Phase Correction of VLBI with WVRs Alan Roy Ute Teuber Helge Rottmann Reinhard Keller

230 R G Krichbaum et al M 87 Pushing VLBI to the Highest Resolution: Some Applications Sgr A* Event Horizon Falcke, Melia, Agol (2000) GR ray tracing 1/r 2 emissivity gas Maximally rotating black hole Include interstellar scattering Convolve to resolution of ideal VLBI array at 1.3 mm and 0.6 mm R schwarzschild 230 GHz Event horizon shadow diameter should be 9.2 Rs = 27 μas in Sgr A* M 87 Jet Collimation (Modelling results) Krichbaum et al GHz 500 GHz

Pushing VLBI to the Highest Resolution: 2003 April 13 HHT – Pico Veleta 4.2 G (record longest baseline)  30 μas resolution Sources: 3C Jy SNR = Jy SNR = 6.4? Demo at 230 GHz 230 GHz500 GHz Pico Veleta – Plateau de Bure First trans-Atlantic fringes at 1.3 mm Krichbaum, Graham, Alef et al., EVN Symp, (2004) 2 days Signal to noise ratio

Pushing VLBI to the Highest Resolution: Coherence Loss due to Troposphere Pico Veleta – Onsala baseline Source: BL Lac Frequency: 86 GHz Coherence Function 7 min 360° VLBI phase time series

The Scanning GHz WVR for Effelsberg Front-end opened Ethernet data acquisition systemTemperature regulation modules Control unit March 16th, 2004

The Scanning GHz WVR for Effelsberg = 18.5 GHz to 26.0 GHz  = 900 MHz Channels = 24 T receiver = 200 K sweep period = 6 s Features  Uncooled (reduce cost)  Scanning (fewer parts, better stability)  Robust implementation (weather-proof, temperature stabilized)  Noise injection for gain stabilization  Beam matched to Effelsberg near-field beam  TCP/IP communication  Web-based data access  Improved version of prototype by Alan Rogers

WVR Performance Requirements Phase Correction Aim:coherence = 0.9 requires  / 20 (0.18 mm rms at = 3.4 mm) after correction Need: thermal noise  14 mK in 3 s Measured: 12 mK = 0.05 mm Need:gain stability 3.9 x in 300 s Measured: 2.7 x Opacity Measurement Aim: correct visibility amplitude to 1 % (1  ) Need:thermal noise  2.7 KMeasured: 12 mK Need:absolute calibration  14 % (1  )Measured: 5 %

Typical Water Line Spectrum

Storm, 2003 Jul 24, 1500 UT

WVR Path Data from 3 mm VLBI, April 2004 Time / UT hours Path length / mm ° 45° 0° Elevation path length elevation

VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 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 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 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 No phase correction EB phase correction Coherence function before & after

VLBI Phase Correction Demo NRAO 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 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 150 Pico Veleta - Effelsberg 86 GHz VLBI 2004 April 17 ● Coherence improves for most scans Coherence function after phase correction at EB divided by CF before phase correction Coherent integration time Improvement factor 360 s240 s120 s0 s

Phase Referencing Errors due to Troposphere ZTD ε ZTD

Absolute Calibration for Astrometry & Geodesy

Conclusion  WVR corrections from Effelsberg improve phase coherence during high-frequency VLBI  Tropospheric delay measured to 10 mm accuracy for astrometry  WVR data recorded at Effelsberg for every EVN session since 2004 See under  Ready to equip high-frequency VLBI array with WVRs Advertisement RadioNet Workshop on Measurement of Atmospheric Water Vapour Theory, Techniques, Astronomical and Geodetic Applications Oct 9-11, 2006, Wettzell, Germany

Scattered Cumulus, 2003 Jul 28, 1300 UT