ALMA Project Book, Chapter 14
View south from Cerro Chajnantor of ALMA site.
Photo: S. Radford, 1994 November.
Revision History:
2001 July 25: Reorganized, merged NRAO and European efforts.
2000 September 6: Updated, removed WBS numbers.
1999 February 4: Updated URLs.
1998 October 16: Reorganized to match WBS,
added section numbers.
1998 July 15: Original version.
14.1 Goals
The ALMA will be located on the high (5000 m) plateau southwest of
Cerro Chajnantor,
Chile,
about 40 km east of the village of San
Pedro de Atacama.
Measurements since 1995 have demonstrated the existence
of exceptional atmospheric conditions,
i. e., transparency and stability, at this site
for observations at millimeter and submillimeter wavelengths.
The goals of continued site characterization and monitoring
are:
-
to identify and quantify site conditions and their influence on 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 History
The ALMA site charaterization program at the the Llano de Chajnantor
grew from the efforts of the
individual partners for their consitutent, predecessor projects.
For the Millimeter Array (MMA) project, the National Radio Astronomy
Observatory (NRAO) conducted extensive measurements to characterize
several candidate sites in the southwest U. S. and on Hawaii from
1986 to 1995.
In 1994, the NRAO chose to study a high (5000 m) plateau
southwest of Cerro Chajnantor, Chile, and installed atmospheric
monitoring equipment in 1995.
These studies led to the recommendation
(NRAO 1998)
of the Llano de Chajnantor as the site for the MMA.
For the Large Southern Array (LSA) project, the European partners
decided to study a lower (3700 m) site near the Salar de Pajonales.
In 1997, the European Southern Observatory (ESO) and the Onsala Space
Observatory (OSO) installed monitoring equipment in this area.
When the LSA and MMA projects were merged to form the ALMA project,
Chajnantor was selected as the site.
In 1998 June, the
European equipment was moved to Chajnantor and installed adjacent
to the NRAO equipment.
The ALMA site charaterization program is carried out by the NRAO
on the US side and, on the European side, by ESO with contributions
from the OSO and the Mullard Radio Astronomy Observatory (MRAO).
ESO provides field support with assistance from the other partners.
14.3 Atmospheric and site studies
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
-
Physical structure of the atmosphere
-
meteorology
-
stratification
-
turbulence
-
Physical characteristics of the site
14.4 Geography, Climatology, and Geology
Located in the Chilean Andes about 40 km ESE of the
village of
San
Pedro de Atacama,
the ALMA site on the Llano de Chajnantor is a
high (5000 m) plateau of about 25 km2.
It is open the west and north-west, but bounded
by several 5600 m peaks in other directions.
The geographic coordinates are 23° 01' S and 67° 45' W
(Radford
1999
&
2000).
The entire area, which includes the Chajnantor
plateau as well as Pampa la Bola,
has been declared a "scientific
preserve" by decree of the Chilean government.
The preserve has been assigned
as a 5 year concession to the official Chilean
science agency, CONICYT.
Climatological conditions in the high Andes of northern
Chile were studied by Schmidt (1997)
and are briefly described by
Otárola, et al. (1998).
Long term and seasonal climate variations have been investigated by
Bustos
et al. (2000) and
Bustos
(2001).
Assessments of seismic and volcanic
hazards at the site were made by
Barrientos
(1996) and
Gardeweg (1996).
In 2000, a geotechnical study of six locations in the science preserve
area was carried out by NRO, NRAO, and ESO to identify
soil characteristics
(Geo
Ambiental 2000).
14.4.1 Maps, digital elevation model, and coordinates
In 1996, 1999, and 2001, contour maps and a digital elevation model
of the science preserve and the adjacent land to
the west were prepared from aerial photographs
(Maps
of the Chajnantor Zone).
These maps, which together cover about 24 × 52 km at 10 m resolution,
are used for planning the array configurations and civil works
and for hydrodynamic studies of airflow over the site.
For compatibility with the published Chilean topographic
maps these maps use the Provisional South American 1956 datum.
These maps were used to identify sites for the compact configurations
(Butler
et al. 2000)
and were the basis for an antenna location mask for configuration designs
(Butler
2001).
Coordinates of installations, roads, and other landmarks
have been recorded with a GPS receiver
(Sakamoto
2001a).
14.5 Infrastructure
For site characaterization and monitoring,
the ALMA operations base on Chajnantor is a group
of four 6 m (20 foot) long ocean shipping containers.
These provide shelter for personnel
and physical support for the instruments.
The first container was installed by the NRAO in 1995 April.
In 1998 June, when the MMA and LSA projects were combined to form the ALMA
project, ESO installed a second container about 15 m north of
the NRAO equipment.
In 1998 October, a third container was installed 1 km west of the other
containers as a launch base for radiosondes.
Most recently, in 2000 December, ESO installed a fourth container
adjacent to the east end
of its earlier container to expand the solar electric power system and
provide additional space for equipment.
14.5.1 Safety program
While working at Chajnantor, all ALMA
staff are governed by the ALMA
safety rules.
These rules include provisions for physical examinations prior to
work at high altitude and for emergency training for
ALMA staff who frequently work at Chajnantor.
The containers contain emergency supplies (first aid kits,
breathing oxygen, food, water, sleeping bags, etc.) and
are equipped with both cellular and satellite phones
that provide permanent communications.
14.5.2 Transportation
Four wheel drive vehicles are required to access to Chajnantor, especially
during inclement weather.
14.5.3 Solar electric power systems
All four containers have arrays of solar panels and battery banks to
supply electrical power.
The system on the NRAO container supplies 24 VDC and 110 VAC at 60 Hz.
Its continuous capacity is about 500 W,
with sufficient reserve capacity to weather
a storm of a few days.
With current instrumentation, this system operates near capacity.
The systems in the two ESO containers
supply 24 VDC and 220 VAC at 50 Hz.
Combined, they have a somewhat larger capacity
than the NRAO system.
The radiosonde container has a small system
(12 VDC and 110 VAC 60 Hz)
adequate to run the radiosonde tracking equipment.
System maintenance includes periodic checks and
refills of battery water.
Although system performance is slowly degrading with age, etc.,
these systems should remain adequate until the
permanent ALMA infrastructure is installed.
14.5.4 Computers and network
All the ALMA site characterization instruments are controlled by
PCs interlinked with an ethernet extending between the containers.
The PC clocks are synchronized to a GPS receiver that provides an
absolute time reference good to about 1 s.
The GPS receiver was also used to determine the position of the
NRAO container (Radford
1999
&
2000).
14.3.3 Communications
Voice and low-speed (<= 4800 baud) data are transmitted over cellular telephones.
In 2001 April, an Inmarsat M4 satellite terminal was deployed
to provide voice and high-speed (64 kbaud ISDN) data communications.
This provides a connection to the NRAO intranet and the internet
for authorized users.
Handheld radios are occasionally used for
communications around the site.
14.5.5 Gases and cryogenic liquids
Liquid helium, liquid nitrogen, and industrial gases can be delivered to
the site by AGA, which has a depot in Calama, about 160 km by road
north east of Chajnantor.
Liquid helium and liquid nitrogen were required to operate the SAO FTS.
14.6 Instrumentation
14.6.1 Atmospheric Transparency and Water Vapor Content
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.
Two radiometers measure the sky brightness temperature in several frequency bands
close to the 183 GHz water vapor line
(Delgado
et al. 1999).
These instruments
were built in a collaborative effort between OSO and MRAO and
were installed at Chajnantor in 1998 and 1999.
One radiometer is deployed at each end of the 300 m baseline of the
11.2 GHz site test interferometers.
The radiometers
are three channel DSB heterodyne receivers with room temperature
mixers.
The shape and intensity of the 183 GHz line are
determined from the calibrated antenna temperatures
for the three channels.
The measured line parameters
are iteratively compared with the predictions
of an atmospheric model to determine the
atmospheric water vapor density, which then is integrated to
obtain the precipitable water vapor (PWV)
and the associated electrical pathlength.
Since the atmosphere is non-dispersive at millimeter frequencies,
electrical pathlength variations translate directly to
phase variations at any given frequency.
Data are recorded every 2 seconds and retrieved about once a month.
After analysis, the PWV time series is posted on
the ESO ALMA
site page
together with the raw data. The data
are also used to study the dynamics of the turbulent layer and,
in conjunction with the 11.2 GHz interferometer data,
atmospheric phase correction schemes.
The NRAO 225 GHz tipping radiometer
(Liu 1987
&
McKinnnon
1987)
is the benchmark instrument for site characterization.
It measures the atmospheric transparency every 10 minutes and the stability
of atmospheric emission every fifth hour.
The data are retrieved weekly over the ISDN link and
daily
and monthly
data summaries are posted on the NRAO web page along with the reduced
data
in machine readable form. Operation is automatic, so current
activity includes maintenance,
including sporadic repair as required, data retrieval, and data analysis.
A tipping photometer was developed by NRAO 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).
An incomplete fourth unit was supplied to the University of New South Wales
in 1999 July for modification prior to remote Antarctic deployment.
In 2000 June, a fifth instrument equipped with a filter wheel and 200,
260, 350, and 1300 µm filters was deployed at Chajnantor.
To investigate the dependence of transparency with altitude, one
unit will be redeployed in late 2001 to 5700 m on Cerro Sairecabur,
about 40 km north of Chajnantor.
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.
To measure the atmospheric emission spectrum at Chajnantor, the
Smithsonian
Astrophysical Observatory
Submillimeter Receiver Lab
deployed a Fourier transform (polarizing Martin-Pupplet)
spectrometer. This cryogenic instrument covers 350 - 3000 GHz with 3 GHz
resolution and a 3° beam
(Paine
et al. 2000). The NRAO container provided a base for
field operations where the FTS recorded data from 1997
October to 2000 August. Subsequently, the instrument was redeployed
to 5700 m on Cerro Sairecabur.
14.6.2 Atmospheric stability
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 11.2 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
Radford, Reiland, & Shillue (1996).
Because the atmosphere is largely non-dispersive
away from line centers, the results can be scaled to millimeter and submillimeter
wavelengths.
Four 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.
A fourth instrument, with a 100 m baseline, was installed at Green Bank in 2000 March.
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.
Holdaway
et al. (1995)
describe the site test interferometer data reduction in detail,
and Holdaway
(1995)
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 ESO interferometer was set up alongside the NRAO 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
(Holdaway
& Radford 1998 &
Robson
et al. 2001).
The interferometers operate autonomously. The data are
retrieved about once a month and analyzed
in Tucson. Monthly
summaries
are posted on the NRAO web page.
Current activity includes operation and maintenance, including
sporadic repair as required, data retrieval, and data analysis.
14.6.2.2 GPS
A geodetic GPS receiver was installed in 1999 by the OSO
to measure the variations in the total radio propagation path delay
as well as the stability of the site coordinates
(Gradinarsky
et al. 2001).
14.6.3 Physical structure of atmosphere
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 learn about the stratification of the
water vapor over Chajnantor and about shear layers that may generate turbulence.
Two surplus radiotheodolites was acquired by NRAO, upgraded by the manufacturer, tested
in Tucson, and deployed at Chajnantor.
In 2001, the radiotheodolites were again upgraded to accomodate switching to
a different type of radiosonde package.
Beginning in 1998 October, balloon
flights have been made whenever appropriate personnel are at the site.
This campaign is a collaboration between NRAO, Cornell, ESO, SAO, and NAOJ. The
balloons are launched from a container placed 1 km west of the main
site.
14.6.3.2 Sodar
Acoustic sounding, or sodar, senses thermal turbulence in the lower atmosphere.
Engineering tests of an ESO sodar unit were made in 1999 November.
We are evaluating our interest in pursuing further measurements.
14.6.3.3 Hydrodynamic models
Calculations of airflow over Chajnantor, with an emphasis on the turbulence generated
by local topography, are being pursued in collaboration with NOAO.
14.6.4 Meteorology
14.6.4.1 Weather stations
ESO and NRAO operate four weather stations near the main containers
and one station near the radiosonde launch container. Additional
stations may be deployed to measure the variation of meterological
parameters across the site.
14.6.4.2 Fast anemometers
To study the wind power spectrum,
a fast vortex anemometer was deployed in 1999.
This reads the wind speed and wind direction
at a rate of 1 Hz.
The results, however, were unexpected, as the power
in fast fluctuations seemed too small.
In early 2001, therefore, a cup anemometer
reading at 10 Hz was deployed.
14.6.4.3 Hygrometer
A high precision, chilled mirror hygrometer will be deployed in
late 2001. This includes a precision barometer and thermometer.
14.6.5 Auxiliary instruments
Two surveillance cameras,
installed on top of the containers, take pictures
of the horizon every two hours. Data are retrieved about
once a month and the
images
posted on the web sites.
14.6.5.2 Seismometer
A seismometer was installed in 1995in collaboration with a group at the University
of Chile (K. Bataille).
The firmware was updated in 2000 July to accomodate GPS date rollover.
14.6.5.3 Subsurface termperature probe
A subsurface temperature probe was operated 1997 June - October and 1998
March - May.
Snyder
et al. (2000) analyzed the data.
14.6.5.4 Lightning detector
In late 1999, a lightning detector was installed at the NRAO container
to quantify the frequency of lightning strike in the area
(Sakamoto et al. in preparation).
14.6.6 Instruments installed by other groups
Several other groups are carrying out site characterization studies or astronomical
experiments nearby. The ALMA project encourages these groups and takes interest in
their results. As needed, ALMA and the other groups coordinate activities.
At Pampa la Bola, about 8 km northeast of the ALMA equipment and 250 m lower,
the NAOJ/NRO has installed:
14.6.6.2 Cornell and partners
Cornell, together with the University of Texas and
the University of Virginia, is making optical seeing (DIMM) measurements in the area.
Campaigns started in 1998 and have been done about every three to six months
(Giovanelli et al.
2001a
&
2001b).
14.6.6.3 CTIO
In collaboration with Cornell, CTIO deployed a weather station and a robotic seeing
monitor during 2000 at Cordon Honar, about 400 m above the Chajnantor plateau.
Since late 1999, a Caltech group has been observing
fluctuations in the Cosmic Background Radiation
(Padin
et al. 2001)
14.6.6.5 MAT
In 1997 and 1998, a group from Princeton and the University of Pennsylvania bserved
fluctuations in the Cosmic Background Radiation from Cerro Toco,
about 5 km north of the ALMA site
(Torbet
et al. 1999,
Miller
et al. 1999).
In 2000, the
Smithsonian
Astrophysical Observatory
Submillimeter Receiver Lab
redeployed their
Fourier
transform spectrometer
to 5700 m on Cerro Sairecabur, about 40 km north of
Chajnantor.
14.6.6.7 Princeton
In 2001, a Princeton group will deploy MINT,
an interferometer to study the Cosmic Background Radiation, on Cerro Toco.
14.7 Results
A number of ALMA Memos and other reports discuss the results of the site
characterization in detail. Some highlights:
14.7.1 Atmospheric Transparency
The 225 GHz atmospheric transparency at Chajnantor has been
measured continuously since 1995 April.
Overall, the median zenith optical depth is 0.06.
The diurnal variation is less than a factor of two,
with higher optical depths in the afternoons.
The lowest transparency occurs from January through March,
when the median optical depth is about twice as high as
it is during the rest of the year.
Year to year, there is considerable variation in the
the severity of the summer season.
Distribution of 225 GHz optical depths.
Diurnal variation of 225 GHz transparency.
Seasonal variation of 225 GHz transparency.
The atmospheric transparency is largely determined by the water vapor content.
The precipitable water vapor (PWV) is measured by the 183 GHz radiometers.
For all of 2000, the median PWV was about 1.8 mm,
but during the winter (May to August) the median was 0.6 mm.
Under normal conditions, the PWV is 2-4 times higher during the
day than at night.
14.7.2 Atmospheric Stability
At Chajnantor, the measured phase stability shows a diurnal variation.
At local noon, the median phase variations are about seven times larger
than at midnight.
This is also when the surface wind speed is highest.
Day time observations will be limited, then, unless phase
correction methods can be employed.
Diurnal variation of phase fluctuations.
Seasonal variations in the phase stability are much
less pronounced than diurnal variations.
The best conditions occur during winter, when the diurnal variations
in phase stability are also less pronounced than in the summer.
There appears to be a seasonal anticorrelation between wind speed
phase stability.
Even though the wind speed is higher in the winter than
in the summer, the phase stability is better in the summer.
It should be possible, therefore, to do high-resolution observations
throughout the day for a large fraction of the winter.
Seasonal variation of phase fluctuations.
14.7.3 Meteorological Conditions
The measured air temperature at Chajnantor shows the expected diurnal and
seasonal variations about an overall median somewhat below freezing, -2.5 °C.
Variation of hourly and monthly median windspeed.
The diurnal variation of the wind speed is quite pronounced in
all seasons with stronger winds in the afternoons. Winters
are windier that summers. Westerly winds prevail almost exclusively
during the winter, while easterly winds prevail 30-50% of the time during summer.
Except during the summer, the relative humidity
remains fairly constant throughout the year, with an average of about 30%.
In summer, altiplanic storms bring
humidity from the Amazon basin into the area.
The daily and yearly maxima, averages, and ranges of the insolation are among
the highest recorded in the world. The average daily maximum reaches almost
1300 W m-2 during summer.
14.7.4 Physical Structure of the Atmosphere
Each radiosonde flight provides a profile of pressure, temperature,
relative humidity, range (horizontal distance from launch site), and wind
speed and direction as the balloon rises through the troposphere.
From these data, inferred parameters including the partial pressure,
density, and total precipitable column of water vapor
are computed after each flight.
Since 1998 there have been about 185
radiosonde
launches from Chajnantor.
Some show the lower troposphere can be quite complex
with the presence of many inversion layers while others
there are no inversion layers present.
The time scale for a significant
change can be of the order of a few hours.
14.7.5 Climate Trends
A 52 year climatological study was conducted in order to investigate
possible long-period cycles. The data used were prepared by the
National Center for Environmental Prediction (NCEP) and the
National Center for Atmospheric Research (NCAR) from historical
weather observations and state of the art climatological models.
The results
(Bustos
et al. 2000)
conclude there are no climatological
trends in Chajnantor, but a few cycles with periods of the order
of several years are seen.
Seasonal cycles are clear and very
strong, with a correlation between the ENSO phenomena and the winter dryness
(Bustos
2001).
14.7.6 Phase Correction Experiment
Atmospheric phase fluctuations during millimetre and submillimetre
interferometric
observations lead to a loss in resolution and a decrease in the signal
strength. A proposed method to correct the phase fluctuations is
to estimate the amount of PWV in the observed column of atmosphere
from measurements close to the strong 183 GHz water vapor line,
and to use this value to determine the excess path delay.
By measuring
the variations of the PWV at each interferometer element,
the relative phase differences can be determined and
used to compensate for phase fluctuation.
At Chajnantor the differential phase derived from
the PWV measured with the 183 GHz radiometers along the same line
of sight as the 11.2 GHz interferometer has been used to correct
the phase variations measured by the interferometer.
The improvement in phase variations after this correction is
applied varies during the day, ranging from very
good, with better than 75% correction, to little or no correction
at all. The degree of success appears correlated
with the height of the turbulence layer.
(Delgado et al.
2000
&
2001
& Delgado 2001).
14.7.7 Height of the turbulence layer
The height of the turbulence layer is an important parameter
because it defines the geometry of the interasecting beams during
the phase correction. For the ALMA antennas, the
astronomical beam will be offset from the 183 GHz water
vapor monitor beam, so the separation of the beams at the
height of the turbulence layer will vary, affecting the
success of the phase correction.
The ESO and NRAO interferometers on Chajnantor share
essentially the same baseline, but observe different
satellites about 5° apart. Lag correlation
of the data from the two interferometers indicates the height
of the turbulent layer
(Holdaway
& Radford 1998).
The results show most of the time the
turbulence layer is less than 500 m above the plateau
&
Robson
et al. 2001).
14.7.8 Comparison between Chajnantor and Pampa La Bola
Within the science preserve, measurements of atmospheric
conditions have been carried out at two particular locations.
The NRAO and the European partners have studied the
Llano de Chajnantor while the Japanese
have investigateded Pampa la Bola, about 8 km northeast of Chajnantor
and 250 m lower.
Sakamoto (2001b)
gives an overall comparison of the two sites.
A comparison of meteorological data since 1996 from the two
adjacent sites of Pampa La Bola and Chajnantor
shows generally similar conditions, with some
differences in seasonal and diurnal variations
(Sakamoto
et al. 2000).
Measurements with the 220 and 225 GHz tipping radiometers indicate
the atmosphere is more transparent at Chajnantor than at Pampa la Bola.
The optical depth at Pampa la Bola is
typically 10% to 50% higher than at Chajnantor.
Side-by-side measurements with both instruments at Chajnantor
in 2001 April--May confirm this is not an instrumental artifact.
Especially for observations at submillimeter wavelengths,
these differences imply Chajnantor enjoys
a significant advantage in observing time or sensitivity
(Radford et al. 2001 in preparation).
The overall distribution of the phase fluctuations is
similar for the two sites, but the median stability is 12%
better at Chajnantor
(Butler
et al. 2001).
At any given time,
however, the phase fluctuations on a 300 m baseline may be quite
different at the two sites, sometimes by a large factor.
Presumably this is the effect of local topography on turbulence.
The wide range of phase stability seen at the two sites at the
same time implies the phase stability may vary significantly
across the array when observing with the larger (> 3 km) configurations
(Holdaway
et al. 1997).
14.7.9 225 GHz Atmospheric Transparency at
Chajnantor, Hawaii, and the South Pole
Measurements of the 225 GHz atmospheric transparency with
functionally identical tipping radiometers at Mauna Kea, the
South Pole, and Chajnantor indicate periods of excellent
observing conditions at all three sites. Conditions at Chajnantor
and the South Pole are better than at Mauna Kea. The first quartile
zenith transparency at Chajnantor and the South Pole are roughly
equal. During the best conditions at Chajnantor, however, the
zenith transparency is better than during the best conditions at
the South Pole
(Radford
& Chamberlin 2000).
14.7.10 Subsurface Temperatures
By monitoring the subsurface temperature at Chajnantor, the
thermal diffusivity of the soil and the damping of diurnal
temperature fluctuations with depth have been measured. The
thermal diffusivity is of roughly the range expected for
sandy soil, and varies daily. For the maximum observed
diffusivity the diurnal temperature swing 1 m deep is only
0.02% of the surface amplitude. Shorter period variations
are damped more strongly. This damping is sufficiently strong
that the overall phase stability of the ALMA optical fibers
may be determined not by the 25 km long buried sections, but
by the shorter lengths above ground
(Snyder
et al. 2000).
14.8 Instrumentation needed for ALMA operations
ALMA will be operated in a service mode and with
flexible scheduling. It will be necessary, therefore, to
constantly monitor the atmospheric conditions
to decide what frequency band should be observed.
It may also be useful to employ
a weather prediction scheme based on satellite data as is
presently done at Paranal.
In addition, current meterological data will be needed
for antenna safety, for the array control system, and
for various calibration schemes.
The instrument suite for a the monitoring station
is yet to be defined, but it might include some or all of these:
- centrally located, high precision meterology sensors for wind, temperature, pressure, and humidity;
- distributed, moderate precision meterology sensors;
- millimeter and submillimeter wavelength tippers;
- an 11 GHz interferometer;
- radiosonde launches;
- a 183 GHz radiometer to monitor the isoplanatic angle;
- a Fourier transform spectrometer, perhaps multibeam;
- a 60 GHz temperature profiler;
- a radar wind profiler;
- a 10 µm cloud monitor; and
- a 20 µm water vapor monitor.
14.9 Site Characterization Reviews
Periodic reviews are held to assess the site
characterization activities and the
data obtained by all groups.
A mailing list (alma-site@nrao.edu) has been set up for
communication among the
ALMA
site characterization group.
Previous meetings include:
14.10 References
14.10.1 Web pages
Several web pages present data and results from
the site characterization program.
14.10.2 Reports
Safety rules for ALMA Personnel on the ALMA 5000 m Site:
NRAO version: http://www.aoc.nrao.edu/~pnapier/mmasafety.htm, 2000 May
ESO version: Volume 4, Appendix C of the Proposal for Phase 2 construction of the ALMA, 2000 December
NRAO 1998 May,
Recommended
Site for the Millimeter Array [also
ps]
Barrientos, S. E., 1996
Seismicity and Seismic Hazard at MMA site, Antofagasta, Chile,
ALMA Memo 250
Bustos, R., 2001,
Summer
Climate over Chajnantor,
ALMA Memo 379
Bustos, R., Delgado, G., Nyman, L.-Å., & Radford, S., 2000,
52
Years of Climatological Data for the Chajnantor Area,
ALMA Memo 333
Butler, B. J., 2001,
An
Antenna Location Mask for Configuration Designs for ALMA,
ALMA Memo 364
Butler, B. J., Radford, S. J. E., & Otárola, A., 2000,
The
Best Sites for the Compact ALMA Configuration,
ALMA Memo 338
Butler, B. J., Radford, S. J. E., Sakamoto, S., & Kohno, K., 2001,
Atmospheric
Phase Stability at Chajnantor and Pampa la Bola,
ALMA Memo 365
Delgado, G., 2001,
Phase Correction of 11.2 GHz Interferometric Measurements Using a Pair of
183 GHz Water Line Radiometers at Chajnantor,
in Astronomical Site Evaluation in the Optical and Radio Range
Astronomical Site Evaluation in the Visible and Radio Range, ASP Conf. Ser.,
ed. Vernin, J., Muñoz-Tuñón, C., &
Benkhaldoun, Z., (San Francisco: ASP) in press
Delgado, G., Nyman, L.-Å., Otárola, A.,
Hills, R., & Robson, Y.,
2001,
Phase
Cross-Correction of a 11.2 GHz Interferometer
and 183 GHz Water Line Radiometers at Chajnantor,
ALMA Memo 361
Delgado, G., Otárola, A., Belitsky, V., & Urbain, D., 1999,
The
Determination of Precipitable Water Vapour at Llano de Chajnantor from
Observations of the 183 GHz Water Line,
ALMA Memo 271
Delgado, G., Otárola, A., Nyman, L.-Å.,
Booth, R., Belitsky, V., & Urbain, D., Hills, R., Robson, Y.,
& Martin-Cocher, P., 2000,
Phase
Correction of Interferometer Data at Mauna Kea and Chajnantor,
ALMA Memo 332
Gardeweg P., M. C., 1996
MMA Site East of San Pedro De Atacama, North Chile;
Volcanic Hazards Assessment and Geologic Setting
ALMA Memo 251
Giovanelli R., Darling, J., Sarazin, M., Yu, J., Harvey, P.,
Henderson, C., Hoffman, W., Keller, L.,
Barry, D., Cordes, J., Eikenberry, S., Gull, G.,
Harrington, J., Smith, J. D., Stacey, G., &
Swain, M., 2001a,
The
Optical/Infrared Astronomical Quality of High Atacama Sites. I.
Preliminary Results of Optical Seeing
PASP 113, 789
Giovanelli R., Darling, J., Henderson, C., Hoffman, W.,
Barry, D., Cordes, J., Eikenberry, S., Gull, G., Keller, L., Smith, J. D., &
Stacey, G., 2001b,
The
Optical/Infrared Astronomical Quality of High Atacama Sites. II.
Infrared Characteristics,
PASP 113, 803
Geo Ambiental Consultores, Ltda., 2000,
Geotechnical
Study, Chajnantor Site, II Region, Chile,
LMSA Memo 2000-004
Gradinarsky, L. P., Johansson, J. M., Elgered, G., & Jarlemark, P., 2001,
(GPS
site testing at Chajnantor in Chile,
Physics and Chemistry of the Earth, 26, 421
Hirota, T., Yamamoto, S., Sekimoto, Y., Kohno, K., Nakai, N., & Kawabe, R., 1998,
Measurements
of the 492 GHz Atmospheric Opacity at Pampa la Bola and Rio Frio in Northern Chile,
PASJ 50, 155
Holdaway, M. A., 1995, Velocity
of Winds Aloft from Site Test Interferometer Data, ALMA Memo 130
Holdaway, M. A., Gordon, M. A., Foster, S. M., Schwab, F. R.,
& Bustos, H., 1996,
Digital
Elevation Models for the Chajnantor Site, ALMA Memo 160
Holdaway, M. A., & Radford, S. J. E., 1998,
Options
for Placement of a Second Site Test Interferometer on Chajnantor, ALMA
Memo 196
Holdaway, M. A., Radford, S. J. E., Owen, F. N., & Foster, S. M.,
1995, Data
Processing for Site Test Interferometers, ALMA Memo 129
Holdaway, M. A., Matsushita, S., & Saito, M., 1997,
Preliminary Phase Stability Comparison of the Chajnantor and Pampa la
Bola sites, ALMA Memo 176
Ishiguro, M., Kanzawa, T., and Kasuga, T., 1990,
Monitoring
of Atmospheric Phase Fluctuations Using Geostationary Satellite Signals, in
Radio Astronomical Seeing, ed. Baldwin, J. E., and Wang S. (Pergamon), p. 60
Kohno, K., Kawabe, R., Ishiguro, M., Kato, T., Otárola, A.,
Booth, R., & Bronfman, L. 1994,
(
Preliminary result of site testing
in northern Chile with a portable 220 GHz radiometer,
NRO Tech. Rept. 42
Liu, Z.-Y., 1987,
225
GHz Atmospheric Receiver - User's Manual,
ALMA Memo 41
Matsuo, H., Sakamoto, A., & Matsushita, S., 1998,
FTS
Measurements of Submillimeter-Wave Atmospheric Opacity at Pampa la Bola
PASJ 50, 359
Matsushita, S., Matsuo, H., Pardo, J. R., & Radford, S. J. E., 1999,
FTS
Measurements of Submillimeter-Wave Atmospheric Opacity at Pampa la Bola II :
Supra-Terahertz Windows and Model Fitting,
PASJ 51, 603
Matsushita, S., Matsuo, H., Sakamoto, A., & Pardo, J. R., 2000,
FTS
measurements of submillimeter opacity and other site testing at Pampa la Bola
Proc. SPIE 4015, 378
McKinnnon, M., 1987,
Measurement
of Atmospheric Opacity Due to Water Vapor at 225 GHz,
ALMA Memo 40
Miller, A. D., Caldwell, R., Devlin, M. J., Dorwart, W. B.,
Herbig, T., Nolta, M. R., Page, L. A., Puchalla, J.,
Torbet, E., & Tran, H. T., 1999,
A
Measurement of the Angular Power Spectrum of the Cosmic Microwave
Background from l = 100 to 400,
ApJ 524, L1
Otárola, A., Delgado, G., Booth, R., Belitsky, V., Urbain, D.,
Radford, S., Hofstadt, D., Nyman, L., Shaver, P., & Hills, R., 1998,
European
site testing at Chajnantor,
ESO Messenger 94, 13
Padin, S., Cartwright, J. K., Mason, B. S., Pearson, T. J.,
Readhead, A. C. S., Shepherd, M. C., Sievers, J.,
Udomprasert, P. S., Holzapfel, W. L., Myers, S. T.,
Carlstrom, J. E., Leitch, E. M., Joy, M., Bronfman, L., & May, J.,
2001,
First
Intrinsic Anisotropy Observations with the Cosmic Background Imager,
ApJ 549, L1-L5
Paine, S., Blundell, R., Papa, D. C., Barrett, J. W., & Radford, S. J. E., 2000,
A
Fourier Transform Spectrometer for Measurement of Atmospheric
Transmission at Submillimeter Wavelengths,
PASP 112, 108
Radford, S. J. E., 1999,
Position
of MMA Equipment on Chajnantor, ALMA Memo 261
Radford, S. J. E., 2000,
Refined
Position of ALMA Equipment on Chajnantor, ALMA Memo 312
Radford, S. J. E., Butler, B. J., Sakamoto, S., & Kohno, K., 2001,
Atmospheric Transparency at Chajnantor and Pampa la Bola,
ALMA Memo in preparation
Radford, S. J. E., & Chamberlin, R. A., 2000,
Atmospheric
Transparency at 225 GHz over Chajnantor, Mauna Kea, and the South Pole,
ALMA Memo 334 rev. 1
Radford, S. J. E., Reiland, G., & Shillue, B., 1996,
Site Test Interferometer, PASP 108, 441
Robson, Y., Hills, R., Richer, J., Delgado, G., Nyman, L.-Å.,
Otárola, A., & Radford, S., 2001,
Phase
Fluctuation at the ALMA Site and the Height of the Turbulent Layer,
ALMA Memo 345
Sakamoto, S., 2001a,
Coordinates
of Roads, Pipelines, and Landmarks Near the ALMA Site,
ALMA Memo 375
Sakamoto, S., 2001b,
Comparison
of the Pampa la Bola and Llano de Chajnantor Sites in Northern Chile,
in Astronomical Site Evaluation in the Optical and Radio Range
Astronomical Site Evaluation in the Visible and Radio Range, ASP Conf. Ser.,
ed. Vernin, J., Muñoz-Tuñón, C., &
Benkhaldoun, Z., (San Francisco: ASP) in press
Sakamoto, S., Handa, K., Kohno, K., Nakai, N., Otárola, A.,
Radford, S. J. E., Butler, B., & Bronfman, L., 2000,
Comparison
of Meteorological Data at the Pampa La Bola and Llano de Chajnantor Sites,
ALMA Memo 322
Sakamoto, S., & Radford, S. J. E. 2001,
Lightning near Cerro Chascón, ALMA Memo, in preparation
Schmidt, D., 1997, Ph. D. thesis, Friedrich-Alexander University, Erlangen-Nürnberg
Snyder, L. A., Radford, S. J. E., & Holdaway, M. A., 2000, Underground
Temperature Fluctuations and Water Drainage at Chajnantor, ALMA Memo 314
Torbet, E., Devlin, M. J., Dorwart, W. B., Herbig, T., Miller, A. D.,
Nolta, M. R., Page, L., Puchalla, J., & Tran, H. T., 1999,
A
Measurement of the Angular Power Spectrum of the Microwave
Background Made from the High Chilean Andes,
ApJ 521, L79
