Gravitation and celestial mechanics investigations with Galileo

The gravitation and celestial mechanics investigations during the cruise phase and Orbiter phase of the Galileo mission depend on Doppler and ranging measurements generated by the Deep Space Network (DSN) at its three spacecraft tracking sites in California, Australia, and Spain. Other investigations which also rely on DSN data, and which like ours fall under the general discipline of spacecraft radio science, are described in a companion paper by Howard et al. (1992). We group our investigations into four broad categories as follows: (1) the determination of the gravity fields of Jupiter and its four major satellites during the orbital tour, (2) a search for gravitational radiation as evidenced by perturbations to the coherent Doppler link between the spacecraft and Earth, (3) the mathematical modeling, and by implication tests, of general relativistic effects on the Doppler and ranging data during both cruise and orbiter phases, and (4) an improvement in the ephemeris of Jupiter by means of spacecraft ranging during the Orbiter phase. The gravity fields are accessible because of their effects on the spacecraft motion, determined primarily from the Doppler data. For the Galilean satellites we will determine second degree and order gravity harmonics that will yield new information on the central condensation and likely composition of material within these giant satellites (Hubbard and Anderson, 1978). The search for gravitational radiation is being conducted in cruise for periods of 40 days centered around solar opposition. During these times the radio link is least affected by scintillations introduced by solar plasma. Our sensitivity to the amplitude of sinusoidal signals approaches 10^-15 in a band of gravitational frequencies between 10^-4 and 10^-3 Hz, by far the best sensitivity obtained in this band to date. In addition to the primary objectives of our investigations, we discuss two secondary objectives: the determination of a range fix on Venus during the flyby on 10 February, 1990, and the determination of the Earth's mass (GM) from the two Earth gravity assists, EGA1 in December 1990 and EGA2 in December 1992.     

Single-phase power distribution system power flow and faultanalysis

Alternative methods for power flow and fault analysis of single-phase distribution systems are presented. The algorithms for both power flow and fault analysis utilize a generalized approach to network modeling. The generalized admittance matrix, formed using elements of linear graph theory, is an accurate network model for all possible single-phase network configurations. Unlike the standard nodal admittance matrix formulation algorithms, the generalized approach uses generalized component models for the transmission line and transformer. The standard assumption of a common node voltage reference point is not required to construct the generalized admittance matrix. Therefore, truly accurate simulation results can be obtained for networks that cannot be modeled using traditional techniques     

The Martian stratigraphy — Short review and perspectives

The main aspects of the Martian stratigraphy have been determined from the detailed study of Mariner 9 and Viking Orbiter images. Three major stratigraphic systems, the Noachian System, the Hesperian System, and the Amazonian System, are inferred from these studies. The global geological evolution of Mars is essentially derived from its stratigraphy. It reveals that tectonism and volcanism were widespread during two major periods (Noachian and Lower Hesperian) and became more localized during the Upper Hesperian and Amazonian periods. The transition between these two major periods occurred about 2 Ga ago, and significant geologic activity could still be present. However, a number of geologic features and processes remain little understood. Future investigations, including complete high resolution imaging and detailed mapping, geochemical mapping, in situ chemical analyses, etc., will be necessary in order to improve our knowledge of the Martian stratigraphy and geologic evolution and are essential to prepare any future Mars Sample Return mission and the Human Exploration of this planet.     

Cross-correlation properties of algebraically constructed Costasarrays

The problem of determining the cross-correlation properties of signals based on algebraically constructed Costas arrays is addressed by examining the discrete cross-correlation of the algebraically constructed Costas arrays for a given construction and dimension. Finding two arrays that minimally correlate implies that the signals based on these arrays also minimally correlate. The properties of finite fields are reviewed, and the major algebraic constructions for Costas arrays are presented, i.e. the Welch construction and the Golomb construction. The discrete cross-correlation properties of the Costas arrays are derived for arrays of the same dimension derived from the same construction. The use of Costas arrays in the signal design problem is discussed, and examples are given to show the cross-correlation of the signals based on the algebraically constructed arrays     

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   

GTD Terrain Reflection Model Applied to ILS Glide Scope

The capability of calculating the reflection of electromagnetic signals from uneven terrain has many applications. One of these is the determination of instrument landing system (ILS) glide slope performance. For this application the wavelength is approximately 1 m, incidence angles are usually near grazing, and the fields are horizontally polarized, so that gross irregularities such as dropoffs and hills are more important than surface roughness. Past approaches used to calculate the ground reflections for this application have been three-dimensional physical optics models which were very cumbersome and time consuming and which neglected important diffraction and shadowing phenomenon; a two-dimensional physical optics model which was faster than the three-dimensional models but ignored many shadowing and transverse terrain variation effects; and a half-plane diffraction model which is applicable only to a specified type of terrain geometry. In this paper a terrain reflection model based on the geometrical theory of diffraction (GTD) is described which can accommodate any piecewise linear terrain profile, requires less computer time than the physical optics models, is capable of including transverse terrain effects, and determines the reflected fields with all important diffraction and blockage effects included.     

An Adaptive Array Signal Processing Algorithm for Communications

An algorithm is described for initial synchronization in a communication system with a digital adaptive array. This algorithm can also be used for message extraction. A set of consecutive complex video samples of the array output is processed to obtain optimum adaptive array weights, based on a least mean square (LMS) error criterion. This computation is performed for each of the possible alternative signals which may be present during an observation interval. The correct synchronization time or message symbol is selected as the one which yields the minimum LMS error. Assuming orthogonality of the alternative codes, a probability distribution for the output of this processor has been derived.     

Ultraviolet spectrometer experiment for the Voyager mission

The Voyager Ultraviolet Spectrometer (UVS) is an objective grating spectrometer covering the wavelength range of 500–1700 Å with 10 Å resolution. Its primary goal is the determination of the composition and structure of the atmospheres of Jupiter, Saturn, Uranus and several of their satellites. The capability for two very different observational modes have been combined in a single instrument. Observations in the airglow mode measure radiation from the atmosphere due to resonant scattering of the solar flux or energetic particle bombardment, and the occultation mode provides measurements of the atmospheric extinction of solar or stellar radiation as the spacecraft enters the shadow zone behind the target. In addition to the primary goal of the solar system atmospheric measurements, the UVS is expected to make valuable contributions to stellar astronomy at wavelengths below 1000 Å.     

Defending a Moving Target Against Missile or Torpedo Attack

Problems involving kinematics of a counterweapon intercepting an attacking missile are well known. However, when the attacker's target, the counterweapon's launch platform, is moving, the problem generally becomes amenable only to trajectory simulation. Such may be the case in defense against torpedo attack against a ship and other situations involving the use of antimissile missiles. This paper derives the kinematic relations among the three bodies (missile, target, and counterweapon) in closed form under the asymptotic approximation of constant-bearing trajectories (collision courses). It is shown that the defending target, even if moving, can still determine the optimum course for its counterweapon when range and speed of the attacking vehicle are unknown. A simple relation is derived for the ratio between the attacker-target range (and time to impact) at counterweapon launch and the range at interception of the attacker by the counterweapon. Normalized curves are presented for some representative cases of defense by a counterweapon of known speed against a torpedo or missile attack on a moving target. The equations are shown to reduce to the familiar form for a stationary target.