Download - 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