Memo Review

Memo: 427 - Antenna Position Calibration
Wright, 2002May16

Reviewer: Robert Lucas

Review:

I like the idea of using the multi-frequency capability to solve 2pi
ambiguities.  This can be very useful if repeatibility of antenna
positioning is not good enough, and certainly when a station is used
for the very first time.

At Plateau de Bure we do not attempt to remove 2$\pi$ ambiguities
munually or by enforcing phase continuity, we just perform a least
squares minimization inside a predefined search box in the space of
baseline coordinates. The minimized function uses the phase
determination closest to that measured. We may also fit a time
dependent polynomial simultaneously with antenna position
offsets. This is not a real help, as instrumental drifts are wery
small and atmospheric phase fluctuations mostly occur faster than we
can correct this way (we spend typically a few minutes on each
source).

We may also fit the axes offsets simultaneously, but we only do this
on very short baselines and good weather.  ALMA antennas can look at
some directions with over-the-top' elevations which should easily
allow to separate this effect from antenna position offsets, and
measure antenna axes offsets in an absolute way (though this is not
really needed, I believe).

One point that would need investigation is whether radiometric
correction should be used to improve phase determination during
antenna position measurements. It is quite possible that the
calibration of that correction scheme would use interferometric
observations of quasars, as mentioned in Bima memo 78, so it may be
not wise to use this phase correction for determination of antenna
positions in that case (except for canceling decorrelation effects
during the integration on each source).

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