7,220 research outputs found
Drag bias feedback for the analytic drag control entry guidance system
The Analytic Drag Control (ADC) entry guidance has been developed and baselined for the space shuttle orbiter entry. A method is presented which corrects the orbiter entry guidance commanded bank angle for biases between navigated drag and guidance computed reference drag. This is accomplished by an integral feedback technique, which uses the drag bias information to adjust the difference between navigated and reference altitude rate used by the ADC guidance. The method improves the capability of the ADC guidance system by compensating for any error source which causes a bias between the navigated drag and reference drag profile. These errors include navigated altitude rate errors, atmosphere dispersions, and roll attitude deadband effects. A discussion of the method and results of digital computer entry simulations is presented
Abort-once-around entry corridor analysis program document
The abort once around entry target corridor analysis program (ABECAP) was studied. The allowable range of flight path angles at entry interface for acceptable entry trajectories from a shuttle abort once around (AOA) situation was established. The solutions thus determined may be shown as corridor plots of entry interface flight path angle versus range from entry interface (EI) to the target
Abort-once-around entry dispersion corridor analysis
Abort-Once-Around (AOA) entry dispersion corridors were determined for Shuttle Mission 3A. These corridors are presented as plots of entry interface flight path angle versus range to target. The methods used to determine the corridors are discussed. Major dispersion sources are discussed and results presented. While specific trajectory inputs and constraints are subject to change, the dispersion corridors show the trends under consideration. The corridors presented show the delta V advantage of the two-burn over the one-burn AOA. The atmospheric dispersion study illustrates the need to target for the seasonal atmosphere. The 40 deg/30 deg angle of attack (alpha) (TPS design) profile did not provide adequate crossrange capability with worst case aerodynamic dispersions. This problem was alleviated with the change to a 38 deg/28 deg alpha profile. The backface temperatures calculated were generally higher than the present limits
Rate-Distortion-Based Physical Layer Secrecy with Applications to Multimode Fiber
Optical networks are vulnerable to physical layer attacks; wiretappers can
improperly receive messages intended for legitimate recipients. Our work
considers an aspect of this security problem within the domain of multimode
fiber (MMF) transmission. MMF transmission can be modeled via a broadcast
channel in which both the legitimate receiver's and wiretapper's channels are
multiple-input-multiple-output complex Gaussian channels. Source-channel coding
analyses based on the use of distortion as the metric for secrecy are
developed. Alice has a source sequence to be encoded and transmitted over this
broadcast channel so that the legitimate user Bob can reliably decode while
forcing the distortion of wiretapper, or eavesdropper, Eve's estimate as high
as possible. Tradeoffs between transmission rate and distortion under two
extreme scenarios are examined: the best case where Eve has only her channel
output and the worst case where she also knows the past realization of the
source. It is shown that under the best case, an operationally separate
source-channel coding scheme guarantees maximum distortion at the same rate as
needed for reliable transmission. Theoretical bounds are given, and
particularized for MMF. Numerical results showing the rate distortion tradeoff
are presented and compared with corresponding results for the perfect secrecy
case.Comment: 30 pages, 5 figures, accepted to IEEE Transactions on Communication
Divergence in seasonal hydrology across northern Eurasia: Emerging trends and water cycle linkages
Discharge from large Eurasia rivers increased during the 20th century, yet much remains unknown regarding details of this increasing freshwater flux. Here, for the three largest Eurasian basins (the Ob, Yenisei, and Lena) we examine the nature of annual and seasonal discharge trends by investigating the flow changes along with those for precipitation, snow depth, and snow water equivalent. On the basis of a multiperiod trend analysis and examination of station data, we propose two characteristic regimes to explain the long‐term discharge increase from these large Eurasian rivers. Over the early decades from approximately 1936 to 1965, annual precipitation correlates well with annual discharge, and positive discharge trends are concurrent with summer/fall discharge increases. The latter decades were marked by a divergence between winter/spring flows, which increased, amid summer/fall discharge declines. A comparison of cold season precipitation (CSP) and spring discharge trends across subbasins of the Ob, Yenisei, and Lena shows limited agreement with one precipitation data set but good agreement (R2 \u3e 0.90) when a second is used. While natural variability in the Arctic system tends to mask these emerging trends, spatial and temporal changes can generally be characterized by increased solid precipitation, primarily to the north, along with a drier hydrography during the warm season
The Evolution of Bias - Generalized
Fry (1996) showed that galaxy bias has the tendency to evolve towards unity,
i.e. in the long run, the galaxy distribution tends to trace that of matter.
Generalizing slightly Fry's reasoning, we show that his conclusion remains
valid in theories of modified gravity (or equivalently, complex clustered dark
energy). This is not surprising: as long as both galaxies and matter are
subject to the same force, dynamics would drive them towards tracing each
other. This holds, for instance, in theories where both galaxies and matter
move on geodesics. This relaxation of bias towards unity is tempered by cosmic
acceleration, however: the bias tends towards unity but does not quite make it,
unless the formation bias were close to unity. Our argument is extended in a
straightforward manner to the case of a stochastic or nonlinear bias. An
important corollary is that dynamical evolution could imprint a scale
dependence on the large scale galaxy bias. This is especially pronounced if
non-standard gravity introduces new scales to the problem: the bias at
different scales relaxes at different rates, the larger scales generally more
slowly and retaining a longer memory of the initial bias. A consistency test of
the current (general relativity + uniform dark energy) paradigm is therefore to
look for departure from a scale independent bias on large scales. A simple way
is to measure the relative bias of different populations of galaxies which are
at different stages of bias relaxation. Lastly, we comment on the possibility
of directly testing the Poisson equation on cosmological scales, as opposed to
indirectly through the growth factor.Comment: 8 pages, 2 figures. References added. Accepted for publication in
Physical Review
Photomechanical Investigation of Structural Behavior of Gyroscope Components. Task IV - Analysis of Initial Redesign of AB5-K8 GYROSCOPE
Photomechanics of structure and materials in redesigned AB5-K8 gyroscope component
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