1. Do we need WVRs for geodetic VLBI? Joerg Wresnik (1), Johannes Boehm (1), Harald Schuh (1), Arthur Niell (2) Workshop on Measurement of Atmospheric Water Vapour: Theory, Techniques, Astronomical and Geodetic Applications Wettzell / Hoellenstein (Germany), October 9-11, 2006 (1) Institute of Geodesy and Geophysics (IGG), TU Vienna, Austria (2) MIT Haystack Observatory, MA, USA

2. VLBI 2010 Working Group 3 of IVS (2004-2005) Tasks: Current and Future Requirements for Geodetic VLBI Systems Design a new VLBI observing system based on small antennas with 1mm accuracy - number, location and configuration of antennas System studies and simulations accuracy of 1mm site position & of 1mm/year velocity continuous measurements for EOP rapid generation & distribution of IVS products Goals:

3. Monte Carlo simulation τ atmosphere clocks wzd & clocks are stochastic processes simulate for station 1 and 2 90-e 2 1

4. Simulation of wzd and clock parameters random walk variances of: 0.1 psec²/s 0.7 psec²/s Simulate the wet zenith delay

5. Simulation of wzd and clock parameters Simulate the clocks random walk Allan standard deviation of: 2∙10 -15 @15min 1∙10 -14 @50min

6. Simulation of wzd and clock parameters Simulate observation errors white noise: 4 psec 8 psec 16 psec

7. Simulation of wzd and clock parameters simulation of 25 identical 24 hour sessions What are the effects of wzd and clocks? Does a “4psec antenna” improve the results? simulate wzd 25 times for each station generate white noise 25 times for each station simulate clocks 25 times for each station 0.1 psec**2/sec 0.7 psec**2/sec 2e-15@15min 1e-14@50min 4 psec 8 psec 16 psec

8. Obs. Schedule: fast antennas The schedule was created by Anthony Searle and Bill Petrachenko

9. Obs. Schedule: high observation density 5760 scans 116308 observations 16 stations 120 baselines 100 scans/hour/station Antenna specification using 16 antennas with the same size as Algonquin but with faster slewing rates azimuth max slewing rate of 18 deg/s azimuth max acceleration of 3.6 deg/s² elevation max slewing rate of 4.5 deg/s elevation max acceleration of 0.9 deg/s² data rate of 48 Gb/s. Scan length 5 sec every 30 sec Source catalogue 50 sources

10. Baseline lengths repeatability 4 psec wet zenith delay 2e-15@15min (ASD) 0.0036 psec**2/sec (PSD) 0.1 psec**2/sec clocks 16 psec white noise if the PSD of wet zenith delay is 0.7 psec**2/sec, 4 psec antennas give the same result as 16 psec antennas by PSD of wet zenithdelay of 0.1 psec**2/s 0.7 psec**2/sec the wet zenith delay is the limiting factor

11. Largest PSD values for wet zenith delay – lowest PSD values for clocks: Conclusions and outlook PSD of wet zenith delays dominates the baseline length repeatability hardly any difference between ‘4, 8 and 16 psec antennas' WVR could improve geodetic VLBI Simulation for wzd station dependence elevation and azimuth dependence Next steps

12. Thanks for your attention