27,229 research outputs found
Satellite time transfer via Tracking and Data Relay Satellite System (TDRSS) and applications
With two geosynchronous relay satellites the tracking and data relay satellite system (TDRSS) can provide nearly worldwide coverage for communication between all near orbiting satellites and the satellite control center at Goddard Space Flight Center. Each future NASA satellite will carry a TDRSS transponder with which the satellite can communicate through a TDRSS to the ground station at White Sands, New Mexico. It is using this system that the ground station master clock time signal can be transmitted to the near Earth orbiting satellite in which a clock may be maintained independently to the accuracy required by the experimenters. The satellite time transfer terminal design concept and the application of the time signal in autonomously operated spacecraft clock are discussed. Some pertinent TDRSS parameters and corrections for the propagation delay measurement as well as the time code used to transfer the time signal are given
A grouped binary time code for telemetry and space applications
A computer oriented time code designed for users with various time resolution requirements is presented. It is intended as a time code for spacecraft and ground applications where direct code compatibility with automatic data processing equipment is of primary consideration. The principal features of this time code are: byte oriented format, selectable resolution options (from seconds to nanoseconds); and long ambiguity period. The time code is compatible with the new data handling and management concepts such as the NASA End-to-End Data System and the Telemetry Data Packetization format
Application of satellite time transfer in autonomous spacecraft clocks
The conceptual design of a spacecraft clock that will provide a standard time scale for experimenters in future spacecraft., and can be sychronized to a time scale without the need for additional calibration and validation is described. The time distribution to the users is handled through onboard computers, without human intervention for extended periods. A group parallel binary code, under consideration for onboard use, is discussed. Each group in the code can easily be truncated. The autonomously operated clock not only achieves simpler procedures and shorter lead times for data processing, but also contributes to spacecraft autonomy for onboard navigation and data packetization. The clock can be used to control the sensor in a spacecraft, compare another time signal such as that from the global positioning system, and, if the cost is not a consideration, can be used on the ground in remote sites for timekeeping and control
Performance of Loran-C chains relative to UTC
The long term performance of the eight Loran-C chains in terms of the Coordinated Universal Time (UTC) of the U.S. Naval Observatory (USNO) and the use of the Loran-C navigation system to maintain the user's clock to a UTC scale, are examined. The atomic time (AT) scale and the UTC of several national laboratories and observatories relative to the international atomic time (TAI) are presented. In addition, typical performance of several NASA tracking station clocks, relative to the USNO master clock, is also presented. Recent revision of the Coordinated Universal Time (UTC) by the International Radio Consultative Committee (CCIR) is given in an appendix
Kinetic Monte Carlo simulation of faceted islands in heteroepitaxy using multi-state lattice model
A solid-on-solid model is generalized to study the formation of Ge pyramid
islands bounded by (105) facets on Si(100) substrates in two dimensions. Each
atomic column is not only characterized by the local surface height but also by
two deformation state variables dictating the local surface tilt and vertical
extension. These deformations phenomenologically model surface reconstructions
in (105) facets and enable the formation of islands which better resemble
faceted pyramids. We demonstrate the model by application to a kinetic limited
growth regime. We observe significantly reduced growth rates after faceting and
a continuous nucleation of new islands until overcrowding occurs.Comment: 7 pages, 5 figure
Leakage and Comparative Advantage Implications of Agricultural Participation in Greenhouse Gas Emission Mitigation
The world is moving toward efforts to reduce greenhouse gas emissions. Net emission reduction efforts may involve the agricultural sector through options such as planting of trees, crop and livestock management changes, and production of biofuels. However, such options can be competitive with domestic food production. In a free trade arena, reduced domestic food production could stimulate increased production and exports in other countries, which are not pursuing similar mitigative courses of action. As a consequence, net emission reductions in implementing countries may be offset by activities stimulated in other countries. In addition producers in countries where agriculture is subject to greenhouse gas mitigation have expressed concern about their competitive position to countries which are not trying to reduce net emissions.Emission Leakage, Agricultural Sector Model, Greenhouse Gas Policy, Mitigation, Carbon Sequestration
Detrended fluctuation analysis on the correlations of complex networks under attack and repair strategy
We analyze the correlation properties of the Erdos-Renyi random graph (RG)
and the Barabasi-Albert scale-free network (SF) under the attack and repair
strategy with detrended fluctuation analysis (DFA). The maximum degree k_max,
representing the local property of the system, shows similar scaling behaviors
for random graphs and scale-free networks. The fluctuations are quite random at
short time scales but display strong anticorrelation at longer time scales
under the same system size N and different repair probability p_re. The average
degree , revealing the statistical property of the system, exhibits
completely different scaling behaviors for random graphs and scale-free
networks. Random graphs display long-range power-law correlations. Scale-free
networks are uncorrelated at short time scales; while anticorrelated at longer
time scales and the anticorrelation becoming stronger with the increase of
p_re.Comment: 5 pages, 4 figure
Chiral quark dynamics and topological charge: The role of the Ramond-Ramond U(1) Gauge Field in Holographic QCD
The Witten-Sakai-Sugimoto construction of holographic QCD in terms of D4
color branes and D8 flavor branes in type IIA string theory is used to
investigate the role of topological charge in the chiral dynamics of quarks in
QCD. The QCD theta term arises from a compactified 5-dimensional Chern-Simons
term on the D4 branes. This term couples the QCD topological charge to the
Ramond-Ramond gauge field of IIA string theory. The nonzero topological
susceptibility of pure-glue QCD can be attributed to the presence of D6 branes,
which constitute magnetic sources of the RR gauge field. The topological charge
of QCD is required, by an anomaly inflow argument, to coincide in space-time
with the intersection of the D6 branes and the D4 color branes. This clarifies
the relation between D6 branes and the coherent, codimension-one topological
charge membranes observed in QCD Monte Carlo calculations. Using
open-string/closed-string duality, we interpret a quark loop (represented by a
D4-D8 open string loop) in terms of closed-string exchange between color and
flavor branes. The role of the RR gauge field in quark-antiquark annihilation
processes is discussed. RR exchange in the s-channel generates a 4-quark
contact term which produces an mass insertion and provides an
explanation for the observed spin-parity structure of the OZI rule. The form of the anomaly emerges naturally. RR exchange in
the t-channel of the scattering amplitude produces a Nambu-Jona
Lasinio interaction which may provide a mechanism for spontaneous breaking of
.Comment: 20 pages, 7 figure
Instability strips of SPB and beta Cephei stars: the effect of the updated OP opacities and of the metal mixture
The discovery of Cephei stars in low metallicity environments, as
well as the difficulty in theoretically explaining the excitation of the
pulsation modes observed in some Cephei and hybrid SPB- Cephei
pulsators, suggest that the ``iron opacity bump'' provided by stellar models
could be underestimated. We analyze the effect of uncertainties in the opacity
computations and in the solar metal mixture, on the excitation of pulsation
modes in B-type stars. We carry out a pulsational stability analysis for four
grids of main-sequence models with masses between 2.5 and 12
computed with OPAL and OP opacity tables and two different metal mixtures.
We find that in a typical Cephei model the OP opacity is 25% larger
than OPAL in the region where the driving of pulsation modes occurs.
Furthermore, the difference in the Fe mass fraction between the two metal
mixtures considered is of the order of 20%. The implication on the excitation
of pulsation modes is non-negligible: the blue border of the SPB instability
strip is displaced at higher effective temperatures, leading to a larger number
of models being hybrid SPB- Cephei pulsators. Moreover, higher overtone
p-modes are excited in Cephei models and unstable modes are found in a
larger number of models for lower metallicities, in particular Cephei
pulsations are also found in models with Z=0.01.Comment: Accepted for publication in MNRAS Letter
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