980 research outputs found
Statistical framework for estimating GNSS bias
We present a statistical framework for estimating global navigation satellite
system (GNSS) non-ionospheric differential time delay bias. The biases are
estimated by examining differences of measured line integrated electron
densities (TEC) that are scaled to equivalent vertical integrated densities.
The spatio-temporal variability, instrumentation dependent errors, and errors
due to inaccurate ionospheric altitude profile assumptions are modeled as
structure functions. These structure functions determine how the TEC
differences are weighted in the linear least-squares minimization procedure,
which is used to produce the bias estimates. A method for automatic detection
and removal of outlier measurements that do not fit into a model of receiver
bias is also described. The same statistical framework can be used for a single
receiver station, but it also scales to a large global network of receivers. In
addition to the Global Positioning System (GPS), the method is also applicable
to other dual frequency GNSS systems, such as GLONASS (Globalnaya
Navigazionnaya Sputnikovaya Sistema). The use of the framework is demonstrated
in practice through several examples. A specific implementation of the methods
presented here are used to compute GPS receiver biases for measurements in the
MIT Haystack Madrigal distributed database system. Results of the new algorithm
are compared with the current MIT Haystack Observatory MAPGPS bias
determination algorithm. The new method is found to produce estimates of
receiver bias that have reduced day-to-day variability and more consistent
coincident vertical TEC values.Comment: 18 pages, 5 figures, submitted to AM
On the "renormalization" transformations induced by cycles of expansion and contraction in causal set cosmology
We study the ``renormalization group action'' induced by cycles of cosmic
expansion and contraction, within the context of a family of stochastic
dynamical laws for causal sets derived earlier. We find a line of fixed points
corresponding to the dynamics of transitive percolation, and we prove that
there exist no other fixed points and no cycles of length two or more. We also
identify an extensive ``basin of attraction'' of the fixed points but find that
it does not exhaust the full parameter space. Nevertheless, we conjecture that
every trajectory is drawn toward the fixed point set in a suitably weakened
sense.Comment: 22 pages, 1 firgure, submitted to Phys. Rev.
Narrative coherence in multiple forensic interviews with child witnesses alleging physical and sexual abuse
This study investigated the narrative coherence of children's accounts elicited in multiple forensic interviews. Transcriptions of 56 police interviews with 28 children aged 3–14 years alleging physical and sexual abuse were coded for markers of completeness, consistency and connectedness. We found that multiple interviews increased the completeness of children's testimony, containing on average almost twice as much new information as single interviews, including crucial location, time and abuse‐related details. When both contradictions within the same interview and across interviews were considered, contradictions were not more frequent in multiple interviews. The frequency of linguistic markers of connectedness remained stable across interviews. Multiple interviews increase the narrative coherence of children's testimony through increasing their completeness without necessarily introducing contradictions or decreasing causal‐temporal connections between details. However, as ‘ground truth’ is not known in field studies, further investigation of the relationship between the narrative coherence and accuracy of testimonies is required
The status of Quantum Geometry in the dynamical sector of Loop Quantum Cosmology
This letter is motivated by the recent papers by Dittrich and Thiemann and,
respectively, by Rovelli discussing the status of Quantum Geometry in the
dynamical sector of Loop Quantum Gravity. Since the papers consider model
examples, we also study the issue in the case of an example, namely on the Loop
Quantum Cosmology model of space-isotropic universe. We derive the
Rovelli-Thiemann-Ditrich partial observables corresponding to the quantum
geometry operators of LQC in both Hilbert spaces: the kinematical one and,
respectively, the physical Hilbert space of solutions to the quantum
constraints. We find, that Quantum Geometry can be used to characterize the
physical solutions, and the operators of quantum geometry preserve many of
their kinematical properties.Comment: Latex, 12 page
Millstone Hill coherent-scatter radar observations of electric field variability in the sub-auroral polarization stream
[1] Coherent backscatter observations with the Millstone Hill UHF radar (MHR) are used to investigate spatial/temporal variations in the ionospheric sub‐auroral polarization stream (SAPS) electric field. For the 440 MHz MHR, coherent amplitude is on average linearly proportional to electric field strength. The use of both main‐beam and sidelobe returns and the great sensitivity of the MHR system permits observations spanning 3° of the SAPS region with 1‐sec temporal and 10‐km spatial resolution. For a moderately disturbed event on May 25, 2000, the SAPS channel moved steadily equatorward. Large‐scale (30 mV/m peak to peak) wave‐like oscillations in the electric field magnitude (200s–300s periodicity) were seen to propagate across the SAPS channel throughout the hour‐long event. It is suggested that such localized electric field intensifications, which exhibit many of the characteristics of the narrow SAID features described in the literature, arise as wavelike perturbations within the SAPS channel
Shape in an Atom of Space: Exploring quantum geometry phenomenology
A phenomenology for the deep spatial geometry of loop quantum gravity is
introduced. In the context of a simple model, an atom of space, it is shown how
purely combinatorial structures can affect observations. The angle operator is
used to develop a model of angular corrections to local, continuum flat-space
3-geometries. The physical effects involve neither breaking of local Lorentz
invariance nor Planck scale suppression, but rather reply on only the
combinatorics of SU(2) recoupling. Bhabha scattering is discussed as an example
of how the effects might be observationally accessible.Comment: 14 pages, 7 figures; v2 references adde
Proper time and Minkowski structure on causal graphs
For causal graphs we propose a definition of proper time which for small
scales is based on the concept of volume, while for large scales the usual
definition of length is applied. The scale where the change from "volume" to
"length" occurs is related to the size of a dynamical clock and defines a
natural cut-off for this type of clock. By changing the cut-off volume we may
probe the geometry of the causal graph on different scales and therey define a
continuum limit. This provides an alternative to the standard coarse graining
procedures. For regular causal lattice (like e.g. the 2-dim. light-cone
lattice) this concept can be proven to lead to a Minkowski structure. An
illustrative example of this approach is provided by the breather solutions of
the Sine-Gordon model on a 2-dimensional light-cone lattice.Comment: 15 pages, 4 figure
The structure of causal sets
More often than not, recently popular structuralist interpretations of
physical theories leave the central concept of a structure insufficiently
precisified. The incipient causal sets approach to quantum gravity offers a
paradigmatic case of a physical theory predestined to be interpreted in
structuralist terms. It is shown how employing structuralism lends itself to a
natural interpretation of the physical meaning of causal sets theory.
Conversely, the conceptually exceptionally clear case of causal sets is used as
a foil to illustrate how a mathematically informed rigorous conceptualization
of structure serves to identify structures in physical theories. Furthermore, a
number of technical issues infesting structuralist interpretations of physical
theories such as difficulties with grounding the identity of the places of
highly symmetrical physical structures in their relational profile and what may
resolve these difficulties can be vividly illustrated with causal sets.Comment: 19 pages, 4 figure
Quantum Dynamics without the Wave Function
When suitably generalized and interpreted, the path-integral offers an
alternative to the more familiar quantal formalism based on state-vectors,
selfadjoint operators, and external observers. Mathematically one generalizes
the path-integral-as-propagator to a {\it quantal measure} on the space
of all ``conceivable worlds'', and this generalized measure expresses
the dynamics or law of motion of the theory, much as Wiener measure expresses
the dynamics of Brownian motion. Within such ``histories-based'' schemes new,
and more ``realistic'' possibilities open up for resolving the philosophical
problems of the state-vector formalism. In particular, one can dispense with
the need for external agents by locating the predictive content of in its
sets of measure zero: such sets are to be ``precluded''. But unrestricted
application of this rule engenders contradictions. One possible response would
remove the contradictions by circumscribing the application of the preclusion
concept. Another response, more in the tradition of ``quantum logic'', would
accommodate the contradictions by dualizing to a space of
``co-events'' and effectively identifying reality with an element of this dual
space.Comment: plainTeX, 24 pages, no figures. To appear in a special volume of {\it
Journal of Physics A: Mathematical and General} entitled ``The Quantum
Universe'' and dedicated to Giancarlo Ghirardi on the occasion of his 70th
birthday. Most current version is available at
http://www.physics.syr.edu/~sorkin/some.papers/ (or wherever my home-page may
be
Spatial Hypersurfaces in Causal Set Cosmology
Within the causal set approach to quantum gravity, a discrete analog of a
spacelike region is a set of unrelated elements, or an antichain. In the
continuum approximation of the theory, a moment-of-time hypersurface is well
represented by an inextendible antichain. We construct a richer structure
corresponding to a thickening of this antichain containing non-trivial
geometric and topological information. We find that covariant observables can
be associated with such thickened antichains and transitions between them, in
classical stochastic growth models of causal sets. This construction highlights
the difference between the covariant measure on causal set cosmology and the
standard sum-over-histories approach: the measure is assigned to completed
histories rather than to histories on a restricted spacetime region. The
resulting re-phrasing of the sum-over-histories may be fruitful in other
approaches to quantum gravity.Comment: Revtex, 12 pages, 2 figure
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