MMA Project Book, Chapter 14 [WBS 11.1]
SITE CHARACTERIZATION AND MONITORING
View south from Cerro Chajnantor of MMA site.
Photo: S. Radford, 1994 November.
Revision History:
1999 February 4: Updated URLs.
1998 October 16: Reorganized to match WBS,
added section numbers.
1998 July 15: Original version.
14.1 Goals
During the project's prehistory, NRAO conducted extensive measurements
to characterize several candidate sites for the Millimeter Array. These
studies culminated in the recommendation
of an array site on the high (5000 m) plateau southwest of Cerro Chajnantor,
Chile,
about 40 km east of the village of San
Pedro de Atacama. The goals of further site characterization and monitoring
are:
-
to identify and quantify site conditions and their influence the instrument
design or operations concepts,
-
to provide a historical record of site conditions to guide priorities for
instrument development and operation,
-
to maintain a continuous presence on the site through development and construction
to the start of operations, and
-
to maintain contact and coordinate efforts with other groups working on
or near the site.
14.2 Areas of interest
At millimeter and submillimeter wavelengths, pressure broadened molecular
spectral lines make the atmosphere a natural limitation to the sensitivity
and resolution of astronomical observations. Tropospheric water vapor is
the principal culprit. The translucent atmosphere both decreases the signal,
by attenuating incoming radiation, and increases the noise, by radiating
thermally. Furthermore, inhomogeneities in the water vapor distribution
cause variations in the electrical path length through the atmosphere.
These variations result in phase errors that degrade the sensitivity and
resolution of images made with both interferometers and filled aperture
telescopes. The site characterization effort addresses these areas:
-
Radiometric properties of the atmosphere
-
stability [11.1.2]
-
transparency [11.1.3]
-
Physical structure of the atmosphere [11.1.4]
-
meteorology
-
stratification
-
turbulence
-
Physical characteristics of the site
14.3 Site Infrastructure [11.1.1]
The MMA operations base on Chajnantor is a 20 foot (6 m) long ocean shipping
container that provides shelter for personnel and physical support for
the instruments. The LSA project has a similar container 15 m north of
the MMA equipment. A third container is installed 1 km west of the MMA
container as a launch base for radiosondes.
14.3.1 Safety program [11.1.1.1]
Site inspections (every six months? year?); inventory, inspection, and,
if necessary, repair or replacement of safety supplies and equipment; identification,
and, if necessary, remediation of safety hazards; training (first aid,
high altitude illness, oxygen therapy, fire safety, industrial safety).
Note safety rules.
14.3.2 Solar power system [11.1.1.2]
An array of solar panels and a battery bank supply electrical power (24
VDC and 110 VAC 60 Hz). The system can supply about 500 W continuously,
with sufficient reserve capacity to weather a storm of a few days. With
current instrumentation, the system operates near capacity. A wind turbine
has been installed on two occasions to augment the solar panels, but it
broke quickly. System maintenance includes a periodic (yearly) check and
refill of battery water.
14.3.3 Communications [11.1.1.3]
Voice and low-speed (<= 9600 baud) data are transmitted by cellular
and satellite (INMARSAT-A) telephones. Handheld radios will be used for
communications on and around the site.
14.3.4 Freight [11.1.1.4]
Expenses of equipment deployment.
14.3.5 Transportation [11.1.1.5]
Vehicles (4WD) for access to Chajnantor.
14.3.6 High resolution digital elevation model [11.1.1.6]
1n 1996, contour maps and digital elevation models were prepared from aerial
photographs (MMA
Memo 160). These cover two 8 × 8 km regions of the Chajnantor
and Pampa la Bola areas at 5-10 m resolution. These maps will be extended
to the entire science reserve (26 × 24 km). They will be used for
hydrodynamic studies of airflow over the site, for planning the array configurations,
and for planning civil works.
14.3.7 Computers and network [11.1.1.7]
All (NRAO) instruments are controlled by PCs running Windows 95. They are
interlinked with ethernet, which extends to the LSA container. The PC clocks
are synchronized to a GPS receiver that provides an absolute time reference
good to about 1 s.
14.3.8 Auxiliary instruments [11.1.1.8]
-
A surveillance camera,
installed on 1997 June 15, takes pictures
of the southwest horizon every two hours. Data are retrieved about
once a month and the images
posted.
-
A subsurface temperature probe was operated 1997 June - October and 1998
March - May. Data analysis is ongoing.
-
A seismometer was installed in 1995. Data are analyzed by Chilean group
(K. Bataille). Status of GPS rollover unknown.
14.3.9 Physiology studies [11.1.1.9]
John West, MD (UCSD) is investigating strategies for improving worker comfort
and performance at high altitude. These include enhancing the oxygen concentration
of the air in working and living quarters (MMA
Memo 191).
14.4 Atmospheric stability [11.1.2]
Inhomogeneities in the distribution of water vapor cause variations in
the electrical path length through the atmosphere. The resulting phase
errors degrade the sensitivity and resolution of observations with both
interferometers and filled aperture telescopes.
The site test interferometers directly measure the tropospheric phase stability.
They observe unmodulated 11.5 GHz beacons broadcast from geostationary
satellites and measure the phase difference between the signals received
by two 1.8 m diameter antennas 300 m apart. Because the atmosphere is non-dispersive
away from line centers, the results can be scaled to millimeter and submillimeter
wavelengths.
Three instruments have been constructed by NRAO's Tucson office. The
first was operated near the VLBA antenna (3720 m) on Mauna Kea, Hawaii,
from 1994 September to 1996 June, then installed at the
VLA in in 1997 May. The second has been operating on Chajnantor (5000 m) near San Pedro
de Atacama, Chile, since 1995 May. A third was built for the LSA project.
ESO installed it at Pajonales in 1997 April and moved it to Chajnantor
in 1998 June.
The design and operation of these instruments are described in Site
Test Interferometer (Radford, Reiland, & Shillue 1996, PASP 108,
441). From the phase time series, we obtain the r. m. s. path fluctuations
on a 300 m baseline, the power law exponent of the phase structure function,
and the velocity at which the turbulent water vapor moves over the array.
MMA
Memo 129 describes the site test interferometer data reduction in detail,
and MMA
Memo 130 illustrates the agreement between two different methods of
deriving the mean velocity of the turbulent water vapor flow in the atmosphere.
In 1998 June, the LSA interferometer was set up alongside the MMA interferometer.
They share essentially the same baseline, but observe different satellites
about 5° apart on the sky. Lag correlation of the data from the two
interferometers will indicate the height of the turbulent layer (see MMA
Memo 196).
The interferometers operate autonomously. Status reports are received
daily and data are retrieved about once a month. The data are analyzed
in Tucson and monthly summaries
are posted. Current activity includes operation and maintenance, including
sporadic repair as required, data retrieval, and data analysis.
14.5 Atmospheric transparency [11.1.3]
Pressure broadened molecular spectral lines, principally of tropospheric
water vapor, make the atmosphere semi-opaque at millimeter and submillimeter
wavelengths. The translucent atmosphere radiates thermally, which increases
the system noise, and attenuates incoming radiation, which decreases the
signal.
The 225 GHz tipping radiometer is the benchmark instrument for site characterization.
It measures the atmospheric transparency every 10 minutes and the stability
of atmospheric emission every fifth hour. Operation is automatic. Daily
and monthly
data summaries are posted. The data are made available to interested parties
in machine readable form. Current activity includes operation and maintenance,
including sporadic repair as required, data retrieval, and data analysis.
A tipping photometer was been developed in collaboration with Carnegie
Mellon University to directly measure the atmospheric transparency at 350
µm wavelength. This instrument is based on an ambient temperature,
pyroelectric detector. The spectral response is defined by a resonant metal
mesh. A compound parabolic (Winston) cone and offset parabolic scanning
mirror together define the 6° beam on the sky. The detector is internally
calibrated with two temperature controlled loads and views the sky through
a woven Gore-tex window. Identical instruments have been deployed on Chajnantor
(1997 October), at the CSO on Mauna Kea (1997 December), and at the South
Pole (1998 January).
The instruments operate autonomously. Status reports are received daily
and data are retrieved about once a month. The data from these instruments
are being analyzed with the aim of making an unbiased comparison of the
three sites. Current activity includes operation and maintenance, including
sporadic repair as required, data retrieval, and data analysis. Further
work includes
-
cross calibration between the submm tipper and other instruments, namely
the 225 GHz tippers, SCUBA, CSO, and AST/RO,
-
continued development of the filter wheel to add 1300 µm and 200
µm channels, and
-
possible deployment on Cerro Toco (5400-5500 m) to investigate the dependence
of transparency with altitude in the area of Chajnantor.
14.5.2 Fourier Transform Spectrometer [11.1.3.3]
To measure the atmospheric emission spectrum at Chajnantor, the Smithsonian
Observatory has deployed a Fourier transform (polarizing Martin-Pupplet)
spectrometer. This cryogenic instrument covers 350 - 3000 GHz with 3 GHz
resolution and a 3° beam. The instrument recorded data for most of
the 1998 winter season. NRAO provides the base for field operations.
14.6 Physical structure of atmosphere [11.1.4]
The vertical profiles of atmospheric water vapor and turbulence may affect
the success of radiometric phase calibration schemes.
Radiosondes carried by weather balloons provide in situ measurements
of pressure, temperature, humidity, and wind speed and direction over the
launch site. From these data we may learn about the stratification of the
water vapor over Chajnantor and about shear layers that may generate turbulence.
A surplus radiotheodolite was acquired, upgraded by the manufacturer, tested
in Tucson, and deployed at Chajnantor. Beginning in 1998 October, balloon
flights will be made whenever appropriate personnel are at the site.
This campaign is a collaboration between NRAO, Cornell, ESO, and SAO. The
balloons will be launched from a container placed 1 km west of the main
site.
14.6.2 Hydrodynamic models [11.1.4.2]
Calculations of airflow over Chajnantor, with emphasis on turbulence generated
by local topography.
14.6.3 Sodar [11.1.4.3]
Acoustic sounding, or sodar, senses thermal turbulence in the lower atmosphere.
Engineering tests of an ESO sodar unit were planned for 1998, but are postponed
indefinately. After these tests, we will evaluate our interest we have
in pursuing further measurements.
14.6.3 Weather stations [11.1.4.3]
Additional weather stations will be deployed to measure the variation of
meteorological parameters over the site.
14.7 Technical planning with collaborators [11.1.5] and neighbors
Several groups are carrying out site characterization studies or astronomical
experiments nearby. NRAO encourages these groups and takes interest in
their results. As needed, NRAO and the other groups coordinates activities.
In 1998 June, the LSA project redeployed its site characterization equipment
to Chajnantor. The LSA equipment, located 15 m north of the MMA equipment,
includes:
-
Several weather stations. These are currently deployed adjacent to the
containers, but will be deployed across the site in the last quarter of
1998.
-
A 12 GHz interferometer. This is set up alongside the MMA interferometer,
sharing essentially the same baseline, but observing different satellites
about 5° apart on the sky. Lag correlation of the data from the two
interferometers will indicate the height of the turbulent layer (see MMA
Memo 196).
-
Dual three channel 183 GHz radiometers. These
(instruments,
designed and
constructed by MRAO, OSO, and ESO, measures the H2O line shape.
They are installed at the ends of the LSA interferometer and look in the
same direction as the interferometer. Variations in the line shape will
then be compared to the phase fluctuations measured with the interferometers.
[11.1.2.2]
-
A single channel 22 GHz radiometer (deployment foreseen in late 1998).
The LSA project provides field support for the MMA site characterization
program.
At Pampa la Bola, about 7 km northeast of the MMA equipment, the LMSA project
has installed:
-
Dual 220 GHz tipping radiometers,
-
A 12 GHz interferometer, and
-
A Fourier Transform Spectrometer (temporary deployments).
The CAT project is making optical seeing (DIMM) measurements. Campaigns
in 1998 May, July, October, etc.
14.7.3 MAT
Observations of fluctuations in the Cosmic Background Radiation by a Princeton/Pennsylvania
group. Campaigns in 1997 and 1998.
14.7.3 CBI
Observations of fluctuations in the Cosmic Background Radiation by a Caltech
group. Deployment foreseen in early 1999.
14.8 Site Characterization Reviews [11.1.6]
Scientific reviews of site characterization data obtained by all groups.
14.8.1 USNC/URSI meeting [11.1.6.1]
At the USNC/URSI National Radio
Science Meeting in 1999 January in Boulder, there will be a session
on Atmospheric Transmission
at Millimeter and Submillimeter Wavelengths. Results from the NRAO
site characterization program will be presented.
14.8.2 Mid-term Review [11.1.6.2]
2000 March
14.8.3 Final Review [11.1.6.3]
2001 March
References
MMA Site Studies
MMA site safety
rules
NRAO 1998 May, Recommended
Site for the Millimeter Array [also ps]
Holdaway, M. A., &. Radford, S. J. E., 1998, Options
for Placement of a Second Site Test Interferometer on Chajnantor, MMA
Memo 196
Holdaway, M. A., Gordon, M. A., Foster, S. M., Schwab, F. R.,
and Bustos, H., 1996, Digital
Elevation Models for the Chajnantor Site, MMA Memo 160
Holdaway, M. A., 1995, Velocity
of Winds Aloft from Site Test Interferometer Data, MMA Memo 130
Holdaway, M. A., Radford, S. J. E., Owen, F. N., & Foster, S. M.,
1995, Data
Processing for Site Test Interferometers, MMA Memo 129
Radford, S. J. E., Reiland, G., & Shillue, B., 1996, Site
Test Interferometer, PASP 108, 441
West, J. B., Powell, F. L., Luks, A. M., 1997, Feasibility
Study of the Use of the White Mountain Research Station (WMRS) Laboratory
to Measure the Effects of 27% Oxygen Enrichment at 5000 m Altitude on Human
Cognitive Function, MMA Memo 191
