Lucia Plank

The New Vienna VLBI Software VieVS

New VLBI (Very Long Baseline Interferometry) data analysis software (called Vienna VLBI Software, VieVS) is being developed at the Institute of Geodesy and Geophysics in Vienna taking into consideration all present and future VLBI2010 requirements. The programming language MATLAB is used, which considerably eases the programming efforts because of many built-in functions and tools. MATLAB is the high-end programming language of the students at the Vienna University of Technology and at many other institutes worldwide. VieVS is equipped with the most recent models recommended by the IERS Conventions. The parameterization with piece-wise linear offsets at integer hours in the least-squares adjustment provides flexibility for the combination with other space geodetic techniques. First comparisons with other VLBI software packages show a very good agreement, and there are plans to add further features to VieVS, e.g. capabilities for Kalman filtering, phase delay solutions, and spacecraft tracking.

Atmospheric Effects on VLBI-Derived Terrestrial and Celestial Reference Frames

We introduce our new terrestrial and celestial reference frames VieTRF10a and VieCRF10a, which have been estimated by the Vienna VLBI Software VieVS using VLBI observations since 1984. Details are provided about the computation, and comparisons are made with VTRF2008 and ICRF2, respectively, in terms of transformation parameters. Furthermore, we reaffirm the essentiality of a proper handling of horizontal tropospheric gradients and point out the systematic effect on the coordinates which arises through the use of constraints. We also assess the impact of two different mapping functions (GMF vs. VMF1) on terrestrial (TRF) and celestial reference frames, showing the scale difference between the TRF of 0.08 ppb which corresponds to 0.5 mm in height change.

Scheduling VLBI observations to satellites with VieVS

Observations of satellites with Very Long Baseline Interferometry (VLBI) radio telescopes provide a variety of new possibilities such as the integration of different geodetic techniques, which is one of the main goals of GGOS, the Global Geodetic Observing System of the IAG. Promising applications can be found, among others, in the field of inter-technique frame ties. With the standard geodetic VLBI scheduling software not being prepared to use satellites as radio sources so far, such observations were complicated due to the need to carefully prepare the required interchange files. The newly developed Satellite Scheduling Module for the Vienna VLBI Software (VieVS) offers a solution to this. It allows the user to prepare VLBI schedule files in a standardized format, providing the possibility to carry out actual satellite observations with standard geodetic antennas, e.g. of the IVS network. First successful observations of GLONASS satellites, based on schedules created with the new VieVS module, took place on the baseline Wettzell-Onsala in January 2014.

The AUSTRAL VLBI observing program

The AUSTRAL observing program was started in 2011, performing geodetic and astrometric very long baseline interferometry (VLBI) sessions using the new Australian AuScope VLBI antennas at Hobart, Katherine, and Yarragadee, with contribution from the Warkworth (New Zealand) 12 m and Hartebeesthoek (South Africa) 15 m antennas to make a southern hemisphere array of telescopes with similar design and capability. Designed in the style of the next-generation VLBI system, these small and fast antennas allow for a new way of observing, comprising higher data rates and more observations than the standard observing sessions coordinated by the International VLBI Service for Geodesy and Astrometry (IVS). In this contribution, the continuous development of the AUSTRAL sessions is described, leading to an improvement of the results in terms of baseline length repeatabilities by a factor of two since the start of this program. The focus is on the scheduling strategy and increased number of observations, aspects of automated operation, and data logistics, as well as results of the 151 AUSTRAL sessions performed so far. The high number of the AUSTRAL sessions makes them an important contributor to VLBI end-products, such as the terrestrial and celestial reference frames and Earth orientation parameters. We compare AUSTRAL results with other IVS sessions and discuss their suitability for the determination of baselines, station coordinates, source coordinates, and Earth orientation parameters.

Simulated VLBI Satellite Tracking of the GNSS Constellation: Observing Strategies

Very Long Baseline Interferometry (VLBI) observations to satellite targets is a promising technique to improve future realizations of terrestrial reference frames (TRF). The high number of available satellites of Global Navigation Satellite Systems (GNSS) provides an attractive existing infrastructure that could be utilized for such observations. The Vienna VLBI Software (VieVS) was extended for the possibilities of scheduling, simulating, and processing VLBI observations to GNSS satellites, allowing to give information on expected accuracies of derived station coordinates. Assuming the GNSS signals to be measured with a precision of 30 ps, we find weekly station position repeatabilities at the centimeter level or better for simulated observations to satellite targets only. Adequate scheduling strategies have to be applied, e.g. in terms of a fast switching between the observed satellites. Even better solutions of about 5 mm in mean 3D position rms after one day are achieved when integrating the satellite observations into standard VLBI sessions to extragalactic radio sources. Further, this combined approach allows the determination of a frame tie between the satellite system and the VLBI system in terms of relative Earth rotation parameters and a scale with a precision of about 1–2 mm at the Earth’s surface.

Results from the regional AUSTRAL VLBI sessions for Southern Hemisphere reference frames

The AUSTRAL observing program is an initiative led by the Australian AuScope VLBI antennas in collaboration with radio telescopes in Warkworth, New Zealand, and Hartebeesthoek, South Africa. In 2014 the number of AUSTRAL sessions increased tremendously. Comparing recent results to the standard products achieved in global VLBI sessions regularly undertaken by the International VLBI Service for Geodesy and Astrometry (IVS), better accuracies in terms of baseline length repeatabilities are found for these regional AUSTRAL sessions. The network of (almost) identical small and fast telescopes as well as the technical equipment at all stations allows for new observing modes and improved operations, as such serving as a testbed for the future VLBI Global Observing System (VGOS). Special AUST-Astro sessions are used for dedicated astrometry of sparsely observed radio sources in the southern sky, as well as for detecting new radio sources for geodesy. In 2015, the AUSTRAL program will be further increased and final steps are now being undertaken for full VGOS compatibility of the three AuScope VLBI antennas. We present the latest results of the AUSTRAL sessions and give an overview of the multiple areas of research they support.

The Effects of Simulated and Observed Quasar Structure on the VLBI Reference Frame

Radio-loud quasars making up the Celestial Reference Frame are dynamic objects with significant structure that changes on timescales of months and years. This is a problem for geodetic VLBI, which has so far largely treated quasars as point sources in analysis. We quantify the effects of various levels of source structure on the terrestrial (TRF) and celestial (CRF) reference frames using the source structure simulator recently implemented in the Vienna VLBI Software (VieVS) package. We find that source structure affects station positions at the level of 0.2–1 mm. While quasar structure contributes only \(\sim \) 10% to the total TRF error budget, which is dominated by tropospheric turbulence; the effect of quasar structure on the CRF is discernible even in present-day observations. Astrophysical properties of quasars are related to their structure and geodetic stability, and we discuss several quasar structure mitigation strategies. These include: (1) astrophysically-based quasar selection techniques; (2) scheduling sources by taking into account source structure; and (3) analyzing geodetic observations using knowledge of source structure. We find that for observed highly variable quasars, flux density is strongly anti-correlated with structure and position stability, suggesting that such quasars should preferentially be observed in their bright phase. We use simulations to investigate new scheduling strategies which avoid unfavourable jet—baseline orientations. Improvement is seen at the millimetre level on the longest baselines when our new scheduling strategy is used in simulations that only include quasar structure. This improvement disappears in the full simulations including the troposphere, because we are compromising sky coverage in order to mitigate source structure effects. This again confirms that, at present, tropospheric turbulence dominates the accuracy of TRF determination. However, the contribution of quasar structure will become more important as tropospheric effects decrease in future broadband observations.

The Processing of Single Differenced GNSS Data with VLBI Software

Space geodetic techniques such as Very Long Baseline Interferometry (VLBI) and Global Navigation Satellite Systems (GNSS) are used for the determination of celestial and terrestrial reference frames and Earth orientation parameters. It is potentially valuable to combine the observations from the different techniques to fully exploit the strengths and unique characteristics of the techniques. Today, discrepancies of locally measured ties between reference points of two techniques and the space geodesy results are a potential issue in the determination of reference frames. To improve the link between the techniques, tests are under way to observe GNSS signals with VLBI radio telescopes directly, and to observe GNSS signals in GNSS antennas with subsequent processing in the VLBI system (“GNSS-VLBI Hybrid System”) including VLBI correlation. In both cases, the GNSS data type is the difference in travel time from the satellite to two ground stations. However, it is still difficult to acquire those observations and thus we apply post-processed phase measurements from a precise point positioning (PPP) solution with the c5++ software to build those difference values which are then used in the Vienna VLBI Software (VieVS). We take seven GNSS sites, exclusively Global Positioning System (GPS) in this study, co-located with CONT11 VLBI sites to validate the models in VieVS for single differenced GNSS data, and estimate geodetic parameters. We find root mean square values of post-fit residuals for the VLBI-like observations of about 3.3 cm, compared to less than 2.0 cm from the GNSS PPP solution. At this stage, we do also find degradation in station coordinate repeatabilities (by a factor of 2 to 8), which is related to the systematic residuals.

Systematic Errors of a VLBI Determined TRF Investigated by Simulations

In this study, we investigate the influence of different analysis setup options for the processing of VLBI measurement data from 2002 until 2010 to derive the terrestrial reference frame (TRF). For estimating the consequent changes of the TRF, the simulation tool of the Vienna VLBI Software (VieVS) is used by applying different a priori models. We show that neglecting atmosphere loading causes a systematic annual scale variation of±0.3 mm, and that the application of the cubic model recommended in the most recent IERS Conventions for the mean pole introduces a scale change of −0.6mm over 8.5years. The effects of antenna thermal deformation on the TRF are±0.5 to 1mm/year in translation and±2 mm/year in scale. No systematic effects are found for the different troposphere mapping functions tested. Besides systematic, annual, and episodic impacts on the coordinates, we focus on possible changes in the scale of the reference frames.