During the development of the Vienna VLBI Software VieVS at the Institute of Geodesy and Geophysics at Vienna University of Technology, a special comparison setup was developed with the goal of easily finding links between deviations of results achieved with different software packages and certain parameters of the observation. The object of comparison is the computed time delay, a value calculated for each observation including all relevant models and corrections that need to be applied in geodetic VLBI analysis. Besides investigating the effects of the various models on the total delay, results of comparisons between VieVS and Occam 6.1 are shown. Using the same methods, a Comparison Campaign of VLBI data analysis software called DeDeCC is about to be launched within the IVS soon

Bohm, Johannes

Plank, Lucia

Schuh, Harald

NASA Technical Reports Server

Lucia Plank et al.: Comparison Campaign of VLBI Data Analysis Software - First Results, IVS 2010 General Meeting
Proceedings, p.217–221
http://ivscc.gsfc.nasa.gov/publications/gm2010/plank.pdf
Comparison Campaign of VLBI Data Analysis Software - First
Results
Lucia Plank, Johannes Bo¨hm, Harald Schuh
Institute of Geodesy and Geophysics, Vienna University of Technology
Contact author: Lucia Plank, e-mail: lucia.plank@tuwien.ac.at
Abstract
During the development of the Vienna VLBI Software VieVS at the Institute of Geodesy and
Geophysics at Vienna University of Technology, a special comparison setup was developed with the
goal of easily finding links between deviations of results achieved with different software packages and
certain parameters of the observation. The object of comparison is the computed time delay, a value
calculated for each observation including all relevant models and corrections that need to be applied in
geodetic VLBI analysis. Besides investigating the effects of the various models on the total delay, results
of comparisons between VieVS and Occam 6.1 are shown. Using the same methods, a Comparison
Campaign of VLBI data analysis software called DeDeCC is about to be launched within the IVS soon.
1. Introduction
The calculation of the theoretical delay is one of the main tasks in geodetic VLBI analysis.
The comparison of the modeled time delays with the observed values together with their partial
derivatives is the basis for the subsequent adjustment procedure. While the results of the adjust-
ment can vary depending on the estimation method and time intervals, the calculated delay relies
on theoretical knowledge and should be identical in all software realizations. In practice, differ-
ent VLBI analysis packages follow different calculation strategies, vary in the usage of correction
models, and are sometimes limited in their ability to adopt the latest IERS Conventions. During
the development of the Vienna VLBI Software (VieVS) (Bo¨hm et al. 2009 [1]) many comparisons
of intermediate results with Occam 6.1 (Titov et al. 2004 [2]) have been performed. While actual
VLBI delay models ensure accuracy up to one picosecond (0.3 mm) other parts of geodetic VLBI
modeling are much more uncertain, and discrepancies at the millimeter level could be found. The
goal of the Delay and partial Derivatives Comparison Campaign (DeDeCC) is to compare the
various software packages used within the IVS in order to detect present deficiencies in the model-
ing part which might lead to systematic errors in a final solution. In the following we want to give
a short insight into the complexity of delay modeling (Section 2), present the main ideas of the
campaign setup (Section 3.1) and show first results of comparisons between VieVS and Occam 6.1
(Section 3.2). This report ends with an outline of upcoming activities within DeDeCC (Section 4).
2. The Computed Time Delay
When radio waves emitted by an extragalactic source are received by two widely separated
antennas on the surface of the Earth, the time epoch when the same wavefront reaches station
1 and station 2 is usually different. This time delay for each baseline is the main observable of
a VLBI experiment and can be accurately determined by the correlation process. On the other
IVS 2010 General Meeting Proceedings 217
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Lucia Plank et al.: Comparison Campaign of VLBI Data Analysis Software
hand, the delay is modeled in the analysis software, which is inspected here. According to Sovers
et al. 1998 [3] the geometric time delay in the barycentric coordinate system is
τbary = (t2 − t1)bary =
~k ·~b/c
1− ~k · β2/c
(1)
where ~k is the source unit vector in the direction of signal propagation, ~b is the baseline between
station 2 and station 1 at the time of signal reception at station 1, β2 is the barycentric velocity of
station 2, and c stands for the velocity of light in vacuum. Considering the fact that the measured
time delay is referred to the Earth’s surface rather than the barycentric system, as well as the
required transformations between the celestial reference frame of the sources and the terrestrial
reference frame in which the stations are determined, equation (1) in practice has to be expanded
for several relativistic terms. A complete formulation of the VLBI time delay is provided with the
IERS consensus model described in Eubanks 1991 [4]. For the calculation of ~b the real station
locations are needed, including corrections due to station velocities, solid Earth tides, pole tide,
and ocean loading as well as atmosphere loading. For the transformation between the terrestrial
and the geocentric reference frame the actual Earth orientation has to be determined beforehand.
Besides the geometric delay described above we have to account for the gravitational bending
delay τgrav due to celestial bodies and for the propagation delay in the troposphere τtrop. Further
corrections due to the ionosphere or clock offsets are often treated separately and are not a matter
of comparison here. Generally, the decision of whether a delay correction is either applied directly
to the observation or taken into account at the modeled delay is up to each software strategy. This
leads to different interpretations of the term computed delay. For clarity, equation (2) shows the
individual parts which are included in the computed delay used in the comparison campaign.
τcomp = τgeom + τgrav + τtrop + τaxis + τthermdef (2)
As shown in Figure 1, the order of magnitude at which the various corrections and models
contribute to the total delay ranges from a few millimeters up to some tens of meters. The
examined time span covers the first three days of DeDeCC1 (see Section 3.1) and refers to a
total delay of approximately ±4,000 kilometers, corresponding to ∼13 milliseconds. After the
geometry, the biggest effect up to 30 meters comes from the troposphere depending on the local
elevation angles. Corrections of some tens of centimeters are made by the antenna axis offsets, the
solid Earth tides, and the gravitational influence of the sun. The antenna thermal correction, the
gravitational effect of the Earth, and the influence of the ocean on Earth rotation and on station
coordinates contribute at the millimeter level.
3. Comparison Campaign
3.1. DeDeCC1
For the first cycle of comparisons, named DeDeCC1, observations of the two stations Westford
and Wettzell of the source 0642+449 are examined. The observations are arranged into fifteen
24-hour sessions, fourteen consecutive days from January 1st to January 14th 2001 and a single
session one year later, on January 1st 2002. The observation interval is 30 minutes. In the reduced
mode, observations with low elevation or beneath the horizon are skipped (every day from 14:00 to
19:30). With an expected agreement at the millimeter level, the usage of identical input parameters
218 IVS 2010 General Meeting Proceedings
Lucia Plank et al.: Comparison Campaign of VLBI Data Analysis Software
−5
0
5
[10
00
 km
]
 
 
−50
0
50
[m
]
 
 
−50
0
50
[cm
]
 
 
0 0.5 1 1.5 2 2.5 3−10
0
10
[m
m]
days 
 
 
total
troposphere
axis offset Sun grav. solid Earth tides
Earth grav. thermdef ocean tides ocean loading
Figure 1. Contribution to the delay at different magnitudes.
and modeling options is essential. Detailed instructions on applied and neglected models, the
observation schedule, and the input coordinates can be found at the website of DeDeCC1. In
order to restrain the effect of the various interpolation methods used in existing software the
Earth orientation parameters, air pressure, and temperature are fixed to constant values, and
atmosphere loading is not considered in DeDeCC.
3.2. VieVS Versus Occam 6.1
A first run of DeDeCC was made with the two software packages available at our institute, the
new VieVS and the formerly used Occam 6.1.
As shown in Figure 2, the calculated delays of VieVS and Occam 6.1 differ by ±2 millimeters
during the inspected time periods. The variations are characterized by a dominant daily period and,
looking at the longer time spans, additional periods of one year and longer (approx. 5 years) can
be detected. Investigations concerning the sources of discrepancies showed that the disagreement
due to inconsistencies in the modeling of solid Earth tides, ocean loading, the troposphere, pole
tide, thermal correction, and antenna axis offset is below 75 micrometers, which is not critical.
The differences more likely originate in the diverse calculation of the Earth rotation angle realized
in both programs. Certainly a comparison with at least one further software package needs to be
done to clarify the reason for the discrepancies.
1http://mars.hg.tuwien.ac.at/∼lplank/DeDeCC/
IVS 2010 General Meeting Proceedings 219
Lucia Plank et al.: Comparison Campaign of VLBI Data Analysis Software
0 2 4 6 8 10 12 14−2
0
2
[m
m]
days since 01/01/2001
 
 
0 0.5 1 1.5 2−2
0
2
[m
m]
years since 01/01/2001
0 1 2 3 4 5 6 7−2
0
2
[m
m]
years since 01/01/2001
(a)
(b)
(c)
Figure 2. Differences VieVS minus Occam 6.1 of the calculated delays for a period of 14 consecutive days
(a), a period of two years with a 24 h session every month (b), and of seven yearly sessions (c).
4. Conclusions and Outlook
So far DeDeCC has helped to detect small bugs and to verify several correction models dur-
ing the development of VieVS. Under the guidance of the IVS Analysis Coordinator and with
the assistance of several IVS analysts, an official Call for Participation in the campaign will be
distributed within the IVS soon. Providing comparisons within DeDeCC1 are successful, further
investigations including the partial derivatives will follow. Recent developments and upcoming
results will be published on the Web (see Section 3.1).
5. Acknowledgements
Lucia Plank is thankful to the Marietta Blau Mobilita¨tsstipendium of the Vienna University
of Technology for subsidizing her participation at the IVS GM2010.
220 IVS 2010 General Meeting Proceedings
Lucia Plank et al.: Comparison Campaign of VLBI Data Analysis Software
References
[1] Boehm, J., S. Boehm, T. Nilsson, A. Pany, L. Plank, H. Spicakova, K. Teke, H. Schuh, The new Vienna
VLBI Software VieVS, In: IAG Symposia Series, Buenos Aires 2009 (to be published)
[2] Titov, O., V. Tesmer, J. Boehm, Occam v.6.0 Software for VLBI Data Analysis, In: International VLBI
Service for Geodesy and Astrometry 2004 General Meeting Proceedings, NASA/CP-2004-212255, N.R.
Vandenberg, K. Baver (eds.), pp 267-271, 2004
[3] Sovers, O. J., J. L. Fanselow, C. S. Jacobs, Astrometry and geodesy with radio interferometry: ex-
periments, models, results, In: Reviews of Modern Physics, Vol. 70, No. 4, pp 1393-1454, October
1998
[4] Eubanks, T. M. (ed.), In: Proceedings of the U. S. Naval Observatory Workshop on Relativistic Models
for Use in Space Geodesy, U. S. Naval Observatory, Washington, D. C., 1991
IVS 2010 General Meeting Proceedings 221


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