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The WVR at Effelsberg Alan RoyReinhard Keller Ute TeuberDave Graham Helge RottmannWalter Alef Thomas Krichbaum
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The Scanning 18-26 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
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The Scanning 18-26 GHz WVR for Effelsberg
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Front-end opened Ethernet data acquisition systemTemperature regulation modules Control unit March 16th, 2004
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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 10 -4 in 300 s Measured: 2.7 x 10 -4 Opacity Measurement Aim: correct visibility amplitude to 1 % (1 ) Need:thermal noise 2.7 KMeasured: 12 mK Need:absolute calibration 14 % (1 )Measured: 5 %
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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 = 0.025 s (0.038 mm rms path length noise for τ int = 3 s)
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Typical Water Line Spectrum
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WVR Panorama of Bonn
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Move to Effelsberg March 20th, 2003
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WVR Panorama of Effelsberg
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Spillover Cal: Skydip with Absorber on Dish 19 to 26 GHz el = 90 ◦ to 0 ◦ detector output 0 V to 0.3 V
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Gain Calibration Measure: hot load sky dip at two elevations noise diode on/off Derive: Tsky Treceiver gain
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WVR Beamwidth: Drift-Scan on Sun 26.0 GHz beamwidth = 1.26 ◦ 18.0 GHz beamwidth = 1.18 ◦
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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)
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Scattered Cumulus, 2003 Jul 28, 1300 UT
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Storm, 2003 Jul 24, 1500 UT
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First Attempt to Validate Phase Correction
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WVR Noise Budget for Phase Correction Thermal noise: 75 mK in the water line strength, April 2003 186 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 2003 4 ◦ = 120 m at 1.5 km and el = 60 ◦ 66 mK to 145 mK Sramek (1990), VLA structure functions 95 mK Sault (2001), ATCA 2001apr27 1700 UT Other? Spillover model errors, cloud liquid water removal, RFI, wet dish, wet horn Total (quadrature):290 mK = 1.3 mm rms
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Move to Focus Cabin March 16th, 2004
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WVR Beam Geometry Beam overlap, April 2003Beam overlap, April 2004
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Optical Alignment using Moon T antenna = 23 K T moon = 220 K at 22 GHz Beam filling factor = 0.114 Beam efficiency = 92 % 23 K
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Spillover Reduction 19 to 26 GHz el = 90 ◦ to 0 ◦ detector output 0 V to 0.3 V
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WVR Path Data from 3 mm VLBI, April 2004 Time / UT hours 1830243642 Path length / mm 0 30 60 90 120 150 180 210 90° 45° 0° Elevation path length elevation
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VLBI Path Comparison, 3 mm VLBI, April 2004
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VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 420 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
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VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 420 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
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VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 420 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
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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 0.0 1.0 2.0 Coherent integration time Improvement factor 360 s240 s120 s0 s
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Cloud Removal NRAO 150 86 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
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VLBI Phase Correction Demo
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Validation of Opacity Measurement
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Path Length & Opacity Statistics at Effelsberg
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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
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Absolute Calibration for Astrometry & Geodesy 120 km
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Opacity Effects and the Mapping Function
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Issues: TCP/IP time overhead
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Issues: Temperature stability 20 mK Physical temperature near LNA vs time T sys vs time 250 mK 3 min
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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
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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 100100010000 Tsys vs time on absorberCalibrate using temp.Calibrate using noise diode 2.0 K 22 h
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Issues: Beam Mismatch at Low Elevation?
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● 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
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● 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 10 -4 in 400 s ● Radiometer sensitivity is 61 mK in 0.025 s integration time http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html Conclusions
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