5,851 research outputs found
NASA Aircraft-Satellite Instrument Calibration Project
Ground based prelaunch calibration of satellite instruments usually is not adequate to provide an accurate characterization of the in-orbit performance of a satellite instrument. This is because the ground calibrations may not simulate the in-orbit environment observations of the satellite sensor, or because the sensor characteristics have changed during launch and in-orbit operations. One technique to obtain a meaningful in-orbit calibration of satellite sensors is to acquire simultaneous observations of an Earth scene with the satellite and a well calibrated aircraft or shuttle sensor which has similar characteristics to the satellite sensor. This is a direct in-orbit calibration technique and is usually called vicarious calibration. The experiment with the control instrument must occur above the sensible atmosphere as measured by the satellite sensor to provide a useful improvement to the calibration of the satellite sensor. Some observations on the experiments are made
Calibration support for the Earth Observing System Project
The Earth Observing System Project (EOS) program guidelines establishes significantly more stringent requirements on calibrations of instruments. This requirement is driven by the need for long-term continuity of acquired data sets and the use of measurements in interdisciplinary investigations. Personnel from the Standards and Calibration Office have been supporting the Program and Project in interpreting these goals into specific requirements. Contributions to EOS have included participation in the Panel of Experts which produced a list of consensus items necessary for accomplishing an accurate calibration and suggested EOS Project Calibration Policy, and drafting the announcement of opportunity and bidders information package positions on instrument calibration and data product validation. Technical staffing was provided to the NASA delegates to the Committee on Earth Orbiting Satellites (club of space-faring nations) for the standing working group on Calibration and Data Validation
The Pulsation Properties of Procyon A
A grid of stellar evolution models for Procyon A has been calculated. These
models include the best physics available to us (including the latest opacities
and equation of state) and are based on the revised astrometric mass of Girard
et al (1996). Models were calculated with helium diffusion and with the
combined effects of helium and heavy element diffusion. Oscillation frequencies
for l=0,1,2 and 3 p-modes and the characteristic period spacing for the g-modes
were calculated for these models. We find that g-modes are sensitive to model
parameters which effect the structure of the core, such as convective core
overshoot, the heavy element abundance and the evolutionary state (main
sequence or shell hydrogen burning) of Procyon A. The p-modes are relatively
insensitive to the details of the physics used to model Procyon A, and only
depend on the evolutionary state of Procyon A. Hence, observations of p-mode
frequencies on Procyon A will serve as a robust test of stellar evolution
models.Comment: 4 pages, to appear in ApJ
Modeling Convective Core Overshoot and Diffusion in Procyon Constrained by Asteroseismic Data
We compare evolved stellar models, which match Procyons mass and position in
the HR diagram, to current ground-based asteroseismic observations. Diffusion
of helium and metals along with two conventional core overshoot descriptions
and the Kuhfuss nonlocal theory of convection are considered. We establish that
one of the two published asteroseismic data reductions for Procyon, which
mainly differ in their identification of even versus odd l-values, is a
significantly more probable and self-consistent match to our models than the
other. The most probable models according to our Bayesian analysis have evolved
to just short of turnoff, still retaining a hydrogen convective core. Our most
probable models include Y and Z diffusion and have conventional core overshoot
between 0.9 and 1.5 pressure scale heights, which increases the outer radius of
the convective core by between 22% to 28%, respectively. We discuss the
significance of this comparatively higher than expected core overshoot amount
in terms of internal mixing during evolution. The parameters of our most
probable models are similar regardless of whether adiabatic or nonadiabatic
model p-mode frequencies are compared to the observations, although, the
Bayesian probabilities are greater when the nonadiabatic model frequencies are
used. All the most probable models (with or without core overshoot, adiabatic
or nonadiabatic model frequencies, diffusion or no diffusion, including priors
for the observed HRD location and mass or not) have masses that are within one
sigma of the observed mass 1.497+/-0.037 Msun
Water vapor in the lower stratosphere measured from aircraft flight
Water vapor in the lower stratosphere was measured in situ by two aluminum oxide hygrometers mounted on the nose of an RB57 aircraft. Data were taken nearly continuously from January to May 1974 from an altitude of approximately 11 km to 19 km as the aircraft flew between 70 deg N and 50 deg S over the land areas in the Western Hemisphere. Pseudomeridional cross sections of water vapor and temperature are derived from the flight data and show mixing ratios predominantly between 2 and 4 micron gm/gm with an extreme range of 1 to 8 micron gm/gm. Measurement precision is estimated by comparing the simultaneously measured values from the two flight hygrometer systems. Accuracy is estimated to be about + or - 40 percent at 19 km. A height-averaged latitudinal cross section of water vapor shows symmetry of wet and dry zones
The nature of p-modes and granulation in HD 49933 observed by CoRoT
Context: Recent observations of HD49933 by the space-photometric mission
CoRoT provide photometric evidence of solar type oscillations in a star other
than our Sun. The first published reduction, analysis, and interpretation of
the CoRoT data yielded a spectrum of p-modes with l = 0, 1, and 2. Aims: We
present our own analysis of the CoRoT data in an attempt to compare the
detected pulsation modes with eigenfrequencies of models that are consistent
with the observed luminosity and surface temperature. Methods: We used the
Gruberbauer et al. frequency set derived based on a more conservative Bayesian
analysis with ignorance priors and fit models from a dense grid of model
spectra. We also introduce a Bayesian approach to searching and quantifying the
best model fits to the observed oscillation spectra. Results: We identify 26
frequencies as radial and dipolar modes. Our best fitting model has solar
composition and coincides within the error box with the spectroscopically
determined position of HD49933 in the H-R diagram. We also show that
lower-than-solar Z models have a lower probability of matching the observations
than the solar metallicity models. To quantify the effect of the deficiencies
in modeling the stellar surface layers in our analysis, we compare adiabatic
and nonadiabatic model fits and find that the latter reproduces the observed
frequencies better.Comment: accepted to be published in A&A, 9 pages, 5 figure
Bayesian Asteroseismology of 23 Solar-Like Kepler Targets
We study 23 previously published Kepler targets to perform a consistent
grid-based Bayesian asteroseismic analysis and compare our results to those
obtained via the Asteroseismic Modelling Portal (AMP). We find differences in
the derived stellar parameters of many targets and their uncertainties. While
some of these differences can be attributed to systematic effects between
stellar evolutionary models, we show that the different methodologies deliver
incompatible uncertainties for some parameters. Using non-adiabatic models and
our capability to measure surface effects, we also investigate the dependency
of these surface effects on the stellar parameters. Our results suggest a
dependence of the magnitude of the surface effect on the mixing length
parameter which also, but only minimally, affects the determination of stellar
parameters. While some stars in our sample show no surface effect at all, the
most significant surface effects are found for stars that are close to the
Sun's position in the HR diagram.Comment: 14 pages, 9 figures, accepted for publication in MNRA
On the detection of Lorentzian profiles in a power spectrum: A Bayesian approach using ignorance priors
Aims. Deriving accurate frequencies, amplitudes, and mode lifetimes from
stochastically driven pulsation is challenging, more so, if one demands that
realistic error estimates be given for all model fitting parameters. As has
been shown by other authors, the traditional method of fitting Lorentzian
profiles to the power spectrum of time-resolved photometric or spectroscopic
data via the Maximum Likelihood Estimation (MLE) procedure delivers good
approximations for these quantities. We, however, show that a conservative
Bayesian approach allows one to treat the detection of modes with minimal
assumptions (i.e., about the existence and identity of the modes).
Methods. We derive a conservative Bayesian treatment for the probability of
Lorentzian profiles being present in a power spectrum and describe an efficient
implementation that evaluates the probability density distribution of
parameters by using a Markov-Chain Monte Carlo (MCMC) technique.
Results. Potentially superior to "best-fit" procedure like MLE, which only
provides formal uncertainties, our method samples and approximates the actual
probability distributions for all parameters involved. Moreover, it avoids
shortcomings that make the MLE treatment susceptible to the built-in
assumptions of a model that is fitted to the data. This is especially relevant
when analyzing solar-type pulsation in stars other than the Sun where the
observations are of lower quality and can be over-interpreted. As an example,
we apply our technique to CoRoT observations of the solar-type pulsator HD
49933.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
- …