18 research outputs found
First In-Orbit Experience of TerraSAR-X Flight Dynamics Operations
TerraSAR-X is an advanced synthetic aperture radar satellite system for scientific and commercial applications that is realized in a public-private partnership between the German Aerospace Center (DLR) and the Astrium GmbH. TerraSAR-X was launched at June 15, 2007 on top of a Russian DNEPR-1 rocket into a 514 km sun-synchronous dusk-dawn orbit with an 11-day repeat cycle and will be operated for a period of at least 5 years during which it will provide high resolution SAR-data in the X-band. Due to the objectives of the interferometric campaigns the satellite has to comply to tight orbit control requirements, which are formulated in the form of a 250 m toroidal tube around a pre-flight determined reference trajectory (see [1] for details). The acquisition of the reference orbit was one of the main and key activities during the Launch and Early Orbit Phase (LEOP) and had to compensate for both injection errors and spacecraft safe mode attitude control thruster activities. The paper summarizes the activities of GSOC flight dynamics team during both LEOP and early Commissioning Phase, where the main tasks have been 1) the first-acquisition support via angle-tracking and GPS-based orbit determination, 2) maneuver planning for target orbit acquisition and maintenance, and 3) precise orbit and attitude determination for SAR processing support. Furthermore, a presentation on the achieved results and encountered problems will be addressed
Empirical modeling of the stellar spectrum of galaxies
An empirical method of modeling the stellar spectrum of galaxies is proposed,
based on two successive applications of Principal Component Analysis (PCA). PCA
is first applied to the newly available stellar library STELIB, supplemented by
the J, H and K magnitudes taken mainly from the 2 Micron All Sky Survey
(2MASS). Next the resultant eigen-spectra are used to fit the observed spectra
of a sample of 1016 galaxies selected from the Sloan Digital Sky Survey Data
Release One (SDSS DR1). PCA is again applied, to the fitted spectra to
construct the eigen-spectra of galaxies with zero velocity dispersion. The
first 9 galactic eigen-spectra so obtained are then used to model the stellar
spectrum of the galaxies in SDSS DR1, and synchronously to estimate the stellar
velocity dispersion, the spectral type, the near-infrared SED, and the average
reddening. Extensive tests show that the spectra of different type galaxies can
be modeled quite accurately using these eigen-spectra. The method can yield
stellar velocity dispersion with accuracies better than 10%, for the spectra of
typical S/N ratios in SDSS DR1.Comment: 34 pages with 18 figures, submitted to A
Synthetic Spectra and Color-Temperature Relations of M Giants
As part of a project to model the integrated spectra and colors of elliptical
galaxies through evolutionary synthesis, we have refined our synthetic spectrum
calculations of M giants. After critically assessing three effective
temperature scales for M giants, we adopted the relation of Dyck et al. (1996)
for our models. Using empirical spectra of field M giants as a guide, we then
calculated MARCS stellar atmosphere models and SSG synthetic spectra of these
cool stars, adjusting the band absorption oscillator strengths of the TiO bands
to better reproduce the observational data. The resulting synthetic spectra are
found to be in very good agreement with the K-band spectra of stars of the
appropriate spectral type taken from Kleinmann & Hall (1986) as well. Spectral
types estimated from the strengths of the TiO bands and the depth of the
bandhead of CO near 2.3 microns quantitatively confirm that the synthetic
spectra are good representations of those of field M giants. The broad-band
colors of the models match the field relations of K and early-M giants very
well; for late-M giants, differences between the field-star and synthetic
colors are probably caused by the omission of spectral lines of VO and water in
the spectrum synthesis calculations. Here, we present four grids of K-band
bolometric corrections and colors -- Johnson U-V and B-V; Cousins V-R and V-I;
Johnson-Glass V-K, J-K and H-K; and CIT/CTIO V-K, J-K, H-K and CO -- for models
having 3000 K < Teff < 4000 K and -0.5 < log g < 1.5. These grids, which have
[Fe/H] = +0.25, 0.0, -0.5 and -1.0, extend and supplement the color-temperature
relations of hotter stars presented in a companion paper (astro-ph/9911367).Comment: To appear in the March 2000 issue of the Astronomical Journal. 60
pages including 15 embedded postscript figures (one page each) and 6 embedded
postscript tables (10 pages total
The Emergent Flux and Effective Temperature of Delta Canis Majoris
New angular diameter determinations for the bright southern F8 supergiant
Delta CMa enable the bolometric emergent flux and effective temperature of the
star to be determined with improved accuracy. The spectral flux distribution
and bolometric flux have been determined from published photometry and
spectrophotometry and combined with the angular diameter to derive the
bolometric emergent flux F = (6.50 plus/minus 0.24) x 10^7 W/m^2 and the
effective temperature Teff = 5818 plus/minus 53 K. The new value for the
effective temperature is compared with previous interferometric and infrared
flux method determinations. The accuracy of the effective temperature is now
limited by the uncertainty in the bolometric flux rather than by the
uncertainty in the angular diameter.Comment: 8 pages, 4 figure
On the Limb Darkening, Spectral Energy Distribution, and Temperature Structure of Procyon
We have fit synthetic visibilities from 3-D (CO5BOLD + PHOENIX) and 1-D
(PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to
high-precision interferometric data from the VLTI Interferometer (K-band) and
from the Mark III interferometer (500 nm and 800 nm). These data sets provide a
test of theoretical wavelength dependent limb-darkening predictions. The work
of Allende Prieto et al. has shown that the temperature structure from a
spatially and temporally averaged 3-D hydrodynamical model produces
significantly less limb darkening at 500 nm relative to the temperature
structure of a 1-D MARCS model atmosphere with a standard mixing-length
approximation for convection. Our direct fits to the interferometric data
confirm this prediction. A 1-D ATLAS 12 model with ``approximate overshooting''
provides the required temperature gradient. We show, however, that 1-D models
cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective
temperatures in the range constrained by the measured bolometric flux and
angular diameter. We find that a good match to the full spectral energy
distribution can be obtained with a composite model consisting of a weighted
average of twelve 1-D model atmospheres based on the surface intensity
distribution of a 3-D granulation simulation. We emphasize that 1-D models with
overshooting may realistically represent the mean temperature structure of
F-type stars like Procyon, but the same models will predict redder colors than
observed because they lack the multicomponent temperature distribution expected
for the surfaces of these stars.Comment: 24 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Accurate fundamental parameters for 23 bright solar-type stars
We combine results from interferometry, asteroseismology and spectroscopy to
determine accurate fundamental parameters of 23 bright solar-type stars, from
spectral type F5 to K2 and luminosity classes III to V. For some stars we can
use direct techniques to determine the mass, radius, luminosity and effective
temperature, and we compare with indirect methods that rely on photometric
calibrations or spectroscopic analyses. We use the asteroseismic information
available in the literature to infer an indirect mass with an accuracy of 4-15
percent. From indirect methods we determine luminosity and radius to 3 percent.
For Teff we find a slight offset of -40+-20 K between the spectroscopic method
and the direct method, meaning the spectroscopic temperatures are too high.
From the spectroscopic analysis we determine the detailed chemical
composition for 13 elements, including Li, C and O. We find no significant
offset between the spectroscopic surface gravity and the value from combining
asteroseismology with radius estimates. From the spectroscopy we also determine
vsini and we present a new calibration of macro- and microturbulence. From the
comparison between the results from the direct and spectroscopic methods we
claim that we can determine Teff, log g, and [Fe/H] with absolute accuracies of
80 K, 0.08 dex, and 0.07 dex. The indirect methods are important to obtain
reliable estimates of the fundamental parameters of relatively faint stars when
interferometry cannot be used. Our study is the first to compare direct and
indirect methods for a large sample of stars, and we conclude that indirect
methods are valid, although slight corrections may be needed.Comment: Accepted by MNRAS. Abstract abridge
The radius and other fundamental parameters of the F9 V star beta Virginis
We have used the Sydney University Stellar Interferometer (SUSI) to measure
the angular diameter of the F9 V star beta Virginis. After correcting for limb
darkening and combining with the revised Hipparcos parallax, we derive a radius
of 1.703 +/- 0.022 R_sun (1.3%). We have also calculated the bolometric flux
from published measurements which, combined with the angular diameter, implies
an effective temperature of 6059 +/- 49 K (0.8%). We also derived the
luminosity of beta Vir to be L = 3.51 +/- 0.08 L_sun (2.1%). Solar-like
oscillations were measured in this star by Carrier et al. (2005) and using
their value for the large frequency separation yields the mean stellar density
with an uncertainty of about 2%. Our constraints on the fundamental parameters
of beta Vir will be important to test theoretical models of this star and its
oscillations.Comment: accepted for publication in MNRAS. Updated reference
The DLR-GSOC Launch Window Software
At the German Space Operations Center of the DLR a new launch window program (LAUWIN) was developed for multi-burn geostationary transfer missions. At the present version, the program processes the following types of constraints for the launch window: - Forbidden Earth or moon shadows. - Maximum duration of Earth or monn shadows. - Forbidden sun aspect angles within a satellite coodinate system. - Forbidden Earth/moon interferences during Earth acquisition scans. - Forbbiden sun colinearities (angles sun-Earth-satellite). - Forbidden Earth-satellite-moon angles