Characterization of the Atmosphere
above a Site for Millimeter
Wavelength Astronomy
Francesco Nasir
Contents
Introduction to the Sardinia radio telescope
project
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
Site testing and monitoring

Instruments
Modelling atmospheric emission/absorption
CIAO L O O P
with ARTS
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frac
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Results

Conclusions
Introduction
Contents:
The Sardinia Radio Telescope (SRT) is a
project managed by Italian National Institute
for Astrophysics (INAF).

It is being built on a plateau (alt. 650 m) in
south-central Sardinia (ITALY) at 30 km from
the city of Cagliari.

It has a 64 m diameter, fully steerable (altaz.), active panels, multi-feed and it should
observe in the frequency range 0.3 – 100
GHz.

Introduction
Location of the Sardinian Radio Telescope (SRT), of
Cagliari’s astronomical observatory (OAC) and airport
Introduction
Offices, South Tower,
Meteo Sensors
Visitor center
North Tower
Introduction
http://www.srt.inaf.it
Introduction
Site Characterization
SRT Project – Organization Chart:
Board of Directors
L. Feretti (Chairman), N. D’Amico, F. Palla, G. Grueff,
G. Tofani, I. Porceddu,F. Mantovani, F. Fusi Pecci
Site and Infrastructure development
P.I. (I. Porceddu)
&
P.I. Working Group
WP1: Local LAN
WP2: Supercomputing
SRT Project Director
SRT Project Scientist
N. D’Amico
(I. Prandoni)
& P.S. Working group
Photographic
documentation
Logistics
Antenna
engineering
services
Radio telescope
P.I. (G. Tofani)
Co-PI. (R. Ambrosini)
& Antenna contracts surveillance
Commission
G.A.I. 1: Active surface
G.A.I. 2: Metrology
WP3: VLBI Terminal
Site Safety
G.A.I. 3: Optics
WP4: Meteo, GPS, and
Site monitoring
WP5: RFI monitoring
WP6: Time & Frequency
Activity coordinated or administrated by an OAC Staff or Associate
Activity coordinated or administrated by an IRA Staff
Activity coordinated or administrated by an IRA Guest at OAC
Activity coordinated or administrated by an OAA Staff or Associate
Site engineering
services
Outsourcing
IRA Administration
G.A.I. 4: Receivers
G.A.I. 5: Servo systems
G.A.I. 6: Software
G.A.I. 7: Antenna plants
OAC Administration
G.A.I. 8: Rx Cntrl
G.A.I. 9: Web page
Site Characterization
- SRT astronomical observations, atmospheric signal absorption
Atmospheric opacity [Np]
Water molecule
maser studies
at 22.23 GHz
for detecting
star forming
regions and
black hole mass
Radio Pulsars at
0.3-1.4 GHz,
supernova
remnants, HII
regions
Thermal and maser
emission of silicon
monoxide at 40 GHz for
detecting high density
gas in nebulas.
Carbon monoxide main
transitions at 110 GHz but
redshifted may be observed at
less than 100 GHz , a useful
tracer for the hydrogen
molecule, the most abundant
molecule in the universe.
Site Characterization
Accurate atmospheric opacity
measurements at different
frequencies , integrated water
vapour (IWV) and integrated cloud
liquid water (ILW) are necessary
to:


a) Characterize the site accurately,
especially at high frequencies
(3mm band): a statistical study of
which astronomical observations
should be performed preferably
during each month of the year.
methods
&
instruments


b) Allow for “Dynamic scheduling”
of the radiotelescope: real time
opacity, IWV and ILW estimates in
order to decide which frequency to
observe. Maybe also nowcasting.
GPS
Local meteorological
station
Microwave radiometer
Radiosondes and
Atmospheric Radiative
Transfer Models
(e.g., ARTS)
Site Characterization
GPS as atmospheric probe:
A geodetic GPS permanent
station is operating at SRT site
since winter 2006.
If antenna coordinates and
satellite ephemerides are
known with sufficient
precision, the path excess or
zenithal total delay (ZTD) is
accurately measured.
IWV is obtained from the ZTD
with 1 mm accuracy by means
of ground meteorological
parameters.
OUTPUT:
- ZWD, ZTD
- IWV
Site Characterization
Local weather:
GPS needs a good weather station.
The SRT weather station was
realized at home using a low cost
Linux micro-computer as datalogger.
Surface Temperature,
humidity and wind
Surface pressure gauge
RS232
micro-computer
USB pen drive
TCP/IP
OUTPUT:
- P, T, RH, Wind
Site Monitoring
Radiometro:
New microwave radiometer
(Radiometrics , MP-3000A) with
35 channels:
- K band (20-30 GHz) useful for
retriving water vapour IWV and
cloud liquid water ILW
- L band (50-60 GHz) useful for
retriving temperature profiles.
- Infrared thermometer for cloud
base detection
- Typical Tb resolution is 0.25 K.
OUTPUT:
- IWV e LWC
- Tb and Opacity at
different frequencies
Site Characterization
Radiosondes:
We used a 50 year radiosonde
dataset (1960 -2010) and a 3
year finer dataset with higher
vertical and time resolution
(1998 – 2001).
The launch site is the cagliari
airport at 30 km from the SRT
site. Launches are routinely
performed by Italian air-force.
Pressure, temperature and
relative humidity were
measured layer by layer up to
30 km height.
OUTPUT:
- T, P, RH profiles
- Post processed IWV
- Post processed ILW (empirical model)
- Post processed opacity (using ARTS)
Integrate profiles for IWV, use
empirical model and profiles
for ILW, use profiles and ARTS
to simulat emission and
absorption properties of the
atmosphere.
Site Characterization
General scheme:
Using radiosonde profiles, Arts (1.0.214) and an
empirical cloud liquid model for site characterization:
Site Characterization
- The radiosonde launch site is at Cagliari airport at sea level.
- The radiotelescope is at 30 km from the airport and at 650 m
of altitude.
- Are the radiosonde soundings representative for the
radiotelescope site atmosphere?
- Evapotraspiration and boundary
layer turbulance make it so that it is
not possible to simply clip off the
first 650 m from the radiosonde
soundings in order to represent the
atmosphere of the radiotelescope
site.
- By clipping off the first 200 of the
radiosonde soundings we found a
good comparison with GPS receiver
IWV at the radiotelescope site (2007
– 2009).
- This is the sensor height used also
in our arts simulations.
Results
Statistics for Water Vapour (IWV):
Results
Statistics for cloud Liquid (ILW):
Results
Statistics for 22 GHz opacity:
Results
Statistics for 100 GHz opacity:
Results
Clear sky versus cloudy sky opacity:
22.23 GHz
At 0.2 Np 10 %
mean relative error
100 GHZ
At 0.2 Np 50 %
mean relative error
Results
Statistics for all quantities and frequencies:
Probability of geophysical values below tresh hold values:
- IWV < 10 mm
- ILW = 0 mm (clear sky)
- Opacity (tau) < 0.15 Np
“Conclusions”
The statistical-climatic study is nearly complete for the Sardinia
Radio Telescope, this could help schedule astronomical observations
at different frequencies in the most appropriate time slots during the
year and thus optimize machine time.

Thank you so
much for Your
Techniques are being studied in order to forecast water vapour
kind and
attention
and other geophysical quantities
so plan ahead astronomical
A real-time monitoring system comprising of a surface
meteorological station, a radiometer and a GPS receiver is being
implemented and tested.
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
observations.