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VLBI Phase Correction with Effelsberg WVR
A. Roy
U. Teuber
H. Rottmann
R. Keller
Troposphere Seen by VLBI at 86 GHz
The Scanning 18-26 GHz WVR for Effelsberg
March 16th, 2004
The Scanning 18-26 GHz WVR for Effelsberg
Front-end opened
Typical Water Line Spectrum
WVR Performance Requirements
Phase Correction
Aim:
coherence = 0.9
requires   / 20 (0.18 mm rms) after correction
Need: thermal noise  14 mK in 3 s
Need: gain stability 3.9 x 10-4 in 300 s
Measured: 12 mK
Measured: 2.7 x 10-4
Opacity Measurement
Aim:
correct visibility amplitude to 1 % (1 )
Need: thermal noise  2.7 K
Need: absolute calibration  14 % (1 )
Measured: 12 mK
Measured: 5 %
WVR Path Data from 3 mm VLBI, April 2004
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
420 s
● Path rms reduced 1.0 mm to 0.34 mm
● Coherent SNR rose 2.1 x
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 reduced 0.85 mm to 0.57 mm
● Coherent SNR rose 1.7 x
420 s
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
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
● Coherence improves for most scans
Cloud Removal
EB WVR path time series
NRAO 150
86 GHz VLBI
2004 April 17
Keep VLBI scan times only
Subtract linear rate
● Cloud contamination shows up as large scatter in the path lengths
Validation of Opacity Measurement
Absolute Calibration for Astrometry & Geodesy
WVR Comparison at Effelsberg
July 12th, 2005
Conclusion
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Effelsberg has a WVR
Phase correction of 3 mm VLBI improves coherence
Opacity measurements are reliable
Absolute calibration agrees within 5 % with GPS and radiosonde
Ready for friendly users
Future
● Connect to EVN calibration pipeline and streamline AIPS import
● Improve cloud separation & line fitting algorithm
● Hardware:
improve temperature stabilization
reduce spillover with new feed?
improve integration time efficiency
better beam overlap: move to prime focus receiver boxes
http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html