I-O map inversion, zero dynamics and flight control

The trajectory control of aircraft in rapid, nonlinear maneuvers is discussed. Based on nonlinear invertibility theory, a control law is derived to independently control roll, pitch, and sideslip angles using rudder, elevator, and aileron. Integral feedback is introduced in order to obtain robustness in the control system to parameter uncertainty. The stability of the zero dynamics is examined. Simulation results are presented to show that in a closed-loop system, precise simultaneous lateral and longitudinal maneuvers can be performed despite the presence of uncertainty in the stability derivatives     

Huygens' Surface Science Package

The design and performance of the Surface Science Package (SSP) on the Huygens probe are discussed. This instrument consists of nine separate sensors that are designed to measure a wide range of physical properties of Titan's lower atmosphere, surface, and sub-surface. By measuring a number of physical properties of the surface it is expected that the SSP will be able to constrain the inferred composition and structure of the moon's near-surface environment. Although the SSP is primarily designed to sense properties of the surface, some of its sensors will also make measurements of the atmosphere along the probe's entry path and will complement the data gathered by other experiments on the Huygens probe. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of generalized guidance laws for homing missiles

It is shown that the optimal guidance problem can be handled as a Bolza-type problem which can be solved by straightforward calculation. The solution proves to be different from that given by Yang, et al. (1989) and is verified to be a minimum performance index solution. Also presented is an alternate unified approach to the solution of the nonlinear system of differential equation of the generalized proportional navigation law discussed earlier by Yang, et al. This unified equation has been solved for all the different cases of proportional navigation for which closed-form solutions have been published by earlier authors     

Sensor Fault Detection in a System with Random Disturbances

Sensor faults are detected in an operating automatic system by a simplified version of the dedicated observer scheme. Control inputs are augmented by a random disturbance of moderate intensity. The dedicated observer in this case is a Kalman filter, driven by a single sensor. This filter provides estimates of the outputs from the other, nonredundant, sensors. A logical combination of these functionally redundant signals with the actual sensor signals provides prompt detection of incipient faults on all instruments without false alarms. The scheme is applied to a simulation of the lateral axis control system of a hydrofoil boat in which four sensors are to be covered by the fault detection scheme. Tests indicate that the scheme is robust with respect to variations in the intensity of the random disturbance.     

Digital Communication Systems in Impulsive Atmospheric Radio Noise

Analyses are presented of the performance of binary and M-ary coherent and noncoherent communication systems operating in the impulsive atmospheric radio noise environment. The receiver is usually a maximum likelihood detector for white Gaussian interference and therefore has the form of a parallel bank of matched filters followed by decision circuitry. By employing a Poisson or generalized Shot noise model for the impulsive noise with a suitable probability density function (pdf), closed-form expressions and bounds of error probabilities for M-ary noncoherent and coherent amplitude-shift keying (ASK), phase-shift keying (PSK), and frequency-shift keying (FSK) systems are obtained and the results discussed.     

Flight Test Evaluation of a New GPS Attitude Determination Algorithm

A new Global Positioning System (GPS) Attitude Determination Algorithm (GADA) is proposed, featuring the capability to keep its accuracy, even when the line-of-sight angle (LOS) of a given satellite vehicle (SV) is below the GPS horizontal antenna plane (HAP). The GADA model has been developed and evaluated through simulations and flight test campaigns, which comprised static and dynamic flight profiles, to best characterize the algorithm performance. As attitude reference a complete flight tests instrumentation (FTI) system was integrated into the testbed for the flight test campaign. The attitude measurements given by GADA and REQUEST algorithms are compared with those given by FTI (i.e., reference system). The results show that GADA accuracy is significantly better than that of REQUEST, for all flight conditions.     

CFAR for homogeneous part of high-resolution imagery

In high-resolution imaging, weak target pixel amplifiers may not be detected in the presence of clutter containing strong nonhomogeneities, when conventional approaches are used. The authors describe a constant false alarm rate (CFAR) approach that avoids the elimination of these significant target returns. The nonhomogeneous clutter as well as the weak target components are detected with this approach. The targets could then be discriminated from the homogeneities by discrimination techniques. It is shown how the lower amplitude components of the background noise and homogeneous clutter (which have Rayleigh statistics) can be detected in the presence of strong homogeneous clutter and targets. The average level of the homogeneous component is then determined using these lower-amplitude components. This CFAR approach avoids having a CFAR on the strong nonhomogeneities as well as the homogeneous component. The avoidance is what yields the ability to detect weak target pixel amplitudes     

Cosmic rays in the outer solar system

A space mission to Jupiter and Saturn, and beyond, provides an opportunity to explore the low energy galactic cosmic rays, which are largely excluded from the inner solar system by the outward sweep of the magnetic fields in the solar wind. The low energy cosmic rays are believed to be responsible for much of the heating of the gaseous disk of the galaxy, so a measurement of their intensity will have far reaching effects on theories of the interstellar gas and the evolution of the galaxy. The nuclear abundances, and in particular the presence or absence of high Z nuclei, will give critical information on the proximity of cosmic ray sources.This is one of the publications by the Science Advisory Group.     

ATS-6 Synchronousi Orbit Trapped Radiation Studies with an Electron-Proton Spectrometer

The University of Minnesota Electron-Proton Spectrometer Experiment consists of two nearly identical detector assemblies. One of these assemblies was mounted in a position fixed on the satellite in the Environmental Measurements Experiments (EME) east direction and the other was rotated so that the spectrometer scanned a range of spatial directions covering 1800 from EME north to EME south through west. Each of the detector assemblies is a magnetic spectrometer containing four gold-silicon surface barrier detectors. This instrument provides a very clean separation between protons and electrons by the combination of pulse height analysis and magnetic deflection. Each detector assembly measures protons in three nominal energy ranges (30-50 keV), (50-160 keV), and (120-514 keV). Electrons also are measured in three energy intervals (30-50 keV), (150-214 keV), and (more than 500 keV). Data are transmitted from the experiment at rates as high as 8 measurements/s. Decreases in the flux of the energetic electrons and protons followed by very rapid increases are frequently observed on the nightside during periods of geomagnetic activity. Separation of temporal and spatial effects is possible using proton gradient information obtained when the detector systems are oppositely directed. Using this technique, the decreases have been interpreted as motion of the trapping region equatorward and Earthward of the satellite. The boundary motion associated with the particle recovery shows a marked local time dependence. Particle increases observed in the evening sector have been interpreted as motion from Earthward and equatorward of Applications Technology Satellite-6 (ATS-6).     

The synthesis of helium and CNO isotopes in massive stars

We present helium and CNO isotopic yields for massive mass-losing stars in the initial mass range 15M M _i 50M . We investigate their dependence on assumptions about mass loss rates, internal mixing processes, and metallicity, and specify the contributions from stellar winds and from supernova ejecta.