Study of isotropic shell survivability by the analytical method

Shells with a through-thickness macrocrack are considered. The algorithms for determining the specific radius r _c in the crack-tip vicinity, the limiting load p _c (the attainment of which makes the crack unstable), and the direction of macrocrack growth θ _c are presented. We study the carrying capacity of a lengthy circular cylindrical shell under the internal pressure and with an arbitrarily oriented macrocrack.     

Deconvolution of Energetic Neutral Atom Images of the Earth's Magnetosphere

A method for deconvolving equatorial pitch angle distributions of energetic neutral atoms from images from the ENA instruments on the IMAGE satellite is described and illustrated using simulated data obtained from calculated models of ring current flux. The unknown flux is expanded in terms of cubic B-splines and Legendre polynomials. The expansion coefficients are obtained from the solution of a linear set of equations that select the smoothest solution that fits the data. In this way, all the information that is contained in the data is extracted but no more.     

Wide Dynamic Range Preselectors

Crowding of the Military communication spectrum has aggravated the need for preselectors capable of receiving a low-level desired signal in the presence of a high-level adjacent channel signal. Described herein are two novel HF preselection approaches which appear eminently suitable to cope with this problem. Applicable to the frequency range of 2 to 30 Mc/s, one preselector employs parametric mixing and the other exploits the low loss properties of superconductivity. In addition to other advantageous features, both preselectors have demonstrated an outstanding dynamic range capability in the order of 140 dB. Each of the preselectors is evaluated as a conventiona1 filter. In addition, performance data of a high quality military receiver is compared with that obtained when the receiver is preceded by each of two preselectors. Considerable stress is placed on the use of crossmodulation characteristics as a more meaningful and precise measure of preselector performance.     

Conical Scanning System for Pioneer Jupiter Spacecraft

The concept, operation, and predicted performance of an RF tracking control system used to point the Pioneer F/G spacecraft at the Earth is described. This system employs a modified conical scanning technique called Conscan. The signal processor, the most interesting unit of the system, is described in detail to show that it approximates a maximum likelihood estimator. The dynamic behavior of the spacecraft and the stability analysis of the system are presented, demonstrating that the system performance is basically determined by the open-loop phase and amplitude errors introduced by the antenna, receiver, and signal processor. A detailed error budget shows that the phase and amplitude errors are small. Finally, closed-loop simulation and test data are presented to verify the error budget.     

Optimum Processing of Unequally Spaced Radar Pulse Trains for Clutter Rejection

A train of radar pulses from one resolution cell can be processed coherently to reject echoes from external clutter and detect targets moving radially with respect to the clutter. Optimum methods of signal processing are defined for systems in which the interpulse spacings are multiply staggered to avoid target blind speeds. Likelihood ratio tests are developed for systems in which the target Doppler frequency is known a priori and for systems employing a bank of filters to cover the target Doppler band. To implement such tests, the N pulses in the train are added with complex weights and the amplitude of the sum compared with a detection threshold. The set of weights which maximizes the average signal-to-clutter ratio is also computed for a single-filter system with unknown target Doppler frequency. When the clutter autocorrelation function is exponential, the clutter covariance matrix can be inverted analytically. This latter result is useful for comparing different interpulse-spacing codes for a particular system application.     

System Design for Improved Extra-Terrestrial Communications

This paper describes a proposed system for improved exo-ionospheric communications. The dynamic magneto-ionic character of the channel is considered, in particular, the multipath situation arising therefrom. An ideal matched filter is found, matched to the multipath structure and the dynamics of the exo-ionospheric channel. The improvement in the signal-to-noise ratio through the matched filter is calculated. It is seen to depend on the quotient of the input signal to the bandwidths of the ``measuring' filter and the ``integrating' filter. Further advantages are shown to accrue from signal processing at the transmitter involving both increases in range, and, in particular, secure coding possibilities.     

Tracking a maneuvering target using sensors of variable quality

In an environment subject to sudden change, the accuracy of tracking and prediction is strongly influenced both by the sensor architecture and by the quality of the sensors. An image-enhanced algorithm is presented for both path following and covariance estimation in applications where the sensors are subject to sudden and unpredictable variation in quality. For an illustrative trajectory, the performance of the algorithm is contrasted with an extended Kalman filter (EKF) and an image-enhanced algorithm based upon the nominal sensors     

Debris swarms seen by SMEI

The large 3° × 60° fields-of-view of the Solar Mass Ejection Imager (SMEI) instruments are oriented on the stabilized Coriolis satellite to image most of the sky each Sun-synchronous orbit. Besides observing coronal mass ejections, the SMEI mission objective, SMEI also has detected a plethora of Earth-orbiting satellites (resident space objects or RSOs) brighter than ∼8th magnitude at a rate of about 1 per minute. Occasionally, SMEI sees an RSO swarm: a sudden onset of a large number of RSOs, many more than the nominal rate, upto dozens detected in a 4-s frame. These swarms usually last for a few minutes. A sample of six such RSO ensembles is analyzed in this paper in which the distance and the direction of the velocity vector for individual objects are estimated. We present the observational evidence indicating that the swarms must be near-field objects traveling in orbits near that of Coriolis, and that the relatively speeds between the objects and Coriolis are low. Further, analyses indicate that the RSOs are quite close (<20 m) and are generally moving radially away from the satellite. The predicted encounter geometries for Coriolis passing through or near a small debris cloud is, generally, quite inconsistent with the observations. The most likely explanation consistent with the observations is that SMEI is seeing debris being ejected from the Coriolis spacecraft itself. An analysis of distance and brightness for a subset of the RSOs indicates that the median diameter of the debris particles is ∼80 μm.     

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvén-speed profile. In a ??=0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.