57 research outputs found
Mapping of Ice Sheet Deep Layers and Fast Outlet Glaciers with Multi-Channel-High-Sensitivity Radar
This dissertation discusses the waveform design, the development of SAR and clutter reduction algorithms for MCRDS radars that are developed at CReSIS to map the ice-sheet bed, deep internal layers and fast-flowing outlet glaciers. It is verified with survey data that the sidelobe level of the designed tapered linear chirp waveform is lower than -60dB for reliable detection of deep ice layers close to the bed. The SAR processing is implemented in f-k domain with motion compensation. Very weak echoes from the deepest parts of Jakobshavn channel are detected for the first time using large synthetic aperture length. A beam-spaced clutter-reduction algorithm is developed to reduce the distributed across-track ice clutter encountered in sounding fast outlet glaciers by estimating the clutter power as a function of depth. On average this method is able to reduce ice clutter by 10dB over Hanning weighting with the MCRDS radar's multi-channel data
Birefringence of GaN/AlGaN optical waveguides
This is the published version. Copyright © 2003 American Institute of PhysicsWe have experimentally studied the birefringence of wurtzite GaNgrown on a sapphire substrate. The measurements were done with single-mode GaN/AlGaN planar optical waveguides on c-plane grownheterostructure films. The refractive indices were found to be different for signal optical field perpendicular or parallel to the crystal c axis (n⊥≠n∥). More importantly, we found an approximately 10% change in index difference Δn=n∥−n⊥ with variation of the waveguide orientation in the a–b plane, and a 60° periodicity was clearly observed. This is attributed to the hexagonal structure of nitride materials
Skip-WaveNet: A Wavelet based Multi-scale Architecture to Trace Firn Layers in Radar Echograms
Echograms created from airborne radar sensors capture the profile of firn
layers present on top of an ice sheet. Accurate tracking of these layers is
essential to calculate the snow accumulation rates, which are required to
investigate the contribution of polar ice cap melt to sea level rise. However,
automatically processing the radar echograms to detect the underlying firn
layers is a challenging problem. In our work, we develop wavelet-based
multi-scale deep learning architectures for these radar echograms to improve
firn layer detection. We show that wavelet based architectures improve the
optimal dataset scale (ODS) and optimal image scale (OIS) F-scores by 3.99% and
3.7%, respectively, over the non-wavelet architecture. Further, our proposed
Skip-WaveNet architecture generates new wavelets in each iteration, achieves
higher generalizability as compared to state-of-the-art firn layer detection
networks, and estimates layer depths with a mean absolute error of 3.31 pixels
and 94.3% average precision. Such a network can be used by scientists to trace
firn layers, calculate the annual snow accumulation rates, estimate the
resulting surface mass balance of the ice sheet, and help project global sea
level rise
GaN-based waveguide devices for long-wavelength optical communications
This is the published version. Copyright © 2003 American Institute of PhysicsRefractive indices of AlxGa1−xN with different Al concentrations have been measured in infrared wavelength regions. Single-mode ridged optical waveguidedevices using GaN/AlGaN heterostructures have been designed, fabricated, and characterized for operation in 1550 nm wavelength window. The feasibility of developing photonic integrated circuits based on III-nitride wide-band-gap semiconductors for fiber-optical communications has been discussed
Efficient Pyramid Channel Attention Network for Pathological Myopia Detection
Pathological myopia (PM) is the leading ocular disease for impaired vision
and blindness worldwide. The key to detecting PM as early as possible is to
detect informative features in global and local lesion regions, such as fundus
tessellation, atrophy and maculopathy. However, applying classical
convolutional neural networks (CNNs) to efficiently highlight global and local
lesion context information in feature maps is quite challenging. To tackle this
issue, we aim to fully leverage the potential of global and local lesion
information with attention module design. Based on this, we propose an
efficient pyramid channel attention (EPCA) module, which dynamically explores
the relative importance of global and local lesion context information in
feature maps. Then we combine the EPCA module with the backbone network to
construct EPCA-Net for automatic PM detection based on fundus images. In
addition, we construct a PM dataset termed PM-fundus by collecting fundus
images of PM from publicly available datasets (e.g., the PALM dataset and ODIR
dataset). The comprehensive experiments are conducted on three datasets,
demonstrating that our EPCA-Net outperforms state-of-the-art methods in
detecting PM. Furthermore, motivated by the recent pretraining-and-finetuning
paradigm, we attempt to adapt pre-trained natural image models for PM detection
by freezing them and treating the EPCA module and other attention modules as
the adapters. The results show that our method with the
pretraining-and-finetuning paradigm achieves competitive performance through
comparisons to part of methods with traditional fine-tuning methods with fewer
tunable parameters.Comment: 12 page
Subglacial topography and geothermal heat flux: potential interactions with drainage of the Greenland ice sheet
This is the published version, also available here: http://dx.doi.org/10.1029/2007GL030046.[1] Many of the outlet glaciers in Greenland overlie deep and narrow trenches cut into the bedrock. It is well known that pronounced topography intensifies the geothermal heat flux in deep valleys and attenuates this flux on mountains. Here we investigate the magnitude of this effect for two subglacial trenches in Greenland. Heat flux variations are estimated for idealized geometries using solutions for plane slopes derived by Lachenbruch (1968). It is found that for channels such as the one under Jakobshavn Isbræ, topographic effects may increase the local geothermal heat flux by as much as 100%
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