736 research outputs found
Accuracy Studies of a Magnetometer-Only Attitude-and-Rate-Determination System
A personal computer based system was recently prototyped that uses measurements from a three axis magnetometer (TAM) to estimate the attitude and rates of a spacecraft using no a priori knowledge of the spacecraft's state. Past studies using in-flight data from the Solar, Anomalous, and Magnetospheric Particles Explorer focused on the robustness of the system and demonstrated that attitude and rate estimates could be obtained accurately to 1.5 degrees (deg) and 0.01 deg per second (deg/sec), respectively, despite limitations in the data and in the accuracies of te truth models. This paper studies the accuracy of the Kalman filter in the system using several orbits of in-flight Earth Radiation Budget Satellite (ERBS) data and attitude and rate truth models obtained from high precision sensors to demonstrate the practical capabilities. This paper shows the following: Using telemetered TAM data, attitude accuracies of 0.2 to 0.4 deg and rate accuracies of 0.002 to 0.005 deg/sec (within ERBS attitude control requirements of 1 deg and 0.0005 deg/sec) can be obtained with minimal tuning of the filter; Replacing the TAM data in the telemetry with simulated TAM data yields corresponding accuracies of 0.1 to 0.2 deg and 0.002 to 0.005 deg/sec, thus demonstrating that the filter's accuracy can be significantly enhanced by further calibrating the TAM. Factors affecting the fillter's accuracy and techniques for tuning the system's Kalman filter are also presented
Statistical Mechanics in the Extended Gaussian Ensemble
The extended gaussian ensemble (EGE) is introduced as a generalization of the
canonical ensemble. The new ensemble is a further extension of the Gaussian
ensemble introduced by J. H. Hetherington [J. Low Temp. Phys. {\bf 66}, 145
(1987)]. The statistical mechanical formalism is derived both from the analysis
of the system attached to a finite reservoir and from the Maximum Statistical
Entropy Principle. The probability of each microstate depends on two parameters
and which allow to fix, independently, the mean energy of the
system and the energy fluctuations respectively. We establish the Legendre
transform structure for the generalized thermodynamic potential and propose a
stability criterion. We also compare the EGE probability distribution with the
-exponential distribution. As an example, an application to a system with
few independent spins is presented.Comment: Revtex 4, 8 pages, 8 figure
Topology of event distribution as a generalized definition of phase transitions in finite systems
We propose a definition of phase transitions in finite systems based on
topology anomalies of the event distribution in the space of observations. This
generalizes all the definitions based on the curvature anomalies of
thermodynamical potentials and provides a natural definition of order
parameters. The proposed definition is directly operational from the
experimental point of view. It allows to study phase transitions in Gibbs
equilibria as well as in other ensembles such as the Tsallis ensemble.Comment: 4 pages, 3 figure
A Generalized Circle Theorem on Zeros of Partition Function at Asymmetric First Order Transitions
We present a generalized circle theorem which includes the Lee-Yang theorem
for symmetric transitions as a special case. It is found that zeros of the
partition function can be written in terms of discontinuities in the
derivatives of the free energy. For asymmetric transitions, the locus of the
zeros is tangent to the unit circle at the positive real axis in the
thermodynamic limit. For finite-size systems, they lie off the unit circle if
the partition functions of the two phases are added up with unequal prefactors.
This conclusion is substantiated by explicit calculation of zeros of the
partition function for the Blume-Capel model near and at the triple line at low
temperatures.Comment: 10 pages, RevTeX. To be published in PRL. 3 Figures will be sent upon
reques
Submillimeter Wave Astronomy Satellite (SWAS) Launch and Early Orbit Support Experiences
The Submillimeter Wave Astronomy Satellite (SWAS) was successfully launched on December 6, 1998 at 00:58 UTC. The two year mission is the fourth in the series of Small Explorer (SMEX) missions. SWAS is dedicated to the study of star formation and interstellar chemistry. SWAS was injected into a 635 km by 650 km orbit with an inclination of nearly 70 deg by an Orbital Sciences Corporation Pegasus XL launch vehicle. The Flight Dynamics attitude and navigation teams supported all phases of the early mission. This support included orbit determination, attitude determination, real-time monitoring, and sensor calibration. This paper reports the main results and lessons learned concerning navigation, support software, star tracker performance, magnetometer and gyroscope calibrations, and anomaly resolution. This includes information on spacecraft tip-off rates, first-day navigation problems, target acquisition anomalies, star tracker anomalies, and significant sensor improvements due to calibration efforts
Extending canonical Monte Carlo methods II
Previously, we have presented a methodology to extend canonical Monte Carlo
methods inspired on a suitable extension of the canonical fluctuation relation
compatible with negative heat capacities .
Now, we improve this methodology by introducing a better treatment of finite
size effects affecting the precision of a direct determination of the
microcanonical caloric curve , as well as
a better implementation of MC schemes. We shall show that despite the
modifications considered, the extended canonical MC methods possibility an
impressive overcome of the so-called \textit{super-critical slowing down}
observed close to the region of a temperature driven first-order phase
transition. In this case, the dependence of the decorrelation time with
the system size is reduced from an exponential growth to a weak power-law
behavior , which is shown in the particular case of
the 2D seven-state Potts model where the exponent .Comment: Version submitted to JSTA
Evaluating patient and provider satisfaction with the use of telemedicine for pediatric pre-anesthetic assessment during the COVID-19 pandemic
SURFACE INDUCED FINITE-SIZE EFFECTS FOR FIRST ORDER PHASE TRANSITIONS
We consider classical lattice models describing first-order phase
transitions, and study the finite-size scaling of the magnetization and
susceptibility. In order to model the effects of an actual surface in systems
like small magnetic clusters, we consider models with free boundary conditions.
For a field driven transition with two coexisting phases at the infinite volume
transition point , we prove that the low temperature finite volume
magnetization m_{\free}(L,h) per site in a cubic volume of size behaves
like
m_\free(L,h)=\frac{m_++m_-}2 + \frac{m_+-m_-}2
\tanh \bigl(\frac{m_+-m_-}2\,L^d\, (h-h_\chi(L))\bigr)+O(1/L),
where is the position of the maximum of the (finite volume)
susceptibility and are the infinite volume magnetizations at
and , respectively. We show that is shifted by an amount
proportional to with respect to the infinite volume transitions point
provided the surface free energies of the two phases at the transition
point are different. This should be compared with the shift for periodic boun\-
dary conditons, which for an asymmetric transition with two coexisting phases
is proportional only to . One also consider the position of
the maximum of the so called Binder cummulant U_\free(L,h). While it is again
shifted by an amount proportional to with respect to the infinite volume
transition point , its shift with respect to is of the much
smaller order . We give explicit formulas for the proportionality
factors, and show that, in the leading term, the relative shift is
the same as that for periodic boundary conditions.Comment: 65 pages, amstex, 1 PostScript figur
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