48 research outputs found
Enhanced GPS Measurements Simulation for Space-oriented Navigation System Design
AbstractAt the stage of preliminary scheme and algorithm design for spaceborne navigation systems, a precise and high-fidelity software global positioning system (GPS) simulator is a necessary and feasible testing facility in laboratory environments, with consideration of the tradeoffs where possible. This article presents a software GPS measurements simulator on the L1 C/A code and carrier signal for space-oriented navigation system design. The simulator, coded in MATLAB language, generates both C/A code pseudorange and carrier phase measurements. Mathematical models in the Earth centered inertial (ECI) frame are formulated to simulate the GPS constellation and to generate GPS measurements. A series of efficient measures are investigated and utilized to rationalize the enhanced simulator, in terms of ephemeris data selection, space ionospheric model and range rate calculation, etc. Such an enhanced simulator has been facilitating our current work for designing a space integrated GPS/inertial navigation system (INS) navigation system. Consequently, it will promote our future research on space-oriented navigation system
Gauss Newton method for solving variational problems of PDEs with neural network discretizaitons
The numerical solution of differential equations using machine learning-based
approaches has gained significant popularity. Neural network-based
discretization has emerged as a powerful tool for solving differential
equations by parameterizing a set of functions. Various approaches, such as the
deep Ritz method and physics-informed neural networks, have been developed for
numerical solutions. Training algorithms, including gradient descent and greedy
algorithms, have been proposed to solve the resulting optimization problems. In
this paper, we focus on the variational formulation of the problem and propose
a Gauss- Newton method for computing the numerical solution. We provide a
comprehensive analysis of the superlinear convergence properties of this
method, along with a discussion on semi-regular zeros of the vanishing
gradient. Numerical examples are presented to demonstrate the efficiency of the
proposed Gauss-Newton method
An efficient greedy training algorithm for neural networks and applications in PDEs
Recently, neural networks have been widely applied for solving partial
differential equations. However, the resulting optimization problem brings many
challenges for current training algorithms. This manifests itself in the fact
that the convergence order that has been proven theoretically cannot be
obtained numerically. In this paper, we develop a novel greedy training
algorithm for solving PDEs which builds the neural network architecture
adaptively. It is the first training algorithm that observes the convergence
order of neural networks numerically. This innovative algorithm is tested on
several benchmark examples in both 1D and 2D to confirm its efficiency and
robustness
Ambiguity Function Method Scheme for Aircraft Attitude Sensor Utilising GPS/GLONASS Carrier Phase Measurement
When the receivers of GPS, GLONASS, COMPASS and other such systems are equipped with multiple antennas, they can give attitude information. Based on the difference carrier phase equations established in local level frame (LLF), a new algorithm is presented to resolve aircraft attitude determination problems in real-time. Presuming that the cycle integer ambiguity is known, the measurement equations have attitude analytical resolutions using single difference (SD) equations of two navigation satellites in-view. Similar with SD process, the doubledifference (DD) measurements are established and analysed. In addition, the SD and DD algorithms are capable of reducing the integer search space into some discrete point space and then the ambiguity function method (AFM) resolves the ambiguity function within the point solutions space. Therefore the procedures have very low computation, thus saving time. The hardware architecture has been realised using multiple GPS/GLONASS OEMs. The experimental results have demonstrated that the proposed approach is effective and can satisfy the requirement of real-time application in cases of GPS, and combined GPS, and GLONASS.Defence Science Journal, 2009, 59(5), pp.466-470, DOI:http://dx.doi.org/10.14429/dsj.59.154
Hybrid Distillation: Connecting Masked Autoencoders with Contrastive Learners
Representation learning has been evolving from traditional supervised
training to Contrastive Learning (CL) and Masked Image Modeling (MIM). Previous
works have demonstrated their pros and cons in specific scenarios, i.e., CL and
supervised pre-training excel at capturing longer-range global patterns and
enabling better feature discrimination, while MIM can introduce more local and
diverse attention across all transformer layers. In this paper, we explore how
to obtain a model that combines their strengths. We start by examining previous
feature distillation and mask feature reconstruction methods and identify their
limitations. We find that their increasing diversity mainly derives from the
asymmetric designs, but these designs may in turn compromise the discrimination
ability. In order to better obtain both discrimination and diversity, we
propose a simple but effective Hybrid Distillation strategy, which utilizes
both the supervised/CL teacher and the MIM teacher to jointly guide the student
model. Hybrid Distill imitates the token relations of the MIM teacher to
alleviate attention collapse, as well as distills the feature maps of the
supervised/CL teacher to enable discrimination. Furthermore, a progressive
redundant token masking strategy is also utilized to reduce the distilling
costs and avoid falling into local optima. Experiment results prove that Hybrid
Distill can achieve superior performance on different benchmarks
AiluRus: A Scalable ViT Framework for Dense Prediction
Vision transformers (ViTs) have emerged as a prevalent architecture for
vision tasks owing to their impressive performance. However, when it comes to
handling long token sequences, especially in dense prediction tasks that
require high-resolution input, the complexity of ViTs increases significantly.
Notably, dense prediction tasks, such as semantic segmentation or object
detection, emphasize more on the contours or shapes of objects, while the
texture inside objects is less informative. Motivated by this observation, we
propose to apply adaptive resolution for different regions in the image
according to their importance. Specifically, at the intermediate layer of the
ViT, we utilize a spatial-aware density-based clustering algorithm to select
representative tokens from the token sequence. Once the representative tokens
are determined, we proceed to merge other tokens into their closest
representative token. Consequently, semantic similar tokens are merged together
to form low-resolution regions, while semantic irrelevant tokens are preserved
independently as high-resolution regions. This strategy effectively reduces the
number of tokens, allowing subsequent layers to handle a reduced token sequence
and achieve acceleration. We evaluate our proposed method on three different
datasets and observe promising performance. For example, the "Segmenter ViT-L"
model can be accelerated by 48% FPS without fine-tuning, while maintaining the
performance. Additionally, our method can be applied to accelerate fine-tuning
as well. Experimental results demonstrate that we can save 52% training time
while accelerating 2.46 times FPS with only a 0.09% performance drop. The code
is available at https://github.com/caddyless/ailurus/tree/main.Comment: Accepted by NeurIPS 202
Manipulating Multiple Order Parameters via Oxygen Vacancies: The case of Eu0.5Ba0.5TiO3-{\delta}
Controlling functionalities, such as magnetism or ferroelectricity, by means
of oxygen vacancies (VO) is a key issue for the future development of
transition metal oxides. Progress in this field is currently addressed through
VO variations and their impact on mainly one order parameter. Here we reveal a
new mechanism for tuning both magnetism and ferroelectricity simultaneously by
using VO. Combined experimental and density-functional theory studies of
Eu0.5Ba0.5TiO3-{\delta}, we demonstrate that oxygen vacancies create Ti3+ 3d1
defect states, mediating the ferromagnetic coupling between the localized Eu
4f7 spins, and increase an off-center displacement of Ti ions, enhancing the
ferroelectric Curie temperature. The dual function of Ti sites also promises a
magnetoelectric coupling in the Eu0.5Ba0.5TiO3-{\delta}.Comment: Accepted by Physical Review B, 201
Precancerous Stem Cells Have the Potential for both Benign and Malignant Differentiation
Cancer stem cells (CSCs) have been identified in hematopoietic and solid tumors. However, their precursors—namely, precancerous stem cells (pCSCs) —have not been characterized. Here we experimentally define the pCSCs that have the potential for both benign and malignant differentiation, depending on environmental cues. While clonal pCSCs can develop into various types of tissue cells in immunocompetent mice without developing into cancer, they often develop, however, into leukemic or solid cancers composed of various types of cancer cells in immunodeficient mice. The progress of the pCSCs to cancers is associated with the up-regulation of c-kit and Sca-1, as well as with lineage markers. Mechanistically, the pCSCs are regulated by the PIWI/AGO family gene called piwil2. Our results provide clear evidence that a single clone of pCSCs has the potential for both benign and malignant differentiation, depending on the environmental cues. We anticipate pCSCs to be a novel target for the early detection, prevention, and therapy of cancers