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

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 GHz, supernova remnants, HII regions Water molecule maser studies at GHz for 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

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 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 ( ) 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 ( ) 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