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The WVR at Effelsberg

The WVR at Effelsberg. Alan Roy Reinhard Keller Ute Teuber Dave Graham Helge Rottmann Walter Alef Thomas Krichbaum. The Scanning 18-26 GHz WVR for Effelsberg.  = 18.5 GHz to 26.0 GHz D  = 900 MHz Channels = 24 T receiver = 200 K sweep period = 6 s

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The WVR at Effelsberg

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  1. The WVR at Effelsberg Alan Roy Reinhard Keller Ute Teuber Dave Graham Helge Rottmann Walter Alef Thomas Krichbaum

  2. The Scanning 18-26 GHz WVR for Effelsberg  = 18.5 GHz to 26.0 GHz D = 900 MHz Channels = 24 Treceiver = 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

  3. The Scanning 18-26 GHz WVR for Effelsberg

  4. The Scanning 18-26 GHz WVR for Effelsberg Front-end opened March 16th, 2004 Ethernet data acquisition system Temperature regulation modules Control unit

  5. 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 K Measured: 12 mK Need: absolute calibration  14 % (1 ) Measured: 5 %

  6. WVR View of Atmospheric Turbulence Zenith sky (clear blue, dry, cold) Absorber 12 h 1 h ● gain stability: 2.7x10-4 over 400 s ● sensitivity: 61 mK for τint = 0.025 s (0.038 mm rms path length noise for τint = 3 s)

  7. Typical Water Line Spectrum

  8. WVR Panorama of Bonn

  9. Move to Effelsberg March 20th, 2003

  10. WVR Panorama of Effelsberg

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

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

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

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

  15. Scattered Cumulus, 2003 Jul 28, 1300 UT

  16. Storm, 2003 Jul 24, 1500 UT

  17. First Attempt to Validate Phase Correction

  18. 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 Tsys of 255 K Elevation noise: 230 mKtypical elevation pointing jitter is 0.1◦ sky brightness gradient = 2.8 K/◦ at el = 30◦ Beam mismatch: 145 mKmeasured 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

  19. Move to Focus Cabin March 16th, 2004

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

  21. Optical Alignment using Moon 23 K Tantenna = 23 K Tmoon = 220 K at 22 GHz Beam filling factor = 0.114 Beam efficiency = 92 %

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

  23. WVR Path Data from 3 mm VLBI, April 2004 210 180 150 120 path length Path length / mm 90 90° 60 elevation Elevation 45° 30 0 0° 18 24 30 36 42 Time / UT hours

  24. VLBI Path Comparison, 3 mm VLBI, April 2004

  25. VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 No phase correction VLBI phase WVR phase EB phase correction path 3.4 mm Coherence function before & after EB+PV phase correction ● Path rms reduced 1.0 mm to 0.34 mm ● Coherent SNR rose 2.1 x 420 s

  26. VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 No phase correction VLBI phase WVR phase EB phase correction path 3.4 mm Coherence function before & after ● Path rms reduced 0.85 mm to 0.57 mm ● Coherent SNR rose 1.7 x 420 s

  27. VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 Before phase correction at EB VLBI phase WVR phase After phase correction at EB path 3.4 mm Coherence function before & after ● Path rms saturated at 0.95 mm ● Coherent SNR decrease 7.5 x 420 s

  28. VLBI Phase Correction Demo Coherence function after phase correction at EB divided by CF before phase correction NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 2.0 Improvement factor 1.0 0.0 0 s 120 s 240 s 360 s Coherent integration time ● Coherence improves for most scans

  29. Cloud Removal EB WVR path time series Keep VLBI scan times only Subtract linear rate ● Cloud contamination shows up as large scatter in the path lengths NRAO 150 86 GHz VLBI 2004 April 17

  30. VLBI Phase Correction Demo

  31. VLBI Phase Correction Demo

  32. Validation of Opacity Measurement

  33. Path Length & Opacity Statistics at Effelsberg

  34. 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 2 mm 1 mm 0 mm 0 h 24 h 0 h 24 h sunset sunrise UT sunrise sunset UT

  35. Absolute Calibration for Astrometry & Geodesy 120 km

  36. Opacity Effects and the Mapping Function

  37. Issues: TCP/IP time overhead

  38. Issues: Temperature stability Physical temperature near LNA vs time 20 mK 3 min Tsys vs time 250 mK

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

  40. Issues: Noise Diode Stability Tsys vs time on absorber Calibrate using temp. Calibrate using noise diode 2.0 K 22 h Structure function of Tsys on absorber 1 K Original data Calibrated with noise diode Tsys rms / K Calibrated with temperature 0.1 K Time / s 100 1000 10000

  41. Issues: Beam Mismatch at Low Elevation?

  42. Future Developments ● 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?

  43. Conclusions ● 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

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