Dynamics of Momentum Biased Spacecraft in a Near-Polar Orbit

The equations of motion of a momentum biased spacecraft are derived in a general form. The spacecraft is assumed to be orbiting in a near-elliptical orbit. An aerodynamic torque model which accounts for the atmospheric superrotation is assumed. The equilibrium attitude angles are obtained in terms of modified Bessel functions. Analytic expressions for the long-term motion of the momentum biased axis are derived for special cases. The analysis is applicable to the Magsat mission.     

A Comparison of Noncoherent and Coherent MTI Improvement Factors

An analysis based on statistical considerations and Monte Carlo simulations indicates that a noncoherent moving target indicator (MTI) using a linear envelope detector differs from one using a square law envelope detector. The square law envelope detector is usually described in the literature because of ease of analysis, and it is commonly stated or implied that the results are the same for the two cases because of the similar spectral characteristics of the detectors. A comparison is made between the two noncoherent MTIs and the coherent MTI in terms of clutter attenuation and MTI improvement factors.     

Maximum Power Transfer for Full-Wave Rectifier Circuits

An analysis is done to determine the maximum power transfer conditions for full-wave rectifier circuits. Potential applications noted are implanted medical instruments, inductive power transfer to weapons, power transfer using space reflectors, and power generation in space. Three types of series impedances are considered: resistive/inductive (RL), resistive/capacitive (RC), and resistive/inductive/capacitive (RLC). The optimum ratio of ac-to-dc voltage output is determined for each type. For the case that involves all three impedance types, the optimum turning condition is also determined. The differential equations describing the circuits are solved in nondimensional form. The solutions involve partial differential equations, closed-form relationships, and simultaneous equations that are solved by numerical methods. The optimum ratio of peak ac-to-dc voltage ranges from 2.0 to 2.8, depending upon the circuit. The optimum turning differs significantly from the usual resonant conditions, especially for low Q.     

Transformer Induced Instability of the Series Resonant Converter

It is shown that the common series resonant power converter is subject to a low frequency oscillation that can lead to the loss of cyclic stability. This oscillation is caused by a low frequency resonant circuit formed by the normal L and C components in series with the magnetizing inductance of the output transformer. Three methods for eliminating this oscillation are presented and analyzed. One of these methods requires a change in the circuit topology during the resonance cycle. This requires a new set of steady state equations which are derived and presented in a normalized form. Experimental results are included which demonstrate the nature of the low frequency oscillation before cyclic stability is lost.     

Frame Synchronization in the Presence of Errors and Erasures

An approach to the problem of achieving frame synchronization in the presence of jamming or other large disturbances is presented. It is assumed that the disturbance can be detected at the receiver, which then erases the corresponding bits and passes this information to the synchronizer. A synchronization algorithm designed to operate in the presence of both erasures and background errors is described and evaluated for a representative set of system parameters. This algorithm is shown to have a significant advantage over an algorithm which does not use the location information inherent in the erasure model.     

A Fast Beamforming Algorithm for Large Arrays

This beamforming algorithm is written specifically for array radars in which the number of array elements K is very large compared with the number of jammers L the radar is designed to suppress. It uses a set of M noise vectors to construct a basis for the jammer component of the antenna output vectors. The component of the quiescent weight vector orthogonal to each basis vector is calculated, renormalized to unit length, and identified as the adapted weight vector. This algorithm is effective in the suppression of many types of jammers. The number of noise samples M required in the construction of the adapted weight vector is approximately equal to L. In the special case of L narrowband noise jammers, for example, a choice of M = L usually reduces the receiver output jammer power to a few dBs above the white noise background. It is permissible to have M相似文献   

An Optimally Integrated Track Recovery System for Aerial Bathymetry

An airborne system has been developed for charting shallow coastal and inland waters. The primary components of this system are an aerial survey camera, a profiling laser radar, an analytical stereo plotter, and a multisensor track recovery system (TRS). The TRS comprises a gimbaled inertial navigation system and a number of auxiliary sensors which acquire redundant position and attitude information. The sensor data are combined postmission using a U-D factorized Kalman filter and modified Bryson-Frazier smoother to compute accurate estimates of the orientation parameters of the survey camera at the times of film exposure. These parameters are used to position each overlapping pair of photographs on the analytical plotter to form a stereo image and corresonding analytical stereomodel from which water depth measurements are made. Flight trial results demonstrate that the TRS can achieve radial position and attitude accuracies which exceed 1 m and 2 arcmin root mean square (rms), respectively, and that this level of performance is sufficient to enable water depth measurements to be made to an accuracy of better than 0.65 m (rms).     

Upstream waves and particles

A brief analysis is made of the interrelation of the intensity of cosmic-ray particles, the column density of gas and the intensity of cosmic γ-rays. It is shown that, locally, γ-ray data enable the calibration of H₂ densities to be inferred from CO data and elsewhere the variation of cosmic-ray intensity with position to be assessed. Finally, the importance of cosmic-ray irradiated molecular clouds in simulating γ-ray ‘sources’ is reiterated.     

Plasma-physical aspects of the solar cycle

Mass motions below the photosphere drive the solar cycle which is associated with variations in the magnetic field structure and accompanying phenomena. In addition to semi-empirical models, dynamo theories have been used to explain the solar cycle. The emergence of magnetic field generated by these mechanisms and its expansion into the corona involves many plasma physical processes. Magnetic buoyancy aids the expulsion of magnetic flux. The corona may respond dynamically or by continually adjusting to a quasi-static force-free or pressure-balanced equilibrium. The formation and disruption of current sheets is significant for the overall structure of the coronal magnetic field and the physics of quiescent prominences. The corona has a fine structure consisting of magnetic loops. The structure and stability of these are important as they are one of the underlying elements which make up the corona.