Download - Background and Status of the Water-Vapour Radiometer at Effelsberg A. Roy U. Teuber R. Keller
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Background and Status of the
Water-Vapour Radiometer at Effelsberg
A. RoyU. TeuberR. Keller
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The Troposphere as Seen from Orbit
Method: Synthetic Aperture Radar (Earth Resources Satellite)Frequency: 9 GHzRegion: GroningenInterferograms by differencing images from different days
5 km
5 km
Internal waves in a homo-genously cloudy troposphere
A frontal zone Convective cells
0 mm
-100 mm
100 mm
Hanssen (1997)
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Troposphere Seen by VLBI at 86 GHz
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Water-Vapour Radiometry Basics
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WVR Performance Requirements
Opacity MeasurementAim: 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 0.07.WVR spec: absolute calibration accuracy 14 % (1 )
thermal noise per measurement 2.7 K.
Tropospheric Phase CorrectionAim: coherence = 0.9 after correction
requires / 20 path length noise after correction, ie 0.18 mm for = 3.4 mmrequires 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 GHztypical water line strength = 35 K
WVR spec: thermal noise 14 mK in 3 sgain stability: 3.9 x 10-4 in 300 s
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The Scanning 18-26 GHz WVR for Effelsberg
= 18 to 26 GHz = 900 MHzNchannel = 25Treceiver = 200 K = 0.025 s per channel = 61 mK per channelsweep period = 5 s
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The Scanning 18-26 GHz WVR for Effelsberg
Front-end opened
Control unit
First light, April 2002, Bonn
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WVR View of Atmospheric Turbulence
Absorber Zenith sky(clear blue, dry, cold)
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)
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WVR Beamwidth: Drift-Scan on Sun
26.0 GHzbeamwidth = 1.26◦
18.0 GHzbeamwidth = 1.18◦
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WVR Beam Overlap Optimization
WVR – 100 m RT Beam Overlap forthree WV profiles
Atmospheric WV Profiles atEssen from Radiosonde launches every 12 h(Data courtesy Dr. S. Crewell, Uni Bonn)
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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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Spillover Cal: Skydip with Absorber on Dish
19 to 26 GHz
el = 90 ◦ to 0 ◦
dete
ctor
ou
tpu
t
0 V
to 0
.3 V
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WVR Control Panel
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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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Scattered Cumulus, 2003 Jul 28, 1300 UT
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Storm, 2003 Jul 24, 1500 UT
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Validation of Opacity Measurement
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Opacity Statistics at Effelsberg
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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 2003186 mK per channel on absorber, scaled to 25 channelsdifference 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 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 20034◦ = 120 m at 1.5 km and el = 60◦
66 mK to 145 mK Sramek (1990), VLA structure functions95 mK Sault (2001), ATCA 2001apr27 1700 UT
Other ? Spillover model errors, cloud liquid waterremoval, 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 2003 Beam overlap, April 2004
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Optical Alignment using Moon
Tantenna = 23 KTmoon = 220 K at 22 GHzBeam filling factor = 0.114
Beam efficiency = 92 %
23 K
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Spillover Reduction
19 to 26 GHz19 to 26 GHz
el = 90 ◦ to 0 ◦
dete
ctor
ou
tpu
t
0 V
to 0
.3 V
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WVR Path Data from 3 mm VLBI, April 2004
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VLBI Path Comparison, 3 mm VLBI, April 2004
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VLBI Phase Correction Demo
Demonstration by Tahmoush & Rogers (2000) 3C 273Hat Creek – Kitt Peak86 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
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Australia Telescope Phase Correction Demo
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● 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 AIPSdata archiveonline (web-based) real-time display
● Investigate limitations on calibration accuracy
● Hardware development: (once usefulness established)temperature stabilization: further improvementsspillover: reduce with new feed?integration time efficiency: Data acquisition system upgradebeam overlap: move to prime focus receiver boxes
Future Developments
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● 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 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