Background and Status of the Water-Vapour Radiometer at Effelsberg A. Roy U. Teuber R. Keller

The Troposphere as Seen from Orbit Method: Synthetic Aperture Radar (Earth Resources Satellite) Frequency: 9 GHz Region: Groningen Interferograms by differencing images from different days 5 km Internal waves in a homo- genously cloudy troposphere A frontal zoneConvective cells 0 mm -100 mm 100 mm Hanssen (1997)

Troposphere Seen by VLBI at 86 GHz

Water-Vapour Radiometry Basics

WVR Performance Requirements Opacity Measurement Aim: measure tropospheric opacity with enough accuracy to correct visibility amplitude to 1 % (1  ) Info: median tropospheric transmission at 22 GHz at Effelsberg in the period 03Jul to 04Mar was WVR spec:absolute calibration accuracy  14 % (1  ) thermal noise per measurement  2.7 K. Tropospheric Phase Correction Aim:coherence = 0.9 after correction requires  / 20 path length noise after correction, ie 0.18 mm for = 3.4 mm requires root-two better at each end of baseline (0.12 mm => 28 mK) Info:tropo path / tropo brightness = 4.5 mm/K at 22.2 GHz typical water line strength = 35 K WVR spec: thermal noise  14 mK in 3 s gain stability: 3.9 x in 300 s

The Scanning GHz WVR for Effelsberg = 18 to 26 GHz  = 900 MHz Nchannel = 25 Treceiver = 200 K  = s per channel  = 61 mK per channel sweep period = 5 s

The Scanning GHz WVR for Effelsberg Front-end opened Control unit First light, April 2002, Bonn

WVR View of Atmospheric Turbulence Absorber Zenith sky (clear blue, dry, cold) 12 h1 h ● gain stability: 2.7x10 -4 over 400 s ● sensitivity: 61 mK for τ int = s (0.038 mm rms path length noise for τ int = 3 s)

WVR Beamwidth: Drift-Scan on Sun 26.0 GHz beamwidth = 1.26 ◦ 18.0 GHz beamwidth = 1.18 ◦

WVR Beam Overlap Optimization WVR – 100 m RT Beam Overlap for three WV profiles Atmospheric WV Profiles at Essen from Radiosonde launches every 12 h (Data courtesy Dr. S. Crewell, Uni Bonn)

Gain Calibration Measure: hot load sky dip at two elevations noise diode on/off Derive: Tsky Treceiver gain

Spillover Cal: Skydip with Absorber on Dish 19 to 26 GHz el = 90 ◦ to 0 ◦ detector output 0 V to 0.3 V

WVR Control Panel

WVR Panorama of Bonn

Move to Effelsberg March 20th, 2003

WVR Panorama of Effelsberg

Scattered Cumulus, 2003 Jul 28, 1300 UT

Storm, 2003 Jul 24, 1500 UT

Validation of Opacity Measurement

Opacity Statistics at Effelsberg

First Attempt to Validate Phase Correction

WVR Noise Budget for Phase Correction Thermal noise: 75 mK in the water line strength, April mK per channel on absorber, scaled to 25 channels difference on-line and off-line channels (34 mK in Feb 2004 due to EDAS hardware & software upgrade) Gain changes: 65 mK in 300 s 2.7x10 -4 multiplies T sys of 255 K Elevation noise:230 mK typical elevation pointing jitter is 0.1 ◦ sky brightness gradient = 2.8 K/ ◦ at el = 30 ◦ Beam mismatch:145 mK measured by chopping with WVR between two sky positions with 4 ◦ throw, Aug ◦ = 120 m at 1.5 km and el = 60 ◦ 66 mK to 145 mK Sramek (1990), VLA structure functions 95 mK Sault (2001), ATCA 2001apr UT Other? Spillover model errors, cloud liquid water removal, RFI, wet dish, wet horn Total (quadrature):290 mK = 1.3 mm rms

Move to Focus Cabin March 16th, 2004

WVR Beam Geometry Beam overlap, April 2003Beam overlap, April 2004

Optical Alignment using Moon T antenna = 23 K T moon = 220 K at 22 GHz Beam filling factor = Beam efficiency = 92 % 23 K

Spillover Reduction 19 to 26 GHz el = 90 ◦ to 0 ◦ detector output 0 V to 0.3 V

WVR Path Data from 3 mm VLBI, April 2004

VLBI Path Comparison, 3 mm VLBI, April 2004

VLBI Phase Correction Demo Demonstration by Tahmoush & Rogers (2000)3C 273 Hat Creek – Kitt Peak 86 GHz VLBI 400 s 4 mm path ● RMS phase noise reduced from 0.88 mm to 0.34 mm after correction. ● Coherent SNR rose by 68 %. VLBI phase WVR phase

Australia Telescope Phase Correction Demo

● Validate phase correction (3 mm VLBI from 2004 April 16-20) ● Validate zenith total delay using geodetic VLBI (July? campaign) ● Software development: (Rottmann, FP6 RadioNet, started May 3) data paths into CLASS and AIPS data archive online (web-based) real-time display ● Investigate limitations on calibration accuracy ● Hardware development: (once usefulness established) temperature stabilization:further improvements spillover: reduce with new feed? integration time efficiency:Data acquisition system upgrade beam overlap: move to prime focus receiver boxes Future Developments

● WVR installation complete; WVR now running continuously ● Data available from Alan Roy ● Opacities agree with those from 100 m RT ● Validation of phase-correction data within weeks ● Web-based display & archive access coming soon ● Radiometer stability is 2.7 x in 400 s ● Radiometer sensitivity is 61 mK in s integration time Conclusions