212 research outputs found
Free Core Nutation observed by VLBI
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. 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. 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.Comment: 5 page
Interaction between celestial and terrestrial reference frames and some considerations for the next VLBI-based ICRF
In this paper we outline several problems related to the realization of the
international celestial and terrestrial reference frames ICRF and ITRF at the
millimeter level of accuracy, with emphasis on ICRF issues. The main topics
considered are: analysis of the current status of the ICRF, mutual impact of
ICRF and ITRF, and some considerations for future ICRF realizations.Comment: Presented at the Journees 2011: Earth rotation, reference systems and
celestial mechanics: Synergies of geodesy and astronomy, Vienna, Austria, Sep
19-2
A Decentralized Processing Schema for Efficient and Robust Real-time Multi-GNSS Satellite Clock Estimation
Real-time multi-GNSS precise point positioning (PPP) requires the support of high-rate satellite clock corrections. Due to the large number of ambiguity parameters, it is difficult to update clocks at high frequency in real-time for a large reference network. With the increasing number of satellites of multi-GNSS constellations and the number of stations, real-time high-rate clock estimation becomes a big challenge. In this contribution, we propose a decentralized clock estimation (DECE) strategy, in which both undifferenced (UD) and epoch-differenced (ED) mode are implemented but run separately in different computers, and their output clocks are combined in another process to generate a unique product. While redundant UD and/or ED processing lines can be run in offsite computers to improve the robustness, processing lines for different networks can also be included to improve the clock quality. The new strategy is realized based on the Position and Navigation Data Analyst (PANDA) software package and is experimentally validated with about 110 real-time stations for clock estimation by comparison of the estimated clocks and the PPP performance applying estimated clocks. The results of the real-time PPP experiment using 12 global stations show that with the greatly improved computational efficiency, 3.14 cm in horizontal and 5.51 cm in vertical can be achieved using the estimated DECE clock
Tidal Love and Shida numbers estimated by geodetic VLBI
AbstractFrequency-dependent Love and Shida numbers, which characterize the Earth response to the tidal forces, were estimated in a global adjustment of all suitable geodetic Very Long Baseline Interferometry (VLBI) sessions from 1984.0 to 2011.0. Several solutions were carried out to determine the Love and Shida numbers for the tidal constituents at periods in the diurnal band and in the long-period band in addition to values of the Love and Shida numbers common for all tides of degree two. Adding up all twelve diurnal tidal waves that were estimated, the total differences in displacement with respect to the theoretical conventional values of the Love and Shida numbers calculated from an Earth model reach 1.73±0.29mm in radial direction and 1.15±0.15mm in the transverse plane. The difference in the radial deformation following from the estimates of the zonal Love numbers is largest for the semi-annual tide Ssa with 1.07±0.19mm
Comparison Campaign of VLBI Data Analysis Software - First Results
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
Prospects for UT1 Measurements from VLBI Intensive Sessions
Very Long Baseline Interferometry (VLBI) Intensives are one-hour single baseline sessions to provide Universal Time (UT1) in near real-time up to a delay of three days if a site is not e-transferring the observational data. Due to the importance of UT1 estimates for the prediction of Earth orientation parameters, as well as any kind of navigation on Earth or in space, there is not only the need to improve the timeliness of the results but also their accuracy. We identify the asymmetry of the tropospheric delays as the major error source, and we provide two strategies to improve the results, in particular of those Intensives which include the station Tsukuba in Japan with its large tropospheric variation. We find an improvement when (1) using ray-traced delays from a numerical weather model, and (2) when estimating tropospheric gradients within the analysis of Intensive sessions. The improvement is shown in terms of reduction of rms of length-of-day estimates w.r.t. those derived from Global Positioning System observation
Estimating integrated water vapor trends from VLBI, GPS,and numerical weather models: sensitivity totropospheric parameterization
©2018. American Geophysical UnionIn this study, we estimate integrated water vapor (IWV) trends from very long baseline interferometry (VLBI) and global navigation satellite systems (GNSS) data analysis, as well as from numerical weather models (NWMs). We study the impact of modeling and parameterization of the tropospheric delay from VLBI on IWV trends. We address the impact of the meteorological data source utilized to model the hydrostatic delay and the thermal deformation of antennas, as well as the mapping functions employed to project zenith delays to arbitrary directions. To do so, we derive a new mapping function, called Potsdam mapping functions based on NWM data and a new empirical model, GFZ‐PT. GFZ‐PT differs from previous realizations as it describes diurnal and subdiurnal in addition to long‐wavelength variations, it provides harmonic functions of ray tracing‐derived gradients, and it features robustly estimated rates. We find that alternating the mapping functions in VLBI data analysis yields no statistically significant differences in the IWV rates, whereas alternating the meteorological data source distorts the trends significantly. Moreover, we explore methods to extract IWV given a NWM. The rigorously estimated IWV rates from the different VLBI setups, GNSS, and ERA‐Interim are intercompared, and a good agreement is found. We find a quite good agreement comparing ERA‐Interim to VLBI and GNSS, separately, at the level of 75%.DFG, 255986470, GGOS-SIM-2: Simulation des "Global Geodetic Observing System
The source structure of 0642+449 detected from the CONT14 observations
The CONT14 campaign with state-of-the-art VLBI data has observed the source
0642+449 with about one thousand observables each day during a continuous
observing period of fifteen days, providing tens of thousands of closure
delays---the sum of the delays around a closed loop of baselines. The closure
delay is independent of the instrumental and propagation delays and provides
valuable additional information about the source structure. We demonstrate the
use of this new "observable" for the determination of the structure in the
radio source 0642+449. This source, as one of the defining sources in the
second realization of the International Celestial Reference Frame (ICRF2), is
found to have two point-like components with a relative position offset of -426
microarcseconds in right ascension and -66 microarcseconds in declination. The
two components are almost equally bright with a flux-density ratio of 0.92. The
standard deviation of closure delays for source 0642+449 was reduced from 139
ps to 90 ps by using this two-component model. Closure delays larger than one
nanosecond are found to be related to the source structure, demonstrating that
structure effects for a source with this simple structure could be up to tens
of nanoseconds. The method described in this paper does not rely on a priori
source structure information, such as knowledge of source structure determined
from direct (Fourier) imaging of the same observations or observations at other
epochs. We anticipate our study to be a starting point for more effective
determination of the structure effect in VLBI observations.Comment: 14 pages, 13 figures, Accepted by Astronomical Journal on 12 Jul,
201
Effects of inter-annual water storage variations on polar motion
This paper focuses on the contribution of inter-annual hydrological mass redistributions to the excitation of polar motion. Variations in hydrological angular momenta are computed from the Land Dynamics Model (LaD) for the period between 1986 January and 2004 May. For validation, the numerical results for the hydrological excitations are compared with respective time-series derived from geodetic observations. In order to provide a comparable reference, the latter are reduced by atmospheric and oceanic effects which are the prominent contributors to polar motion excitation on subdiurnal to decadal timescales. Both the hydrological and the geodetic excitation series are low-pass filtered by means of a Vondrak filter in order to remove the dominant annual oscillations. For the comparison of hydrological and geodetic excitations, wavelet scalograms and cross-scalograms along with the respective normed coherency are computed. Analyses reveal that the hydrological mass redistributions deduced from LaD contribute to polar motion excitation at retrograde periods around 4 yr, although the signal energy is smaller in the hydrological excitations than that in the residual geodetic excitations.Facultad de Ciencias Astronómicas y Geofísica
Retrieving Precipitable Water Vapor From Shipborne Multi‐GNSS Observations
©2019. American Geophysical UnionPrecipitable water vapor (PWV) is an important parameter for climate research and a crucial factor to achieve high accuracy in satellite geodesy and satellite altimetry. Currently Global Navigation Satellite System (GNSS) PWV retrieval using static Precise Point Positioning is limited to ground stations. We demonstrated the PWV retrieval using kinematic Precise Point Positioning method with shipborne GNSS observations during a 20‐day experiment in 2016 in Fram Strait, the region of the Arctic Ocean between Greenland and Svalbard. The shipborne GNSS PWV shows an agreement of ~1.1 mm with numerical weather model data and radiosonde observations, and a root‐mean‐square of ~1.7 mm compared to Satellite with ARgos and ALtiKa PWV. An improvement of 10% is demonstrated with the multi‐GNSS compared to the Global Positioning System solution. The PWV retrieval was conducted under different sea state from calm water up to gale. Such shipborne GNSS PWV has the promising potential to improve numerical weather forecasts and satellite altimetry
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