4,510 research outputs found

    Forced motion near black holes

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    We present two methods for integrating forced geodesic equations in the Kerr spacetime, which can accommodate arbitrary forces. As a test case, we compute inspirals under a simple drag force, mimicking the presence of gas. We verify that both methods give the same results for this simple force. We find that drag generally causes eccentricity to increase throughout the inspiral. This is a relativistic effect qualitatively opposite to what is seen in gravitational-radiation-driven inspirals, and similar to what is observed in hydrodynamic simulations of gaseous binaries. We provide an analytic explanation by deriving the leading order relativistic correction to the Newtonian dynamics. If observed, an increasing eccentricity would provide clear evidence that the inspiral was occurring in a non-vacuum environment. Our two methods are especially useful for evolving orbits in the adiabatic regime. Both use the method of osculating orbits, in which each point on the orbit is characterized by the parameters of the geodesic with the same instantaneous position and velocity. Both methods describe the orbit in terms of the geodesic energy, axial angular momentum, Carter constant, azimuthal phase, and two angular variables that increase monotonically and are relativistic generalizations of the eccentric anomaly. The two methods differ in their treatment of the orbital phases and the representation of the force. In one method the geodesic phase and phase constant are evolved together as a single orbital phase parameter, and the force is expressed in terms of its components on the Kinnersley orthonormal tetrad. In the second method, the phase constants of the geodesic motion are evolved separately and the force is expressed in terms of its Boyer-Lindquist components. This second approach is a generalization of earlier work by Pound and Poisson for planar forces in a Schwarzschild background.Comment: 28 pages, 2 figures, submitted to Phys. Rev. D; v2 has minor changes for consistency with published version, plus a new section discussing the relative advantages of the two approache

    High-rate GPS clock corrections from CODE: support of 1Hz applications

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    GPS zero-difference applications with a sampling rate up to 1Hz require corresponding high-rate GPS clock corrections. The determination of the clock corrections in a full network solution is a time-consuming task. The Center for Orbit Determination in Europe (CODE) has developed an efficient algorithm based on epoch-differenced phase observations, which allows to generate high-rate clock corrections within reasonably short time (<2h) and with sufficient accuracy (on the same level as the CODE rapid or final clock corrections, respectively). The clock determination procedure at CODE and the new algorithm is described in detail. It is shown that the simplifications to speed up the processing are not causing a significant loss of accuracy for the clock corrections. The high-rate clock corrections have in essence the same quality as clock corrections determined in a full network solution. In order to support 1Hz applications 1-s clock corrections would be needed. The computation time, even for the efficient algorithm, is not negligible, however. Therefore, we studied whether a reduced sampling is sufficient for the GPS satellite clock corrections to reach the same or only slightly inferior level of accuracy as for the full 1-s clock correction set. We show that high-rate satellite clock corrections with a spacing of 5s may be linearly interpolated resulting in less than 2% degradation of accurac

    Fano Lineshapes Revisited: Symmetric Photoionization Peaks from Pure Continuum Excitation

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    In a photoionization spectrum in which there is no excitation of the discrete states, but only the underlying continuum, we have observed resonances which appear as symmetric peaks, not the commonly expected window resonances. Furthermore, since the excitation to the unperturbed continuum vanishes, the cross section expected from Fano's configuration interaction theory is identically zero. This shortcoming is removed by the explicit introduction of the phase shifted continuum, which demonstrates that the shape of a resonance, by itself, provides no information about the relative excitation amplitudes to the discrete state and the continuum.Comment: 4 pages, 3 figure

    Validating ocean tide loading models using GPS

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    Abstract.: Ocean tides cause periodic deformations of the Earth's surface, also referred to as ocean tide loading (OTL). Tide-induced displacements of the Earth's crust relying on OTL models are usually taken into account in GPS (Global Positioning System) data analyses. On the other hand, it is also possible to validate OTL models using GPS analyses. The following simple approach is used to validate OTL models. Based on a particular model, instantaneous corrections of the site coordinates due to OTL are computed. Site-specific scale factors, f, for these corrections are estimated in a standard least-squares adjustment process of GPS observations together with other relevant parameters. A resulting value of f close to unity indicates a good agreement of the model with the actual site displacements. Such scale factors are computed for about 140 globally distributed IGS (International GPS Service) tracking sites. Three OTL models derived from the ocean tide models FES95.2.1, FES99, and GOT00.2 are analyzed. As expected, the most reliable factors are estimated for sites with a large loading effect. In general, the scaling factors have a value close to unity and no significant differences between the three ocean tide models could be observed. It is found that the validation approach is easy to apply. Without requiring much additional effort for a global and self-consistent GPS data analysis, it allows detection of general model misfits on the basis of a large number of globally distributed sites. For detailed validation studies on OTL models, the simultaneous estimation of amplitudes and phases for the main contributing partial tides within a GPS parameter adjustment process would provide more detailed answer

    Phase center modeling for LEO GPS receiver antennas and its impact on precise orbit determination

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    Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations, where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses. Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations. We use GPS data of the aforementioned LEOs of the year 2007 together with the absolute antenna modeling to assess the presently achieved accuracy from state-of-the-art reduced-dynamic LEO POD strategies for absolute and relative navigation. Near-field multipath and cross-talk with active GPS occultation antennas turn out to be important and significant sources for systematic carrier phase measurement errors that are encountered in the actual spacecraft environments. We assess different methodologies for the in-flight determination of empirical phase pattern corrections for LEO receiver antennas and discuss their impact on POD. By means of independent K-band measurements, we show that zero-difference GRACE orbits can be significantly improved from about 10 to 6mm K-band standard deviation when taking empirical phase corrections into account, and assess the impact of the corrections on precise baseline estimates and further applications such as gravity field recovery from kinematic LEO position

    satFRET: estimation of Forster resonance energy transfer by acceptor saturation

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    We demonstrate theoretically and experimentally the quantification of Förster resonance energy transfer (FRET) by direct and systematic saturation of the excited state of acceptor molecules. This version of acceptor depletion methods for FRET estimation, denoted as “satFRET” is reversible and suitable for time-resolved measurements. The technique was investigated theoretically using the steady-state solution of the differential equation system of donor and acceptor molecular states. The influence of acceptor photobleaching during measurement was included in the model. Experimental verification was achieved with the FRET-pair Alexa 546- Alexa 633 loaded on particles in different stoichiometries and measured in a confocal microscope. Estimates of energy transfer efficiency by excited state saturation were compared to those obtained by measurements of sensitised emission and acceptor photobleaching. The results lead to a protocol that allows time-resolved FRET measurements of fixed and living cells on a conventional confocal microscope. This procedure was applied to fixed Chinese hamster ovary cells containing a cyan fluorescent protein and yellow fluorescent protein pair. The time resolution of the technique was demonstrated in a live T cell activation assay comparing the FRET efficiencies measured using a genetically encoded green and red fluorescent protein biosensor for GTP/GDP turnover to those measured by acceptor photobleaching of fixed cells

    Public Price Reporting, Marketing Channel Selection, and Price Discovery: The Perspective of Cow/Calf Producers in the Dakotas

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    Cow/calf producers operating in the Dakotas were surveyed on their price discovery strategies, marketing channel preferences, and their perceptions of how regime change in the public price reporting system for fed cattle affected the beef industry in general and the cow/calf industry in particular. Survey results indicate cow/calf producers consider local institutions (auction barns, etc.) to be more reliable for price discovery than regional or national institutions (futures market, USDA public price reports, satellite auctions, etc.). The auction barn marketing channel is the preferred channel for marketing cattle and is considered the most reliable source of market information by producers. Dakota cow/calf producers perceive livestock mandatory price reporting as benefiting the beef industry in general, but consider public price reports to be less reliable than local sources of market information.beef supply chain, cow-calf marketing, marketing channel, price discovery, public price reporting, Livestock Production/Industries,

    The Effect of Early vs. Normal Calf Weaning on Feedlot Performance and Herd Management: A Cross-Discipline Case Study

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    A study of early vs. normal weaning of calves concludes that early weaning improves feedlot production efficiency, reducing per day and per pound feedlot production costs. Early weaned steers finished higher thus feedlot profitability was not affected. Early weaning has a positive affect on cow health and pasture utilization rates.Livestock Production/Industries,
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