John Gipson

THE SECOND REALIZATION OF THE INTERNATIONAL CELESTIAL REFERENCE FRAME BY VERY LONG BASELINE INTERFEROMETRY

We present the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30 years of Very Long Baseline Interferometry observations. ICRF2 contains precise positions of 3414 compact radio astronomical objects and has a positional noise floor of ∼40 μas and a directional stability of the frame axes of ∼10 μas. A set of 295 new “defining” sources was selected on the basis of positional stability and the lack of extensive intrinsic source structure. The positional stability of these 295 defining sources and their more uniform sky distribution eliminates the two greatest weaknesses of the first realization of the International Celestial Reference Frame (ICRF1). Alignment of ICRF2 with the International Celestial Reference System was made using 138 positionally stable sources common to both ICRF2 and ICRF1. The resulting ICRF2 was adopted by the International Astronomical Union as the new fundamental celestial reference frame, replacing ICRF1 as of 2010 January 1.

Very long baseline interferometry determination of neglected tidal terms in high-frequency Earth orientation variation

I present a model of high-frequency earth orientation parameter (HF-EOP) variation derived from 15 years of very long baseline interferometry (VLBI) data. This model uses twice as much data as previously published VLBI models. I estimate the coefficients of HF-EOP at all tides with a magnitude greater than 5 mm in the tidal potential. This includes sidebands of the larger tides K1, O1, M2, and N2. I compare this model with other empirical models derived from VLBI and satellite laser ranging (SLR) data and also with predictions for HF-EOP variation derived from models of the ocean. This model has the best agreement of any VLBI model with the independent SLR results: The RMS level of agreement of the coefficients is 1.2 μs in UT1 and 9 microseconds of arc (μas) in polar motion (PM). This model also has the best agreement of any VLBI or SLR model with the predictions of the best ocean model: The RMS level of agreement of the coefficients are 1.1 μs in UT1 and 6.5 μas in PM. I also compare the predictions of this model with the hourly measurements of EOP from the CONT94 VLBI campaign. The residuals are 8.9 μs in UT1 and 222 μas in PM, which are consistent with the formal errors of the measurements.

A bound on the rheology of continental lithosphere using very long baseline interferometry : the velocity of south China with respect to Eurasia

Very long baseline interferometric (VLBI) measurements using the antenna near Shanghai, China, between 1987 and 1994 show that this site moves at 8 ± 0.5 mm/yr (1σ) in the direction N116.5°E (±4. 1°) with respect to VLBI sites in Europe on the Eurasia plate. Independent measurements of the site at Shanghai with respect to sites on nearly all major plates (Eurasia, North America, Pacific, Australia, and Africa) determine this velocity. South China translates east-southeast relative to Eurasia more slowly than 20% of Indias roughly 50 mm/yr convergence rate with Eurasia and hence accommodates only a small fraction of Indias penetration into Eurasia. Apparently, crustal thickening absorbs most of Indias convergence. The relatively low speed of south China supports treatments of continental lithosphere as a thin viscous sheet whose strength is governed primarily by power law creep in the upper mantle, and not by friction in the crust.

Atmospheric pressure loading parameters from very long baseline interferometry observations

Atmospheric mass loading produces a primarily vertical displacement of the Earths crust. This displacement is correlated with surface pressure and is large enough to be detected by very long baseline interferometry (VLBI) measurements. Using the measured surface pressure at VLBI stations, we have estimated the atmospheric loading term for each station location directly from VLBI data acquired from 1979 to 1992. Our estimates of the vertical sensitivity to change in pressure range from 0 to −0.6 mm/mbar depending on the station. These estimates agree with inverted barometer model calculations (Manabe et al., 1991; vanDam and Herring, 1994) of the vertical displacement sensitivity computed by convolving actual pressure distributions with loading Greens functions. The pressure sensitivity tends to be smaller for stations near the coast, which is consistent with the inverted barometer hypothesis. Applying this estimated pressure loading correction in standard VLBI geodetic analysis improves the repeatability of estimated lengths of 25 out of 37 baselines that were measured at least 50 times. In a root-sum-square (rss) sense, the improvement generally increases with baseline length at a rate of about 0.3 to 0.6 ppb depending on whether the baseline stations are close to the coast. For the 5998-km baseline from Westford, Massachusetts, to Wettzell, Germany, the rss improvement is about 3.6 mm out of 11.0 mm. The average rss reduction of the vertical scatter for inland stations ranges from 2.7 to 5.4 mm.

Tidal station displacements

Theoretical modeling of the station displacements produced by tidal deformations of the Earth due to lunisolar gravitational forces is a necessary part of the analysis of space geodetic data. To attain the accuracies demanded by the precision of the data, a generalized version of the Love number formalism has to be used wherein the classical Love and Shida numbers are replaced by sets of Love number parameters with values that depend on the frequency of the tidal excitation. This paper presents theoretical expressions for the contributions from the various Love number parameters to the station displacement vector and a scheme for efficient computation of these contributions, taking into account the frequency dependence of these parameters as well as the imaginary parts that are present when the effects of mantle anelasticity are taken into account. A brief discussion of the permanent tide which arises from the zero frequency component of the tide-generating potential is included.

Very long baseline interferometry and active rotations of crustal blocks in the Western Transverse Ranges, California

Changes in baseline vectors between very long baseline interferometry (VLBI) receiving stations in the Western Transverse Ranges imply that east-west blocks of crust in this region rotate clockwise about vertical axes with respect to the Pacific and North American plates. Minimum apparent rotations, given by the ratios between components of velocity perpendicular to baseline vectors and the lengths of the baselines, imply minimum current rotation rates of a few degrees per million years. The relevant VLBI receivers lie on different crustal blocks that are separated by major active faults. Both geologic and other geodetic observations imply north-south convergence between such blocks at several millimeters per year. Corrections to perpendicular components of velocity for such relative movements between blocks yield likely clockwise rotation rates of 6°/m.y. ± 2°/m.y., which are indistinguishable from the average rate inferred from paleomagnetic declinations of rocks in the Western Transverse Ranges with ages less than 15 m.y. Thus, rotation seems to have occurred continuously and apparently with only small variations in rate during a period when the tectonics of southern California changed dramatically. This apparent independence of the rotation rate on the changing surface kinematics is consistent (1) with such rotation being a manifestation of continuous deformation at depth in the lower crust and upper mantle, (2) with weak faults separating upper-crustal blocks, and (3) with the important resistance to continental deformation lying in the upper mantle and/or lower crust.

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

Site displacement due to variation in Earth rotation

We study site displacement caused by changes in the rate of rotation of the Earth and in the orientation of the Earths pole. We include small effects which have been previously neglected, most notably those caused by polar motion induced ocean loading. Using very long baseline interferometry (VLBI) data we estimate the real and imaginary Love numbers associated with vertical and horizontal deformations caused by polar motion. The estimates of the real part of the Love numbers are consistent with theoretical predictions, although the imaginary parts are too large. Including the loading effect improves the agreement of the real part.

Tropospheric delay ray tracing applied in VLBI analysis

Tropospheric delay modeling error continues to be one of the largest sources of error in VLBI (very long baseline interferometry) analysis. For standard operational solutions, we use the VMF1 elevation-dependent mapping functions derived from European Centre for Medium-Range Weather Forecasts data. These mapping functions assume that tropospheric delay at a site is azimuthally symmetric. As this assumption is not true, we have instead determined the ray trace delay along the signal path through the troposphere for each VLBI quasar observation. We determined the troposphere refractivity fields from the pressure, temperature, specific humidity, and geopotential height fields of the NASA Goddard Space Flight Center Goddard Earth Observing System version 5 numerical weather model. When applied in VLBI analysis, baseline length repeatabilities were improved compared with using the VMF1 mapping function model for 72% of the baselines and site vertical repeatabilities were better for 11 of 13 sites during the 2 week CONT11 observing period in September 2011. When applied to a larger data set (2011–2013), we see a similar improvement in baseline length and also in site position repeatabilities for about two thirds of the stations in each of the site topocentric components.

IVS Observation of ICRF2-Gaia Transfer Sources

The second realization of the International Celestial Reference Frame (ICRF2), which is the current fundamental celestial reference frame adopted by the International Astronomical Union, is based on Very Long Baseline Interferometry (VLBI) data at radio frequencies in X band and S band. The European Space Agency’s Gaia mission, launched on 2013 December 19, started routine scientific operations in 2014 July. By scanning the whole sky, it is expected to observe ∼500,000 Quasi Stellar Objects in the optical domain an average of 70 times each during the five years of the mission. This means that, in the future, two extragalactic celestial reference frames, at two different frequency domains, will coexist. It will thus be important to align them very accurately. In 2012, the Laboratoire d’Astrophysique de Bordeaux (LAB) selected 195 sources from ICRF2 that will be observed by Gaia and should be suitable for aligning the radio and optical frames: they are called ICRF2-Gaia transfer sources. The LAB submitted a proposal to the International VLBI Service (IVS) to regularly observe these ICRF2-Gaia transfer sources at the same rate as Gaia observes them in the optical realm, e.g., roughly once a month. We describe our successful effort to implement such a program and report on the results. Most observations of the ICRF2-Gaia transfer sources now occur automatically as part of the IVS source monitoring program, while a subset of 37 sources requires special attention. Beginning in 2013, we scheduled 25 VLBI sessions devoted in whole or in part to measuring these 37 sources. Of the 195 sources, all but one have been successfully observed in the 12 months prior to 2015 September 01. Of the sources, 87 met their observing target of 12 successful sessions per year. The position uncertainties of all of the ICRF2-Gaia transfer sources have improved since the start of this observing program. For a subset of 24 sources whose positions were very poorly known, the uncertainty has decreased, on average, by a factor of four. This observing program is successful because the two main goals were reached for most of the 195 ICRF2-Gaia transfer sources: observing at the requested target of 12 successful sessions per year and improving the position uncertainties to better than 200 μas for both R.A. and decl. However, scheduling some of the transfer sources remains a challenge because of network geometry and the weakness of the sources, and this will be one focus of future sessions used in this ongoing program.