The Exact Cramer-Rao Bound for Gaussian Autoregressive Processes

An explicit expression is derived for the Cramer-Rao bound(CRB)on unbiased estimates of the parameters of autoregressiveprocesses, given a finite number of measurements. The expressionconverges to the well-known asymptotic form of the CRB when thenumber of measurements tends to infinity. The behavior of thebound is illustrated by some numerical examples.     

Software intensive systems safety analysis

Two important elements in the avionics suite of modern aircraft are: the flight control system (FCS) and the flight management system (FMS). The FCS provides the capability to stabilize and control the aircraft, while the FMS is responsible for flight planning and navigation. A clear trend in the aerospace industry is to place greater reliance on software systems, and many FCS and FMS subsystems are implemented primarily in software. For example, within the FCS is the flight guidance system (FGS) that generates roll and pitch guidance commands. Similarly, within the FMS is the vertical navigation (VNAV) function that acts like a third crew member in the cockpit, ordering mode change requests and resetting target altitude values to enable the aircraft to track the vertical flight plan. We have developed formal, executable models of the requirements for the mode logic of a FGS and for portions of the VNAV functionality. We have also conducted a comprehensive software safety analysis on the FGS mode logic model, and are completing the analysis of the VNAV model. This analysis uses as its starting point several "traditional" safety analysis techniques such as a functional hazard assessment (FHA), a fault tree analysis (FTA), and a failure mode effects analysis (FMEA). However, we are also using formal methods techniques known as model checking and theorem proving to verify the presence of safety properties in the model. This paper summarizes the (now completed) safety analysis that was performed on the FGS model, and highlights the similarities and differences with the (still on-going) safety analysis of the FMS model. In particular, we summarize progress made to date in the use of formal methods to verify the presence of the required safety properties in the models themselves.     

Space-borne technology and inversion techniques for radar

This presents recent progress of the state-of-the-art of space-borne radar technology in case of either earth-orbiting or planetary-orbiting satellites and space probes, respectively. In addition to that, recent progress is also discussed concerning specific inversion techniques to evaluate radar measurements; i.e., the art of deriving the relevant physical quantities to be determined such as terrain and depth profiles for planetary surface and/or subsurface structures ((3D)-profiles) from radar data measured depending on signal frequency, aspect angle, polarization, etc., as a function of time.     

Numerical prediction and wind tunnel experiment for a pitching unmanned combat air vehicle

The low-speed flowfield for a generic unmanned combat air vehicle (UCAV) is investigated both experimentally and numerically. A wind tunnel experiment was conducted with the Boeing 1301 UCAV at a variety of angles of attack up to 70 degrees, both statically and with various frequencies of pitch oscillation (0.5, 1.0, and 2.0 Hz). In addition, pitching was performed about three longitudinal locations on the configuration (the nose, 35% MAC, and the tail). Solutions to the unsteady, laminar, compressible Navier–Stokes equations were obtained on an unstructured mesh to match results from the static and dynamic experiments. The computational results are compared with experimental results for both static and pitching cases. Details about the flowfield, including vortex formation and interaction, are shown and discussed, including the non-linear aerodynamic characteristics of the vehicle.     

Maps of hydrogen isotopes at low altitudes in the inner zone from sampex observations

The PET instrument aboard the SAMPEX satellite has provided us with long-term intra-calibrated observations of geomagnetically trapped protons and deuterons in the inner zone, suitable for use in constraining the low-altitude portions of radiation belt models being developed as successors to AP-8. These observations have been summarized elsewhere (Looper et al., 1996). Here we report a detection of geomagnetically-trapped tritium at energies from 14 to 35 MeV/nuc below L = 1.2, at about 1/8 the flux of deuterium previously reported at that location and at similar energy per nucleon. We also demonstrate the utility of the SAMPEX/PET observations for measuring the east-west anisotropy in the trapped particle flux at low altitudes, which is due to displacement of particle gyrocenters from the position of observation in a region of strong flux gradients. This anisotropy is implicitly ignored in omnidirectional radiation-flux models, but it can be important to mission planners considering how to distribute shielding over the surface of oriented spacecraft in low Earth orbit.     

Effect of track structure and radioprotectors on the induction of oncogenic transformation in murine fibroblasts by heavy ions

The oncogenic potential of high-energy ⁵⁶Fe particles (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at the Brookhaven National Laboratory was examined utilizing the mouse C3H 10T1/2 cell model. The dose-averaged LET for high-energy ⁵⁶Fe is estimated to be 143 keV/μm with the exposure conditions used in this study. For ⁵⁶Fe ions, the maximum relative biological effectiveness (RBE_max) values for cell survival and oncogenic transformation were 7.71 and 16.5 respectively. Compared to 150 keV/μm ⁴He nuclei, high-energy ⁵⁶Fe nuclei were significantly less effective in cell killing and oncogenic induction. The prostaglandin E₁ analog misoprostol, an effective oncoprotector of C3H 10T1/2 cells exposed to X rays, was evaluated for its potential as a radioprotector of oncogenic transformation with high-energy ⁵⁶Fe. Exposure of cells to misoprostol did not alter ⁵⁶Fe cytotoxicity or the rate of ⁵⁶Fe-induced oncogenic transformation.     

Ground assisted rendezvous with geosynchronous satellites for the disposal of space debris by means of Earth-oriented tethers

Previous studies have shown that extended length Earth-oriented tethers in the geosynchronous (GEO) region can be used to re-orbit satellites to disposal orbits. One such approach involves the extension of a GEO based tether, collection of a debris object, and retraction of the tether, which transfers the retracted configuration to a higher energy orbit for debris disposal. The re-extension of the tether after debris disposal returns the configuration to the near-GEO altitude. The practical feasibility of such a system depends on the ability to collect GEO debris objects, attach them to a deployed tether system, and retract the tethers for transfer to the disposal orbits.This study addresses the collection and delivery of debris objects to the deployed tether system in GEO. The investigation considers the number, type and the characteristics of the debris objects as well as the collection tug that can be ground controlled to detect, rendezvous and dock with the debris objects for their delivery to the tethers system.A total of more than 400 objects are in drift orbits crossing all longitudes either below or above the geostationary radius. More than 130 objects are also known to librate around the stable points in GEO with periods of libration up to five or more years. A characterization of the position and velocity of the debris objects relative to the collection tug is investigated. Typical rendezvous performance requirements for uncooperative GEO satellites are examined, and the similarities with other approaches such as the ESA's CX-OLEV commercial mission proposal to extend the life of geostationary telecommunication satellites are noted.     

New evidence for strong non-thermal effects in Tycho’s supernova remnant

For the case of Tycho’s supernova remnant (SNR), we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is very close to the shock radius. Therefore a consistent explanation of these observations can be given in terms of efficient CR acceleration which makes the medium more compressible.     

Tissue responses to low protracted doses of high LET radiations or photons: early and late damage relevant to radio-protective countermeasures.

Early and late murine tissue responses to single or fractionated low doses of heavy charged particles, fission-spectrum neutrons or gamma rays are considered. Damage to the hematopoietic system is emphasized, but results on acute lethality, host response to challenge with transplanted leukemia cells and life-shortening are presented. Low dose rates per fraction were used in some neutron experiments. Split-dose lethality studies (LD 50/30) with fission neutrons indicated greater accumulation of injury during a 9 fraction course (over 17 days) than was the case for gamma-radiation. When total doses of 96 or 247 cGy of neutrons or gamma rays were given as a single dose or in 9 fractions, a significant sparing effect on femur CFU-S depression was observed for both radiation qualities during the first 11 days, but there was not an earlier return to normal with dose fractionation. During the 9 fraction sequence, a significant sparing effect of low dose rate on CFU-S depression was observed in both neutron and gamma-irradiated mice. CFU-S content at the end of the fractionation sequence did not correlate with measured LD 50/30. Sustained depression of femur and spleen CFU-S and a significant thrombocytopenia were observed when a total neutron dose of 240 cGy was given in 72 fractions over 24 weeks at low dose rates. The temporal aspects of CFU-S repopulation were different after a single versus fractionated neutron doses. The sustained reduction in the size of the CFU-S population was accompanied by an increase in the fraction in DNA synthesis. The proliferation characteristics and effects of age were different for radial CFU-S population closely associated with bone, compared with the axial population that can be readily aspirated from the femur. In aged irradiated animals, the CFU-S proliferation/redistribution response to typhoid vaccine showed both an age and radiation effect. After high single doses of neutrons or gamma rays, a significant age- and radiation-related deficiency in host defense mechanisms was detected by a shorter mean survival time following challenge with transplantable leukemia cells. Comparison of dose-response curves for life shortening after irradiation with fission-spectrum neutrons or high energy silicon particles indicated high initial slopes for both radiation qualities at low doses, but for higher doses of silicon, the effect per Gy decreased to a value similar to that for gamma rays. The two component life-shortening curve for silicon particles has implications for the potential efficacy of radioprotectants. Recent studies on protection against early and late effects by aminothiols, prostaglandins, and other compounds are discussed.     

Aeronautical use of semiconductor lasers for short optical pulses

In this paper, we investigate the feasible schemes to generate periodic optical pulses of width between several picoseconds (ps) and tens ps. Gain-switched semiconductor laser diodes are shown to be more suitable for avionics applications than mode-locked laser diodes. In the experiment, we use a low-cost Fabry-Perot laser diode to generate short optical pulse streams. A simple optical injection-locking scheme is then used to reduce the timing jitter and frequency chirp in such laser diodes, which in turn can improve the transmission performance of the generated optical pulses. It is expected that the use of gain-switched semiconductor lasers can meet the requirement of high-speed airborne communication networks or ground supporting systems at airports. Moreover, we discuss the possibility of using the pulsed semiconductor lasers to generate millimetre-wave (mm-wave) signals for future applications to airborne high-resolution, mm-wave radars.