Effect of Observation Errors on Satellite Attitude Estimation

A study of the effect of observation errors on the best least squares estimate of satellite attitude based on two sets of direction measurements, where one set contains two independent measurements which are corrupted by zero-mean normally distributed additive errors, is summarized. Using Monte-Carlo techniques, the statistics of the estimated rotation of the satellite, from a known reference, were obtained. It was found that one of the parameterizations of the rotation, namely, the vector z, is best suited for estimation, since ?z is normally distributed with zero mean and its standard deviation is independent of the two direction measurements.     

ARTEMIS Mission Design

The ARTEMIS mission takes two of the five THEMIS spacecraft beyond their prime mission objectives and reuses them to study the Moon and the lunar space environment. Although the spacecraft and fuel resources were tailored to space observations from Earth orbit, sufficient fuel margins, spacecraft capability, and operational flexibility were present that with a circuitous, ballistic, constrained-thrust trajectory, new scientific information could be gleaned from the instruments near the Moon and in lunar orbit. We discuss the challenges of ARTEMIS trajectory design and describe its current implementation to address both heliophysics and planetary science objectives. In particular, we explain the challenges imposed by the constraints of the orbiting hardware and describe the trajectory solutions found in prolonged ballistic flight paths that include multiple lunar approaches, lunar flybys, low-energy trajectory segments, lunar Lissajous orbits, and low-lunar-periapse orbits. We conclude with a discussion of the risks that we took to enable the development and implementation of ARTEMIS.     

An Automatic Data Processing System for Laser Anemometers

An automatic data processing system for the evaluation of statistically occurring Doppler-difference bursts is described. In addition to the higher accuracy compared with the data acquisition procedures known so far, this technique allows the Doppler-difference method to be used as an operational instrument for investigations into turbulent flows, especially in the case of low particle density and extremely small turbulence degrees.     

Survey of High-Power Microwave Filters

A summary of research and development work on high-power microwave filters is given here: first, design considerations in high-power microwave filters?power handling capacity, waveform distortion, multimode suppression, and stop and pass band characteristics; second, a discussion of filtering devices and techniques in two broad categories-dominant mode filters and harmonic filters (discussion covers methods of operation and significant characteristics of each filter developed to date); and third, a discussion of applications and limitations of filters in various systems and interference situations. Also included are the availability, cost, size, weight, etc. as functions of frequency for the various filter types.     

Landmark Navigation Rule, a New Navigation Device

Two new methods of landmark navigation were recently presented. The landmarks are assumed to be within sight of the navigator but with unknown positions. Both methods require computations which are time consuming when a computer is not available. This correspondence presents the concept of a new navigation device called ?landmark navigation rule? which eliminates all the computation effort required in both new methods. The device is simple in construction, lightweight, and consumes no power.     

Theory of Operation of an Elastically Supported Tuned Gyroscope

Detailed derivation of the transfer function for a multigimbal, elastically supported, tuned gyro is presented and comparison made between its characteristics and those of a classical two-axis, free-rotor gyro. Knowledge of the gyro transfer function is necessary for the purpose of servo analysis of the system in which the gyro is used; also, the transfer function is a basis of evaluation of errors caused by angular inputs that occur at twice spin frequency.     

Pitch angle diffusion as seen by CRRES

Lifetimes and pitch angle distributions of radiation belt electrons injected into the slot region are determined from CRRES/MEA particle flux measurements. The dominant loss mechanism is presumed to be pitch angle scattering due to whistler waves, for which a theoretical formulation is available. The empirical lifetimes are compared to those from recent calculations, which rely on model whistler wave parameters. CRRES measurements of cyclotron-frequency electric fields are also presented and compared to the wave model.