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