The WVR at Effelsberg Alan Roy Ute Teuber Helge Rottmann Thomas Krichbaum Reinhard Keller Dave Graham Walter Alef
The Scanning 18-26 GHz WVR for Effelsberg ν = 18.5 GHz to 26.0 GHz Δν = 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 18-26 GHz WVR for Effelsberg
The Scanning 18-26 GHz WVR for Effelsberg Front-end opened March 16th, 2004 Ethernet data acquisition system Temperature regulation modules Control unit
WVR Performance Requirements Phase Correction Aim: coherence = 0.9 requires λ/ 20 (0.18 mm rms at λ = 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 σ) Need: thermal noise 2.7 K Measured: 12 mk Need: absolute calibration 14 % (1 σ) Measured: 5 %
WVR View of Atmospheric Turbulence Absorber Zenith sky (clear blue, dry, cold) 280 Tantenna vs Time 57 Tantenna vs Time 10 2 Allan Variance of Tantenna 279 56 Antenna temperature / K 278 277 276 275 274 273 272 271 270 Absorber, on roof in Effelsberg 03apr09 1800 to apr10 0600 UT 64800 86400 108000 time / s Antenna temperature / K 55 54 53 52 51 50 49 48 Zenith, clear sky, on roof in Bonn 02apr05 1105 to 1205 UT 47000 47600 48200 48800 49400 50000 time / s Allan Variance 10 3 10 4 zenith sky Absorber, on roof in Effeslberg 03apr09 1800 to apr10 0600 UT absorber 10 5 10 100 1000 10000 100000 time / s 12 h 1 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)
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 detector output 0 V to 0.3 V el = 90 to 0 19 to 26 GHz
Gain Calibration Measure: hot load sky dip at two elevations noise diode on/off detector output / V 10 9 8 7 6 5 4 3 2 Calibration Data from 04feb10 absorber + noise diode absorber 23.6d elevation 41.8d elevation 1 40 0 18 19 20 21 22 23 24 25 26 frequency / GHz Derive: Tsky Treceiver gain Tsky at zenith, derived from 04feb10 cal data 300 Treceiver, derived from 04feb10 cal data 0.020 Gain, derived from 04feb10 cal data 30 250 0.015 Tsky / K 20 Treceiver / K 200 150 gain / V/K 0.010 0.005 10 100 50 0.000 0 18 19 20 21 22 23 24 25 26 frequency / GHz 0 18 19 20 21 22 23 24 25 26 frequency / GHz 0.005 18 19 20 21 22 23 24 25 26 frequency / GHz
WVR Beamwidth: Drift-Scan on Sun Drift scan through sun Detector voltage 9 8 7 6 5 4 08.04.2002 1200 UT WVR in Bonn YIG freq = 26.0 GHz FWHM = 1.26d 26.0 GHz beamwidth = 1.26 3 Detector voltage 2 0 1 2 3 7 05.04.2002 0954 UT WVR in Bonn YIG freq = 18.0 GHz 6 FWHM = 1.18d 18.0 GHz beamwidth = 1.18 5 0 1 2 3 Sun position / degrees
WVR Beam Overlap Optimization 2000 m Altitude 4.2 Water Vapour Density / g/m3 Water vapour density / kg m^ 3 0.01 0.005 0 0 2 4 6 8 10 Altitude / km WVR 100 m RT Beam Overlap for three WV profiles Beam overlap 0.3 0.2 Atmospheric WV Profiles at Essen from Radiosonde launches every 12 h (courtesy Dr. S. Crewell, Uni Cologne) (1) (2) (3) 0 m 12.3 0.1 1 = low altitude water vapour 2 = mean water vapour profile 3 = high altitude water vapour 0 0 1 2 3 4 Half power beamwidth / degrees
Scattered Cumulus, 2003 Jul 28, 1300 UT
Storm, 2003 Jul 24, 1500 UT
First Attempt to Validate Phase Correction 300 WVR Path Correction vs Time, 03apr30 0640 UT 250 path correction / mm 200 150 100 50 0 0 60 120 180 240 300 360 420 time / s (start = 10392003 s) 180 WVR Predicted Phase and VLBI Measured Phase, 03apr30 0640 UT phase at 86 GHz / degrees 120 60 0 60 120 180 0 60 120 180 240 300 360 420 time / s (start = 10392003 s)
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
Move to Focus Cabin March 16th, 2004
WVR Beam Geometry 1500 m Beam overlap, April 2003 Beam overlap, April 2004
Optical Alignment using Moon 60 Azimuth Scans across Moon 2004mar30 60 Elevation Scans across Moon 2004mar30 Antenna Temperature / kelvin 50 40 Antenna Temperature / kelvin 50 40 23 K 30 30 2 1 0 1 2 Azimuth Offset / degrees 2 1 0 1 2 Elevation Offset / degrees T antenna = 23 K T moon = 220 K at 22 GHz Beam filling factor = 0.114 Beam efficiency = 92 %
19 to 26 GHz 19 to 26 GHz Spillover Reduction el = 90 to 0 detector output 0 V to 0.3 V
WVR Path Data from 3 mm VLBI, April 2004 210 Path Length and Elevation vs Time, 2004apr16 17 180 Path length / mm 150 Path length / mm 120 90 60 30 path length elevation 90 45 Elevation 0 0 18 24 30 36 42 time / UT Time / UT hours 0
VLBI Path Comparison, 3 mm VLBI, April 2004 VLBI Phase and EB WVR Path Length Comparison, 2004apr17 86.8 DOY: 108 UT: 00:40 to 00:47 Baseline: Pico Veleta Effeslberg Path length / mm 83.4 VLBI phase + constant EB WVR path 80 0.66666 0.68333 0.69999 0.71666 0.73332 0.74999 0.76666 0.78332 time / UT
VLBI Phase Correction Demo No phase correction EB phase correction 180 Baseline 1-3 Scan 7 NRAO150.UV.CL10 90 0-90 -180 45720.5 s 45840.5 s 45960.5 s 180 Baseline 1-3 Scan 7 NRAO150.UV.CL12 VLBI phase WVR phase NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 path 3.4 mm 90 0-90 -180 45720.5 s 45840.5 s 45960.5 s Coherence function before & after 1.0 Baseline 1-3 Scan 7 Coherence Functions EB+PV phase correction 180 Baseline 1-3 Scan 7 NRAO150.UV.CL14 0.8 0.6 90 0.4 0 0.2-90 0.0 0 s 120 s 240 s 360 s -180 45720.5 s 45840.5 s 45960.5 s 420 s Path rms reduced 1.0 mm to 0.34 mm Coherent SNR rose 2.1 x
VLBI Phase Correction Demo 180 Baseline 1-3 Scan 11../NRAO150.UV.CL13 No phase correction 90 0 180 Baseline 1-3 Scan 12../NRAO150.UV.CL13 90 0 NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17-90 -90 VLBI phase -180 51720.5 s 51840.5 s 51960.5 s -180 52920.5 s 53040.5 s 53160.5 s WVR phase 180 Baseline 1-3 Scan 11../NRAO150.UV.CL15 EB phase correction 90 0 180 Baseline 1-3 Scan 12../NRAO150.UV.CL15 90 0 path 3.4 mm -90-90 -180 51720.5 s 51840.5 s 51960.5 s -180 52920.5 s 53040.5 s 53160.5 s 1.0 Baseline 1-3 Scan 11 Coherence Functions Coherence function before../nrao150.uv.cl15 & after../nrao150.uv.cl13 0.8 1.0 Baseline 1-3 Scan 12 Coherence Functions 0.8../NRAO150.UV.CL15../NRAO150.UV.CL13 0.6 0.6 0.4 0.4 0.2 0.0 0 s 120 s 240 s 360 s 0.2 0.0 0 s 120 s 240 s 360 s Path rms reduced 0.85 mm to 0.57 mm Coherent SNR rose 1.7 x 100 d Baseline 1-3 Scan 11 Structure Functions 100 d Baseline 1-3 Scan 12 Structure Functions 420 s
VLBI Phase Correction Demo Before phase correction at EB 180 Baseline 1-3 Scan 21../NRAO150.UV.CL13 CLOUDED 90 0 NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 180 Baseline 1-3 Scan 22../NRAO150.UV.CL13 90 0-90 -90 VLBI phase -180 64920.5 s 65040.5 s 65160.5 s -180 WVR phase 66120.5 s 66240.5 s 66360.5 s After phase correction at EB 180 Baseline 1-3 Scan 21../NRAO150.UV.CL15 CLOUDED 180 Baseline 1-3 Scan 22../NRAO150.UV.CL15 90 0 90 0 path 3.4 mm -90-90 -180 64920.5 s 65040.5 s 65160.5 s -180 66120.5 s 66240.5 s 66360.5 s Coherence function before & after 1.0 Baseline 1-3 Scan 21 Coherence Functions 0.8../NRAO150.UV.CL15../NRAO150.UV.CL13 1.0 Baseline 1-3 Scan 22 Coherence Functions 0.8../NRAO150.UV.CL15../NRAO150.UV.CL13 0.6 0.6 0.4 0.2 0.0 0 s 120 s 240 s 360 s Baseline 1-3 Scan 21 Structure Functions 0.4 0.2 Path rms saturated at 0.95 mm Coherent SNR decrease 7.5 x 0.0 0 s 120 s 240 s 360 s 420 s Baseline 1-3 Scan 22 Structure Functions
VLBI Phase Correction Demo Coherence function after phase correction at EB divided by CF before phase correction Improvement factor 2.4 2.2 2.0 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 Baseline 1-3 Corrected/Uncorrected Coherence Functions NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 0.4 0.2 0.0 0 s 0.0 0 s 120 s 240 s 360 s 120 s 240 s 360 s Coherent integration time Baseline 1-3 Corrected/Uncorrected Structure Functions 2.4 2.2 Coherence improves for most scans 2.0 1.8
Cloud Removal EB WVR path time series Keep VLBI scan times only Subtract linear rate 200 WVR path length vs time, EB, 2004 Apr 17 200 WVR path length vs time, EB, 2004 Apr 17 WVR path length vs time, EB, 2004 Apr 17 190 180 190 180 Kept data during VLBI scans 40 30 Kept data during VLBI scans Fit and subtracted linear slope from each scan Path / mm 170 160 150 140 Path / mm 170 160 150 140 Path / mm 20 10 0 130 130 120 120 10 110 110 20 100 40000 50000 60000 70000 Time / seconds 100 40000 50000 60000 70000 Time / seconds 40000 50000 60000 70000 Time / seconds NRAO 150 86 GHz VLBI 2004 April 17 Cloud contamination shows up as large scatter in the path lengths
VLBI Phase Correction Demo
VLBI Phase Correction Demo
Validation of Opacity Measurement Comparison of Opacity Measured with 100 m RT and WVR, 2004feb13 0.08 100 m RT RCP 100 m RT LCP opacity WVR 0.07 13 14 15 16 17 18 19 UT / hours
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 2 mm 1 mm 0 mm 0 h 24 h sunrise UT sunset 0 h sunrise UT sunset 24 h
Absolute Calibration for Astrometry & Geodesy 120 km
Opacity Effects and the Mapping Function
Issues: TCP/IP time overhead
Issues: Temperature stability Physical temperature near LNA vs time 20 mk 3 min T sys vs time 250 mk
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 Strong thermal coupling Temperature vs time Temperature vs time 0.7 C 5.5 C 0.75 days 2.5 days
Issues: Noise Diode Stability Tsys vs time on absorber Calibrate using temp. Calibrate using noise diode 2.0 K 22 h Tsys rms / K Structure function of Tsys on absorber 1 K Original data Calibrated with noise diode Calibrated with temperature 0.1 K Time / s 100 1000 10000
Issues: Beam Mismatch at Low Elevation?
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: reduce Tsys? spillover: integration time efficiency: beam overlap: better insulation, regulation Cooling? reduce with new feed? Data acquisition system upgrade move to prime focus receiver boxes?
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