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Spacecraft attitude sensor
A system for sensing the attitude of a spacecraft includes a pair of optical scanners having a relatively narrow field of view rotating about the spacecraft x-y plane. The spacecraft rotates about its z axis at a relatively high angular velocity while one scanner rotates at low velocity, whereby a panoramic sweep of the entire celestial sphere is derived from the scanner. In the alternative, the scanner rotates at a relatively high angular velocity about the x-y plane while the spacecraft rotates at an extremely low rate or at zero angular velocity relative to its z axis to provide a rotating horizon scan. The positions of the scanners about the x-y plane are read out to assist in a determination of attitude. While the satellite is spinning at a relatively high angular velocity, the angular positions of the bodies detected by the scanners are determined relative to the sun by providing a sun detector having a field of view different from the scanners
Constraints on scalar diffusion anomaly in three-dimensional flows having bounded velocity gradients
This study is concerned with the decay behaviour of a passive scalar
in three-dimensional flows having bounded velocity gradients. Given an
initially smooth scalar distribution, the decay rate of the
scalar variance is found to be bounded in terms of controlled
physical parameters. Furthermore, in the zero diffusivity limit, ,
this rate vanishes as if there exists an
independent of such that for
. This condition is satisfied if in the limit ,
the variance spectrum remains steeper than for large wave
numbers . When no such positive exists, the scalar field may be
said to become virtually singular. A plausible scenario consistent with
Batchelor's theory is that becomes increasingly shallower for
smaller , approaching the Batchelor scaling in the limit
. For this classical case, the decay rate also vanishes, albeit
more slowly -- like , where is the Prandtl or Schmidt
number. Hence, diffusion anomaly is ruled out for a broad range of scalar
distribution, including power-law spectra no shallower than . The
implication is that in order to have a -independent and non-vanishing
decay rate, the variance at small scales must necessarily be greater than that
allowed by the Batchelor spectrum. These results are discussed in the light of
existing literature on the asymptotic exponential decay , where is independent of .Comment: 6-7 journal pages, no figures. accepted for publication by Phys.
Fluid
A Basis for Interactive Schema Merging
We present a technique for merging the schemas of heterogeneous databases that generalizes to several different data models, and show how it can be used in an interactive program that merges Entity-Relationship diagrams. Given a collection of schemas to be merged, the user asserts the correspondence between entities and relationships in the various schemas by defining "isa" relations between them. These assertions are then considered to be elementary schemas, and are combined with the elementary schemas in the merge. Since the method defines the merge to be the join in an information ordering on schemas, it is a commutative and associative operation, which means that the merge is defined independent of the order in which schemas are presented. We briefly describe a prototype interactive schema merging tool that has been built on these principles. Keywords: schemas, merging, semantic data models, entity-relationship data models, inheritance 1 Introduction Schema merging is the proble..
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Ancient Cosmic Dust from Triassic Halite
We describe the discovery of fossil micrometeorites in ancient Triassic rock salt; the first to be found in salt and the oldest complete micrometeorites found to date. We present an estimated flux rate of micrometeorites to Earth at this time
The Great Eruption of Eta Carinae
During the years 1838-1858, the very massive star {\eta} Carinae became the
prototype supernova impostor: it released nearly as much light as a supernova
explosion and shed an impressive amount of mass, but survived as a star.1 Based
on a light-echo spectrum of that event, Rest et al.2 conclude that "a new
physical mechanism" is required to explain it, because the gas outflow appears
cooler than theoretical expectations. Here we note that (1) theory predicted a
substantially lower temperature than they quoted, and (2) their inferred
observational value is quite uncertain. Therefore, analyses so far do not
reveal any significant contradiction between the observed spectrum and most
previous discussions of the Great Eruption and its physics.Comment: To appear in Nature, a brief communication arising in response to
Rest et al. 2012. Submitted to Nature February 17, 201
Suppression of Dephasing of Optically Trapped Atoms
Ultra-cold atoms trapped in an optical dipole trap and prepared in a coherent
superposition of their hyperfine ground states, decohere as they interact with
their environment. We demonstrate than the loss in coherence in an "echo"
experiment, which is caused by mechanisms such as Rayleigh scattering, can be
suppressed by the use of a new pulse sequence. We also show that the coherence
time is then limited by mixing to other vibrational levels in the trap and by
the finite lifetime of the internal quantum states of the atoms
Dirac Quantization of the Pais-Uhlenbeck Fourth Order Oscillator
As a model, the Pais-Uhlenbeck fourth order oscillator with equation of
motion
is a quantum-mechanical prototype of a field theory containing both second and
fourth order derivative terms. With its dynamical degrees of freedom obeying
constraints due to the presence of higher order time derivatives, the model
cannot be quantized canonically. We thus quantize it using the method of Dirac
constraints to construct the correct quantum-mechanical Hamiltonian for the
system, and find that the Hamiltonian diagonalizes in the positive and negative
norm states that are characteristic of higher derivative field theories.
However, we also find that the oscillator commutation relations become singular
in the limit, a limit which corresponds to a prototype
of a pure fourth order theory. Thus the particle content of the theory cannot be inferred from that of the
theory; and in fact in the limit we find that all of
the negative norm states move off shell, with the
spectrum of asymptotic in and out states of the equal frequency theory being
found to be completely devoid of states with either negative energy or negative
norm. As a byproduct of our work we find a Pais-Uhlenbeck analog of the zero
energy theorem of Boulware, Horowitz and Strominger, and show how in the equal
frequency Pais-Uhlenbeck theory the theorem can be transformed into a positive
energy theorem instead.Comment: RevTeX4, 20 pages. Final version, to appear in Phys. Rev.
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