6,985 research outputs found
Multipath errors in range rate measurement by a TDRS/VHF - GRARR
Range rate errors due to multipath reflection are calculated for a tracking and data relay satellite system using the VHF Goddard range and range rate (GRARR) system. At VHF the reflection is primarily specular, and the strength of the multipath relative to the direct path can be modeled in terms of the geometry and the surface characteristics, specifically the root-mean-square (rms) ocean wave height. The uplink and downlink multipath introduces phase jitter on the GRARR carrier and subcarrier. The derivation of these effects is reviewed leading to an expression for the rms range rate error. The derivation assumed the worst-case orbital configurations in which there was very little relative specular Doppler. This means that the specular multipath interference was not attenuated by the carrier and subcarrier PLL transfer functions. Curves of range rate error are presented as a function of grazing angle with wave height 0.3 to 0.7 meters and spacecraft altitude 100 to 700 miles as parameters
Multipath performance of a TDRS system employing wideband FM VHF signals
An approximate theoretical analysis is presented for the effects of specular reflection multipath on the performance of a one-way tracking and data relay satellite-to-user link. The analysis pertains to the wideband FM system employing a sinusoidal subcarrier to achieve spectrum spreading. Bounds on multipath effects are derived for receivers with and without limiters and for data modulated on the carrier or the subcarrier. For data modulation, performance is evaluated in terms of an additive lowpass signal at the data detection filter. Doppler and range tracking performance is evaluated in terms of root-mean-square (RMS) error of carrier frequency in a carrier PLL and rms phase jitter in a subcarrier PLL
Multipath error in range rate measurement by PLL-transponder/GRARR/TDRS
Range rate errors due to specular and diffuse multipath are calculated for a tracking and data relay satellite (TDRS) using an S band Goddard range and range rate (GRARR) system modified with a phase-locked loop transponder. Carrier signal processing in the coherent turn-around transponder and the GRARR reciever is taken into account. The root-mean-square (rms) range rate error was computed for the GRARR Doppler extractor and N-cycle count range rate measurement. Curves of worst-case range rate error are presented as a function of grazing angle at the reflection point. At very low grazing angles specular scattering predominates over diffuse scattering as expected, whereas for grazing angles greater than approximately 15 deg, the diffuse multipath predominates. The range rate errors at different low orbit altutudes peaked between 5 and 10 deg grazing angles
Low temperature terahertz spectroscopy of n-InSb through a magnetic field driven metal-insulator transition
We use fiber-coupled photoconductive emitters and detectors to perform
terahertz (THz) spectroscopy of lightly-doped n-InSb directly in the cryogenic
(1.5 K) bore of a high-field superconducting magnet. We measure transmission
spectra from 0.1-1.1 THz as the sample is driven through a metal-insulator
transition (MIT) by applied magnetic field. In the low-field metallic state,
the data directly reveal the plasma edge and magneto-plasmon modes. With
increasing field, a surprisingly broad band (0.3-0.8 THz) of low transmission
appears at the onset of the MIT. This band subsequently collapses and evolves
into the sharp 1s -> 2p- transition of electrons `frozen' onto isolated donors
in the insulating state.Comment: 4 pages, 3 figure
Critical Currents of Josephson-Coupled Wire Arrays
We calculate the current-voltage characteristics and critical current
I_c^{array} of an array of Josephson-coupled superconducting wires. The array
has two layers, each consisting of a set of parallel wires, arranged at right
angles, such that an overdamped resistively-shunted junction forms wherever two
wires cross. A uniform magnetic field equal to f flux quanta per plaquette is
applied perpendicular to the layers. If f = p/q, where p and q are mutually
prime integers, I_c^{array}(f) is found to have sharp peaks when q is a small
integer. To an excellent approximation, it is found in a square array of n^2
plaquettes, that I_c^{array}(f) \propto (n/q)^{1/2} for sufficiently large n.
This result is interpreted in terms of the commensurability between the array
and the assumed q \times q unit cell of the ground state vortex lattice.Comment: 4 pages, 4 figure
Arp 65 interaction debris: massive HI displacement and star formation
Context: Pre-merger interactions between galaxies can induce significant
changes in the morphologies and kinematics of the stellar and ISM components.
Large amounts of gas and stars are often found to be disturbed or displaced as
tidal debris. This debris then evolves, sometimes forming stars and
occasionally tidal dwarf galaxies. Here we present results from our HI study of
Arp 65, an interacting pair hosting extended HI tidal debris. Aims: In an
effort to understand the evolution of tidal debris produced by interacting
pairs of galaxies, including in situ star and tidal dwarf galaxy formation, we
are mapping HI in a sample of interacting galaxy pairs. The Arp 65 pair is one
of them. Methods: Our resolved HI 21 cm line survey is being carried out using
the Giant Metrewave Radio Telescope (GMRT). We used our HI survey data as well
as available SDSS optical, Spitzer infra-red and GALEX UV data to study the
evolution of the tidal debris and the correlation of HI with the star-forming
regions within it. Results: In Arp 65 we see a high impact pre-merger
interaction involving a pair of massive galaxies (NGC 90 and NGC 93) that have
a stellar mass ratio of ~ 1:3. The interaction, which probably occurred ~ 1.0
-- 2.5 10 yr ago, appears to have displaced a large fraction of
the HI in NGC 90 (including the highest column density HI) beyond its optical
disk. We also find extended ongoing star formation in the outer disk of NGC 90.
In the major star-forming regions, we find the HI column densities to be ~ 4.7
10 cm or lower. But no signature of star formation was
found in the highest column density HI debris, SE of NGC 90. This indicates
conditions within the highest column density HI debris remain hostile to star
formation and it reaffirms that high HI column densities may be a necessary but
not sufficient criterion for star formation.Comment: Accepted in A&
Diurnal variation of upper tropospheric humidity and its relations to convective activities over tropical Africa
Diurnal variations of upper tropospheric humidity (UTH) as well as middle tropospheric humidity (MTH) were examined in conjunction with the diurnal cycle of convection over tropical Africa and the adjacent tropical Atlantic Ocean using Meteosat-8 measurements. Cloud and humidity features were also tracked to document the diurnal variations of humidity and clouds in the Lagrangian framework. <br><br> A distinct diurnal variation of UTH (and MTH) is noted over regions where tropical deep convective cloud systems are commonly observed. The amplitude of the UTH diurnal variation is larger over land, while its variations over convectively inactive subtropical regions are much smaller. The diurnal variation of UTH tends to reach a maximum during nighttime over land, lagging deep convection and high cloud whose maxima occurred in the late afternoon and evening, respectively. It was revealed that these diurnal variations over the African continent are likely associated with continental-scale daytime solar heating and topography, in which topographically-induced signals develop earlier around the mid-afternoon and merge into stronger and broader continental-scale convection clusters later, forming a precipitation maximum in the late afternoon. It was also revealed that advection effect on the diurnal variation appears to be insignificant
Mass accretion rates of clusters of galaxies: CIRS and HeCS
We use a new spherical accretion recipe tested on N-body simulations to
measure the observed mass accretion rate (MAR) of 129 clusters in the Cluster
Infall Regions in the Sloan Digital Sky Survey (CIRS) and in the Hectospec
Cluster Survey (HeCS). The observed clusters cover the redshift range of
and the mass range of . Based on three-dimensional mass profiles of simulated
clusters reaching beyond the virial radius, our recipe returns MARs that agree
with MARs based on merger trees. We adopt this recipe to estimate the MAR of
real clusters based on measurements of the mass profile out to .
We use the caustic method to measure the mass profiles to these large radii. We
demonstrate the validity of our estimates by applying the same approach to a
set of mock redshift surveys of a sample of 2000 simulated clusters with a
median mass of as well as a sample
of 50 simulated clusters with a median mass of : the median MARs based on the caustic mass profiles of
the simulated clusters are unbiased and agree within with the median
MARs based on the real mass profile of the clusters. The MAR of the CIRS and
HeCS clusters increases with the mass and the redshift of the accreting
cluster, which is in excellent agreement with the growth of clusters in the
CDM model.Comment: 25 pages, 19 figures, 7 table
Husimi Maps in Lattices
We build upon previous work that used coherent states as a measurement of the
local phase space and extended the flux operator by adapting the Husimi
projection to produce a vector field called the Husimi map. In this article, we
extend its definition from continuous systems to lattices. This requires making
several adjustments to incorporate effects such as group velocity and multiple
bands. Several phenomena which uniquely occur in lattice systems, like
group-velocity warping and internal Bragg diffraction, are explained and
demonstrated using Husimi maps. We also show that scattering points between
bands and valleys can be identified in the divergence of the Husimi map
- …