1.5 s frequency switching time anyway, and the residual phases are dominated by temporal changes, not by the distance between the calibrator and the target source. So, we could make the 90 GHz system twice as bad, and get like a 5 or 10 percent increase in the residual phase errors on most observations. Similarly, fast switching doesn't depend critically on the height of the turbulent layer: the height effects the distance between the cal source and target source's lines of site, but as stated above, the residual phase errors are dominated by the temporal changes in the atmosphere. If ALL the assumptions are wrong in a bad direction, the results could be significantly over-optimistic. I think 0.5, 1, and 2 km turbulent heights would be fine to use for revised calculations. 18msec == OK, that was the time required to achieve the required phase errors on the calibrator. In the simulations, I probably should have done something like t_cal := max(t_cal, 0.5s) I don't think anyone is going to try to justify deleting 183 GHz WVR from the project, certainly not me.