do we need wvrs for geodetic vlbi?

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