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Doppler W-band polarization diversity space-borne radar simulator for wind studies
CloudSat observations are used in combination with collocated European Centre for Medium-Range
Weather Forecasts (ECMWF) reanalysis to simulate spaceborne W-band Doppler observations from slant-looking
radars. The simulator also includes cross-polarization effects
which are relevant if the Doppler velocities are derived from
polarization diversity pulse pair correlation. A specific conically scanning radar configuration (WIVERN), recently proposed to the ESA-Earth Explorer 10 call that aims to provide
global in-cloud winds for data assimilation, is analysed in
detail in this study.
One hundred granules of CloudSat data are exploited to investigate the impact on Doppler velocity estimates from three
specific effects: (1) non-uniform beam filling, (2) wind shear
and (3) crosstalk between orthogonal polarization channels
induced by hydrometeors and surface targets. Errors associated with non-uniform beam filling constitute the most important source of error and can account for almost 1 m s−1
standard deviation, but this can be reduced effectively to less
than 0.5 m s−1 by adopting corrections based on estimates
of vertical reflectivity gradients. Wind-shear-induced errors
are generally much smaller (∼ 0.2 m s−1
). A methodology
for correcting these errors has been developed based on estimates of the vertical wind shear and the reflectivity gradient. Low signal-to-noise ratios lead to higher random errors
(especially in winds) and therefore the correction (particularly the one related to the wind-shear-induced error) is less
effective at low signal-to-noise ratio. Both errors can be underestimated in our model because the CloudSat data do not
fully sample the spatial variability of the reflectivity fields,
whereas the ECMWF reanalysis may have smoother velocity fields than in reality (e.g. they underestimate vertical wind
shear).
The simulator allows for quantification of the average
number of accurate measurements that could be gathered by
the Doppler radar for each polar orbit, which is strongly impacted by the selection of the polarization diversity H − V
pulse separation, Thv. For WIVERN a selection close to 20 µs
(with a corresponding folding velocity equal to 40 m s−1
)
seems to achieve the right balance between maximizing the
number of accurate wind measurements (exceeding 10 % of
the time at any particular level in the mid-troposphere) and
minimizing aliasing effects in the presence of high winds.
The study lays the foundation for future studies towards
a thorough assessment of the performance of polar orbiting
wide-swath W-band Doppler radars on a global scale. The
next generation of scanning cloud radar systems and reanalyses with improved resolution will enable a full capture of the
spatial variability of the cloud reflectivity and the in-cloud
wind fields, thus refining the results of this study
Roche tomography of the secondary stars in CVs
The secondary stars in cataclysmic variables (CVs) are key to our
understanding of the origin, evolution and behaviour of this class of
interacting binary. In seeking a fuller understanding of these objects, the
challenge for observers is to obtain images of the secondary star. This goal
can be achieved through Roche tomography, an indirect imaging technique that
can be used to map the Roche-lobe-filling secondary. The review begins with a
description of the basic principles that underpin Roche tomography, including
methods for determining the system parameters. Finally, we conclude with a look
at the main scientific highlights to date, including the first unambiguous
detection of starspots on AE Aqr B, and consider the future prospects of this
technique.Comment: 4 pages, 4 figures. Accepted for publication in A
The Secondary Star in Cataclysmic Variables and Low Mass X-ray Binaries
We critically re-examine the available data on the spectral types, masses and
radii of the secondary stars in cataclysmic variables (CVs) and low-mass X-ray
binaries (LMXBs), using the new catalogue of Ritter & Kolb (1998) as a starting
point. We find there are 55 reliable spectral type determinations and only 14
reliable mass determinations of CV secondary stars (10 and 5, respectively, in
the case of LMXBs). We derive new spectral type-period, mass-radius,
mass-period and radius-period relations, and compare them with theoretical
predictions. We find that CV secondary stars with orbital periods shorter than
7-8 hours are, as a group, indistinguishable from main sequence stars in
detached binaries. We find it is not valid, however, to estimate the mass from
the spectral type of the secondary star in CVs or LMXBs. We find that LMXB
secondary stars show some evidence for evolution, with secondary stars which
are slightly too large for their mass. We show how the masses and radii of the
secondary stars in CVs can be used to test the validity of the disrupted
magnetic braking model of CV evolution, but we find that the currently
available data are not sufficiently accurate or numerous to allow such an
analysis. As well as considering secondary star masses, we also discuss the
masses of the white dwarfs in CVs, and find mean values of M_1 = 0.69+/-0.13
M_sun below the period gap, and M_1 = 0.80+/-0.22 M_sun above the period gap.Comment: 18 pages, 5 figure
Tight Lower Bounds on the Contact Distance Distribution in Poisson Hole Process
In this letter, we derive new lower bounds on the cumulative distribution
function (CDF) of the contact distance in the Poisson Hole Process (PHP) for
two cases: (i) reference point is selected uniformly at random from
independently of the PHP, and (ii) reference point is located at
the center of a hole selected uniformly at random from the PHP. While one can
derive upper bounds on the CDF of contact distance by simply ignoring the
effect of holes, deriving lower bounds is known to be relatively more
challenging. As a part of our proof, we introduce a tractable way of bounding
the effect of all the holes in a PHP, which can be used to study other
properties of a PHP as well.Comment: To appear in IEEE Wireless Communications Letter
Coexistence of RF-powered IoT and a Primary Wireless Network with Secrecy Guard Zones
This paper studies the secrecy performance of a wireless network (primary
network) overlaid with an ambient RF energy harvesting IoT network (secondary
network). The nodes in the secondary network are assumed to be solely powered
by ambient RF energy harvested from the transmissions of the primary network.
We assume that the secondary nodes can eavesdrop on the primary transmissions
due to which the primary network uses secrecy guard zones. The primary
transmitter goes silent if any secondary receiver is detected within its guard
zone. Using tools from stochastic geometry, we derive the probability of
successful connection of the primary network as well as the probability of
secure communication. Two conditions must be jointly satisfied in order to
ensure successful connection: (i) the SINR at the primary receiver is above a
predefined threshold, and (ii) the primary transmitter is not silent. In order
to ensure secure communication, the SINR value at each of the secondary nodes
should be less than a predefined threshold. Clearly, when more secondary nodes
are deployed, more primary transmitters will remain silent for a given guard
zone radius, thus impacting the amount of energy harvested by the secondary
network. Our results concretely show the existence of an optimal deployment
density for the secondary network that maximizes the density of nodes that are
able to harvest sufficient amount of energy. Furthermore, we show the
dependence of this optimal deployment density on the guard zone radius of the
primary network. In addition, we show that the optimal guard zone radius
selected by the primary network is a function of the deployment density of the
secondary network. This interesting coupling between the two networks is
studied using tools from game theory. Overall, this work is one of the few
concrete works that symbiotically merge tools from stochastic geometry and game
theory
Joint Uplink and Downlink Coverage Analysis of Cellular-based RF-powered IoT Network
Ambient radio frequency (RF) energy harvesting has emerged as a promising
solution for powering small devices and sensors in massive Internet of Things
(IoT) ecosystem due to its ubiquity and cost efficiency. In this paper, we
study joint uplink and downlink coverage of cellular-based ambient RF energy
harvesting IoT where the cellular network is assumed to be the only source of
RF energy. We consider a time division-based approach for power and information
transmission where each time-slot is partitioned into three sub-slots: (i)
charging sub-slot during which the cellular base stations (BSs) act as RF
chargers for the IoT devices, which then use the energy harvested in this
sub-slot for information transmission and/or reception during the remaining two
sub-slots, (ii) downlink sub-slot during which the IoT device receives
information from the associated BS, and (iii) uplink sub-slot during which the
IoT device transmits information to the associated BS. For this setup, we
characterize the joint coverage probability, which is the joint probability of
the events that the typical device harvests sufficient energy in the given time
slot and is under both uplink and downlink signal-to-interference-plus-noise
ratio (SINR) coverage with respect to its associated BS. This metric
significantly generalizes the prior art on energy harvesting communications,
which usually focused on downlink or uplink coverage separately. The key
technical challenge is in handling the correlation between the amount of energy
harvested in the charging sub-slot and the information signal quality (SINR) in
the downlink and uplink sub-slots. Dominant BS-based approach is developed to
derive tight approximation for this joint coverage probability. Several system
design insights including comparison with regularly powered IoT network and
throughput-optimal slot partitioning are also provided
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