Characterization of the Atmosphere above a Site for Millimeter

Wavelength Astronomy

Francesco Nasir

frac

CIAO L O O P

Contents

Introduction to the Sardinia radio telescope project

Site testing and monitoring

Instruments

Modelling atmospheric emission/absorption with ARTS

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 (alt-az.), 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

North Tower

Offices, South Tower,Meteo Sensors Visitor center

Introduction

http://www.srt.inaf.it

Introduction

Introduction

Site Characterization

Board of Directors L. Feretti (Chairman), N. D’Amico, F. Palla, G. Grueff,

G. Tofani, I. Porceddu,F. Mantovani, F. Fusi Pecci

SRT Project Director

N. D’Amico

G.A.I. 1: Active surface

Radio telescope

P.I. (G. Tofani)Co-PI. (R. Ambrosini)

& Antenna contracts surveillance Commission

Site and Infrastructure development

P.I. (I. Porceddu)&

P.I. Working Group

SRT Project Scientist

(I. Prandoni)

& P.S. Working group

G.A.I. 2: Metrology

G.A.I. 3: Optics

G.A.I. 4: Receivers

G.A.I. 5: Servo systems

G.A.I. 6: Software

G.A.I. 7: Antenna plants

G.A.I. 8: Rx Cntrl

G.A.I. 9: Web page

WP1: Local LAN

WP2: Supercomputing

WP3: VLBI Terminal

WP4: Meteo, GPS, and Site monitoring

WP5: RFI monitoring

WP6: Time & Frequency

Antenna engineering services

Site Safety

Site engineering services

Photographic documentation

IRA Administration

Logistics

Outsourcing

OAC Administration

Activity coordinated or administrated by an IRA Staff

Activity coordinated or administrated by an OAC Staff or Associate

Activity coordinated or administrated by an IRA Guest at OAC

Activity coordinated or administrated by an OAA Staff or Associate

SRT Project – Organization Chart:

Radio Pulsars at 0.3-1.4 GHz, supernova

remnants, HII regions

Water molecule maser studies at 22.23 GHzfor detecting star forming regions and

black hole mass

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.

Thermal and maser emission of silicon

monoxide at 40 GHz for detecting high density

gas in nebulas.

Atmospheric opacity [Np]

- SRT astronomical observations, atmospheric signal absorption

Site Characterization

Site Characterization

GPS

Local meteorological station

Microwave radiometer

Radiosondes and Atmospheric Radiative Transfer Models (e.g., ARTS)

methods &

instruments

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.

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 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 data-logger.

Surface pressure gauge

micro-computer

TCP/IP

USB pen drive

Surface Temperature, humidity and wind

RS232

OUTPUT:- P, T, RH, Wind

Site Characterization

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.

Radiometro:

OUTPUT:- IWV e LWC- Tb and Opacity at different frequencies

Site Monitoring

Site Characterization

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.

Integrate profiles for IWV, use empirical model and profiles for ILW, use profiles and ARTS to simulat emission and absorption properties of the atmosphere.

OUTPUT:- T, P, RH profiles- Post processed IWV- Post processed ILW (empirical model) - Post processed opacity (using ARTS)

Radiosondes:

Using radiosonde profiles, Arts (1.0.214) and an empirical cloud liquid model for site characterization:

Site Characterization

General scheme:

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.

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:100 GHZ22.23 GHz

At 0.2 Np 50 % mean relative error

At 0.2 Np 10 % 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

Results

“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. A real-time monitoring system comprising of a surface meteorological station, a radiometer and a GPS receiver is being implemented and tested.

Techniques are being studied in order to forecast water vapour and other geophysical quantities and so plan ahead astronomical observations.

Thank you so

much for Your kind attention