295 research outputs found
Long term frequency stability analysis of the GPS NAVSTAR 6 Cesium clock
Time domain measurements, taken between the NAVSTAR 6 Spacecraft Vehicle (SV) and the Vandenberg Global Positioning System (GPS) Monitor Site, by a pseudo random noise receiver, were collected over an extended period of time and analyzed to estimate the long term frequency stability of the NAVSTAR 6 onboard frequency standard, referenced to the Vandenberg MS frequency standard. The technique employed separates the clock offset from the composite signal by first applying corrections for equipment delays, ionospheric delay, tropospheric delay, Earth rotation and the relativistic effect. The data are edited and smoothed using the predicted SV ephemeris to calculate the geometric delay. Then all available passes from each of the four GPS monitor stations, are collected at 1-week intervals and used to calculate the NAVSTAR orbital elements. The procedure is then completed by subtracting the corrections and the geometric delay, using the final orbital elements, from the composite signal, thus leaving the clock offset and random error
Submicrosecond comparisons of time standards via the Navigation Technology Satellites (NTS)
An interim demonstration was performed of the time transfer capability of the NAVSTAR GPS system using a single NTS satellite. Measurements of time difference (pseudo-range) are made from the NTS tracking network and at the participating observatories. The NTS network measurements are used to compute the NTS orbit trajectory. The central NTS tracking station has a time link to the Naval Observatory UTC (USNO,MC1) master clock. Measurements are used with the NTS receiver at the remote observatory, the time transfer value UTC (USNO,MC1)-UTC (REMOTE, VIA NTS) is calculated. Intercomparisons were computed using predicted values of satellite clock offset and ephemeus
Submicrosecond comparison of international clock synchronization by VLBI and the NTS satellite
The intercontinental clock synchronization capabilities of Very Long Baseline Interferometry (VLBI) and the Navigation Technology Satellite (NTS) were compared using both methods to synchronize the Cesium clocks at the NASA Deep Space Net complexes at Madrid, Spain and Goldstone, California. Verification of the accuracy of both systems was examined. The VLBI experiments used the Wideband VLBI Data Acquisition System developed at the NASA Jet Propulsion Laboratory. The NTS Satellites were designed and built by the Naval Research Laboratory used with NTS Timing Receivers developed by the Goddard Space Flight Center. The two methods agreed at about the one-half microsecond level
On-orbit frequency stability analysis of the GPS NAVSTAR-1 quartz clock and the NAVSTARs-6 and -8 rubidium clocks
An on-orbit frequency stability performance analysis of the GPS NAVSTAR-1 quartz clock and the NAVSTARs-6 and -8 rubidium clocks is presented. The clock offsets were obtained from measurements taken at the GPS monitor stations which use high performance cesium standards as a reference. Clock performance is characterized through the use of the Allan variance, which is evaluated for sample times of 15 minutes to two hours, and from one day to 10 days. The quartz and rubidium clocks' offsets were corrected for aging rate before computing the frequency stability. The effect of small errors in aging rate is presented for the NAVSTAR-8 rubidium clock's stability analysis. The analysis includes presentation of time and frequency residuals with respect to linear and quadratic models, which aid in obtaining aging rate values and identifying systematic and random effects. The frequency stability values were further processed with a time domain noise process analysis, which is used to classify random noise process and modulation type
Prediction and statistics of pseudoknots in RNA structures using exactly clustered stochastic simulations
Ab initio RNA secondary structure predictions have long dismissed helices
interior to loops, so-called pseudoknots, despite their structural importance.
Here, we report that many pseudoknots can be predicted through long time scales
RNA folding simulations, which follow the stochastic closing and opening of
individual RNA helices. The numerical efficacy of these stochastic simulations
relies on an O(n^2) clustering algorithm which computes time averages over a
continously updated set of n reference structures. Applying this exact
stochastic clustering approach, we typically obtain a 5- to 100-fold simulation
speed-up for RNA sequences up to 400 bases, while the effective acceleration
can be as high as 100,000-fold for short multistable molecules (<150 bases). We
performed extensive folding statistics on random and natural RNA sequences, and
found that pseudoknots are unevenly distributed amongst RNAstructures and
account for up to 30% of base pairs in G+C rich RNA sequences (Online RNA
folding kinetics server including pseudoknots : http://kinefold.u-strasbg.fr/
).Comment: 6 pages, 5 figure
RNA Folding and Large N Matrix Theory
We formulate the RNA folding problem as an matrix field theory.
This matrix formalism allows us to give a systematic classification of the
terms in the partition function according to their topological character. The
theory is set up in such a way that the limit yields the
so-called secondary structure (Hartree theory). Tertiary structure and
pseudo-knots are obtained by calculating the corrections to the
partition function. We propose a generalization of the Hartree recursion
relation to generate the tertiary structure.Comment: 29 pages (LaTex), 13 figures (eps). Missing paragraph and figure
adde
RNA secondary structure formation: a solvable model of heteropolymer folding
The statistical mechanics of heteropolymer structure formation is studied in
the context of RNA secondary structures. A designed RNA sequence biased
energetically towards a particular native structure (a hairpin) is used to
study the transition between the native and molten phase of the RNA as a
function of temperature. The transition is driven by a competition between the
energy gained from the polymer's overlap with the native structure and the
entropic gain of forming random contacts. A simplified Go-like model is
proposed and solved exactly. The predicted critical behavior is verified via
exact numerical enumeration of a large ensemble of similarly designed
sequences.Comment: 4 pages including 2 figure
CentroidFold: a web server for RNA secondary structure prediction
The CentroidFold web server (http://www.ncrna.org/centroidfold/) is a web application for RNA secondary structure prediction powered by one of the most accurate prediction engine. The server accepts two kinds of sequence data: a single RNA sequence and a multiple alignment of RNA sequences. It responses with a prediction result shown as a popular base-pair notation and a graph representation. PDF version of the graph representation is also available. For a multiple alignment sequence, the server predicts a common secondary structure. Usage of the server is quite simple. You can paste a single RNA sequence (FASTA or plain sequence text) or a multiple alignment (CLUSTAL-W format) into the textarea then click on the ‘execute CentroidFold’ button. The server quickly responses with a prediction result. The major advantage of this server is that it employs our original CentroidFold software as its prediction engine which scores the best accuracy in our benchmark results. Our web server is freely available with no login requirement
Helicon Plasma Injector and Ion Cyclotron Acceleration Development in the VASIMR Experiment
In the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) radio frequency (rf) waves both produce the plasma and then accelerate the ions. The plasma production is done by action of helicon waves. These waves are circular polarized waves in the direction of the electron gyromotion. The ion acceleration is performed by ion cyclotron resonant frequency (ICRF) acceleration. The Advanced Space Propulsion Laboratory (ASPL) is actively developing efficient helicon plasma production and ICRF acceleration. The VASIMR experimental device at the ASPL is called VX-10. It is configured to demonstrate the plasma production and acceleration at the 10kW level to support a space flight demonstration design. The VX-10 consists of three electromagnets integrated into a vacuum chamber that produce magnetic fields up to 0.5 Tesla. Magnetic field shaping is achieved by independent magnet current control and placement of the magnets. We have generated both helium and hydrogen high density (>10(exp 18) cu m) discharges with the helicon source. ICRF experiments are underway. This paper describes the VX-10 device, presents recent results and discusses future plans
Translocation of structured polynucleotides through nanopores
We investigate theoretically the translocation of structured RNA/DNA
molecules through narrow pores which allow single but not double strands to
pass. The unzipping of basepaired regions within the molecules presents
significant kinetic barriers for the translocation process. We show that this
circumstance may be exploited to determine the full basepairing pattern of
polynucleotides, including RNA pseudoknots. The crucial requirement is that the
translocation dynamics (i.e., the length of the translocated molecular segment)
needs to be recorded as a function of time with a spatial resolution of a few
nucleotides. This could be achieved, for instance, by applying a mechanical
driving force for translocation and recording force-extension curves (FEC's)
with a device such as an atomic force microscope or optical tweezers. Our
analysis suggests that with this added spatial resolution, nanopores could be
transformed into a powerful experimental tool to study the folding of nucleic
acids.Comment: 9 pages, 5 figure
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