62,547 research outputs found
On-board processing concepts for future satellite communications systems
The initial definition of on-board processing for an advanced satellite communications system to service domestic markets in the 1990's is discussed. An exemplar system with both RF on-board switching and demodulation/remodulation baseband processing is used to identify important issues related to system implementation, cost, and technology development. Analyses of spectrum-efficient modulation, coding, and system control techniques are summarized. Implementations for an RF switch and baseband processor are described. Among the major conclusions listed is the need for high gain satellites capable of handling tens of simultaneous beams for the efficient reuse of the 2.5 GHz 30/20 frequency band. Several scanning beams are recommended in addition to the fixed beams. Low power solid state 20 GHz GaAs FET power amplifiers in the 5W range and a general purpose digital baseband processor with gigahertz logic speeds and megabits of memory are also recommended
Recent progress towards predicting aircraft ground handling performance
Capability implemented in simulating aircraft ground handling performance is reviewed and areas for further expansion and improvement are identified. Problems associated with providing necessary simulator input data for adequate modeling of aircraft tire/runway friction behavior are discussed and efforts to improve tire/runway friction definition, and simulator fidelity are described. Aircraft braking performance data obtained on several wet runway surfaces are compared to ground vehicle friction measurements. Research to improve methods of predicting tire friction performance are discussed
Recent Progress Towards Predicting Aircraft Ground Handling Performance
The significant progress which has been achieved in development of aircraft ground handling simulation capability is reviewed and additional improvements in software modeling identified. The problem associated with providing necessary simulator input data for adequate modeling of aircraft tire/runway friction behavior is discussed and efforts to improve this complex model, and hence simulator fidelity, are described. Aircraft braking performance data obtained on several wet runway surfaces is compared to ground vehicle friction measurements and, by use of empirically derived methods, good agreement between actual and estimated aircraft braking friction from ground vehilce data is shown. The performance of a relatively new friction measuring device, the friction tester, showed great promise in providing data applicable to aircraft friction performance. Additional research efforts to improve methods of predicting tire friction performance are discussed including use of an instrumented tire test vehicle to expand the tire friction data bank and a study of surface texture measurement techniques
An Attempt to Calculate Energy Eigenvalues in Quantum Systems of Large Sizes
We report an attempt to calculate energy eigenvalues of large quantum systems
by the diagonalization of an effectively truncated Hamiltonian matrix. For this
purpose we employ a specific way to systematically make a set of orthogonal
states from a trial wavefunction and the Hamiltonian. In comparison with the
Lanczos method, which is quite powerful if the size of the system is within the
memory capacity of computers, our method requires much less memory resources at
the cost of the extreme accuracy.
In this paper we demonstrate that our method works well in the systems of
one-dimensional frustrated spins up to 48 sites, of bosons on a chain up to 32
sites and of fermions on a ladder up to 28 sites. We will see this method
enables us to study eigenvalues of these quantum systems within reasonable
accuracy.Comment: 17pages, 4figures(eps-files
Ground State Properties of the Doped 3-Leg t-J Ladder
Results for a doped 3-leg t-J ladder obtained using the density matrix
renormalization group are reported. At low hole doping, the holes form a dilute
gas with a uniform density. The momentum occupation of the odd band shows a
sharp decrease at a large value of k_F similar to the behavior of a lightly
doped t-J chain, while the even modes appear gapped. The spin-spin correlations
decay as a power law consistent with the absence of a spin gap, but the pair
field correlations are negligible. At larger doping we find evidence for a spin
gap and as x increases further we find 3-hole diagonal domain walls. In this
regime there are pair field correlations and the internal pair orbital has
d_x^2-y^2 - like symmetry. However, the pair field correlations appear to fall
exponentially at large distances.Comment: 14 pages, 11 postscript figure
The X-ray absorption spectrum of 4U1700-37 and its implications for the stellar wind of the companion HD153919
The first high resolution non-dispersive 2-60 KeV X-ray spectra of 4U1700-37 is presented. The continuum is typical of that found from X-ray pulsars; that is a flat power law between 2 and 10 keV and, beyond 10 keV, an exponential decay of characteristic energy varying between 10 and 20 keV. No X-ray pulsations were detected between 160 ms and 6 min with an amplitude greater than approximately 2%. The absorption measured at binary phases approximately 0.72 is comparable to that expected from the stellar wind of the primary. The gravitational capture of material in the wind is found to be more than enough to power the X-ray source. The increase in the average absorption after phi o approximately 0.5 is confirmed. The minimum level of adsorption is a factor of 2 or 3 lower than that reported by previous observers, which may be related to a factor of approximately 10 decline in the average X-ray luminosity over the same interval. Short term approximately 50% variations in adsorption are seen for the first time which appear to be loosely correlated with approximately 10 min flickering activity in the X-ray flux. These most likely originate from inhomogeneities in the stellar wind of the primary
The design, development, and flight test results of the Boeing 737 aircraft antennas for the ICAO demonstration of the TRSB microwave landing system
The Research Support Flight System, a modified Boeing 737, was used to evaluate the performance of several aircraft antennas and locations for the Time Reference Scanning Beam (TRSB) Microwave Landing System (MLS). These tests were conducted at the National Aviation Facilities Experimental Center (NAFEC), Atlantic City, New Jersey on December 18, 1975. The flight tests measured the signal strength and all pertinent MLS data during a straight-in approach, a racetrack approach, and ICAO approach profiles using the independent antenna-receiver combinations simultaneously on the aircraft. Signal drop-outs were experienced during the various approaches but only a small percentage could be attributed to antenna pattern effects
Fixed Point of the Finite System DMRG
The density matrix renormalization group (DMRG) is a numerical method that
optimizes a variational state expressed by a tensor product. We show that the
ground state is not fully optimized as far as we use the standard finite system
algorithm, that uses the block structure B**B. This is because the tensors are
not improved directly. We overcome this problem by using the simpler block
structure B*B for the final several sweeps in the finite iteration process. It
is possible to increase the numerical precision of the finite system algorithm
without increasing the computational effort.Comment: 6 pages, 4 figure
SHEEP: The Search for the High Energy Extragalactic Population
We present the SHEEP survey for serendipitously-detected hard X-ray sources
in ASCA GIS images. In a survey area of deg, 69 sources were
detected in the 5-10 keV band to a limiting flux of erg
cm s. The number counts agree with those obtained by the similar
BeppoSAX HELLAS survey, and both are in close agreement with ASCA and BeppoSAX
2-10 keV surveys. Spectral analysis of the SHEEP sample reveals that the 2-10
and 5-10 keV surveys do not sample the same populations, however, as we find
considerably harder spectra, with an average assuming no
absorption. The implication is that the agreement in the number counts is
coincidental, with the 5-10 keV surveys gaining approximately as many hard
sources as they lose soft ones, when compared to the 2-10 keV surveys. This is
hard to reconcile with standard AGN ``population synthesis'' models for the
X-ray background, which posit the existence of a large population of absorbed
sources. We find no evidence of the population hardening at faint fluxes, with
the exception that the few very brightest objects are anomalously soft. 53 of
the SHEEP sources have been covered by ROSAT in the pointed phase. Of these 32
were detected. An additional 3 were detected in the RASS. As expected the
sources detected with ROSAT are systematically softer than those detected with
ASCA alone, and of the sample as a whole (truncated).Comment: 36 pages, 7 figs, to appear in Ap
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