[evlatests] Master Pad position; EVLA phase vs.elevation.

[Vivek Dhawan]

Some progress from the latest round of antenna position fits.

Now that the antennas are about to move again, we know where
they are. In particular, the master pad position was far off,
and so we could not assess the true phase stability of EA23.
I found the position using a combination of L, C, and X band
data, with IF phase differences to resolve ambiguity.

The antenna phase changes as a function of elevation are now
reduced to about +_1 radian (on EA16, worst case) or less.
EA23, 24 and 18 are better than that by a factor of 3 to 4.

I propose we modify the usual baseline observations file to
use widely separated IF bands (e.g. 8.3 & 8.8 GHz at X band;
4.5 and 5 GHz at C band). I will do a test of this at the next
opportunity. Is there any reason not to convert to this mode
in routine operations?

In tandem, AIPS task LOCIT, which fits for position offsets
from phase residuals, can be augmented to work on IF differ-
ences as well its usual mode. Eric Greisen is thinking about
this.

Vivek.

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Antenna position changes, 2007 January 10.

delta   X,cm     Y       Z

EA13    2.50,  -0.5,    0.6

EA14    2.00,  -0.2,   -0.5

EA16    1.35,   1.5,   -1.3

EA18    0.4,   -0.7,   -0.3

EA23  -11.9,   -4.4,  -27.1

EA26   -0.6,    1.2,    0.8

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