8,331 research outputs found
Method and apparatus for relative navigation using reflected GPS signals
A method and system to passively navigate an orbiting moving body towards an orbiting target using reflected GPS signals. A pair of antennas is employed to receive both direct signals from a plurality of GPS satellites and a second antenna to receive GPS signals reflected off an orbiting target. The direct and reflected signals are processed and compared to determine the relative distance and position of the orbiting moving body relative to the orbiting target
Probing the Galactic Potential with Next-Generation Observations of Disk Stars
Near-future surveys promise a dramatic improvement in the number and
precision of astrometric, photometric and spectroscopic measurements of stars
in the Milky Way's disk. We examine the impact of such surveys on our
understanding of the Galaxy by "observing" particle realizations of
non-axisymmetric disk distributions orbiting in an axisymmetric halo with
appropriate errors and then attempting to recover the underlying potential
using a Markov Chain Monte Carlo (MCMC) approach. We demonstrate that the
azimuthally averaged gravitational force field in the Galactic plane--and
hence, to a lesser extent, the Galactic mass distribution--can be tightly
constrained over a large range of radii using a variety of types of surveys so
long as the error distribution of the measurements of the parallax, proper
motion and radial velocity are well-understood and the disk is surveyed
globally. One advantage of our method is that the target stars can be selected
non-randomly in real or apparent-magnitude space to ensure just such a global
sample without biasing the results. Assuming we can always measure the
line-of-sight velocity of a star with at least 1 km/s precision, we demonstrate
that the force field can be determined to better than ~1% for Galactocentric
radii in the range R=4-20 kpc We conclude that near-future surveys, like SIM
Lite, Gaia, and VERA, will provide the first precise mapping of the
gravitational force field in the region of the Galactic disk.Comment: 41 pages and 10 figures, accepted for publication in Ap
The DWARF project: Eclipsing binaries - precise clocks to discover exoplanets
We present a new observational campaign, DWARF, aimed at detection of
circumbinary extrasolar planets using the timing of the minima of low-mass
eclipsing binaries. The observations will be performed within an extensive
network of relatively small to medium-size telescopes with apertures of ~20-200
cm. The starting sample of the objects to be monitored contains (i) low-mass
eclipsing binaries with M and K components, (ii) short-period binaries with sdB
or sdO component, and (iii) post-common-envelope systems containing a WD, which
enable to determine minima with high precision. Since the amplitude of the
timing signal increases with the orbital period of an invisible third
component, the timescale of project is long, at least 5-10 years. The paper
gives simple formulas to estimate suitability of individual eclipsing binaries
for the circumbinary planet detection. Intrinsic variability of the binaries
(photospheric spots, flares, pulsation etc.) limiting the accuracy of the
minima timing is also discussed. The manuscript also describes the best
observing strategy and methods to detect cyclic timing variability in the
minima times indicating presence of circumbinary planets. First test
observation of the selected targets are presented.Comment: 12 pages, 2 figures, submitted to Astron. Nachrichte
On the Completeness of Reflex Astrometry on Extrasolar Planets near the Sensitivity Limit
We provide a preliminary estimate of the performance of reflex astrometry on
Earth-like planets in the habitable zones of nearby stars. In Monte Carlo
experiments, we analyze large samples of astrometric data sets with low to
moderate signal-to-noise ratios. We treat the idealized case of a single planet
orbiting a single star, and assume there are no non-Keplerian complications or
uncertainties. The real case can only be more difficult. We use periodograms
for discovery and least-squares fits for estimating the Keplerian parameters.
We find a completeness for detection compatible with estimates in the
literature. We find mass estimation by least squares to be biased, as has been
found for noisy radial-velocity data sets; this bias degrades the completeness
of accurate mass estimation. When we compare the true planetary position with
the position predicted from the fitted orbital parameters, at future times, we
find low completeness for an accuracy goal of 0.3 times the semimajor axis of
the planet, even with no delay following the end of astrometric observations.
Our findings suggest that the recommendation of the ExoPlanet Task Force
(Lunine et al. 2008) for "the capability to measure convincingly wobble
semi-amplitudes down to 0.2 as integrated over the mission lifetime," may
not be satisfied by an instrument characterized by the noise floor of the Space
Interferometry Mission, as. An important,
unsolved, strategic challenge for the exoplanetary science program is figuring
out how to predict the future position of an Earth-like planet with accuracy
sufficient to ensure the efficiency and success of the science operations for
follow-on spectroscopy, which would search for biologically significant
molecules in the atmosphere.Comment: v2: 16 pages, 4 figures; ApJ accepte
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