Johannes Böhm

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.

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.

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.

Path Delays in the Neutral Atmosphere

This part describes the effects of the troposphere—strictly speaking the neutral atmosphere—on the propagation delay of space geodetic signals. A theoretical description of this tropospheric propagation delay is given as well as strategies for correcting for it in the data analysis of the space geodetic observations. The differences between the tropospheric effects for microwave techniques, like the Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry (VLBI), and those for optical techniques, like Satellite Laser Ranging (SLR), are discussed. Usually, residual tropospheric delays are estimated in the data analysis, and the parameterization needed to do so is presented. Other possibilities of correcting for the tropospheric delays are their calculation by ray-tracing through the fields of numerical weather models and by utilizing water vapor radiometer measurements. Finally, we shortly discuss how space geodetic techniques can be used in atmospheric analysis in meteorology and climatology.

Atmospheric effects in space geodesy

Foreword - Preface - Acknowledgements - Geodetic and atmospheric background - Ionospheric effects on microwave signals - Path delays in the neutral atmosphere - Atmospheric pressure loading - Atmospheric effects on gravity space missions - Atmospheric effects on Earth rotation

Comparison of Ray-Tracing Packages for Troposphere Delays

A comparison campaign to evaluate and compare troposphere delays from different ray-tracing software was carried out under the umbrella of the International Association of Geodesy Working Group 4.3.3 in the first half of 2010 with five institutions participating: the GFZ German Research Centre for Geosciences (GFZ), the Groupe de Recherche de Geodesie Spatiale, the National Institute of Information and Communications Technology (NICT), the University of New Brunswick, and the Institute of Geodesy and Geophysics of the Vienna University of Technology. High-resolution data from the operational analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) for stations Tsukuba (Japan) and Wettzell (Germany) were provided to the participants of the comparison campaign. The data consisted of geopotential differences with respect to mean sea level, temperature, and specific humidity, all at isobaric levels. Additionally, information about the geoid undulations was provided, and the participants computed the ray-traced total delays for 5° elevation angle and every degree in azimuth. In general, we find good agreement between the ray-traced slant factors from the different solutions at 5° elevation if determined from the same pressure level data of the ECMWF. Standard deviations and biases are at the 1-cm level (or significantly better for some combinations). Some of these discrepancies are due to differences in the algorithms and the interpolation approaches. If compared with slant factors determined from ECMWF native model level data, the biases can be significantly larger.

Very Long Baseline Interferometry for Geodesy and Astrometry

Very Long Baseline Interferometry (VLBI) is a microwave-based space geodetic technique that measures the difference in arrival times of signals from a radio source by cross correlation. Most commonly the observed radio sources are extragalactic objects but beacons from satellites have also been used. VLBI plays a unique role in the practical realization and maintenance of the international celestial reference frame (ICRF) and contributes significantly to the international terrestrial reference frame (ITRF), in particular for its scale. It is the only technique that provides the full set of Earth orientation parameters, which are indispensable for positioning and navigation on Earth and in space. In addition, VLBI allows access to valuable information concerning interactions within the Earth system. In particular, direct measurements of nutation parameters and of the Earth rotation angle (UT1–UTC) are uniquely provided by VLBI. Furthermore, several other geodynamic, atmospheric, and astronomical parameters can be derived from the long history of VLBI measurements starting in the late 1970s. In 1999, the International Association of Geodesy (IAG) accepted the international VLBI Service for Geodesy and Astrometry (IVS) as an official IAG service and the IVS was also approved as a service of the International Astronomical Union (IAU). Since then, the coordination of world-wide VLBI observation and analysis has improved significantly, leading to valuable results for the wider scientific community. Since 2005, the IVS has been working on a new VLBI system in terms of hardware, software, and operational procedures, known as VLBI2010. The IVS recommended a review of all current VLBI systems and processes from antennas to analysis and outlined the path to the next-generation system with unprecedented new capabilities envisaged: 1 mm position and 0.1 mm/year velocity accuracy on global scales, continuous measurements to obtain uninterrupted time series of station positions and Earth orientation parameters, and a turnaround time from the observations to initial geodetic results of less than 24 h. This new system will be realized in the coming years.

Recent Progress in the VLBI2010 Development

From October 2003 to September 2005, the International VLBI Service for Geodesy and Astrometry (IVS) examined current and future requirements for geodetic VLBI, including all components from antennas to analysis. IVS Working Group 3 “VLBI 2010”, which was tasked with this effort, concluded with recommendations for a new generation of VLBI systems. These recommendations were based on the goals of achieving 1 mm measurement accuracy on global baselines, performing continuous measurements for time series of station positions and Earth orientation parameters, and reaching a turnaround time from measurement to initial geodetic results of less than 24 h. To realize these recommendations and goals, along with the need for low cost of construction and operation, requires a complete examination of all aspects of geodetic VLBI including equipment, processes, and observational strategies. Hence, in October 2005, the IVS VLBI2010 Committee (V2C) commenced work on defining the VLBI2010 system specifications. In this paper we give a summary of the recent progress of the VLBI2010 project

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.

Sub-Diurnal Earth Rotation Variations Observed by VLBI

Sub-Diurnal Earth Rotation Variations Observed by VLBI We analyse sub-diurnal Earth rotation variations obtained from the continuous VLBI experiments CONT02, CONT05, and CONT08. We find that the Earth rotation parameters estimated from these campaigns contain signals with periods ±12 hours, +24 hours, and in CONT02 also -8 hours, which cannot be explained by the current IERS sub-diurnal pole model. We investigate if these signals could be caused by atmospheric excitations, but find that these excitations are too small.