Hana Krásná

GPT2: Empirical slant delay model for radio space geodetic techniques

Up to now, state-of-the-art empirical slant delay modeling for processing observations from radio space geodetic techniques has been provided by a combination of two empirical models. These are GPT (Global Pressure and Temperature) and GMF (Global Mapping Function), both operating on the basis of long-term averages of surface values from numerical weather models. Weaknesses in GPT/GMF, specifically their limited spatial and temporal variability, are largely eradicated by a new, combined model GPT2, which provides pressure, temperature, lapse rate, water vapor pressure, and mapping function coefficients at any site, resting upon a global 5° grid of mean values, annual, and semi-annual variations in all parameters. Built on ERA-Interim data, GPT2 brings forth improved empirical slant delays for geophysical studies. Compared to GPT/GMF, GPT2 yields a 40% reduction of annual and semi-annual amplitude differences in station heights with respect to a solution based on instantaneous local pressure values and the Vienna mapping functions 1, as shown with a series of global VLBI (Very Long Baseline Interferometry) solutions.

New VLBI2010 scheduling strategies and implications on the terrestrial reference frames

In connection with the work for the next generation VLBI2010 Global Observing System (VGOS) of the International VLBI Service for Geodesy and Astrometry, a new scheduling package (Vie_Sched) has been developed at the Vienna University of Technology as a part of the Vienna VLBI Software. In addition to the classical station-based approach it is equipped with a new scheduling strategy based on the radio sources to be observed. We introduce different configurations of source-based scheduling options and investigate the implications on present and future VLBI2010 geodetic schedules. By comparison to existing VLBI schedules of the continuous campaign CONT11, we find that the source-based approach with two sources has a performance similar to the station-based approach in terms of number of observations, sky coverage, and geodetic parameters. For an artificial 16 station VLBI2010 network, the source-based approach with four sources provides an improved distribution of source observations on the celestial sphere. Monte Carlo simulations yield slightly better repeatabilities of station coordinates with the source-based approach with two sources or four sources than the classical strategy. The new VLBI scheduling software with its alternative scheduling strategy offers a promising option with respect to applications of the VGOS.

Free core nutation observed by VLBI

Aims. The signature of free core nutation (FCN) is found in the motion of the celestial intermediate pole in the celestial reference frame and in the resonance behaviour of the frequency-dependent Earth tidal displacement in its diurnal band. We focus on estimation of the FCN parameters, i.e. the period and amplitude. Methods. We run several global adjustments of 27 years of very long baseline interferometry (VLBI) data (1984.0–2011.0) to determine the FCN period from partial derivatives of the VLBI observables with respect to the FCN as contained in the nutation of the celestial intermediate pole and in the solid Earth tidal displacement in the diurnal band. Finally, we estimate the FCN period by a global adjustment from both phenomena simultaneously, which has not been done before. Results. We find that our estimate of the FCN period of −431.18 ± 0.10 sidereal days slightly deviates from the conventional value of −431.39 sidereal days. Additionally, we present our empirical model of the FCN with variable amplitude and phase compatible with the estimated period.

Atmospheric Pressure Loading

Loading of the Earth’s crust due to variations of global atmosphere pressure can displace the positions of geodetic sites by more than 1 cm both vertically and horizontally on annual to sub-diurnal time scales, and thus has to be taken into account in the analysis of space geodetic observations. This part of the book discusses methods for the calculation of the displacements. In particular, it summarizes the simple approach with regression coefficients between surface pressure and the vertical displacement and the more rigorous geophysical approach with load Love numbers and Green’s functions. Furthermore, we describe the special treatment of the thermal tides (S1 and S2), the importance of the reference pressure, as well as the inverted barometer hypothesis for the oceans. Finally, we present space geodetic results with the application of those correction models for the analysis of Very Long Baseline Interferometry observations.

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.

Non-linear VLBI station motions and their impact on the celestial reference frame and Earth orientation parameters

The increasing accuracy and growing time span of Very Long Baseline Interferometry (VLBI) observations allow the determination of seasonal signals in station positions which still remain unmodelled in conventional analysis approaches. In this study we focus on the impact of the neglected seasonal signals in the station displacement on the celestial reference frame and Earth orientation parameters. We estimate empirical harmonic models for selected stations within a global solution of all suitable VLBI sessions and create mean annual models by stacking yearly time series of station positions which are then entered a priori in the analysis of VLBI observations. Our results reveal that there is no systematic propagation of the seasonal signal into the orientation of celestial reference frame but position changes occur for radio sources observed non-evenly over the year. On the other hand, the omitted seasonal harmonic signal in horizontal station coordinates propagates directly into the Earth rotation parameters causing differences of several tens of microarcseconds.

Tropospheric delay modelling and the celestial reference frame at radio wavelengths

Aims. We examine the relationship between Very Long Baseline Interferometry (VLBI) tropospheric delay modelling and source positions. In particular, the effect of a priori ray-traced slant delays on source declination is investigated. Methods. We estimated source coordinates as global positions from 5830 geodetic VLBI sessions incorporating about 10 million group delay measurements. This data set was used for the International Celestial Reference Frame 3 (ICRF3) prototype solutions as of December 2016. Results. We report on a significant bias in source declination of about 50 μ as, which can be found between a normal solution and a solution where a priori ray-traced slant delays are used. More traditional tropospheric delay modelling techniques, such as a priori gradients, are tested as well. Significant differences of about 30 μ as in declination can only be found when absolute constraints are used for a priori gradient models. Further, we find that none of these models decrease the declination bias between ICRF3 prototype solutions and ICRF2.

Comparison of VieVS and Solve UT1 results from VLBI measurements

Comparison of different geodetic VLBI analysis software packages is highly important to ensure that they work properly. Two of the data software packages used by the geodetic VLBI community are Solve and the Vienna VLBI Software (VieVS) packages. In our study we investigate UT1 estimates from VieVS and Solve for both 1-hour IVS Intensives and 24 hour R1s and R4s with various configurations. We attempted to synchronize the settings of the two software packages as much as possible without extensive software changes. In this configuration the weighted mean difference of the UT1 estimates from the IVS intensives was 7.77 μs with an WRMS scatter of 7.14 μs. The weighted mean difference for the 24 hour sessions was -1.66 μs with an WRMS scatter of 5.46 μs. We also investigated the effect of changing other aspects of the solution configurations. In general the resulting differences were small except for imposing a tight clock constraint which significantly worsened the solutions.

Measurement of the solar system acceleration using the Earth scale factor

We propose an alternative method to detect the secular aberration drift induced by the solar system acceleration due to the attraction to the Galaxy centre. This method is free of the individual radio source proper motion caused by intrinsic structure variation. We developed a procedure to estimate the scale factor directly from very long baseline interferometry (VLBI) data analysis in a source-wise mode within a global solution. The scale factor is estimated for each reference radio source individually as a function of astrometric coordinates (right ascension and declination). This approach splits the systematic dipole effect and uncorrelated motions on the level of observational parameters. We processed VLBI observations from 1979.7 to 2016.5 to obtain the scale factor estimates for more than 4,000 reference radio sources. We show that the estimates highlight a dipole systematics aligned with the direction to the centre of the Galaxy. With this method we obtained a Galactocentric acceleration vector with an amplitude of 5.2

Analysis Strategies for the Densification of the ICRF with VLBA Calibrator Survey Sources

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