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

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:

Monte Carlo simulation

τ

atmosphere

clocks

wzd & clocks are stochastic processes simulate for station 1 and 2

90-e

2

1

Simulation of wzd and clock parameters

random walk variances of: 0.1 psec²/s

0.7 psec²/s

Simulate the wet zenith delay

11112222 )()( wnclemfwwzdwnclemfwwzdco

Simulation of wzd and clock parameters

11112222 )()( wnclemfwwzdwnclemfwwzdco

Simulate the clocks

random walkAllan standard deviation of: 2∙10-15@15min

1∙10-14@50min

Simulation of wzd and clock parameters

11112222 )()( wnclemfwwzdwnclemfwwzdco

Simulate observation errors

white noise: 4 psec

8 psec16 psec

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 wzd25 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

Obs. Schedule: fast antennas

The schedule was created by Anthony Searle and Bill Petrachenko

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

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

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

Thanks for your attention