25 research outputs found
Adapting Covariance Propagation to Account for the Presence of Modeled and Unmodeled Maneuvers
This paper explores techniques that can be used to adapt the standard linearized propagation of an orbital covariance matrix to the case where there is a maneuver and an associated execution uncertainty. A Monte Carlo technique is used to construct a final orbital covariance matrix for a 'prop-burn-prop' process that takes into account initial state uncertainty and execution uncertainties in the maneuver magnitude. This final orbital covariance matrix is regarded as 'truth' and comparisons are made with three methods using modified linearized covariance propagation. The first method accounts for the maneuver by modeling its nominal effect within the state transition matrix but excludes the execution uncertainty by omitting a process noise matrix from the computation. The second method does not model the maneuver but includes a process noise matrix to account for the uncertainty in its magnitude. The third method, which is essentially a hybrid of the first two, includes the nominal portion of the maneuver via the state transition matrix and uses a process noise matrix to account for the magnitude uncertainty. The first method is unable to produce the final orbit covariance except in the case of zero maneuver uncertainty. The second method yields good accuracy for the final covariance matrix but fails to model the final orbital state accurately. Agreement between the simulated covariance data produced by this method and the Monte Carlo truth data fell within 0.5-2.5 percent over a range of maneuver sizes that span two orders of magnitude (0.1-20 m/s). The third method, which yields a combination of good accuracy in the computation of the final covariance matrix and correct accounting for the presence of the maneuver in the nominal orbit, is the best method for applications involving the computation of times of closest approach and the corresponding probability of collision, PC. However, applications for the two other methods exist and are briefly discussed. Although the process model ("prop-burn-prop") that was studied is very simple - point-mass gravitational effects due to the Earth combined with an impulsive delta-V in the velocity direction for the maneuver - generalizations to more complex scenarios, including high fidelity force models, finite duration maneuvers, and maneuver pointing errors, are straightforward and are discussed in the conclusion
A VALIDATION STUDY OF THE ROBERTSON-BERGER METER
An evaluation of the Robertson-Berger meter was done in order to address the question of whether instrumental and calibration factors can cause what might be interpreted as a change in the ground level solar ultraviolet-B (UV-B) flux. The evaluation consisted of reviewing information about the instrumentation and components in the published literature, a review of the records and procedures in both operations and calibrations, and examination of two instruments including temperature tests of them. It is shown that the instrument is basically stable and that the calibration procedures did not support data drift. There is a slight dependance of the two instruments upon temperature, 0.3°C/y and 0.6°C/y, which is not sufficient to lead to the reported UV-B trends of the order of 0.7 %/y. There is negligible temperature drift in the control electronics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73331/1/j.1751-1097.1992.tb09614.x.pd
Carrier thermal escape in families of InAs/InP self-assembled quantum dots
We investigate the thermal quenching of the multimodal photoluminescence from
InAs/InP (001) self-assembled quantum dots. The temperature evolution of the
photoluminescence spectra of two samples is followed from 10 K to 300 K. We
develop a coupled rate-equation model that includes the effect of carrier
thermal escape from a quantum dot to the wetting layer and to the InP matrix,
followed by transport, recapture or non-radiative recombination. Our model
reproduces the temperature dependence of the emission of each family of quantum
dots with a single set of parameters. We find that the main escape mechanism of
the carriers confined in the quantum dots is through thermal emission to the
wetting layer. The activation energy for this process is found to be close to
one-half the energy difference between that of a given family of quantum dots
and that of the wetting layer as measured by photoluminescence excitation
experiments. This indicates that electron and holes exit the InAs quantum dots
as correlated pairs
Diffusive hidden Markov model characterization of DNA looping dynamics in tethered particle experiments
In many biochemical processes, proteins bound to DNA at distant sites are
brought into close proximity by loops in the underlying DNA. For example, the
function of some gene-regulatory proteins depends on such DNA looping
interactions. We present a new technique for characterizing the kinetics of
loop formation in vitro, as observed using the tethered particle method, and
apply it to experimental data on looping induced by lambda repressor. Our
method uses a modified (diffusive) hidden Markov analysis that directly
incorporates the Brownian motion of the observed tethered bead. We compare
looping lifetimes found with our method (which we find are consistent over a
range of sampling frequencies) to those obtained via the traditional
threshold-crossing analysis (which can vary depending on how the raw data are
filtered in the time domain). Our method does not involve any time filtering
and can detect sudden changes in looping behavior. For example, we show how our
method can identify transitions between long-lived, kinetically distinct states
that would otherwise be difficult to discern
Definitive Evidence of Interlayer Coupling Between (Ga,Mn)As Layers Separated by a Nonmagnetic Spacer
We have used polarized neutron reflectometry to study the structural and
magnetic properties of the individual layers in a series of
(Al,Be,Ga)As/(Ga,Mn)As/GaAs/(Ga,Mn)As multilayer samples. Structurally, we
observe that the samples are virtually identical except for the GaAs spacer
thickness (which varies from 3-12 nm), and confirm that the spacers contain
little or no Mn. Magnetically, we observe that for the sample with the thickest
spacer layer, modulation doping by the(Al,Be,Ga)As results in (Ga,Mn)As layers
with very different temperature dependent magnetizations. However, as the
spacer layer thickness is reduced, the temperature dependent magnetizations of
the top an bottom (Ga,Mn)As layers become progressively more similar - a trend
we find to be independent of the crystallographic direction along which spins
are magnetized. These results definitively show that (Ga,Mn)As layers can
couple across a non-magnetic spacer, and that such coupling depends on spacer
thickness.Comment: Submitted to Physical Review
Photometric and Spectroscopic Studies of Massive Binaries in the Large Magellanic Cloud. I. Introduction and Orbits for Two Detached Systems: Evidence for a Mass Discrepancy?
The stellar mass-luminosity relation is poorly constrained by observations
for high mass stars. We describe our program to find eclipsing massive binaries
in the Magellanic Clouds using photometry of regions rich in massive stars, and
our spectroscopic follow-up to obtain radial velocities and orbits. Our
photometric campaign identified 48 early-type periodic variables, of which only
15 (31%) were found as part of the microlensing surveys. Spectroscopy is now
complete for 17 of these systems, and in this paper we present analysis of the
first two, LMC 172231 and ST2-28, simple detached systems of late-type O dwarfs
of relatively modest masses. Our orbit analysis yields very precise masses (2%)
and we use tomography to separate the components and determine effective
temperatures by model fitting, necessary for determining accurate (0.05-0.07
dex) bolometric luminosities in combination with the light-curve analysis. Our
approach allows more precise comparisons with evolutionary theory than
previously possible. To our considerable surprise, we find a small, but
significant, systematic discrepancy: all of the stars are slightly
under-massive, by typically 11% (or over-luminous by 0.2 dex) compared to that
predicted by the evolutionary models. We examine our approach for systematic
problems, but find no satisfactory explanation. The discrepancy is in the same
sense as the long-discussed and elusive discrepancy between the masses measured
from stellar atmosphere analysis with the stellar evolutionary models, and
might suggest that either increased rotation or convective overshooting is
needed in the models. Additional systems will be discussed in future papers of
this series, and will hopefully confirm or refute this trend.Comment: Accepted by the Astrophysical Journa
Experimental investigation of random noise-induced beam degradation in high-intensity accelerators using a linear Paul trap
A random noise-induced beam degradation that could affect intense beam transport over long propagation distances has been experimentally investigated by making use of the transverse beam dynamics equivalence between an alternating-gradient focusing system and a linear Paul trap system. For the present study, machine imperfections in the quadrupole focusing lattice are considered, which are emulated by adding small random noise on the voltage waveform of the quadrupole electrodes in the Paul trap. It is observed that externally driven noise continuously increases the rms radius, transverse emittance, and nonthermal tail of the trapped charge bunch almost linearly with the duration of the noise. The combined effects of collective modes and colored noise are also investigated and compared with numerical simulationsclose3