The IVS Analysis Center at the Onsala Space Observatory
Rüdiger Haas, and Hans-Georg Scherneck
Onsala Space Observatory (OSO)
We give a brief overview of
the activities of the IVS Analysis Center
at the Onsala Space Observatory during 2003.
Some examples of achieved results and
related to earth rotation, loading phenomena,
and atmospheric research are presented.
The IVS Analysis Center at the Onsala Space Observatory (OSO)
is active in research concentrating on
a number of particular topics that
are relevant to space geodesy and geosciences.
These research topics are important for geodetic VLBI
and are investigated using VLBI observations
and corresponding analysis programs.
In the following we will briefly address some of these topics
and present some examples of performed and ongoing analyses.
For the future we plan to continue
VLBI related research along those lines,
concentrating on particular research topics.
There are no plans at OSO for a routine analysis of global
VLBI data in a service sense.
We analysed the combined IVS time series
of earth orientation parameters 
to study El-Niño/Southern Oscilation (ENSO).
The IVS UT1 values were processed similar to the description in
and using the effective atmospheric angular momentum
functions as calculated from NCEP/NCAR reanalyses
The resulting excess length-of-day (dLOD) values
are shown in Figure 1
together with the Multivariate ENSO Index (MEI)
There is a clear correlation between the two time series and
both clearly show the ENSO events during the last 23 years.
We analysed the CONT02 VLBI observations and determined
polar motion and UT1 values with a time resolution of 1 hour.
Figure 2 displays the time series and their
Variations with 12 hour and 24 hour periodicity are detectable
both in polar motion and UT1.
A variation with an 8 hour period is detected marginally
above the significance limit in Yp.
Further investigations are ongoing.
Lower blue curve, left scale:
Excess in length of day (dLOD) in ms derived from
combined IVS EOP values .
Top red curve, right scale:
Multivariate ENSO index (MEI) .
Both time series clearly show the ENSO events
during the last 23 years.
The automatic ocean tide loading provider
has been maintained during 2003.
On the website http://www.oso.chalmers.se/loading
users can chose between 11 different ocean tide models
that are available to calculate ocean tide loading
parameters for site positions that can be
The parameters are provided in several formats,
and are sent to the user via e-mail.
Polar motion and UT1 for CONT02.
Upper plots in red: time series of Xp (left column), Yp (middle column)
and UT1 (right column) with a time resolution of 1 hour.
Lower plots in blue:
spectra of the above time series.
Periods of 8, 12, 24, and 48 hours are indicated with red vertical lines.
Time series of atmospheric loading predictions that are
based on global convolution of atmopheric pressure fields
are available for most of the VLBI databases since 1990
on the website
The analysis of the purely European geodetic VLBI observations
has continued and results for crustal motion
have been published in 2003 .
Additionally to the CONT02 VLBI observations
we performed simultaneous observations with the
Onsala IGS permanent GPS equipment,
two water vapor radiometers (WVR), and
a rain radar (RR) at the observatory.
The water vapor radiometer Astrid was operated
in sky mapping mode, while the water vapor radiometer
Konrad followed the VLBI observation schedule
and was perfoming tip-curve measurements during
slewing times of the VLBI telescope.
Figure 3 shows preliminary results for the
zenith wet delay (ZWD) derived from observations
with the four collocated techniques.
These prelimiary results show a reasonably good agreement
between the ZWD derived from the different techniques
and have been communicated
the EGS-AGU-EUG Joint Assembly 2003 
and the 16th Working Meeting on European VLBI
for Geodesy and Astrometry .
However, the rain radar measurements indicate that there are still
some WVR-data that were influenced by rain and should be
removed from the time series.
Furthermore, there are still some unexplained biases
and thus the investigations are continuing.
We investigated trends in the tropospheric
water vapor content based on
ZWD data determined from observations with the
three collocated techniques VLBI, GPS, and WVR
at Onsala and radiosondes (RS) at Landvetter
airport 37 km away from the observatory.
Data for more than 23 years are available from
VLBI, WVR and RS, while GPS data are available
Each of the techniques has specific
advantages and disadvantages in terms of
stability and time resolution.
Therefore, a combination of the
results of the individual techniques
appears to be a promising approach.
We developed strategies to assess
trends in tropospheric water vapor
and to combine the results of the
four independent techniques in order to
determine robust results that
can be useful for climate related research
Equivalent zenith wet delays in cm and rain rate in mm/hr
at Onsala during CONT02.
Top to bottom the time series are:
radiometer Konrad (red, offset by +30 cm),
radiometer Astrid (blue, offset by +20 cm),
VLBI (black, offset by +10 cm),
GPS (green, no offset),
rain radar (magenta, offset by -5 mm/hr).
We continued our activity in the
IVS Pilot Project - Tropospheric Parameters.
Tropospheric parameters for all VLBI stations observing
in the IVS R1 and R4 networks were submitted to the IVS.
Figure 4 shows histograms of the ZWD for
four stations that are located in different climate zones:
Ny-Ålesund - polar, Wettzell - temperate,
Hartrao - dry, Fortaleza - tropical.
A dependence of the amount of water vapor
on the climate zone is obvious.
Histograms of zenith wet delay (ZWD) values for four
stations located in four different climate regions.
The histograms include results
from the IVS R1 and R4 experiments during 2002 and 2003.
Steinforth, C., and Nothnagel, A.:
Official IVS EOP results,
Combined series IVS03Q3
Gipson, J. M., and Ma, C.:
Signature of El Nino in length of day as measured by VLBI.
IERS(1998) Technical Note No 26.
Salstein, D. A., and Rosen, R. D.:
Global momentum and energy signals from reanalysis systems.
Preprints, 7th Conf. on Climate Variations,
American Meteorological Society, Boston, MA, 344-348.
Multivariate ENSO Index (MEI) webpage
Scherneck, H.-G., and Bos, M.:
Ocean Tide and Atmospheric Loading.
In: N. R. Vandenberg and K. D. Baver (Eds.):
IVS 2002 General Meeting Proceedings,
NASA/CP-2002-210002, 205-214, 2002.
Haas, R., Nothnagel, A., Campbell, J., and Gueguen, E.:
Recent crustal movements observed with the European
geodetic VLBI network: Geodetic analysis and results.
Journal of Geodynamics, 35(4-5),
Haas, R., Gradinarsky, L., Elgered, G., and Johansson, J.:
Studying atmospheric properties at the Onsala Space
Observatory during the CONT02 experiment.
Geophys. Res. Abstr., Vol. 5,
Elgered, G., and Haas, R.:
The Geodetic VLBI Network Station at the
Onsala Space Observatory - Activities During 2002.
In: W. Schwegmann and V. Thorandt (Eds.):
Proc. of the 16th Working Meeting
on European VLBI for Geodesy and Astrometry,
Bundesamt für Kartographie und Geodäsie,
Haas, R., Elgered, G., Gradinarsky, L., and Johansson, J.:
Assessing Long Term Trends in the Atmospheric Water Vapor Content by
Combining Data From VLBI, GPS, Radiosondes and Microwave Radiometry.
Proc. of the 16th Working Meeting
European VLBI for Geodesy and Astrometry,
W. Schwegmann and V. Thorandt (eds.),
Bundesamt für Geodäsie und Kartographie,