Duality Symmetries in Orbifold Models

We derive the duality symmetries relevant to moduli dependent gauge coupling constant threshold corrections, in Coxeter ${\bf Z_N}$ orbifolds. We consider those orbifolds for which the point group leaves fixed a 2-dimensional sublattice $\Lambda_2$, of the six dimensional torus lattice $\Lambda_6$, where $\Lambda_6$ cannot be decomposed as $\Lambda_2 \bigoplus{\Lambda_4}.$Comment: 13 pages, QMW--TH--93/21, SUSX--TH--93/1

Modular Symmetries, Threshold Corrections And Moduli For Z2×Z2Z_2 \times Z_2 Orbifolds

${\bf Z}_2\times {\bf Z}_2$ Coxeter orbifolds are constructed with the property that some twisted sectors have fixed planes for which the six-torus can not be decomposed into a direct sum ${\bf T}^2\bigoplus{\bf T}^4$ with the fixed plane lying in ${\bf T}^2$. The string loop threshold corrections to the gauge coupling constants are derived, and display symmetry groups for the $T$ and $U$ moduli that are subgroups of the full modular group $PSL(2,Z)$. The effective potential for duality invariant gaugino condensate in the presence of hidden sector matter is constructed and minimized for the values of the moduli. The effect of Wilson lines on the modular symmetries is also studied.Comment: QMW--TH--94/18, 12 page

Anisotropic Solutions For Orbifold Moduli From Duality Invariant Gaugino Condensates

The values of the $T$ and $U$ moduli are studied for those ${\bf Z}_N$ Coxeter orbifolds with the property that some of the twisted sectors have fixed planes for which the six-torus ${\bf T}^6$ can not be decomposed into a direct sum ${\bf T}^2\bigoplus {\bf T}^4$ with the fixed plane lying in ${\bf T}^2$. Such moduli in general transform under a subgroup of the modular group $SL(2,Z).$ The moduli are determined by minimizing the effective potential derived from a duality invariant gaugino condensate.Comment: QMW--TH--94/11, SUSX--TH--94/11, 16 page

Modular Symmetries in ZNZ_N Orbifold Compactified String Theories with Wilson lines

Target space modular symmetries relevant to string loop threshold corrections are studied for $Z_N$ orbifold compactified string theories containing Wilson line background fields.Comment: SUSX--TH--93/17, QMW--TH--93/31, 12 page

A concept for reducing oceanic separation minima through the use of a TCAS-derived CDTI

A concept for using a cockpit display of traffic information (CDTI), as derived from a modified version of the Traffic Alert and Collision Avoidance System 2 (TCAS 2), to support reductions in air traffic separation minima for an oceanic track system is presented. The concept, and the TCAS modifications required to support it, are described. The feasibility of the concept is examined from a number of standpoints, including expected benefits, maximum alert rates, and possible transition strategies. Various implementation issues are analyzed. Pilot procedures are suggested for dealing with alert situations. Possible variations of the concept are also examined. Finally, recommendations are presented for other studies and simulation experiments which can be used to further verify the feasibility of the concept

Study of shuttle imaging microwave system antenna. Volume 1: Conceptual design

A detailed preliminary design and complete performance evaluation are presented of an 11-channel large aperture scanning radiometer antenna for the shuttle imaging microwave system (SIMS) program. Provisions for interfacing the antenna with the space shuttle orbiter are presented and discussed. A program plan for hardware development and a rough order of magnitude (ROM) cost are also included. The conceptual design of the antenna is presented. It consists of a four-meter diameter parabolic torus main reflector, which is a graphite/epoxy shell supported by a graphite/epoxy truss. A rotating feed wheel assembly supports six Gregorian subreflectors covering the upper eight frequency channels from 6.6 GHz through 118.7 GHz, and two three-channel prime forms feed assemblies for 0.6, 1.4, and 2.7 GHz. The feed wheel assembly also holds the radiometers and power supplies, and a drive system using a 400 Hz synchronous motor is described. The RF analysis of the antenna is performed using physical optics procedures for both the dual reflector Gregorian concept and the single reflector prime focus concept. A unique aberration correcting feed for 2.7 GHz is analyzed. A structural analysis is also included. The analyses indicate that the antenna will meet system requirements