[asac] [almanews] ALMA Memos 345 and 346 Released

[Carolyn White]

LALMA Memo 345

Phase Fluctuation at the ALMA Site and the Height of the Turbulent Layer

Yasmin Robson, et al.

January 24, 2001

Phase compensation schemes are needed for sub-mm and mm inter-
ferometry if we are to obtain high resolution images with the proposed
ALMA telescope. The presence of uctuating water vapour in the at-
mosphere is the main culprit as it effectively distorts the phase of the
signals arriving at each element of the interferometer.

There are two possible methods of correcting for this phase distor-
tion. The first involves fast position-switching between the astronom-
ical source and a calibrator of known phase. The second uses a ra-
diometer close to the antenna to measure the amount of water vapour
along the line of sight. The expected phase distortion is then calcu-
lated and used to correct the astronomical signal. No matter which
technique is used, it is important to know the altitude at which most
of the uctuating water vapour is to be found, so that we can set lim-
its on the offset between the astronomical and calibration beams. If
the calibration beam does not sample a similar column of water vapour
as the astronomical beam, then the phase correction will be inaccurate.

We describe a method for obtaining the height at which the bulk of
the turbulence occurs, using two site-testing interferometers pointing in
dierent directions. The time-lag between the interferometers is found
by cross-correlating the phase data and then converted to a height us-
ing measurements of wind speed and direction.

Our results show that much of the turbulence is close to the ground,
usually within the first 500 m. This gives confidence in the use of Wa-
ter Vapour Radiometers (WVR) as a means of phase compensation, as there
would be a high degree of overlap between the astronomical and calibration
beams.

View a PDF version of ALMA Memo #345 at
   http://www.alma.nrao.edu/memos/html-memos/alma345/memo345.pdf

Download postscript version of ALMA Memo #345 from
   http://www.alma.nrao.edu/memos/html-memos/alma345/memo345.ps
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ALMA Memo 346

Spatial Distribution of Near-Surface Soil Resistivity 
in the Cerro Chascon Science Preserve

Seiichi Sakamoto, and Tomohiko Sekiguchi

January 27, 2001


We examined near-surface soil resistivity of twenty-one locations in the
Cerro Chascon science preserve area in the Wenner method with a fixed
electrode spacing of 2 m.  There were systematic differences in soil
resistivity near the surface (down to a few meters): The values in the
Pampa La Bola area were around 1000 Ohm-m, whereas those in the Llano de
Chajnantor area were much higher and > 4400 Ohm-m in most locations
topographically suitable for antenna pads.

Special treatment may be needed for each pad to meet the Chilean
regulation for grounding.  A factor $\sim$ 1.3 difference expected from
the 1.8deg C temperature difference between the Llano de Chajnantor and
Pampa La Bola areas is not enough to explain all of the measured
difference.  The trend that quebradas systematically had higher values
than their adjacent arroyos supports the idea that the difference
primarily reflects probable difference of water content due to local
topography and drainage.

View a PDF version of ALMA Memo #346 at
   http://www.alma.nrao.edu/memos/html-memos/alma346/memo346.pdf

Download postscript version of ALMA Memo #346 from
   http://www.alma.nrao.edu/memos/html-memos/alma346/memo346.ps
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