3,870 research outputs found
Autonomous Spacecraft Navigation With Pulsars
An external reference system suitable for deep space navigation can be
defined by fast spinning and strongly magnetized neutron stars, called pulsars.
Their beamed periodic signals have timing stabilities comparable to atomic
clocks and provide characteristic temporal signatures that can be used as
natural navigation beacons, quite similar to the use of GPS satellites for
navigation on Earth. By comparing pulse arrival times measured on-board a
spacecraft with predicted pulse arrivals at a reference location, the
spacecraft position can be determined autonomously and with high accuracy
everywhere in the solar system and beyond. The unique properties of pulsars
make clear already today that such a navigation system will have its
application in future astronautics. In this paper we describe the basic
principle of spacecraft navigation using pulsars and report on the current
development status of this novel technology.Comment: 22 pages, 12 figures, 2 tables, to be published in the proceedings of
the workshop "Relativistic Positioning Systems and their Scientific
Applications", held on 19-21 Sept. 2012, Brdo near Kranj, Sloveni
Adaptive dual-comb spectroscopy in the green region
Dual-comb spectroscopy is extended to the visible spectral range with a
set-up based on two frequency-doubled femtosecond ytterbium-doped fiber lasers.
The dense rovibronic spectrum of iodine around 19240 cm-1 is recorded within 12
ms at Doppler-limited resolution with a simple scheme that only uses
free-running femtosecond lasers
The flora of Istria: Juncaceae
A checklist, key and distribution were prepared for the Juncaceae family after research into the Istrian flora was completed. According to the checklist, the Juncaceae family in Istria includes two genera (Juncus and Luzula) and 27 taxa (species, subspecies and varieties). During research in 2003 and 2004, two new taxa were discovered in the Istrian
flora: Juncus hybridus Brot. in ephemeral and wet locations along the Premantura Peninsula and Juncus littoralis C.A. Mey ssp. tommasinii (Parl.) Arcang. at the mouth of the RaĆĄa River
Identification of the SlmA Active Site Responsible for Blocking Bacterial Cytokinetic Ring Assembly over the Chromosome
Bacterial cells use chromosome-associated division inhibitors to help coordinate the processes of DNA replication and segregation with cytokinesis. SlmA from Escherichia coli, a member of the tetracycline repressor (TetR)âlike protein family, is one example of this class of regulator. It blocks the assembly of the bacterial cytokinetic ring by interfering with the polymerization of the tubulin-like FtsZ protein in a manner that is dramatically stimulated upon specific DNA binding. Here we used a combination of molecular genetics and biochemistry to identify the active site of SlmA responsible for disrupting FtsZ polymerization. Interestingly, this site maps to a region of SlmA that in the published DNAâfree structure is partially occluded by the DNA-binding domains. In this conformation, the SlmA structure resembles the drug/inducer-bound conformers of other TetRâlike proteins, which in the absence of inducer require an inward rotation of their DNA-binding domains to bind successive major grooves on operator DNA. Our results are therefore consistent with a model in which DNA-binding activates SlmA by promoting a rotational movement of the DNA-binding domains that fully exposes the FtsZ-binding sites. SlmA may thus represent a special subclass of TetRâlike proteins that have adapted conformational changes normally associated with inducer sensing in order to modulate an interaction with a partner protein. In this case, the adaptation ensures that SlmA only blocks cytokinesis in regions of the cell occupied by the origin-proximal portion of the chromosome where SlmA-binding sites are enriched
Schoenus nigricans (Cyperaceae) xerophytic grasslands on the NE Adriatic islands Cres and Krk (Croatia)
Several xerophytic grasslands on the NE Adriatic islands Cres and Krk (Croatia) are dominated by tussock-forming Schoenus nigricans (Cyperaceae). All vegetation types reported are characterised by the degradation of the vegetation cover due to grazing. Phytosociologically, the vegetation is classified as the Danthonio-Scorzoneretum villosae subass. schoenetosum nigricantis H-i} 57 of the Scorzonerion villosae H-i} 49 alliance within the order Scorzonero-Chrysopogonetalia, but partly shows transitions to the
Bromo-Chrysopogonetum grylli H-i} 60.We differentiated three variants: more xerophytic sites (on Cres), sites characterised by alternating soil humidity (mainly on Krk), and advanced succession stages. Physiognomically all variants are dominated by Schoenus
nigricans tussocks, which are fostered by sheep grazing and the summer-dry conditions on the compacted soils. Comparison with Central European more humid habitats suggests the hypothesis of an ecotypic differentiation of Schoenus nigricans within its European
distribution area
Enhancing the Performance of the T-Peel Test for Thin and Flexible Adhered Laminates
Symmetrically bonded thin and flexible T-peel specimens, when tested on
vertical travel machines, can be subject to significant gravitational loading;
with the associated asymmetry and mixed-mode failure during peeling. This can
cause erroneously high experimental peel forces to be recorded which leads to
uncertainty in estimating interfacial fracture toughness and failure mode. To
overcome these issues, a mechanical test fixture has been designed for use with
vertical test machines, that supports the unpeeled portion of the test specimen
and suppresses parasitic loads due to gravity from affecting the peel test. The
mechanism, driven by the test machine cross-head, moves at one-half of the
velocity of the cross-head such that the unpeeled portion always lies in the
plane of the instantaneous center of motion. Several specimens such as bonded
polymeric films, laminates, and commercial tapes were tested with and without
the fixture, and the importance of the proposed T-peel procedure has been
demonstrated
Raman-induced Kerr-effect dual-comb spectroscopy
We report on the first demonstration of nonlinear dual-frequency-comb
spectroscopy. In multi-heterodyne femtosecond Raman-induced Kerr-effect
spectroscopy, the Raman gain resulting from the coherent excitation of
molecular vibrations by a spectrally-narrow pump is imprinted onto the
femtosecond laser frequency comb probe spectrum. The birefringence signal
induced by the nonlinear interaction of these beams and the sample is
heterodyned against a frequency comb local oscillator with a repetition
frequency slightly different from that of the comb probe. Such time-domain
interference provides multiplex access to the phase and amplitude Raman spectra
over a broad spectral bandwidth within a short measurement time. Experimental
demonstration, at a spectral resolution of 200 GHz, a measurement time of 293
{\mu}s and a sensitivity of 10^-6, is given on liquid samples exhibiting a C-H
stretch Raman shift.Comment: 7 pages, 4 figure
A test of speculative arbitrage : is the cross-section of volatility invariant?
We derive testable implications of Kyle and Obizhaevaâs (2016) notion of âbet invarianceâ for the cross-section of trade-time volatilities. We jointly develop theoretical foundations of âno speculative arbitrageâ whose implications incorporate those of bet invariance. Our proposed test circumvents the unobservable nature of âbets.â Utilizing a large sample of U.S. stocks post decimilization, we show that using realized volatilities rather than expected volatilities introduces noise that substantially biases the tests. This leads us to use estimates of normalized volatilities based on running 24 month windows. We ïŹnd strong support for no speculative arbitrage at a moment in time, but not across time
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