3,851 research outputs found
Subnanosecond GPS-based clock synchronization and precision deep-space tracking
Interferometric spacecraft tracking is accomplished by the Deep Space Network (DSN) by comparing the arrival time of electromagnetic spacecraft signals at ground antennas separated by baselines on the order of 8000 km. Clock synchronization errors within and between DSN stations directly impact the attainable tracking accuracy, with a 0.3-nsec error in clock synchronization resulting in an 11-nrad angular position error. This level of synchronization is currently achieved by observing a quasar which is angularly close to the spacecraft just after the spacecraft observations. By determining the differential arrival times of the random quasar signal at the stations, clock offsets and propagation delays within the atmosphere and within the DSN stations are calibrated. Recent developments in time transfer techniques may allow medium accuracy (50-100 nrad) spacecraft tracking without near-simultaneous quasar-based calibrations. Solutions are presented for a worldwide network of Global Positioning System (GPS) receivers in which the formal errors for DSN clock offset parameters are less than 0.5 nsec. Comparisons of clock rate offsets derived from GPS measurements and from very long baseline interferometry (VLBI), as well as the examination of clock closure, suggest that these formal errors are a realistic measure of GPS-based clock offset precision and accuracy. Incorporating GPS-based clock synchronization measurements into a spacecraft differential ranging system would allow tracking without near-simultaneous quasar observations. The impact on individual spacecraft navigation-error sources due to elimination of quasar-based calibrations is presented. System implementation, including calibration of station electronic delays, is discussed
Peer-review in a world with rational scientists: Toward selection of the average
One of the virtues of peer review is that it provides a self-regulating
selection mechanism for scientific work, papers and projects. Peer review as a
selection mechanism is hard to evaluate in terms of its efficiency. Serious
efforts to understand its strengths and weaknesses have not yet lead to clear
answers. In theory peer review works if the involved parties (editors and
referees) conform to a set of requirements, such as love for high quality
science, objectiveness, and absence of biases, nepotism, friend and clique
networks, selfishness, etc. If these requirements are violated, what is the
effect on the selection of high quality work? We study this question with a
simple agent based model. In particular we are interested in the effects of
rational referees, who might not have any incentive to see high quality work
other than their own published or promoted. We find that a small fraction of
incorrect (selfish or rational) referees can drastically reduce the quality of
the published (accepted) scientific standard. We quantify the fraction for
which peer review will no longer select better than pure chance. Decline of
quality of accepted scientific work is shown as a function of the fraction of
rational and unqualified referees. We show how a simple quality-increasing
policy of e.g. a journal can lead to a loss in overall scientific quality, and
how mutual support-networks of authors and referees deteriorate the system.Comment: 5 pages 4 figure
A micromechanics based constitutive model for fibre reinforced cementitious composites
A new constitutive model for fibre reinforced cementitious composites based on micromechanical solutions is proposed. The model employs a two-phase composite based on the Eshelby matrix-inclusion solution and the Mori-Tanaka homogenization scheme and also simulates directional microcracking. An exterior point Eshelby based criterion is employed to model crack-initiation in the matrix-inclusion interface. Microcrack surfaces are assumed to be rough and able to regain contact under both normal and shear displacements. Fibres are included into the formulation in both cracked and uncracked conditions. Once cracks start to develop, the crack-bridging action of fibres is simulated using a local constitutive equation that accounts for the debonding and pull-out of fibre groups with different orientations. It is shown that the combination of the rough microcrack and fibre-bridging sub-models allows microcracking behaviour deriving from both tensile and compressive loads to be modelled in a unified manner. This ability to model tensile and compressive behaviour using the same micromechanical mechanisms is considered to be a particularly attractive feature of the formulation, which removes the need for multi-parameter triaxial yield surfaces and evolution functions that bedevil many competitor models. The model is successfully validated using a series of examples based on experimental test data
Entanglement between static and flying qubits in a semiconducting carbon nanotube
Entanglement can be generated by two electrons in a spin-zero state on a
semiconducting single-walled carbon nanotube. The two electrons, one weakly
bound in a shallow well in the conduction band, and the other injected into the
conduction band, are coupled by the Coulomb interaction. Both transmission and
entanglement are dependent on the well characteristics, which can be controlled
by a local gate, and on the kinetic energy of the injected electron. Regimes
with different degrees of electron correlation exhibit full or partial
entanglement. In the latter case, the maximum entanglement can be estimated as
a function of width and separation of a pair of singlet-triplet resonances.Comment: 17 pages and 12 figures, accepted to J. Phys. Cond. Ma
X-ray photoemission spectroscopy determination of the InN/yttria stabilized cubic-zirconia valence band offset
The valence band offset of wurtzite InN(0001)/yttria stabilized cubic-zirconia (YSZ)(111) heterojunctions is determined by x-ray photoemission spectroscopy to be 1.19±0.17 eV giving a conduction band offset of 3.06±0.20 eV. Consequently, a type-I heterojunction forms between InN and YSZ in the straddling arrangement. The low lattice mismatch and high band offsets suggest potential for use of YSZ as a gate dielectric in high-frequency InN-based electronic devices
Evidence of spontaneous spin polarized transport in magnetic nanowires
The exploitation of the spin in charge-based systems is opening revolutionary
opportunities for device architecture. Surprisingly, room temperature
electrical transport through magnetic nanowires is still an unresolved issue.
Here, we show that ferromagnetic (Co) suspended atom chains spontaneously
display an electron transport of half a conductance quantum, as expected for a
fully polarized conduction channel. Similar behavior has been observed for Pd
(a quasi-magnetic 4d metal) and Pt (a non-magnetic 5d metal). These results
suggest that the nanowire low dimensionality reinforces or induces magnetic
behavior, lifting off spin degeneracy even at room temperature and zero
external magnetic field.Comment: 4 pages, 3 eps fig
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