Modelling and performance analysis of four and eight element TCAS

This semi-annual report describes the work performed during the period September 1989 through March 1990. The first section presents a description of the effect of the engines of the Boeing 737-200 on the performance of a bottom mounted eight-element traffic alert and collision avoidance system (TCAS). The second section deals exclusively with a four element TCAS antenna. The model obtained to simulate the four element TCAS and new algorithms developed for studying its performance are described. The effect of location on its performance when mounted on top of a Boeing 737-200 operating at 1060 MHz is discussed. It was found that the four element TCAS generally does not perform as well as the eight element TCAS III

Performance of traffic-alert collision avoidance (TCAS) antennas in the presence of scatterers

The performance of two TCAS systems is studied in the presence of electromagnetic scatterers. TCAS is an aircraft mounted angle of arrival (AOA) system, which estimates the bearing of a signal transmitted from a mode-S transponder on another nearby aircraft (intruder). Two systems are studied: (1) Comparison of Relative Amplitude system (CRA), and (2) Spiral Phase Antenna (SPA). The CRA antenna receives the reply via four switched beams. The bearing is estimated by comparing the amplitudes of the received signal. The SPA is based on the phase interferometer, which utilizes the received phase via sum and difference beams. The AOA is computed by comparing the reply with similar values on a calibration table, which is generated by modeling the TCAS antenna on the bare fuselage of a Boeing 727-200. The antenna patterns for the TCAS are found via high frequency methods based on the Uniform Geometric theory of Diffraction (UTD). By minimizing the standard deviation of the bearing error in a specified angular sector, optimal locations for top and bottom mounted TCAS antennas are found on the Boeing 727-200, 737-300 and 747-200 airframes. It will be shown that the overall bearing errors of the amplitude system are consistently smaller than the spiral phase TCAS. The effect of two types of nearby scatterers--antennas, and engine inlets--is studied. The AT741 L-band blade, DMC60-1 VHF Communication antenna were chosen as being representative antenna interference examples. Models are derived for the blades via a moment method analysis followed by a least squares procedure to synthesize the scattering patterns. Studies were conducted to estimate the minimum separation between the two antennas for acceptable operation. It will be shown that the spiral phase TCAS is adversely affected by the presence of a blade antenna. The amplitude system does not suffer from this limitation, especially for the forward look angles which are of most interest here. A model to represent the inlet scattering is based on the multiple scattering method and UTD. The engine on top of the B727-200 fuselage is modeled by a terminated circular waveguide. Then, the effect of moving the antenna forward on the fuselage is studied. It is again shown that the performance of the amplitude system is superior

Probation, credibility and justice

This paper explores the difficulties that arise for probation agencies or those that deliver community sanctions in developing and maintaining their credibility in prevailing ‘late-modern’ social conditions. It begins by questioning the limits of the pursuit and promise of ‘public protection’ as a source of credibility, and then proceeds to examine the emergence of an alternative strategy – based principally on reparation and ‘payback’ – in Scotland, arguing that these Scottish developments have much to say to the emerging debates in England and Wales (and elsewhere) about the ‘rehabilitation revolution’ and the proper use of imprisonment. The paper provides a critical account of the development and meaning of the Scottish version of ‘payback’, linking it to some important philosophical and empirical studies that may help to steer the development of payback away from a ‘merely punitive’ drift. In the conclusion, I argue that probation agencies and services need to engage much more deeply and urgently with their roles as justice services, rather than as ‘mere’ crime reduction agencies

Effects of habitat and livestock on nest productivity of the Asian houbara Chlamydotis macqueenii in Bukhara Province, Uzbekistan

To inform population support measures for the unsustainably hunted Asian houbara Chlamydotis macqueenii (IUCN Vulnerable) we examined potential habitat and land-use effects on nest productivity in the Kyzylkum Desert, Uzbekistan. We monitored 177 nests across different semi-arid shrub assemblages (clay-sand and salinity gradients) and a range of livestock densities (0–80 km-2). Nest success (mean 51.4%, 95% CI 42.4–60.4%) was similar across four years; predation caused 85% of those failures for which the cause was known, and only three nests were trampled by livestock. Nesting begins within a few weeks of arrival when food appears scarce, but later nests were more likely to fail owing to the emergence of a key predator, suggesting foraging conditions on wintering and passage sites may be important for nest productivity. Nest success was similar across three shrub assemblages and was unrelated to landscape rugosity, shrub frequency or livestock density, but was greater with taller mean shrub height (range 13–67 cm) within 50 m. Clutch size (mean = 3.2 eggs) and per-egg hatchability in successful nests (87.5%) did not differ with laying date, shrub assemblage or livestock density. We therefore found no evidence that livestock density reduced nest productivity across the range examined, while differing shrub assemblages appeared to offer similar habitat quality. Asian houbara appear well-adapted to a range of semi-desert habitats and tolerate moderate disturbance by pastoralism. No obvious in situ mitigation measures arise from these findings, leaving regulation and control as the key requirement to render hunting sustainable

On the trace of the antipode and higher indicators

We introduce two kinds of gauge invariants for any finite-dimensional Hopf algebra H. When H is semisimple over C, these invariants are respectively, the trace of the map induced by the antipode on the endomorphism ring of a self-dual simple module, and the higher Frobenius-Schur indicators of the regular representation. We further study the values of these higher indicators in the context of complex semisimple quasi-Hopf algebras H. We prove that these indicators are non-negative provided the module category over H is modular, and that for a prime p, the p-th indicator is equal to 1 if, and only if, p is a factor of dim H. As an application, we show the existence of a non-trivial self-dual simple H-module with bounded dimension which is determined by the value of the second indicator.Comment: additional references, fixed some typos, minor additions including a questions and some remark