Modelling the neutral gas environment of comets with special application to P/Halley

A technique has been developed which allows relatively accurate modelling of cometary gas production from nothing more than a visible light curve. Application to P/Halley suggests the production rate of parent molecules will be about 2.6 × 10²⁹ per second on March 10, 1986, for example. The uncertainties and intrinsic limitations in this approach are outlined. The theory is then extended to predictions of abundance of other gaseous species, and a photometric model of these gases provided. Combined with the dust model of N. Divine, preliminary predictions of the luminance of P/Halley as seen in any direction from inside the coma or outside can be provided for λλ3000–7000.     

Optical fiber wavelength division multiplexing

During flight, aircraft avionics transmit and receive RF signals to/from antennas over coaxial cables. As the density and complexity of onboard avionics increases, the electromagnetic interference (EMI) environment degrades proportionately, leading to decreasing signal-to-noise ratios (SNRs) and potential safety concerns. The coaxial cables are inherently lossy, limiting the RF signal bandwidth while adding considerable weight. To overcome these limitations, we have investigated a fiber optic communications link for aircraft that utilizes wavelength division multiplexing (WDM) to support the simultaneous transmission of multiple signals (including RF) over a single optical fiber. Optical fiber has many advantages over coaxial cable, particularly lower loss, greater bandwidth, and immunity to EMI. In this paper, we demonstrate that WDM can be successfully used to transmit multiple RF signals over a single optical fiber with no appreciable signal degradation. We investigate the transmission of FM and AM analog modulated signals, as well as FSK digital modulated signals, over a fiber optic link (FOL) employing WDM. We present measurements of power loss, delay, SNR, carrier-to-noise ratio (CNR), total harmonic distortion (THD), and bit error rate (BER). Our experimental results indicate that WDM is a fiber optic technology suitable for avionics applications.     

Fly-by-wire T & E challenge [aircraft test pilot handling compensation]

Systems developers and testers have always assumed that human compensation is measurable, or, at least, that a cognizant and trained tester is able to identify and detect compensation. More than one study conducted at the Wright-Patterson large amplitude multi-mode aerospace research simulator (LAMARS) facility indicates that this is not necessarily true. Test pilots were able to compensate sufficiently to fly and meet defined performance standards on intentionally crippled aircraft flight control designs. These flight control systems were designed to trigger pilot-induced oscillations, but, in most cases, test pilots could compensate sufficiently to prevent pilot-induced oscillations and to control the simulated aircraft. Anecdotally, this points to a colossal deficiency in the test of highly augmented aircraft systems that has been borne out by multiple aircraft accidents in actual aircraft designs: natural pilot compensation is sufficient to allow faulty designs to reach production and operational service while hiding critical handling qualities cliffs that can lead to loss of an aircraft. This observation, if applied across the gamut of human factors experimentation, has vast ramifications for test and evaluation and development of all human interface systems.     

Kinetics of gas-Grain Reactions in the Solar Nebula

Thermochemical equilibrium calculations predict gas phase, gas-grain, and solid phase reactions as a function of pressure and temperature in the solar nebula. However, chemical reactions proceed at different rates, which generally decrease exponentially with decreasing temperature. At sufficiently low temperatures (which vary depending on the specific reaction) there may not have been enough time for the predicted equilibrium chemistry to have taken place before the local environment cooled significantly or before the gaseous solar nebula was dispersed. As a consequence, some of the high temperature chemistry established in sufficiently hot regions of the solar nebula may be quenched or frozen in without the production of predicted low temperature phases. Experimental studies and theoretical models of three exemplary low temperature reactions, the formation of troilite (FeS), magnetite (Fe₃O₄), and hydrous silicates, have been done to quantify these ideas. A comparison of the chemical reaction rates with the estimated nebular lifetime of 0.1-10 million years indicates that troilite formation proceeded to completion in the solar nebula. Magnetite formation was much slower and only thin magnetite rims could have formed on metal grains. Hydrous silicate formation is predicted to be even slower, and hydrous silicates in meteorites and interplanetary dust particles probably formed later on the parent bodies of these objects, instead of in the solar nebula. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reciprocal radar waveforms

Digitally coded radar waveforms can be used to obtain large time-bandwidth products (pulse compression ratios). It is demonstrated that periodic radar waveforms with zero sidelobes or almost zero sidelobes can be defined. A perfect periodic code is a periodic code whose autocorrelation function has zero sidelobes and whose amplitude is uniform (maximum power efficiency=1). An asymptotically perfect periodic code has the property that as the number of elements in the code goes to infinity the autocorrelation function of the code has zero sidelobes and its power efficiency is one. The authors introduce a class of radar waveforms that are either perfect or asymptotically perfect codes. These are called reciprocal codes because they can be derived through a linear transformation of known codes. The aperiodic performance of the reciprocal code is examined