30,503 research outputs found
Statistical framework for estimating GNSS bias
We present a statistical framework for estimating global navigation satellite
system (GNSS) non-ionospheric differential time delay bias. The biases are
estimated by examining differences of measured line integrated electron
densities (TEC) that are scaled to equivalent vertical integrated densities.
The spatio-temporal variability, instrumentation dependent errors, and errors
due to inaccurate ionospheric altitude profile assumptions are modeled as
structure functions. These structure functions determine how the TEC
differences are weighted in the linear least-squares minimization procedure,
which is used to produce the bias estimates. A method for automatic detection
and removal of outlier measurements that do not fit into a model of receiver
bias is also described. The same statistical framework can be used for a single
receiver station, but it also scales to a large global network of receivers. In
addition to the Global Positioning System (GPS), the method is also applicable
to other dual frequency GNSS systems, such as GLONASS (Globalnaya
Navigazionnaya Sputnikovaya Sistema). The use of the framework is demonstrated
in practice through several examples. A specific implementation of the methods
presented here are used to compute GPS receiver biases for measurements in the
MIT Haystack Madrigal distributed database system. Results of the new algorithm
are compared with the current MIT Haystack Observatory MAPGPS bias
determination algorithm. The new method is found to produce estimates of
receiver bias that have reduced day-to-day variability and more consistent
coincident vertical TEC values.Comment: 18 pages, 5 figures, submitted to AM
Towards Real-Time Detection and Tracking of Spatio-Temporal Features: Blob-Filaments in Fusion Plasma
A novel algorithm and implementation of real-time identification and tracking
of blob-filaments in fusion reactor data is presented. Similar spatio-temporal
features are important in many other applications, for example, ignition
kernels in combustion and tumor cells in a medical image. This work presents an
approach for extracting these features by dividing the overall task into three
steps: local identification of feature cells, grouping feature cells into
extended feature, and tracking movement of feature through overlapping in
space. Through our extensive work in parallelization, we demonstrate that this
approach can effectively make use of a large number of compute nodes to detect
and track blob-filaments in real time in fusion plasma. On a set of 30GB fusion
simulation data, we observed linear speedup on 1024 processes and completed
blob detection in less than three milliseconds using Edison, a Cray XC30 system
at NERSC.Comment: 14 pages, 40 figure
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