Engineering topics

Forsyth, P. A.

Publications and source records attributed to Forsyth, P. A..

3 records

NASA NTRS1994

Performance issues for iterative solvers in device simulation

Due to memory limitations, iterative methods have become the method of choice for large scale semiconductor device simulation. However, it is well known that these methods still suffer from reliability problems. The linear systems which appear in numerical simulation of semiconductor devices are notoriously ill-conditioned. In order to produce robust algorithms for practical problems, careful attention must be given to many implementation issues. This paper concentrates on strategies for developing robust preconditioners. In addition, effective data structures and convergence check issues are also discussed. These algorithms are compared with a standard direct sparse matrix solver on a variety of problems.

Fan, Qing↗

NASA NTRS1993

Analysis of rocket beacon transmissions for computerized reconstruction of ionospheric densities

Three methods are described to obtain ionospheric electron densities from transionospheric, rocket-beacon TEC data. First, when the line-of-sight from a ground receiver to the rocket beacon is tangent to the flight trajectory, the electron concentration can be obtained by differentiating the TEC with respect to the distance to the rocket. A similar method may be used to obtain the electron-density profile if the layer is horizontally stratified. Second, TEC data obtained during chemical release experiments may be interpreted with the aid of physical models of the disturbed ionosphere to yield spatial maps of the modified regions. Third, computerized tomography (CT) can be used to analyze TEC data obtained along a chain of ground-based receivers aligned along the plane of the rocket trajectory. CT analysis of TEC data is used to reconstruct a 2D image of a simulated equatorial plume. TEC data is computed for a linear chain of nine receivers with adjacent spacings of either 100 or 200 km. The simulation data are analyzed to provide an F region reconstruction on a grid with 15 x 15 km pixels. Ionospheric rocket tomography may also be applied to rocket-assisted measurements of amplitude and phase scintillations and airglow intensities.

Bernhardt, P. A.↗

NASA NTRS1987

Ionospheric electron-content measurements during the second space-plasma negative-ion experiment (SPINEX-2)

The second space-plasma negative-ion experiment (SPINEX-2), a chemical-release active experiment to investigate negative-ion effects in the ionospheric F region, is described by Mendillo et al. (1982). This paper describes the electron-content measurements in somewhat more detail than would be appropriate there. The circumstances of the experiment, particularly the use of a vehicle with a very high spin rate, presented some unusual challenges during interpretation of the electron-content data. These are described. The resulting profiles show clearly that the chemical release caused a very significant 'hole' in the ionosphere. Under certain fairly realistic assumptions, the actual number of free electrons removed from the region of the peak of the ionospheric F layer is estimated to be about 4 x 10 to the 25th. The same assumptions lead to a simple radial distribution of the depleted region about the rocket trajectory in the neighborhood of the release.

Fulford, J. A.↗