The WVR at Effelsberg Alan RoyReinhard Keller Ute TeuberDave Graham Helge RottmannWalter Alef Thomas Krichbaum

The Scanning GHz WVR for Effelsberg = 18.5 GHz to 26.0 GHz  = 900 MHz Channels = 24 T receiver = 200 K sweep period = 6 s rms = 61 mK per channel Features  Uncooled (reduce cost)  Scanning (fewer parts, better stability)  Robust implementation (weather-proof, temperature stabilized)  Noise injection for gain stabilization  Beam matched to Effelsberg near-field beam  TCP/IP communication  Web-based data access  Improved version of prototype by Alan Rogers

The Scanning GHz WVR for Effelsberg

Front-end opened Ethernet data acquisition systemTemperature regulation modules Control unit March 16th, 2004

WVR Performance Requirements Phase Correction Aim:coherence = 0.9 requires  / 20 (0.18 mm rms at = 3.4 mm) after correction Need: thermal noise  14 mK in 3 s Measured: 12 mK = 0.05 mm Need:gain stability 3.9 x in 300 s Measured: 2.7 x Opacity Measurement Aim: correct visibility amplitude to 1 % (1  ) Need:thermal noise  2.7 KMeasured: 12 mK Need:absolute calibration  14 % (1  )Measured: 5 %

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)

Typical Water Line Spectrum

WVR Panorama of Bonn

Move to Effelsberg March 20th, 2003

WVR Panorama of Effelsberg

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

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

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 (courtesy Dr. S. Crewell, Uni Cologne)

Scattered Cumulus, 2003 Jul 28, 1300 UT

Storm, 2003 Jul 24, 1500 UT

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 Time / UT hours Path length / mm ° 45° 0° Elevation path length elevation

VLBI Path Comparison, 3 mm VLBI, April 2004

VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April s 3.4 mm path ● Path rms reduced 1.0 mm to 0.34 mm ● Coherent SNR rose 2.1 x WVR phase VLBI phase No phase correction EB phase correction Coherence function before & after EB+PV phase correction

VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April s 3.4 mm path ● Path rms reduced 0.85 mm to 0.57 mm ● Coherent SNR rose 1.7 x WVR phase VLBI phase No phase correction EB phase correction Coherence function before & after

VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April s 3.4 mm path ● Path rms saturated at 0.95 mm ● Coherent SNR decrease 7.5 x WVR phase VLBI phase Before phase correction at EB After phase correction at EB Coherence function before & after

VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 ● Coherence improves for most scans Coherence function after phase correction at EB divided by CF before phase correction Coherent integration time Improvement factor 360 s240 s120 s0 s

Cloud Removal NRAO GHz VLBI 2004 April 17 ● Cloud contamination shows up as large scatter in the path lengths EB WVR path time seriesKeep VLBI scan times onlySubtract linear rate

VLBI Phase Correction Demo

Validation of Opacity Measurement

Path Length & Opacity Statistics at Effelsberg

Path Length Stability at Effelsberg RMS path fluctuation over 120 s vs hour of day - July RMS path fluctuation over 120 s vs hour of day - December 0 mm 2 mm 1 mm 0 h24 h 0 h UT sunrise sunset

Absolute Calibration for Astrometry & Geodesy 120 km

Opacity Effects and the Mapping Function

Issues: TCP/IP time overhead

Issues: Temperature stability 20 mK Physical temperature near LNA vs time T sys vs time 250 mK 3 min

Issues: Temperature stability Solution: weaken thermal coupling between Peltier and RF plate Effects: No more 3 min temperature oscillation Worse long-term temperature stability  Strong thermal coupling Weak thermal coupling Temperature vs time 5.5 C 0.7 C 2.5 days 0.75 days

Issues: Noise Diode Stability Calibrated with noise diode Calibrated with temperature Original data Structure function of Tsys on absorber Tsys rms / K 0.1 K 1 K Time / s Tsys vs time on absorberCalibrate using temp.Calibrate using noise diode 2.0 K 22 h

Issues: Beam Mismatch at Low Elevation?

● Software development: (Helge Rottmann, RadioNet) data paths into JIVE correlator, AIPS and CLASS improve calibration accuracy (allow for opacity effects) ● Hardware development: temperature stabilization:better insulation, regulation reduce Tsys?Cooling? spillover: reduce with new feed? integration time efficiency:Data acquisition system upgrade beam overlap: move to prime focus receiver boxes? Future Developments

● WVR running continuously ● Phase correction of 3 mm VLBI has been demonstrated (but in four experiments WVR made things worse.) ● Opacities agree with those from 100 m RT ● Zenith wet delays agree with GPS & radiosonde within 10 mm ● Web-based display & archive access available ● Radiometer stability is 2.7 x in 400 s ● Radiometer sensitivity is 61 mK in s integration time Conclusions