315 research outputs found
Performance Assessment of Navigation Using Carrier Doppler Measurements from Multiple LEO Constellations
The goal of this work is to characterize a novel navigation method which uses carrier Doppler shift measurements from LEO satellites. An ever-growing reliance on the GNSS has coincided with an increase in ways it can be degraded or denied, whether naturally occurring or man-made. These potentially disastrous threats to traditional navigation and timing have necessitated new technologies to augment GNSS in the case of an outage. LEO constellations, whose size and higher signal power make them potentially useful for navigation, are one technology that has been explored. The navigation algorithms detailed in this research use Doppler measurements from 8 or more LEO satellites to simultaneously solve for position, clock offset, velocity, and clock offset rate. Through simulation, a user-satellite geometry analysis is conducted for a number of emerging LEO constellations, as well as navigation simulations with the same constellations. Results are presented which show promise from both a satellite geometry perspective and PVT solution convergence perspective
Microwave cavity light shining through a wall optimization and experiment
It has been proposed that microwave cavities can be used in a photon
regeneration experiment to search for hidden sector photons. Using two isolated
cavities, the presence of hidden sector photons could be inferred from a 'light
shining through a wall' phenomenon. The sensitivity of the experiment has
strong a dependence on the geometric construction and electromagnetic mode
properties of the two cavities. In this paper we perform an in depth
investigation to determine the optimal setup for such an experiment. We also
describe the results of our first microwave cavity experiment to search for
hidden sector photons. The experiment consisted of two cylindrical copper
cavities stacked axially inside a single vacuum chamber. At a hidden sector
photon mass of 37.78 micro eV we place an upper limit on the kinetic mixing
parameter chi = 2.9 * 10^(-5). Whilst this result lies within already
established limits our experiment validates the microwave cavity `light shining
through a wall' concept. We also show that the experiment has great scope for
improvement, potentially able to reduce the current upper limit on the mixing
parameter chi by several orders of magnitude.Comment: To be published in PR
High Resolution Flicker-Noise-Free Frequency Measurements of Weak Microwave Signals
Amplification is usually necessary when measuring the frequency instability
of microwave signals. In this work, we develop a flicker noise free frequency
measurement system based on a common or shared amplifier. First, we show that
correlated flicker phase noise can be cancelled in such a system. Then we
compare the new system with the conventional by simultaneously measuring the
beat frequency from two cryogenic sapphire oscillators with parts in 10^15
fractional frequency instability. We determine for low power, below -80 dBm,
the measurements were not limited by correlated noise processes but by thermal
noise of the readout amplifier. In this regime, we show that the new readout
system performs as expected and at the same level as the standard system but
with only half the number of amplifiers. We also show that, using a standard
readout system, the next generation of cryogenic sapphire oscillators could be
flicker phase noise limited when instability reaches parts in 10^16 or betterComment: Accepted for publication in IEEE Transactions on Microwave Theory &
Technique
A Fuzzy approach for detecting anomalous behaviour in e-mail traffic
This paper investigates the use of fuzzy inference for detection of abnormal changes in email traffic communication behaviour. Several communication behaviour measures and metrics are defined for extracting information on the traffic communication behaviour of email users. The information from these behaviour measures is then combined using a hierarchy of fuzzy inference systems, to provide an abnormality rating for overall changes in communication behaviour of suspect email accounts. The use of fuzzy inference is then demonstrated with a case study investigating the email traffic behaviour of a person’s email accounts from the Enron email corpus
Detrapping and retrapping of free carriers in nominally pure single crystal GaP, GaAs and 4H-SiC semiconductors under light illumination at cryogenic temperatures
We report on extremely sensitive measurements of changes in the microwave
properties of high purity non-intentionally-doped single-crystal semiconductor
samples of gallium phosphide, gallium arsenide and 4H-silicon carbide when
illuminated with light of different wavelengths at cryogenic temperatures.
Whispering gallery modes were excited in the semiconductors whilst they were
cooled on the coldfinger of a single-stage cryocooler and their frequencies and
Q-factors measured under light and dark conditions. With these materials, the
whispering gallery mode technique is able to resolve changes of a few parts per
million in the permittivity and the microwave losses as compared with those
measured in darkness. A phenomenological model is proposed to explain the
observed changes, which result not from direct valence to conduction band
transitions but from detrapping and retrapping of carriers from impurity/defect
sites with ionization energies that lay in the semiconductor band gap.
Detrapping and retrapping relaxation times have been evaluated from comparison
with measured data.Comment: 7 pages, 6 figure
Modified permittivity observed in bulk Gallium Arsenide and Gallium Phosphide samples at 50 K using the Whispering Gallery mode method
Whispering Gallery modes in bulk cylindrical Gallium Arsenide and Gallium
Phosphide samples have been examined both in darkness and under white light at
50 K. In both samples we observed change in permittivity under light and dark
conditions. This results from a change in the polarization state of the
semiconductor, which is consistent with a free electron-hole
creation/recombination process. The permittivity of the semiconductor is
modified by free photocarriers in the surface layers of the sample which is the
region sampled by Whispering Gallery modes.Comment: 8 pages, 3 figure
The Application of a High Resolution Nested Hydrodynamic Model to Quantification of Power Output from Tidal Turbine Arrays
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv
An Approach to Optmising the Harvesting of Tidal Energy
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv
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