11 research outputs found
Three-Dimensional Electromagnetic Scattering from Layered Media with Rough Interfaces for Subsurface Radar Remote Sensing
The objective of this dissertation is to develop forward scattering models for active microwave remote sensing of natural features represented by layered media with rough interfaces. In particular, soil profiles are considered, for which a model of electromagnetic scattering from multilayer rough surfaces with/without buried random media is constructed.
Starting from a single rough surface, radar scattering is modeled using the stabilized extended boundary condition method (SEBCM). This method solves the long-standing instability issue of the classical EBCM, and gives three-dimensional full wave solutions over large ranges of surface roughnesses with higher computational e±ciency than pure numerical solutions, e.g., method of moments (MoM). Based on this single surface solution, multilayer rough surface scattering is modeled using the scattering matrix approach and the model is used for a comprehensive sensitivity analysis of the total ground scattering as a function of layer separation, subsurface statistics, and sublayer dielectric properties.
The buried inhomogeneities such as rocks and vegetation roots are considered for the first time in the forward scattering model. Radar scattering from buried random media is modeled by the aggregate transition matrix using either the recursive transition matrix approach for spherical or short-length cylindrical scatterers, or the generalized iterative extended boundary condition method we developed for long cylinders or root-like cylindrical clusters. These approaches take the field interactions among scatterers into account with high computational efficiency. The aggregate transition matrix is transformed to a scattering matrix for the full solution to the layered-medium problem. This step is based on the near-to-far field transformation of the numerical plane wave expansion of the spherical harmonics and the multipole expansion of plane waves. This transformation consolidates volume scattering from the buried random medium with the scattering from layered structure in general. Combined with scattering from multilayer rough surfaces, scattering contributions from subsurfaces and vegetation roots can be then simulated. Solutions of both the rough surface scattering and random media scattering are validated numerically, experimentally, or both. The experimental validations have been carried out using a laboratory-based transmit-receive system for scattering from random media and a new bistatic tower-mounted radar system for field-based surface scattering measurements.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91459/1/xduan_1.pd
X band model of Venus atmosphere permittivity
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94912/1/rds5697.pd
Assessing the Privacy Benefits of Domain Name Encryption
As Internet users have become more savvy about the potential for their
Internet communication to be observed, the use of network traffic encryption
technologies (e.g., HTTPS/TLS) is on the rise. However, even when encryption is
enabled, users leak information about the domains they visit via DNS queries
and via the Server Name Indication (SNI) extension of TLS. Two recent proposals
to ameliorate this issue are DNS over HTTPS/TLS (DoH/DoT) and Encrypted SNI
(ESNI). In this paper we aim to assess the privacy benefits of these proposals
by considering the relationship between hostnames and IP addresses, the latter
of which are still exposed. We perform DNS queries from nine vantage points
around the globe to characterize this relationship. We quantify the privacy
gain offered by ESNI for different hosting and CDN providers using two
different metrics, the k-anonymity degree due to co-hosting and the dynamics of
IP address changes. We find that 20% of the domains studied will not gain any
privacy benefit since they have a one-to-one mapping between their hostname and
IP address. On the other hand, 30% will gain a significant privacy benefit with
a k value greater than 100, since these domains are co-hosted with more than
100 other domains. Domains whose visitors' privacy will meaningfully improve
are far less popular, while for popular domains the benefit is not significant.
Analyzing the dynamics of IP addresses of long-lived domains, we find that only
7.7% of them change their hosting IP addresses on a daily basis. We conclude by
discussing potential approaches for website owners and hosting/CDN providers
for maximizing the privacy benefits of ESNI.Comment: In Proceedings of the 15th ACM Asia Conference on Computer and
Communications Security (ASIA CCS '20), October 5-9, 2020, Taipei, Taiwa
Cross-Linked and Biodegradable Polymeric System as a Safe Magnetic Resonance Imaging Contrast Agent
Owing
to the low efficacy of clinically used small-molecule gadolinium (Gd)-based
magnetic resonance imaging (MRI) agents, we designed and explored
biodegradable macromolecular conjugates as MRI contrast agents. The
linear polymeric structure and core-cross-linked formulation possessed
different characteristics and features, so we prepared and comparatively
studied the two kinds of Gd-based <i>N</i>-(2-hydroxypropyl)
methacrylamide (HPMA) polymeric systems (the core-cross-linked pHPMA–DOTA-Gd
and the linear one) using the clinical agent diethylene-triamine pentaacetic
acid-GdÂ(III) (DTPA-Gd) as a control. This study was aimed to find
the optimal polymeric formulation as a biocompatible and efficient
MRI contrast agent. The high molecular weight (MW, 181 kDa) and core-cross-linked
copolymer was obtained via the cross-linked block linear copolymer
and could be degraded to low-MW segments (29 kDa) in the presence
of glutathione (GSH) and cleaned from the body. Both core-cross-linked
and linear pHPMA–DOTA-Gd copolymers displayed 2–3-fold
increased relaxivity (<i>r</i><sub>1</sub> value) than that
of DTPA-Gd. Animal studies demonstrated that two kinds of macromolecular
systems led to much longer blood circulation time, higher tumor accumulation,
and much higher signal intensity compared with the linear and clinical
ones. Finally, in vivo and in vitro toxicity studies indicated that
the two macromolecular agents had great biocompatibility. Therefore,
we performed preliminary but important studies on the Gd-based HPMA
polymeric systems as biocompatible and efficient MRI contrast agents
and found that the biodegradable core-cross-linked pHPMA–DOTA-Gd
copolymer might have greater benefits for the foreground