Transcript Kein Folientitel

Background and Status of the
Water-Vapour Radiometer at Effelsberg
A. Roy
U. Teuber
R. Keller
The Troposphere as Seen from Orbit
Method: Synthetic Aperture Radar (Earth Resources Satellite)
Frequency: 9 GHz
Region: Groningen
Interferograms by differencing images from different days
100 mm
0 mm
5 km
-100 mm
5 km
A frontal zone
Convective cells
Internal waves in a homogenously cloudy troposphere
Hanssen (1997)
Troposphere Seen by VLBI at 86 GHz
Water-Vapour Radiometry Basics
WVR Performance Requirements
Opacity Measurement
Aim:
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 Correction
Aim:
Info:
coherence = 0.9 after correction
requires   / 20 path length noise after correction, ie 0.18 mm for  = 3.4 mm
requires root-two better at each end of baseline (0.12 mm => 28 mK)
tropo path / tropo brightness = 4.5 mm/K at 22.2 GHz
typical water line strength = 35 K
WVR spec:
thermal noise  14 mK in 3 s
gain stability: 3.9 x 10-4 in 300 s
The Scanning 18-26 GHz WVR for Effelsberg
 = 18 to 26 GHz
D = 900 MHz
Nchannel = 25
Treceiver = 200 K
Dt = 0.025 s per channel
 = 61 mK per channel
sweep period = 5 s
The Scanning 18-26 GHz WVR for Effelsberg
Front-end opened
First light, April 2002, Bonn
Control unit
WVR View of Atmospheric Turbulence
Absorber
12 h
Zenith sky
(clear blue, dry, cold)
1h
● 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)
WVR Beamwidth: Drift-Scan on Sun
26.0 GHz
beamwidth = 1.26◦
18.0 GHz
beamwidth = 1.18◦
WVR Beam Overlap Optimization
Atmospheric WV Profiles at
Essen from Radiosonde
launches every 12 h
(Data courtesy Dr. S. Crewell, Uni Bonn)
WVR – 100 m RT
Beam Overlap for
three WV profiles
Gain Calibration
Measure: hot load
sky dip at two elevations
noise diode on/off
Derive: Tsky
Treceiver
gain
detector output
0 V to 0.3 V
Spillover Cal: Skydip with Absorber on Dish
WVR Control Panel
WVR Panorama of Bonn
Move to Effelsberg
March 20th, 2003
WVR Panorama of Effelsberg
Scattered Cumulus, 2003 Jul 28, 1300 UT
Storm, 2003 Jul 24, 1500 UT
Validation of Opacity Measurement
Opacity Statistics at Effelsberg
First Attempt to Validate Phase Correction
WVR Noise Budget for Phase Correction
Thermal noise:
75 mK in the water line strength, April 2003
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 2003
◦
◦
4 = 120 m at 1.5 km and el = 60
Sramek (1990), VLA structure functions
Sault (2001), ATCA 2001apr27 1700 UT
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)
66 mK to 145 mK
95 mK
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 2003
Beam overlap, April 2004
Optical Alignment using Moon
23 K
Tantenna = 23 K
Tmoon = 220 K at 22 GHz
Beam filling factor = 0.114
Beam efficiency = 92 %
detector output
0 V to 0.3 V
Spillover Reduction
WVR Path Data from 3 mm VLBI, April 2004
VLBI Path Comparison, 3 mm VLBI, April 2004
VLBI Phase Correction Demo
Demonstration by Tahmoush & Rogers (2000)
3C 273
Hat Creek – Kitt Peak
86 GHz VLBI
path
4 mm
VLBI phase
WVR phase
400 s
● RMS phase noise reduced from 0.88 mm to 0.34 mm after correction.
● Coherent SNR rose by 68 %.
Australia Telescope Phase Correction Demo
Future Developments
● 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 AIPS
data archive
online (web-based) real-time display
● Investigate limitations on calibration accuracy
● Hardware development:
temperature stabilization:
spillover:
integration time efficiency:
beam overlap:
(once usefulness established)
further improvements
reduce with new feed?
Data acquisition system upgrade
move to prime focus receiver boxes
Conclusions
● 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