VLBI Phase Correction with Effelsberg WVR
A. RoyU. TeuberH. RottmannR. Keller
Troposphere Seen by VLBI at 86 GHz
March 16th, 2004
The Scanning 18-26 GHz WVR for Effelsberg
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 Measured: 12 mKNeed: 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 mKNeed: absolute calibration 14 % (1 ) Measured: 5 %
WVR Path Data from 3 mm VLBI, April 2004
VLBI Phase Correction Demo
NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17
420 s
3.4 mm
path
● Path rms reduced 1.0 mm to 0.34 mm● Coherent SNR rose 2.1 x
WVR phase
VLBI phase
No phase correction
EB phase correction
Coherence function before & after
EB+PV phase correction
VLBI Phase Correction Demo
NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17
420 s
3.4 mm
path
● Path rms reduced 0.85 mm to 0.57 mm● Coherent SNR rose 1.7 x
WVR phase
VLBI phase
Before phase correction at EB
After phase correction at EB
Coherence function before & after
VLBI Phase Correction Demo
NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17
420 s
3.4 mm
path
● Path rms saturated at 0.95 mm● Coherent SNR decrease 7.5 x
WVR phase
VLBI phase
Before phase correction at EB
After phase correction at EB
Coherence function before & after
VLBI Phase Correction Demo
NRAO 150Pico Veleta - Effelsberg86 GHz VLBI2004 April 17
● Coherence improves for most scans
Coherence function after phase correction at EB divided by CF before phase correction
Cloud Removal
NRAO 15086 GHz VLBI2004 April 17
● Cloud contamination shows up as large scatter in the path lengths
EB WVR path time series Keep VLBI scan times only Subtract linear rate
Validation of Opacity Measurement
Absolute Calibration for Astrometry & Geodesy
WVR Comparison at Effelsberg
July 12th, 2005
● Connect to EVN calibration pipeline and streamline AIPS import● Improve cloud separation & line fitting algorithm● Hardware:
improve temperature stabilizationreduce spillover with new feed?improve integration time efficiencybetter beam overlap: move to prime focus receiver boxes
http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html
Conclusion
● 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
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