MacMillan D.S.,NVI, Inc.
Journal of Geodesy | Year: 2017
Continuous (CONT) VLBI campaigns have been carried out about every 3 years since 2002. The basic idea of these campaigns is to acquire state-of-the-art VLBI data over a continuous time period of about 2 weeks to demonstrate the highest accuracy of which the current VLBI system is capable. In addition, these campaigns support scientific studies such as investigations of high-resolution Earth rotation, reference frame stability, and daily to sub-daily site motions. The size of the CONT networks and the observing data rate have increased steadily since 1994. Performance of these networks based on reference frame scale precision and polar motion/LOD comparison with global navigation satellite system (GNSS) earth orientation parameters (EOP) has been substantially better than the weekly operational R1 and R4 series. The precisions of CONT EOP and scale have improved by more than a factor of two since 2002. Polar motion precision based on the WRMS difference between VLBI and GNSS for the most recent CONT campaigns is at the 30 (Formula presented.)as level, which is comparable to that of GNSS. The CONT campaigns are a natural precursor to the planned future VLBI observing networks, which are expected to observe continuously. We compare the performance of the most recent CONT campaigns in 2011 and 2014 with the expected performance of the future VLBI global observing system network using simulations. These simulations indicate that the expected future precision of scale and EOP will be at least 3 times better than the current CONT precision. © 2017 Springer-Verlag Berlin Heidelberg
Nothnagel A.,University of Bonn |
Artz T.,University of Bonn |
Behrend D.,NVI, Inc. |
Malkin Z.,Saint Petersburg State University
Journal of Geodesy | Year: 2016
The International VLBI Service for Geodesy and Astrometry (IVS) regularly produces high-quality Earth orientation parameters from observing sessions employing extensive networks or individual baselines. The master schedule is designed according to the telescope days committed by the stations and by the need for dense sampling of the Earth orientation parameters (EOP). In the pre-2011 era, the network constellations with their number of telescopes participating were limited by the playback and baseline capabilities of the hardware (Mark4) correlators. This limitation was overcome by the advent of software correlators, which can now accommodate many more playback units in a flexible configuration. In this paper, we describe the current operations of the IVS with special emphasis on the quality of the polar motion results since these are the only EOP components which can be validated against independent benchmarks. The polar motion results provided by the IVS have improved continuously over the years, now providing an agreement with IGS results at the level of 20–25 (Formula presented.)as in a WRMS sense. At the end of the paper, an outlook is given for the realization of the VLBI Global Observing System. © 2016 Springer-Verlag Berlin Heidelberg
Eriksson D.,Cornell University |
Macmillan D.S.,NVI, Inc. |
Gipson J.M.,NVI, Inc.
Journal of Geophysical Research B: Solid Earth | Year: 2015
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. ©2014. American Geophysical Union. All Rights Reserved.
Zavala R.T.,Us Naval Observatory |
Hummel C.A.,Hemisphere |
Boboltz D.A.,Us Naval Observatory |
Ojha R.,Us Naval Observatory |
And 5 more authors.
Astrophysical Journal Letters | Year: 2010
Interacting binaries typically have separations in the milliarcsecond regime, and hence it has been challenging to resolve them at any wavelength. However, recent advances in optical interferometry have improved our ability to discern the components in these systems and have now enabled the direct determination of physical parameters. We used the Navy Prototype Optical Interferometer to produce for the first time images resolving all three components in the well-known Algol triple system. Specifically, we have separated the tertiary component from the binary and simultaneously resolved the eclipsing binary pair, which represents the nearest and brightest eclipsing binary in the sky. We present revised orbital elements for the triple system, and we have rectified the 180° ambiguity in the position angle of Algol C. Our directly determined magnitude differences and masses for this triple star system are consistent with earlier light curve modeling results. © 2010 The American Astronomical Society. All rights reserved.
Petrov L.,NASA |
Kovalev Y.Y.,U.S. National Radio Astronomy Observatory |
Fomalont E.B.,U.S. National Radio Astronomy Observatory |
Gordon D.,NVI, Inc.
Astronomical Journal | Year: 2011
This paper presents accurate absolute positions from a 24 GHz Very Long Baseline Array (VLBA) search for compact extragalactic sources in an area where the density of known calibrators with precise coordinates is low. The goals were to identify additional sources suitable for use as phase calibrators for galactic sources, determine their precise positions, and produce radio images. In order to achieve these goals, we developed a new software package, PIMA, for determining group delays from wide-band data with much lower detection limits. With the use of PIMA, we have detected 327 sources out of 487 targets observed in three 24 hr VLBA experiments. Among the 327 detected objects, 176 are within 10° of the Galactic plane. This VGaPS catalog of source positions, plots of correlated flux density versus projected baseline length, contour plots, as well as weighted CLEAN images, and calibrated visibility data are available on the Web in FITS format. Approximately one-half of objects from the 24 GHz catalog were observed at dual-band 8.6 GHz and 2.3 GHz experiments. Position differences at 24 GHz versus 8.6/2.3 GHz for all but two objects on average are strictly within reported uncertainties. We found that for two objects with complex structures, positions at different frequencies correspond to different components of a source. © 2011. The American Astronomical Society. All rights reserved..
Gipson J.,NVI, Inc. |
Baver K.,NVI, Inc.
Journal of Geodesy | Year: 2016
The primary purpose of the International very long baseline interferometry (VLBI) Service for Geodesy and Astrometry Intensive sessions is the rapid estimation of UT1-TAI. Improving the robustness and the precision of the UT1 estimates from the Intensives is an important goal. The INT01 series, which usually uses the Kokee–Wettzell baseline and runs on weekdays, is the most regular IVS Intensive series. The United States Naval Observatory which schedules these sessions traditionally used a small list of strong sources. In 2009, the authors requested and received the use of nine IVS R&D sessions for the evaluation of a new strategy which draws on all sources mutually visible on the Kokee–Wettzell baseline. Analysis of these sessions was sufficiently promising that in July 2010, USNO began to alternate the use of the original and the new strategy in scheduling the INT01 sessions to assess the operational effectiveness of the proposed strategy. In this paper, we summarize our analysis of the R&D sessions, and we also analyze 2 years of operational INT01 sessions. Considered in toto, the new strategy performs as well as, or better than, the original strategy in terms of several measures of robustness and precision. Furthermore, the RMS difference of the UT1 estimates from the 1 h operational INTO1 sessions and concurrently run 24 h VLBI sessions is 21.0 (Formula presented.) s, compared to 30.7 (Formula presented.) s using the standard strategy, indicating that the new strategy is, on average, 30 % more accurate. © 2015, Springer-Verlag Berlin Heidelberg.
Eriksson D.,Chalmers University of Technology |
MacMillan D.S.,NVI, Inc.
Journal of Geodesy | Year: 2014
Variations in continental water storage lead to loading deformation of the crust with typical peak-to-peak variations at very long baseline interferometry (VLBI) sites of 3-15 mm in the vertical component and 1-2 mm in the horizontal component. The hydrology signal at VLBI sites has annual and semi-annual components and clear interannual variations. We have calculated the hydrology loading series using mass loading distributions derived from the global land data assimilation system (GLDAS) hydrology model and alternatively from a global grid of equal-area gravity recovery and climate experiment (GRACE) mascons. In the analysis of the two weekly VLBI 24-h R1 and R4 network sessions from 2003 to 2010 the baseline length repeatabilities are reduced in 79 % (80 %) of baselines when GLDAS (GRACE) loading corrections are applied. Site vertical coordinate repeatabilities are reduced in about 80 % of the sites when either GLDAS or GRACE loading is used. In the horizontal components, reduction occurs in 70-80 % of the sites. Estimates of the annual site vertical amplitudes were reduced for 16 out of 18 sites if either loading series was applied. We estimated loading admittance factors for each site and found that the average admittances were 1.01 ± 0.05 for GRACE and 1.39 ± 0.07 for GLDAS. The standard deviations of the GRACE admittances and GLDAS admittances were 0.31 and 0.68, respectively. For sites that have been observed in a set of sufficiently temporally dense daily sessions, the average correlation between VLBI vertical monthly averaged series and GLDAS or GRACE loading series was 0.47 and 0.43, respectively. © 2014 Springer-Verlag Berlin Heidelberg.
Agency: National Science Foundation | Branch: | Program: SBIR | Phase: Phase II | Award Amount: 500.00K | Year: 2012
This Small Business Innovation Research (SBIR) Phase II project focuses on the development of a novel, environment-friendly adsorbent material with low life-cycle cost for the removal of Arsenic from the water stream. The proposed material solution is based on a novel nanoporous geopolymer composite designed specifically for this application and manufactured via a patented, sustainable, and energy- and materials-efficient production process. During Phase II, the superior performance of the new media demonstrated at lab scale in Phase I will be further validated at the pilot scale. The anticipated results include 1) successful pilot production of the novel adsorbent to validate the cost advantages of materials; 2) successful validation of superior performance of the nanoporous composite at a pilot testing scale; and 3) further improvement of the product cost by using lower cost precursors. The broader impacts of this research are (1) providing to the environmental remediation industry a new class of materials and novel platform technology that may be expanded to removing other water contaminants. The novel material will be offered to replace the existing sorbent media and will provide performance/cost benefits for residential and commercial systems with additional environmental advantages; (2) enabling the development of new fresh water sources currently unusable due to high Arsenic content in the US and developing countries; and (3) transforming the nanoporous materials production technology with a broad spectrum of critical clean tech applications, including energy efficiency (insulation and catalyst), energy generation, and energy storage.
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 99.76K | Year: 2010
We propose to investigate and develop an inexpensive system to determine: 1)VLBI antenna properties such as axis-offset, non-intersection of axis and antenna reference point; 2) Gravitational and/or Thermal deformation; 3) The eccentricity vector from the VLBI antenna to nearby GPS receivers. The proposed system involves using several GPS receivers in close proximity driven by a common clock. Because of their proximity, atmosphere and ionosphere errors are common, and vanish when we take the difference of measurements from different receivers to a common a satellite. Satellite clock errors also vanish in this difference. Since the receivers use a common clock, receiver clock error also vanishes. The remaining error source is GPS multipath which can be mitigated.
Gordon D.,NVI, Inc.
Journal of Geodesy | Year: 2016
The geodetic VLBI community began using VLBA antennas in 1989 for geodesy and astrometry. We examine how usage of the VLBA has improved the celestial reference frame, the terrestrial reference frame, and Earth orientation parameters. Without the VLBA, ICRF2 would have had only 1011 sources instead of 3414. ICRF3 will contain at least 4121 sources, with approximately 70 % or more coming exclusively from VLBA astrometry and geodesy sessions. The terrestrial reference frame is also more stable and precise due to VLBA geodesy sessions. Approximately two dozen geodesy stations that have participated in VLBA sessions show average position formal errors that are (Formula presented.)13–14 % better in the horizontal components and (Formula presented.)5 % better in the vertical component than would be expected solely from the increased number of observations. Also the Earth orientation parameters obtained from the RDV sessions represent the most accurate EOP series of any of the long-term VLBI session types. © 2016 Springer-Verlag Berlin Heidelberg