[asac][almanews] ALMA Memo 503 Released

[Stacy Oliver]

ALMA Memo # 503

Antenna Position Determination: Observational Methods and Atmospheric 
Limits

John Conway  (Onsala Space Observatory, Sweden)

2004-09-03

We discuss the accuracy to which ALMA antenna relative positions can be 
determined via astronomical observations of phase and delay toward 
multiple strong calibrators. We show that delay induced phase gradients 
across the bandpass can be used to resolve turn ambiguities so that 
accurate delays can estimated from the phase. At low frequencies this 
demands only modest stability of the bandpass phase. For this and other 
reasons we argue that 90GHz is the best frequency for position 
calibration observations. The proposed specification for short time 
instrumental phase stability is adequate for antenna position 
determination. We discuss in detail the effect of the wet troposphere 
and derive how position errors scale with baseline length in the case of 
single-baseline calibration. We then generalise to a full calibration of 
the whole array. It is found that the resulting position errors between 
two antennas is the same as if these two antennas participated in there 
own single baseline calibration. We find that because of the geometry 
and the need to solve for instrumental phase that even on short 
baselines the rms error on the vertical or z-component is twice as large 
as for the x and y components. In addition for >1km baselines while the 
x and y rms errors rapidly saturate the z components rms errors continue 
to increase. Some uncertainly in estimating errors on long baselines 
comes from our lack of knowledge of the outer scale of turbulence at the 
site. The effects of systematic gradients in the zenith wet or dry delay 
and methods of calibration are briefly considered.
We propose that when in the intermediate 'zoom' array configurations an 
initial calibration of the moved antennas is made in late afternoon 
lasting 30minutes. Later in the early hours of the morning, when phase 
stability is best, we propose a 30 - 60 minute calibration of the whole 
array. Because of the need to apply phase corrections for antenna 
positions retro-actively even continuum data should always be stored in 
spectral line mode with channel widths <1 GHz. Final pipelining for the 
highest dynamic range imaging may have to wait for up to 12 hours until 
good antenna positions are obtained. With good 'a priori' positioning of 
antennas on pads and/or the acceptance of delayed pipelining as the norm 
after reconfiguration the first late-afternoon calibration might be 
avoided. For the smallest configurations we expect that the troposphere 
will not be a limitation on achieving the proposed goal of 100 microns 
relative positioning on all baselines. For larger configurations we 
estimate that while most baselines will achieve the target accuracy 
those baselines to recently moved antennas will have much larger errors. 
Further work is required to understand the effects of this on imaging 
and astrometry.

View a pdf version of ALMA Memo #503.
http://www.alma.nrao.edu/memos/html-memos/alma503/memo503.pdf
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